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Commonly used austenitic stainless steels (ASSs) have some limitation in sliding wear conditions due to their relatively low yield strength and hardness. To improve the wear resistance, three kinds of microstructure (coarse grain (CG), heterogeneous structure (HS), and ultrafine grain (UFG)) are prepared, to investigate the grain size on the dry sliding tribological behavior, as well as wear mechanisms of Fe-18Cr-8Ni ASSs at room temperature (RT) and cryogenic temperature (−120 °C). The results indicate that at RT the UFG specimen exhibits the lowest wear rate during the wear tests, where the wear mechanisms are mainly oxidation wear and abrasive wear. While, when tested at −120 °C, the CG specimen exhibits the best wear resistance compared with that of the other two specimens due to its superior plastic deformation ability and strain hardening ability. Moreover, the HS specimen exhibits the lowest coefficient of friction (CoF), which is due to the abrasive particles generated on the contact surface provide a certain level of friction reduction, while the wear rate increases as these particles serve as third-party abrasives, which further removing material. • The ultrafine grained structure exhibits the best wear resistance at RT. • The wear resistance of coarse grained structure improves at −120 °C. • The heterogeneous structure exhibits the lowest CoF at −120 °C. [ABSTRACT FROM AUTHOR]

Irradiation damage and thermal aging greatly affect the phase boundary microstructure and stress corrosion cracking of austenitic stainless steel weld metals (ASSWMs) in water-cooled nuclear reactors. However, the effects of irradiation plus thermal aging (I + A) on the phase boundary segregation and phase changes remain unclear. Phase changes and elemental segregation at the carbide and carbide-free phase boundaries of 308 L ASSWMs after I + A treatment were investigated using atom probe tomography and transmission electron microscopy. The I + A treatment induced Si depletion at the phase interfaces of δ-ferrite/austenite (δ/γ) and δ-ferrite/carbide (δ/C) and also induced Ni enrichment and Cr depletion with concentrations having the order Ni/Cr(δ/γ) > Ni/Cr(δ/C) > Ni/Cr(γ/C). The solute-defect binding model and the vacancy mechanism were applied to explain the phase boundary segregation. Furthermore, the I + A treatment affected microstructural evolution near the phase boundaries; this reduced spinodal decomposition and inhibited G-phase and Ni/Si-rich-cluster formation. The phase separation of δ-ferrite near δ/γ and δ/C phase boundaries differed with the distance from the boundary, forming a gradient microstructure from phase boundaries to δ-ferrite internally. The gradual precipitation of the G-phase was observed beyond about 15 and 10 nm from the δ/γ and δ/C interfaces, respectively; moreover, the growth of α'-phase in the δ-ferrite was accelerated with increasing distance from the δ/γ and δ/C interfaces. [Display omitted] • I + A treatment induced Ni enrichment and Cr depletion at the phase boundaries. • I + A treatment enhanced spinodal decomposition within δ-ferrite, but reduced it near the phase boundary. • The G phases gradually precipitated at ∼ 15 and ∼ 10 nm away from the δ/γ and δ/C interfaces, respectively. • No G phase precipitated near phase boundary because of Ni deficiency for G-phase formation near δ/γ and δ/C interfaces. • The formation of Ni/Si-rich clusters near the phase boundary was inhibited after I + A treatment. [ABSTRACT FROM AUTHOR]

In engineering applications where high temperatures are a concern, the quest for structural components exhibiting robust thermomechanical stability is paramount. This study delves into the thermomechanical stability of 316 L steel fabricated via laser powder bed fusion (LPBF), with a particular emphasis on understanding the role of dislocation cell structures. In this study, deliberate rolling deformation followed by annealing at elevated temperatures was performed. The rolling deformation resulted in the introduction of dislocations and deformation twins in LPBF 316 L. Notably, deformation twins were observable in LPBF 316 L annealed at 800 °C, but they vanished at 1050 °C. Dislocation recovery was evident in pre-deformed samples following annealing at both temperatures. However, even after annealing at 800 °C, the grain and dislocation cell structures of heavily pre-deformed samples exhibited considerable stability. Conversely, during annealing at 1050 °C, substantial recrystallization occurred in pre-deformed samples, with the degree of recrystallization correlating with the extent of plastic deformation. In contrast, the grain structure of non-deformed samples remained unchanged after annealing at 1050 °C. This suggests that the dislocations induced by rolling serves as dislocation-stored energy, thus promoting recrystallization. The enduring stability observed in the grain and cell structures of heavily pre-deformed samples at 800 °C, as well as the high stability of these structures in non-deformed samples at 1050 °C, can be attributed to the presence of dislocation cell structure with low-energy contributing to retarded recrystallization. • The LPBF 316 L has enhanced thermomechanical stability due to the formation of low energy dislocation cell structure. • The existence of a dislocation cell structure reduces stress localization, thus enhancing the RX resistance of LPBF 316 L. • The high-density dislocations introduced by LPBF alone is insufficient to trigger RX of LPBF 316 L even up to 1050 °C. [ABSTRACT FROM AUTHOR]

Austenitic stainless steel is a promising candidate structural material for direct contact components with lead‑bismuth eutectic (LBE) in lead-cooled fast reactor (LFR). However, the preferential dissolution of Ni from austenitic steels at low oxygen concentrations will lead to the precipitation of Ni-containing phases in the low-temperature parts of reactors, which has a risk of pipe clogging. In this paper, three samples, comprising one extracted from a corrosion tank after prolonged operation and two prepared, respectively, by quenching in liquid nitrogen and cooling in the furnace, were obtained to investigate the effect of cooling rate on the precipitation characteristics and growth mechanism of Bi 3 Ni that forms by the reaction of dissolved Ni and Bi in LBE. The results showed that rod-like Bi 3 Ni was found in all three Ni-containing LBE samples. As the cooling rate increased, the width of Bi 3 Ni decreased. The Bi 3 Ni in all three samples displayed a pseudo-hexagonal prismatic shape and had a typical faceted growth character; the cooling rate had no effect on the morphology of them. The crystallographic analysis showed that the growth axis of Bi 3 Ni was in the 〈010〉 direction, and the prism planes were composed of {001} and {101}. In addition, the precipitation of Bi 3 Ni inevitably increased the content of Pb 7 Bi 3 , which would in turn enhance the volumetric expansion of solid LBE samples. For the growth mechanism, Bi 3 Ni phases were controlled by the screw dislocation mechanism, with the step growth rate of the dislocation core being greater than that of the outer edges. Given the low cooling rate and low Ni content in the LBE coolant of reactors, the precipitation of Bi 3 Ni with a rod-like morphology seems to be inevitable. • Three Ni-containing LBE samples were prepared with different cooling rates. • Regardless of the cooling rate, Bi 3 Ni in all three samples displayed a pseudo-hexagonal prismatic shape. • The Bi 3 Ni grew along the <010> direction, and the prism planes were composed of {001} and {101}. • Considering the low cooling rate and low Ni content, the precipitation of rod-like Bi 3 Ni seems to be inevitable. [ABSTRACT FROM AUTHOR]

Since cross rolling promotes the transformation of austenite to deformation-induced α΄-martensite and increases the dislocation density in the retained austenite, the reversion/recrystallization annealing after cross rolling might be more effective to achieve finer grain sizes in metastable austenitic stainless steels. Accordingly, the effect of cold rolling route and annealing for grain refinement and improvement of mechanical properties of AISI 316 L stainless steel was systematically investigated in the present work. It was found and formulated that by increasing strain, the grain refinement efficiency during annealing is improved, the effect being more pronounced if the cold rolling step is applied by cross rolling instead of unidirectional rolling. The evaluation of tensile properties revealed that all cold rolling and annealing routes lead to significant improvements of mechanical properties in terms of strength and plasticity, the influence of cross rolling being more prominent again. Moreover, the cross rolling and annealing route led to a remarkable strength-ductility balance, which revealed the opportunities that this route can offer for the improvement of mechanical properties of austenitic stainless steels. Accordingly, the results of the present work may shed light on the further utilization of cold rolling route and annealing treatment. [Display omitted] • Finer annealed grains obtained by cross rolling compared to unidirectional rolling. • Grain size formula based on reduction in thickness and rolling route was proposed. • Excellent strength-ductility balance was obtained by cross rolling and annealing. [ABSTRACT FROM AUTHOR]

The microstructure of 316 L austenitic stainless steel, welded with Co-based wires (Stellite 6 and Stellite 12), is closely related to its phase transformation, fracture, and cryogenic mechanical properties. The analysis involved tensile tests, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD). The results indicated that significant two-stage train hardening occurred in the matrix specimen at cryogenic temperatures, with a tensile strength of 1494 MPa compared to 578 MPa at room temperatures. This phenomenon was primarily attributed to the formation of numerous martensite particles, accompanied by fine twin lamellae. Ductile fracture predominated in the failure process of matrix specimens, under all temperature conditions. Co-based welding wires demonstrated strong metallurgical bonding with matrix materials. Specifically, the specimen welded with Stellite 6 exhibited an excellent combination of yield strength (596 MPa), tensile strength (887 MPa), and elongation (16%) at cryogenic temperatures. The cryogenic failure mechanism of the welded specimens involved a dimple and quasi-cleavage composite fracture. This work offers effective guidance for the use of 316 L austenitic stainless steel as a valve material in cryogenic environments. • Tensile tests of 316 L austenitic stainless steel and those welded with Co-based wires were conducted at the cryogenic temperature of −180 °C. • Martensite is formed after deformation, accompanied by fine twin lamellae, particularly at −180 °C. • The yield and tensile strengths of the matrix specimens were 452 MPa and 1494 MPa at −180 °C, respectively, compared to 213 MPa and 578 MPa at room temperature. • The specimen welded with Stellite 6 exhibited an excellent combination of a yield strength of 596 MPa, a tensile strength of 887 MPa, and an elongation of 16% at −180 °C. [ABSTRACT FROM AUTHOR]

High-throughput image segmentation of atomic resolution electron microscopy data poses an ongoing challenge for materials characterization. In this paper, we investigate the application of the polyhedral template matching (PTM) method, a technique widely employed for visualizing three-dimensional (3D) atomistic simulations, to the analysis of two-dimensional (2D) atomic resolution electron microscopy images. This technique is complementary with other atomic resolution data reduction techniques, such as the centrosymmetry parameter, that use the measured atomic peak positions as the starting input. Furthermore, since the template matching process also gives a measure of the local rotation, the method can be used to segment images based on local orientation. We begin by presenting a 2D implementation of the PTM method, suitable for atomic resolution images. We then demonstrate the technique's application to atomic resolution scanning transmission electron microscopy images from close-packed metals, providing examples of the analysis of twins and other grain boundaries in FCC gold and martensite phases in 304 L austenitic stainless steel. Finally, we discuss factors, such as positional errors in the image peak locations, that can affect the accuracy and sensitivity of the structural determinations. [Display omitted] • We develop the 2D-PTM technique to analyze atomic resolution electron microscopy images. • 2D-PTM matches local atomic peak positions to predefined templates. • The technique segments images into local structural regions and maps local rotations. • We demonstrate the 2D-PTM for polycrystalline Au and martensite phases in stainless steel. • Our analysis addresses sensitivity of the 2D-PTM to positional errors in peak location. [ABSTRACT FROM AUTHOR]

The effect of V on the high-temperature oxidation performance of austenitic stainless steel has been investigated by the constant-temperature oxidation weight gain approach, while the morphology, composition and elemental distribution of the oxide film are further analyzed by scanning electron microscopy and energy dispersive spectrum, transmission electron microscope and X-ray powder diffractometer as well as X-ray photoelectron spectroscopy, revealing the mechanism of the effect of V on the high-temperature oxidation performance of austenitic stainless steel. As a result, the addition of V has a significant adverse effect on the oxidation resistance of steel. Noteworthy, vanadium pentoxide generated during the oxidation process increases the internal stress of the surface oxide layer, reduces the integrity of the oxide layer and promotes oxygen penetration into the matrix. Simultaneously, the Z-phase formed at the grain boundaries of the matrix inhibits the diffusion of Cr ions, resulting in the inability to form an effective protective layer of Cr 2 O 3 oxidation on the surface. • The addition of element V affects the oxidizing properties of austenitic stainless steels at 850 °C. • The growth of V 2 O 5 causes cracking and dispersion of the oxide layer. • The Z-phase hinders the formation of protective oxide film chromium oxide. [ABSTRACT FROM AUTHOR]

In this work, the effects of strain rate on dynamic hot deformation behavior and microstructure evolution of Fe-Mn-Cr-N steel were studied under quasi-static and dynamic loading conditions using a Gleeble-3800 thermal-mechanical simulator and Split-Hopkinson pressure bar (SHPB) system. The stress-strain curves show that the yield and flow stresses are both significantly dependent on strain rate, also the strain rate sensitivity increases from 0.050 at a low strain rate (0.001–0.1 s−1) to 1.507 at a high strain rate (4500–5500 s−1). At a low strain rate (0.001–0.1 s−1), the microstructure evolution is dominated by dislocation slip and dynamic recovery (DRV). When strain rates are increased to 2500–3500 s−1, dislocation slip and mechanical twining together coordinate plastic deformation. When the strain rate was 4500–5500 s−1, the dynamic recrystallization (DRX) based on twinning formed in the deformation bands, and its fraction increased with the increasing strain rate due to adiabatic temperature rise (ΔT). The high micro-hardness value was obtained mainly because of the strain rate hardening (2500–3500 s−1), but the decrease at the strain rate of 4500–5500 s−1 is primarily attributed to recrystallization softening after deformation. The modified Johnson-Cook (J-C) model considering adiabatic temperature rise agrees with experimental results. It can be used to predict the plastic deformation behavior of Fe-Mn-Cr-N steel under high strain rate loading. • The effects of strain rate on dynamic hot deformation behavior and microstructure evolution of Fe-Mn-Cr-N steel were studied under quasi-static and dynamic loading conditions. • There are three distinct zones of strain rate sensitivity, with m values increasing from 0.050 to 0.480 and 1.507. • At a low strain rate (0.001∼0.1s-1), the microstructure evolution is dominated by dislocation slip and DRV. When strain rates are increased to 2500∼3500s-1, dislocation slip and mechanical twining together coordinate plastic deformation. When the strain rate was 4500∼5500s-1, the DRX based on twinning is activated. • The modified J-C model can macroscopically predict the flow behaviors during deformation with a high strain rate, which takes into account the effect of adiabatic temperature rise. [ABSTRACT FROM AUTHOR]

δ-ferrite is usually found in the 300 series austenitic stainless steels (ASS), but the role of δ-ferrite on corrosion behaviors exposed to oxygen-controlled lead–bismuth eutectic (LBE) has not yet been elucidated. The corrosion behaviors of AISI 316 ASS containing δ-ferrite in the oxygen-saturated static LBE at 550 °C were studied in terms of their solution-treated and accelerated-aging treated conditions. The δ-ferrite and decomposed δ-ferrite exhibit the superior resistance to oxidation attack compared to the austenite, and the δ-ferrite possesses a better corrosion resistance than the decomposed δ-ferrite. Compared with the austenite, the higher diffusivities of Cr and Si in δ-ferrite cause a reduction in the critical concentration required for the formation of chromium oxide and Si-rich oxide. A mixture of Cr 2 O 3 oxide and Fe-Cr-Si spinel oxide developed on the surface of δ-ferrite can effectively impede the metal transfer, leading to the superior resistance to oxidation attack. The retained δ-ferrite and M 23 C 6 carbides in the decomposed δ-ferrite oxidizes independently into FeCr 2 O 4 and Cr 2 O 3 oxides, respectively. The presence of Cr 2 O 3 scale around the retained δ-ferrite decreases its oxidation rate, resulting in the non-uniform oxide phenomenon. • δ-ferrite and decomposed δ-ferrite exhibit superior resistance to oxidation attack as compared to austenite. • δ-ferrite possesses a better corrosion resistance than the decomposed δ-ferrite. • A mixture of Cr 2 O 3 and Fe-Cr-Si spinel oxide developed on δ-ferrite can impede the metal transfer, leading to the superior corrosion resistance. • The retained δ-ferrite and M 23 C 6 carbides in decomposed δ-ferrite oxidize independently, resulting in the non-uniform oxide phenomenon. [ABSTRACT FROM AUTHOR]

The solidification behavior and crystallographic texture of 316L austenitic stainless steel builds fabricated via laser-wire directed energy deposition additive manufacturing (AM) were investigated. Shielding gas set-up and build type (single-track vs. multi-track) were varied, which led to changes in the solidification conditions and the final build compositions. Primary δ-ferrite solidification is predicted based on equilibrium thermodynamic predictions of the bulk feedstock composition. However, the increased solidification velocity of the AM process promotes a solidification mode transition from primary δ-ferrite to primary austenite. Skeletal δ-ferrite and lathy δ-ferrite associated with primary δ-ferrite solidification and interdendritic δ-ferrite associated with primary austenite solidification were observed among the laser-wire directed energy deposition walls. Skeletal and interdendritic δ-ferrite exhibited a (001)δ // (001)γ parallel relationship with austenite and a {001} solidification texture aligned with the build direction for both austenite and δ-ferrite. Lathy δ-ferrite exhibited a Kurdjumov-Sachs orientation relationship, (101)δ//(1 1 ¯ 1)γ, with austenite due to austenite nucleation in the solid-state. It is shown that the solidification behavior of AM 316L is accurately represented by as-built composition data and a dendrite growth model, which combined, encompasses the compositional and solidification condition impacts on the solidification pathway. • Composition & solidification conditions impact microstructure of AM stainless steel. • δ-ferrite morphologies form in AM components depending on solidification pathways. • Solidification mode can be predicted using as-built material composition. • Dendrite growth model effectively used to predict shift in solidification modes. [ABSTRACT FROM AUTHOR]

Nitrogen addition is known to be one of the most effective ways of strengthening austenitic stainless steels while maintaining good properties of the steels. This study investigates Nb addition as an additional strategy to strengthen a high nitrogen austenitic stainless steel. It was found that Nb-containing steels developed Z -phase precipitates during homogenization and hot-rolling. Their fraction and size hardly changed by subsequent aging at 1050 °C. They effectively delayed grain coarsening and fine grains were maintained throughout the aging. The yield strengths of Nb-containing steels were always higher than those of Nb-free steel by 30–83 MPa. Quantitative analyses revealed that the major strengthening mechanism was grain boundary strengthening. The contribution from precipitation hardening was small because the precipitates were not sufficiently fine and the negative contribution from solid solution strengthening due to N consumption by the precipitation was also insignificant. • Nb addition generated Z -phase and suppressed grain coarsening. • Z-phase formation slightly weakened solid solution strengthening. • Precipitation hardening by Z-phase was insignificant due to its size. • Nb contribution to yield strength of austenitic stainless steel was quantified. • Nb addition increased yield strength mostly by grain boundary strengthening. [ABSTRACT FROM AUTHOR]

A widely used approach for extending the fatigue life of engineering steels is through the introduction of compressive residual stress (CRS). The thermal stabilities of the introduced residual stress may determine their effectiveness in high-temperature application, which had been extensively studied by ex-situ technology in the past two decades. Here, the in-situ heating techniques of synchrotron XRD, XRD and TEM were used to study the evolution of CRS and corresponding microstructural evolution in a gradient nanostructured (GNS) austenitic stainless steel. As the temperature increased upto 500 °C, the magnitude of CRS decreased continuously by 63% when the phase composed of 100% strain-induced α'-martensite in the GNS layer, while it was decreased by 23% in the dual-phases composed of 60% α' and 40% γ. This result was attributed to the fact that the CRS introduced by the α'-martensite belongs to the type III micro-stress and was stored in the dislocations, which shows a high recovery trend upon heating. However, the CRS in the dual-phase structures is the type II intergranular stress, which is relatively stable upon heating. The current work suggested the importance of α'-martensite on the stabilizing of CRS, which may guide the materials design for high-temperature application. • Thermal stability of RS in GNS steel was investigated by in-situ XRD and TEM. • RS relaxation in single α' phase material is higher than the dual-phase of α' + γ. • Compressive RS of the dual-phase (60%α' + 40%γ) is stable upon heating. • RS relaxation in 100% α'-bcc attributed to the dislocation annihilation at 500 °C. • The stable of RS in dual-phase is due to the stable phases at 500 °C. [ABSTRACT FROM AUTHOR]

Except the excellent oxidation and corrosion resistance at elevated temperatures, the high temperature microstructure stability of high Si austenitic stainless steels is also of attention. In this work, the microstructure and the corresponding mechanical properties evolutions of a high Si austenitic stainless steel during long-term aging at 510 °C were investigated. The results showed that Si has promoted the formation of a multi-phase, composed by G phase and ferrite. It is revealed that G phase precipitates at grain boundaries prior to ferrite and it is because G phase formation has changed the composition in the surrounding austenite that ferrite was induced. Such ferrite is proved to be responsible for the dramatically decreased impact toughness during aging, since the impact toughness could be rejuvenated by only reducing the ferrite content through high temperature annealing. • G phase precipitates prior to ferrite and it is because G phase formation has changed the composition in the surrounding austenite that ferrite was induced. • The impact toughness could be rejuvenated by only reducing the ferrite content through high temperature annealing. • Si is not only a G phase and ferrite forming element, but also enhances the precipitation driving force for the ferrite. [ABSTRACT FROM AUTHOR]

Dynamic strain aging (DSA) has long been seen in the solution-treated austenitic stainless steels, but DSA effect is affected after long-term exposure to high temperature in the fourth-generation nuclear reactors. This study investigated the effect of precipitates on the DSA effect of 316 austenitic stainless steel after accelerated aging at 750 °C for 10, 100 and 1000 h, which is used to simulate the long-term microstructural evolution at service temperature. A gradual reduction in the DSA effect is found with an increase in the grain boundary precipitates after aging for 10 and 100 h. The frequency of the interactions between dislocations and grain boundaries decreases because moving dislocations are blocked by the precipitates formed at the grain boundaries. Meanwhile, the grain boundary precipitates occupy part of the grain boundaries, reducing the probability of dislocation emission from grain boundary. As the aging time prolongs to 1000 h, the newly-formed intragranular σ phase acts as an obstacle to the dislocation movement, resulting in an increase in waiting time (t w-AG1000). Also, Cr atoms can be drained from mobile dislocation into σ phase, leading to an increase in the aging time (t a-AG1000) required for pinning the arrested dislocations. As a result, the critical strain for the occurrence of DSA significantly increases, leading to the absence of DSA phenomenon. • Grain boundary M 23 C 6 carbide and η phase form preferentially after aging at 750 °C for 10 h, and σ phase and η phase form in the grain besides the grain boundary precipitates after 1000 h. • A gradual reduction in the DSA effect with an increase in the grain boundary precipitates is induced by changing the dislocation-grain boundary interactions. • The dislocation-intragranular σ phase interactions increase the aging time required for pinning the arrested dislocations, leading to the absence of DSA effect. [ABSTRACT FROM AUTHOR]

The effect of recrystallization annealing on the grain boundary assembly of a 316 L-type austenitic stainless steel produced by selective laser melting was studied. The primary recrystallization readily developed upon heating to 1373–1473 K. Using the orientation imaging microscopy, the recrystallization development could be evaluated by means of the grain orientation spread and/or the kernel average misorientation. The recrystallization kinetics was mainly controlled by the grain nucleation rate rather than the grain growth rate. The recrystallized microstructures were characterized by large fractions of special Σ3n CSL (coincident site lattice) grain boundaries above 0.7 that was associated with a high number density of annealing twins. The change in the fraction of Σ3n SCL boundaries could be expressed by unique linear function of the recrystallized fraction irrespective of annealing temperature. The development of Σ3n CSL boundaries interrupted the network of ordinary grain boundaries, when the number of triple junctions with 3 ordinary boundaries decreased and that with 1 ordinary and 2 Σ3n CSL boundaries increased. An increase in the fraction of triple junctions with 1 ordinary and 2 Σ3n SCL boundaries clearly correlated with the recrystallized fraction. • Recrystallization developed at 1373–1473 K in a 316 L steel produced by 3D printing. • The fraction of twin-related boundaries increased above 0.7 during recrystallization. • The fraction of special boundaries linearly increased with the recrystallized fraction. • Numerous twin-related boundaries interrupted the network of ordinary grain boundaries. • The fraction of triple junctions with 2 or 3 special boundaries increased to 0.5. [ABSTRACT FROM AUTHOR]

The creep-resistant austenitic stainless steel Alloy 709 (Fe-20Cr-25Ni (wt%) based steel) is being investigated as a candidate structural material for the next generation fast neutron reactors at service temperature of 500–550 °C. However, the study of microstructural evolution of Alloy 709 during aging is lacking. In this study, thus, the microstructure of Alloy 709 has been investigated using electron microscopy in the as-received state and after static aging at 550, 650 and 750 °C. The results show that the prominent precipitate in the as-received Alloy 709 is Nb(CN), with the rod-like Z phase (CrNbN) observed very occasionally. After aging at 550 °C even up to 2000 h, no significant microstructure change was observed, which means that Alloy 709 is fairly stable at 550 °C. Aging at 650 °C produced globular M 23 C 6 phase on grain boundaries, plate-like M 23 C 6 carbides at twin boundaries and in the grain interior, and blocky M 23 C 6 carbide nucleated on Nb(CN). Fine dispersoid Z phases were found on dislocations after aging at 650 °C for 500 h; their amount increases with aging time and temperature. (Cr,Mo) 3 (Ni,Fe) 2 SiN θ phase forms at grain boundaries after aging at 650 °C for 1000 h. After aging at 750 °C, θ phase nucleated on M 23 C 6 carbide and a transformation of M 23 C 6 to θ phase was found, which suggests that θ phase is the more stable. Aging at 550 °C promotes the segregation of Cr and Mo to grain boundaries whereas clear Cr depletion was observed at 650 °C due to the precipitation of Cr-rich M 23 C 6 carbides at grain boundaries. Such depletion nearly disappears at 750 °C. The implications of aging for the subsequent mechanical behaviour of Alloy 709 are discussed briefly. • No obvious microstructure change was observed after aging at 550°C even up to 2000h. • Aging at 650°C produced M23C6 phase while θ phase forms after aging for 1000 h. • At 750°C, θ phase nucleated on M23C6 and a transformation of M23C6 to θ phase was found, suggesting that θ phase is the more stable. • Fine dispersoid Z phases were found on dislocations after aging at 650 and 750°C. [ABSTRACT FROM AUTHOR]

A study on elemental composition, phase composition, microstructure, nanohardness and fracture micromechanisms of the surface layers obtained by ion-plasma treatment of a stable CrNiMo austenitic stainless steel with different initial microstructures has been carried out. In cold-rolled specimens, containing a high dislocation density and deformation-induced low- and high-angle boundaries and possessing structural elements of 330 nm in size, interstitial concentration (carbon and nitrogen) is much higher than that for cold-rolled and annealed specimens with a similar size of structural elements. After ion-plasma treatment, a surface nanohardness of the pre-deformed specimens is higher by 8 GPa than one for specimens annealed after cold rolling. Deformation-assisted microstructure accumulates carbon more likely than nitrogen under ion-plasma treatment. These results provide experimental support for decisive role of high dislocation density and deformation-assisted well-developed microstructure in accumulation and bulk diffusion of interstitials under ion-plasma treatment of steel. Unlabelled Image • Pre-deformation stimulates accumulation of interstitials under ion-plasma treatment. • Deformation-assisted defects suppress bulk diffusion of carbon under IPT. • Deformation-associated microstructure favors decomposition of austenite under IPT. • As-deformed specimens possess higher surface nanohardness after IPT compared to annealed one. [ABSTRACT FROM AUTHOR]

A high-N austenitic stainless steel (HNASS) was fabricated using a novel approach, in which duplex stainless steel powders were firstly gas nitrided in the temperature range of 700–900 °C and then consolidated using spark plasma sintering, resulting in an FCC matrix with average grain sizes of 1.6–8.2 μm. The Cr 2 N precipitates transformed from cellular precipitates (lamellar within the grains) to intergranular precipitation (equiaxed along the grain boundaries) as the nitriding temperature rose from 700 °C to 750 °C, and the N content rose from 0.68 wt% to 1.71 wt%. The 700 °C-treated alloy exhibits a yield strength of 783 MPa, a high tensile strength of 1091 MPa, and an elongation of 41% while the 900 °C-treated alloy exhibits an ultra-high compressive strength of 3017 MPa, and hardness of 520 HV. These values dramatically exceed the published values of similar HNASSs. Quantitative calculations showed that these superior mechanical properties could be attributed to increased solid solution and grain boundary strengthening. Furthermore, it can be inferred that while the equiaxed Cr 2 N phase increased the yield strength, it greatly degraded the fracture toughness. • A novel method, nitriding and followed spark plasma sintering, was used to fabricate HNASS. • The morphology of Cr 2 N phase changed from lamellar (within the grains) to equiaxed (along the grain boundaries). • The alloy features superior mechanical properties of yield strength (783 MPa) and ultimate tensile strength (1091 MPa). • Solid solution and grain boundary strengthening are dominant strengthening mechanisms. [ABSTRACT FROM AUTHOR]

This work focuses on the role of boron as a sintering additive in powder metallurgy processing of the AISI 316L austenitic stainless steel. Boron effect was investigated regarding densification, microstructure, hardness and micro-scale abrasion resistance. Samples were prepared with 0.6 wt% and 0.8 wt% of boron and sintered at 1240 °C. The microstructure of the sintered bodies was characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), and Rietveld refinement of X-ray diffraction (XRD) patterns. Differential scanning calorimetry (DSC) was used to study the behavior of both the free and boron-containing 316L samples at solid and liquid states. The hardness of austenitic matrix and precipitated borides was determined by instrumented indentation tests. EDS and XRD results revealed the formation of M 23 (B,C) 6 carboboride and hard M 2 B borides, which were concentrated mainly in intergranular sites. The formation of M 2 B borides favored material densification through the development of a liquid phase during sintering, promoting an increase in density, hardness and micro-scale abrasion resistance of the AISI 316L steel. • Pro-eutectic Cr-rich and eutectic Mo-rich borides are formed in B-alloyed ASS 316L. • Orthorhombic Cr-rich and tetragonal Mo-rich were found by XRD Rietveld refinement. • Hardness of borides and austenitic matrix was accessed by depth-sensing indentation. • Boron addition enhanced density, hardness and micro-abrasion resistance of the ASS 316L. [ABSTRACT FROM AUTHOR]

Understanding mechanisms of the nucleation and transformation sequence is of critical importance to control temperature-induced martensitic transformations and hence retained austenite. In the present work, quasi-in-situ Electron Backscattering Diffraction and Transmission Electron Microscopy were employed to investigate the nucleation and transition in a prototype austenitic stainless steel. Detailed evidence was provided, that nucleation could occur on new defects generated during cryogenic treatment, and not just on pre-existing defects. Moreover, transformation follows both austenite → ε-martensite → α′-martensite and austenite → α′-martensite routes, their associated mechanisms, and crystallographic characteristics were analysed. These new insights could provide further guidelines for the optimization and modelling of martensitic transformation. Unlabelled Image • The temperature-induced martensitic nucleation behaviour and transformation process were investigated in stainless steel. • New generated defects were found and they could provide the martensitic nucleation sites. • For γ→ε→α' sequence, the detailed transformation process was found as γ→faulted γ→ε→α'. • For γ→α' sequence, the detailed transformation process was found as γ→faulted γ→stacking faults bundles→α'. • The ε and α' martensitic transformation followed S N and K S relationship respectively. [ABSTRACT FROM AUTHOR]

The irradiation-induced precipitation and inverse coarsening of G-phase in austenitic stainless steel weld metal were studied by using transmission electron microscopy and atom probe tomography. Following 0.5-dpa (displacement per atom) irradiation, Ni/Si/Mn/P-enriched G-phase was observed in delta-ferrite. By increasing the irradiation dose from 0.5 to 3 dpa, coarsening of G-phase occurred at first, followed by an obvious inverse coarsening. The segregation of Ni and Si atoms at α/α′ interface due to spinodal decomposition was directly observed, which contributed to the G-phase precipitation. The inverse coarsening of G-phase was caused by irradiation recoil and could be interpreted by the cell model. • Irradiation induced precipitation of Ni/Si/Mn/P-rich and Cr-depleted G-phase. • Coarsening and inverse coarsening of G-phase occurred by increasing dose. • Segregation of solute atoms at α/α′ interface contributed to G-phase precipitation. • Inverse coarsening of G-phase was dominated by irradiation recoil. [ABSTRACT FROM AUTHOR]

Abstract A complete understanding of 'cyclic plasticity induced transformation' of austenitic stainless steels is documented in this article through different experiments, their critical analysis and comprehensive literature review. The formation and nucleation micro–mechanisms of deformation induced martensites (i.e., ϵ (hcp) and α / (bcc)) have been thoroughly examined by analytical transmission electron microscope after cyclic plastic deformation of AISI 304LN austenitic stainless steel at various total strain amplitudes (Δ ϵ t) under ambient environment. Magnetic measurements are performed for all fatigue failed specimens at different strain amplitudes. The measured characteristic of magnetization is sensitive to employed strain amplitude, corresponding deformed microstructural features and cyclic plastic response of the material. As strain amplitude increases, the extent of α / (bcc) martensite increases. It has also been found that deformation induced martensites can nucleate at a number of sites through multiplicity of mechanisms with different transformation sequences, i.e., γ (f c c) → ϵ (h c p) , γ (f c c) → ϵ (h c p) → α / (b c c) , γ (f c c) → deformation twin → α / (b c c) and γ (f c c) → α / (b c c). It is also investigated that these martensites can nucleate at a number of sites, such as: shear band intersections, isolated shear bands, grain boundary triple junctions, shear band–grain boundary intersections, dislocation piled–up etc., wherever total interaction energy favours to form. Deformation induced dislocation sub–structures are also characterized to show that cyclic plasticity towards higher strain amplitude loading results in the organization of more uniform dislocation cell structure. Dislocation sub–structure evolution (i.e., arrays, planar, veins, patches, ladders, wall, labyrinth, maze and cell) is also used to explain the shape of hardening/softening curves of the material for all strain amplitudes. The proposed study also quantitatively connects the strain amplitude dependency of the formation of deformation induced martensite and corresponding organizations, configurations and patterning of dislocation sub–structures. Two-dimensional striation spacing and crack density quantified from the fatigued fracture surfaces are also observed to predict the nature of variation in cyclic plastic response of the material as a function of strain amplitudes. Graphical Abstract [ABSTRACT FROM AUTHOR]

Abstract The microstructural evolution, strength, ductility change, and precipitates in the base metal and coarse-grain heat-affected zone (CGHAZ) of high nitrogen austenitic stainless steel with different thermal cycles were investigated. The research results indicate that the grain size of austenite and the precipitates reduced while the strength improved with increasing cooling rate. Furthermore, the shape and formation location of the precipitates that were proved to be M 23 C 6 and Cr 2 N by transmission electron microscopy (TEM) varied with the cooling rate. M 23 C 6 precipitated from the catenary to the round type and from the grain boundary to the grain interior with increasing cooling rate, causing the transformation of the fracture mode from an intergranular fracture to a transcrystalline fracture. Cr 2 N precipitated cellularly and was detrimental to the mechanical properties. The low cooling rate (<5 °C/s) deteriorated the strength and ductility owing to the mass of intermetallic compounds, while the properties of the simulated specimens above 5 °C/s were better than those of the base metal. However, the ductility was the best at 5 °C/s and the fracture surface consisted of many deep dimples. Meanwhile, the grains deformed and increased as the cooling rate changed from 1 °C/s to 30 °C/s according to electron backscatter diffraction (EBSD). Further, the results of Taylor factor are consistent with the variation in ductility. Highlights • The improved strengths are obtained with the increase of cooling rate. • Change of precipitates with cooling rate is quantitatively analyzed. • The mode between microstructure and mechanical property is established. • The fracture behavior and strengthening mechanism are studied. • Taylor factor can predict the change of the elongation and explained ductility change. [ABSTRACT FROM AUTHOR]

Abstract Isothermal aging treatments in the temperature range from 550 to 1150 °C for the aging time range from 2 to 2880 min were carried out on 20Cr-24Ni-6Mo super-austenitic stainless steel plates, which were processed by conventional casting and twin-roll strip casting (TRSC). Precipitation behaviors were comparatively studied, from which Time-Temperature-Precipitation (TTP) curves were developed. The results show that two TTP curves basically exhibited C-shapes and each TTP curve was the superposition of sigma and Laves precipitation kinetics curves. When the temperature was higher than about 825 °C, TTP curves were mainly sigma precipitation kinetics curves. On the contrary, TTP curves were mainly sigma and Laves precipitation kinetics curves. Aging temperatures of the precipitation sensitive interval were about in the range from 875 to 1025 °C. The nose temperature of conventional specimen was about 925 °C and the corresponding incubation time was about 2 min. However, the nose temperature of cast strip (CS) specimen was about 950 °C and the corresponding incubation time was about 4 min. TTP curve of CS specimens moved to the upper right as compared to that of conventional specimens, indicating that the precipitation in CS specimens was delayed. By analyzing the compositional distribution in the conventional and CS specimens after solution treatment, it was found that after the completion of solution treatment, the distribution of alloying elements in the CS plate was more homogeneous than that in conventional plate. Therefore, the conventional specimen was more susceptible to the formation of precipitates in aging treatments. Highlights • Precipitation kinetics for conventional and cast strip (CS) specimens was established. • TTP curves of conventional and CS specimens basically exhibited C-shapes. • Each TTP curve was the superposition of σ and Laves precipitation kinetics curves. • The sensitive precipitation processes were about 925 °C & 2 min for conventional specimens and 950 °C & 4 min for CS specimens. • σ was detected at 650–1050 °C, while Laves was mainly detected below 825 °C. [ABSTRACT FROM AUTHOR]

This study investigates the mechanical properties and deformation behavior of 2205 duplex stainless steel (DSS) through a two-stage solution (TSS) process. The TSS process involves heat treatment at 1380 °C for 20 min, followed by 1000–1200 °C for 20 min. This process achieves a favorable balance between strength and plasticity, resulting in a yield strength ranging from 585 to 607 MPa, an ultimate tensile strength ranging from 796 to 802 MPa, and a fracture elongation ranging from 28.4% to 32.8%. The activation and transmission of slip systems within the constituent phases, as well as the interaction between dislocations and different interfaces, are examined via in-situ electron backscatter diffraction, in-situ scanning electron microscopy, and transmission electron microscopy. The results indicate that slip transfer is the primary mechanism by which austenite and ferrite adapt to coordinated deformation in DSS 2205. Furthermore, the possibility of dislocation slip across boundaries is assessed using the Luster-Morris parameter (m'). When m' exceeds 0.81, activated dislocations within the austenite grains can traverse the Kurdjumov-Sachs (K S) and Nishiyama-Wassermann (N W) interfaces, promoting coordinated dislocation slip in the ferrite matrix. Dislocation motion remains uninhibited at the ferrite-austenite interface, while accumulation at the ferrite grain boundaries leads to localized stress concentration. • In situ studies were conducted to investigate the microstructure evolution and deformation behavior of 2205 duplex stainless steel with equiaxed austenitic domains. • Slip transfer across ferrite and austenite grain boundaries was investigated in duplex stainless steel using slip trace analysis. • Multiple slip systems were activated on the surface grains, with no strict correlation to Schmid factors, but were constrained by the surrounding grains due to cross-grain slip transfer. • Dislocations tend to pile up at the ferrite grain boundaries, while slip transfer occurs at the austenite grain boundaries and ferrite-austenite phase boundaries. [ABSTRACT FROM AUTHOR]

Deformation-induced martensitic transformation (γ-austenite → ε-martensite/α'-martensite) in austenitic steels has garnered significant interest owing to its transformation-induced plasticity effect. To elucidate the orientation-dependent intricate γ/ε/α' phase microstructure at deformation-induced ε-ε intersection, a single crystal of 316 austenitic stainless steel was compressed along the [001] γ axis at a cryogenic temperature (173 K). Electron backscattered diffraction analysis was employed to reveal the deformed microstructure on the (110) γ surface. A hierarchical triple phase structure was discovered at ε – ε intersection, where the γ rotated 90° from the matrix (γ 90), { 10 1 ¯ 2 } ε-twin, and α' phases coexist. Depending on the operative shear angle with a common intersection axis, either 90° (Type I) or 30° (Type II), three distinct atomic rearrangements of the intersection volume were observed: γ 90 was present at Type I intersection, and α'-phase was developed at Type II intersection, respectively. { 10 1 ¯ 2 } ε-twin also occurred at Type I intersection, serving as an accommodation mechanism alongside the intersection γ 90. EBSD results confirm the Shoji–Nishiyama (S N), Kurdjumov–Sachs (K S), and Burgers (B) orientation relationships within the complex γ – ε – α' triple phase structures at ε – ε intersection. Transformation paths for three intersection products were visualized by the unified tetrahedron model, considering T/2 or T/3 γ-twinning shear as an intermediate state. Moreover, a novel scheme is proposed to index the α'-martensite crystallographic variants at ε-ε intersections by establishing a correlation between the Bain distortion and double shear process. [Display omitted] • A hierarchical γ 90 /ε twin /α' triple-phase structure is discovered at ε-ε intersection. • Two types of ε-ε intersection are established, 90° (Type I) and 30° (Type II). • 90° rotated austenite (γ 90) and { 10 1 ¯ 2 }ε-twin are formed at Type I intersection. • α'-martensite is observed at Type II crossing shear intersection volume. • Unified tetrahedron model is built to visualize complex intersection structures. [ABSTRACT FROM AUTHOR]

This study employs a continuous scanning electron beam to deposit AlCoCrFeNi composite cladding on the surface of 253MA austenitic stainless steel to enhance its surface properties. The characteristics were examined through a scanning electron microscope (SEM), backscattered electron diffraction (EBSD), and white light interferometric three-dimensional surface profile instrument. This study investigated the effect of electron beam process parameters on the surface organization and mechanical properties of 253MA steel. The sample surface was divided into four zones: overlay, bonding, thermal influence, and substrate, according to the results. Coarse columnar and island-shaped crystals composed the overlay zone, while equiaxed austenite grains primarily constituted the substrate. With the increase of the beam current, the microstructure's grain size decreased, the substructures increased, and the BCC phase transformed into the FCC phase. The grain size slightly decreased, substructures changed marginally, and the BCC phase increased as the scanning speed increased. Additionally, the microhardness and mechanical properties of the overlay layer increased significantly, and wear volume and wear rate decreased progressively with the increase in beam current and scanning speed. At a beam current of 18 mA and a scanning speed of 200 mm/min, the surface hardness of 253MA steel reached the maximum value of 520 HV. When the beam current was 24 mA, and the scanning speed was 200 mm/min, the wear volume and wear rate of the overlay sample were minimal, accounting for 37.4% of the original sample's, and its wear resistance was 2.67 times more than that of the substrate. • Unique microstructure in cladding zone: coarse and island-shaped crystals; equiaxed austenite matrix. • Beam current refines grains, enhances substructure, transforms BCC to FCC phase. • Scanning speed causes minor grain refinement, limited substructure changes, increased BCC phase. • Higher beam current and scanning speed boost microhardness, mechanical properties, wear resistance. [ABSTRACT FROM AUTHOR]

Cu precipitation behavior of a Cr-Ni-Mn austenitic stainless steel containing a small amount of Cu during aging between 923 and 1023 K for up to 24 h was investigated by atom probe tomography with cluster-finding and interfacial segregation analyses. At 923 K, Cu precipitation started at 20 min of aging, forming uniformly dispersed nano-sized precipitates and the number density of precipitates reached the maximum at 2 h of aging, although the volume fraction of precipitates continued to increase with increasing aging time up to 24 h. The precipitate size steadily increased with increasing aging time and temperature. The growth rate of Cu precipitates observed in this study is significantly lower than that in previous studies of other austenitic stainless steels, although the isothermal growth behavior obeys Lifshitz-Slyozov-Wagner theory. It was noted that both Ni and Mn were enriched near precipitate/matrix interfaces, reducing the interfacial energy as a result. This interfacial segregation seems to effectively reduce the growth rate of Cu precipitates during aging, which was significantly lower than that reported for other austenitic steels. • Cu precipitation behavior in an austenitic stainless steel was investigated using APT. • The highest number density of Cu precipitates was obtained at 2 h of aging at 923 K. • Ni and Mn segregation occurred near interfaces between Cu precipitates and matrix. • Growth rate of Cu precipitates was relatively low compared with other austenitic steels. • The relatively slow growth is attributed to lower interfacial energy due to the segregation. [ABSTRACT FROM AUTHOR]

The ability to characterise the three-dimensional microstructure of multiphase materials is essential for understanding the interaction between phases and their associated materials properties. Here, laboratory-based diffraction-contrast tomography (lab-based DCT), a recently-established materials characterization technique that can determine grain phases, morphologies, positions and orientations in a voxel-based reconstruction method, was used to map part of a dual-phase steel alloy sample. To assess the resulting microstructures produced by the lab-based DCT technique, an electron backscatter diffraction (EBSD) map was collected within the same sample volume. To identify the two-dimensional (2D) slice of the three-dimensional (3D) lab-based DCT reconstruction that best corresponded to the 2D EBSD map, a novel registration technique based solely on grain-averaged orientations was developed – this registration technique requires very little a priori knowledge of dataset alignment and can be extended to other techniques that only recover grain-averaged orientation data such as far-field 3D X-ray diffraction microscopy. Once the corresponding 2D slice was identified in the lab-based DCT dataset, comparisons of phase balance, grain size, shape and texture were performed between lab-based DCT and EBSD techniques. More complicated aspects of the microstructural morphology such as grain boundary shape and grains less than a critical size were poorly reproduced by the lab-based DCT reconstruction, primarily due to the difference in resolutions of the technique compared with EBSD. However, lab-based DCT is shown to accurately determine the centre-of-mass position, orientation, and size of the large grains for each phase present, austenite and martensitic ferrite. The results reveals a complex ferrite grain network of similar crystal orientations that are absent from the EBSD dataset. Such detail demonstrates that lab-based DCT, as a technique, shows great promise in the field of multi-phase material characterization. [Display omitted] • A simultaneous lab-based DCT reconstruction, applied to a dual-phase alloy, is shown. • A metastable austenitic stainless steel is studied with lab-based DCT and EBSD. • A new registration method identifies common grains between DCT and EBSD datasets. • Complex microstructures can skew lab-based DCT measurements of grain size and shape. • Lab-based DCT can describe 3D texture and macrozones missed by 2D-EBSD. [ABSTRACT FROM AUTHOR]

This study explicates the microstructural evolution and mechanical response of the joints of 304 stainless steel developed through micro-deformation diffusion bonding. The diffusion-bonded interface comprised the refined grains embedded with the intergranular M 23 C 6 carbides and complex oxides, which was induced by dynamic recrystallization at 950 °C. The refined grains persisted at the bonding temperature of 950 °C due to the dynamic equilibrium of dislocations and pinning effect of intergranular particles. As the bonding temperature was raised to 1000 °C, the combination-grow up of grains was initiated caused by sufficient thermal activation and complete dissolution of M 23 C 6 carbides. The interface was entirely migrated at 1050 and 1100 °C, and notably, the migration of interfacial grain boundaries played a greater role in diffusion bonding with an increase of bonding temperature. The interface with refined grains permitted a high joint strength (or impact load), while was incompetent to enhancing plasticity and particularly impact toughness. The migrated interface, in contrast, was provided with exceptional plasticity and impact toughness, attributed to its enhanced resistance to crack propagation. The joint prepared at 1050 °C for 60 min exhibited the optimum combination of ultimate tensile strength, fracture elongation, and impact toughness, and the ductile fracture was observed passing through the substrate instead of along the interface. In addition, the comprehensive performance of joints was degraded when the grains were excessively coarsened at 1100 °C for 60 min. • The joints of 304 stainless steel were effectively diffusion-bonded under micro-deformation conditions. • The microstructural evolution was characterized comprehensively, and the corresponding mechanism was explicated. • The mechanical response of the joints was evaluated via tensile and impact tests, indicating highly sensitive to the microstructural characteristics. [ABSTRACT FROM AUTHOR]

In this study, the Computational Fluid Dynamics (CFD)-Finite Element Method (CFD-FEM) were used to simulate the transient stress response of the structure surface under thermal striping environment, and a uniaxial random cyclic loading (RCL) method was used to reveal the mechanical properties and initial microstructure evolution of 316H austenitic stainless steel (SS) under random stress induced by thermal striping. The experimental results showed that 316H exhibited a lower saturation ratcheting strain rate at 334.44 MPa, along with stronger cyclic hardening effects and resistance to plastic accumulation at loading rate of 100 Hz. Random loading tests with different cycles were performed at σ peak = 334.44 MPa and f R C L = 100 Hz , and ratcheted specimens were characterized to investigate the effect of the micro-mechanism on cyclic softening/hardening. The geometrically necessary dislocation (GND) density increases and then decreases with cyclic loading, which is mainly attributed to the deformation twin (DT) and multi-dislocation slip mechanisms. The deformation mechanism during the ratcheting cycle from 1000 to 3000 cycles is mainly dominated by the twinning reduction, where the dislocation migration accelerated the transformation from dislocation walls to subgrain boundaries; the twinning increase dominates from 3000 to 9000 cycles, where the subgrain boundary grew into boundaries with high angle misorientations. • Significant tensile ratchet deformation occurred when the peak stress was >274.14 MPa. • The cyclic hardening and suppressed ratcheting strain were observed due to asymmetric peak and valley loading. • The critical cycles of ratcheting strain growth rate decrease with the loading frequency. • Dislocations sunk by grain boundaries under random cyclic loading. • Twinning decreases at low cycles and increases at high cycles during RCL deformation. [ABSTRACT FROM AUTHOR]

In the presented research, the intergranular elastic interaction and the second-order plastic incompatibility stress in textured ferritic and austenitic steels were investigated by means of diffraction. The lattice strains were measured inside the samples by the multiple reflection method using high energy X-rays diffraction during uniaxial in situ tensile tests. Comparing experiment with various models of intergranular interaction, it was found that the Eshelby-Kröner model correctly approximates the X-ray stress factors (XSFs) for different reflections hkl and scattering vector orientations. The verified XSFs were used to investigate the evolution of the first and second-order stresses in both austenitic and ferritic steels. It was shown that considering only the elastic anisotropy, the non-linearity of sin2ψ plots cannot be explained by crystallographic texture. Therefore, a more advanced method based on elastic-plastic self-consistent modeling (EPSC) is required for the analysis. Using such methodology the non-linearities of cos2φ plots were explained, and the evolutions of the first and second-order stresses were determined. It was found that plastic deformation of about 1–2% can completely exchange the state of second-order plastic incompatibility stresses. [Display omitted] • The Eshelby-Kröner XSF agrees best with the experimental results. • First and second-order stresses can be simultaneously determined. • Second-order stresses are generated or modified only during plastic deformation. [ABSTRACT FROM AUTHOR]

In this study, the effects of laser shock peening (LSP) and surface mechanical attrition treatment (SMAT) were comparatively investigated on the oxidation behaviors of 20Cr-25Ni-Nb stainless steel in a high-temperature (650 °C) CO 2 environment up to 1000 h. The results show that the oxidation resistance of stainless steel can be improved by LSP but deteriorated by SMAT. LSP promoted the formation of a thin and uniform Cr-rich oxide layer on the 20Cr-25Ni-Nb stainless steel instead of a thick duplex oxide layer with Fe-rich outer layer and Cr-rich inner layer which appeared on the untreated sample. For the SMAT-treated sample, besides the oxide scale, a thick internal oxidation zone (IOZ) and oxides spallation were also observed. The formation of a Cr-rich oxide layer was facilitated by the enhancement of the Cr diffusion within the LSP-induced hardened layer. The unsatisfying oxidation performance of the SMAT-treated sample is due to the surface micro-cracks and significant grain refinement which synergistically contributed to the visible carburization and oxidation in a high-temperature CO 2 environment. • LSP improves the oxidation resistance of austenitic stainless steel in a high-temperature CO 2 environment. • The LSP-induced microstructural evolution promotes the formation of a single chromia layer by enhancing diffusion of Cr. • SMAT deteriorates the resistance of austenitic stainless steel to oxidation and carburization in a CO 2 environment. • SMAT-induced micro-cracks and ultra-fine grains synergistically contribute to the deterioration of oxidation performance. [ABSTRACT FROM AUTHOR]

Abstract We have performed an in-depth characterisation of the microstructure evolution of 20Cr-25Ni Nb-stabilised austenitic stainless steel during isothermal annealing at 930 °C using scanning and transmission electron microscopy. This steel grade is used as cladding material in advanced gas-cooled fission reactors, due to its resistance to thermal creep and water corrosion. The initial deformed microstructure undergoes recrystallisation via a strain-induced boundary migration mechanism, attaining a fully recrystallised microstructure after 120 s of annealing. The transition from low-to-high grain boundaries has already occurred after 15 s, together with an increase in the cube grain orientation at the expense of the S texture component. After 120 s, the grain boundary migration induces the formation of new fine Nb(C,N) particles, whereas the pre-existing particles become enriched in Ni and Si. The resulting particle population limits the grain growth in the austenitic matrix, based on the Zener pinning model, resulting in relatively small recrystallised austenite grains and a high density of high-angle and special coincidence-lattice-site grain boundaries, together with a large number of particle/matrix interfaces. Graphical Abstract Unlabelled Image Highlights • At 930 °C recrystallisation of deformed 20/25 Nb steel is completed after 120 s. • Recrystallisation proceeds by strain induced boundary migration. • An increase of cube oriented grains at the expense of S oriented is observed. • The boundary migration induced the precipitation of niobium carbides. • The grain size is limited by the particle dispersion according to Zener pinning. [ABSTRACT FROM AUTHOR]

Abstract The present study aims to investigate the impact of cold deformation on structure-property relationship in phase reversion-induced stainless steels. The commercial 18Cr-8Ni stainless steel was cold rolled at room temperature to 30%, 60% and 90% thickness reduction, respectively and subsequently annealed at 900 °C for 1–100 min. Evolution of phases in selected samples was identified and quantified by X-ray diffraction together with the corresponding microstructural characterization through optical, scanning and transmission electron microscopy, and electron backscatter diffraction. Mechanical properties of selected samples were determined by the tensile test. The results indicated that parts of metastable austenite transformed into strain-induced martensite during cold rolling process, and the volume fraction of martensite increased with increase in cold rolling reduction. The kinetics of reversion transformation from martensite to austenite was fastest in 90% cold-rolled sample during annealing treatment. The austenite grain size for the 90% cold-rolled sample after annealing was smallest and most uniform, which induced highest yield strength and elongation combination. Highlights • Parts of metastable austenite transformed into strain-induced martensite in 18Cr-8Ni stainless steel during cold-rolled process. • The kinetics of reversion transformation from martensite to austenite was fastest in 90% cold-rolled sample during annealing treatment. • The austenite grain size for the 90% cold-rolled sample after annealing was smallest and most uniform, which induced highest yield strength and elongation combination. [ABSTRACT FROM AUTHOR]

Abstract This study deals with the susceptibility to intergranular corrosion (IGC) in type 304 austenitic stainless steel from a crystallographic viewpoint. An IGC test was conducted using oxalic acid and sensitized stainless steel, whose crystal orientation was analyzed by electron backscattered diffraction (EBSD). GBs were classified according to the misorientation of adjacent grains, and subclassified using the Σ value based on the coincidence site lattice (CSL) model. The width of corroded groove was measured for each GB category, and the effect of GB category on the susceptibility to IGC was examined. Then, the area of the unit cell of CSL boundaries was used to characterize the susceptibility to IGC of CSL boundaries. It was found that no IGC occurred in low-angle GBs with a misorientation of <10°, while IGC occurred in some of the low-angle GBs with misorientations of 10°–15°. In large-angle GBs with misorientations of >15°, the width of corroded groove at each Σ boundary exhibited large scatter. The relationship between the width and unit cell area was characterized by a sigmoid curve, and the susceptibility to IGC was well described by the unit cell area. Highlights • Intergranular corrosion (IGC) in stainless steel was crystallographically discussed. • Grain boundaries (GBs) were classified based on the coincidence site lattice (CSL). • Influence of the Σ value on their IGC susceptibility had a wide range scatter. • IGC susceptibility was characterized by the Σ value and unit cell area of CSL GBs. [ABSTRACT FROM AUTHOR]

Abstract The present work investigated the effects of strong carbide-forming elements M (M = Nb, Ti, V, Ta) on the phase precipitation and oxidation resistance at 1073 K in alumina-forming austenitic (AFA) stainless steels (Fe-20Ni-(18–23)Cr-2.5Al-2.3Mo-0.08C-Nb/Ti/V/Ta wt%). The designed alloy rods were solid-solutioned at 1523 K for 1.5 h, and then aged at 1073 K for 24 h or oxidized at 1073 K for 500 h, respectively. Besides primary MC carbides, B2-NiAl, Fe 2 M, and σ-FeCr phases would be precipitated from the γ-austenitic matrix in these aged AFA alloys. Furthermore, the precipitation of σ-FeCr could be accelerated by the B2-NiAl formation, which is related to the γ stability. The formation of MC and Fe 2 M can be controlled via adjusting the ratio of M/C. The oxidation resistance of these AFA alloys can be improved effectively by V and Ta addition due to the formation of compact Al 2 O 3 protective scales. This work will provide a new approach to obtain a good combination of creep strength and oxidation resistance through co-alloying with Nb/Ta/V and controlling the M/C ratio within 1.0–2.0 (in molar ratio) simultaneously. Highlights • Effects of Nb/Ti/V/Ta on phase precipitation at 1073 K in AFA alloys are studied. • The precipitation of σ-FeCr can be accelerated by the formation of B2-NiAl. • The additions of V and Ta can improve the oxidation resistance of AFA alloys. • The precipitation of σ-FeCr will deteriorate the oxidation resistance. • Good performance can be achieved by M co-alloying and an appropriate M/C ratio. [ABSTRACT FROM AUTHOR]

In this work, the role of grain boundary engineering (GBE) on the weld heat-affected-zone (HAZ) liquation cracking resistance of austenitic stainless steel AISI 316Ti was investigated. Standard wrought-processed alloy 316Ti was cold rolled to 5% strain and subsequently annealed at 1373 K for 30 min to achieve the optimum grain boundary character distribution (GBCD). The GBE samples were found to consist of a significantly higher fraction (72%) of low Ʃ coincident site lattice (CSL) boundaries as compared to the as received (AR) samples (45%). To study the liquation cracking behavior, single-track longitudinal varestraint tests was performed on AR and GBE samples. Microstructural examination revealed that the grain coarsening and the liquation events were less prominent in the GBE samples as compared to the AR samples. It is believed that the reduced segregation of impurities to the special boundaries (coherent Ʃ3 twins, in particular) present in the GBE samples is responsible for the observed improvement in the HAZ liquation cracking resistance. [ABSTRACT FROM AUTHOR]

A micro-mechanism responsible for the occurrence of post-dynamic recrystallization (DRX) grain growth during deformation at T > 0.8T m is described. Thermo-mechanical testing, microstructural characterization and diffraction studies reveal the link between slip and grain growth. If deformation is imparted in a single stage, grains grow by a cooperative growth process involving grain rotation and alignment of slip planes. This mechanism changes when deformation is interrupted and controlled intermediate soaking is performed before re-loading. A premeditated control of mechanism is shown to yield favourable microstructural and mechanical properties. A strategy to apply this control to industrial hot deformation is outlined. [ABSTRACT FROM AUTHOR]

Plastic deformation of surface grains has been observed by electron backscatter diffraction technique during in situ tensile testing of a high-nickel austenitic stainless steel. The evolution of low- and high-angle boundaries as well as the orientation changes within individual grains has been studied. The number of low-angle boundaries and their respective misorientation increases with increasing strain and some of them also evolve into high-angle boundaries leading to grain fragmentation. The annealing twin boundaries successively lose their integrity with increasing strain. The changes in individual grains are characterized by an increasing spread of orientations and by grains moving towards more stable orientations with 〈111〉 or 〈001〉 parallel to the tensile direction. No deformation twins were observed and deformation was assumed to be caused by dislocation slip only. [ABSTRACT FROM AUTHOR]

Through-thickness microstructural variations and their synergistic influence on degree of sensitization in grain boundary engineering (GBE) treated 304L stainless steel was compared with solution-annealed and marginally cold-worked (~ 3% thickness reduction) specimens. The solution-annealed specimen has shown higher degree of sensitization due to the presence of lower fraction of Σ3 (as well as other low Σ CSL) boundaries and dominant random high-angle boundaries (RHAGBs) connectivity. However, it has shown thickness-independent sensitization behavior as grain boundary character distribution, residual strain and grain boundary network topology towards thickness direction are similar. Owing to higher residual strain, as revealed by local misorientation analysis, the cold-worked specimen showed maximum degree of sensitization (higher than solution-annealed condition) at surface. The GBE specimen has shown much lower degree of sensitization at surface due to higher-fraction of Σ3 boundaries and discontinuous RHAGBs connectivity. However, increasing fraction of RHAGBs and their connectivity, as confirmed by twinning-related domain and fractal analysis, cause more sensitization at interior of this specimen. In spite of this, the degree of sensitization at the interior of GBE specimen is lower than the solution-annealed or cold-worked specimen. Even though the residual strain in GBE specimen is higher than the solution-annealed condition, the lower degree of sensitization indicates that the grain boundary character distribution and grain boundary network topology have higher implication than residual strain in controlling sensitization. [ABSTRACT FROM AUTHOR]

This paper reports an in-situ observation on the twin boundary evolution and crack initiation behavior during the tensile test in 316L stainless steel by means of the electron backscattered diffraction (EBSD) method. The results indicated that the percentage of the twin boundaries gradually reduced with increasing elongation and the crack initiated at the twin boundary after a definite elongation. The difference in Schmid factor value between two grains besides twin boundaries continuously increased with increase of elongation, which led to movement of the dislocations within grains to twin boundaries. High density of the dislocations led to a larger local misorientation value difference between two sides of the twin boundaries, which indicated that those twin boundaries accumulated more strains during the tensile. As a result, the twin lost their coincidence site lattice with austenite grains, and twin boundaries transformed to random high angle ones, and the finally led to initiation of the crack at the high angle boundary where the twin boundary transformed. [ABSTRACT FROM AUTHOR]

In this study, fatigue crack growth rates in weld center and Heat Affected Zone (HAZ) of laser-MIG hybrid welded joint are compared using standard compact tension (CT) specimens. Electron backscatter diffraction (EBSD) is applied to analyze crack tip transformation zone by characterizing phases, plastic strains and crystallographic orientations. This study provides interesting insights into the growth of fatigue cracks in austenite steel welded joint at micro-scale. Analysis of the plastic strain and Schmid factor indicates that mechanical energy provided by external load transforms into internal crystal energy firstly, which is stored within grains in the form of strain energy, then partly relieves to provide mechanical driving force for martensite transformation. With plastic strain accumulating within individual grain, dislocations begin to glide on slip system resulting in formation of persistent slip bands (PSB), then metastable austenite grains transform into martensite. Thus, strain-induced martensite transformation increases resistance to crack growth by dissipating energy and crack closure effect. Different crack growth rate in weld center and HAZ is attributed to variant volume of strain-induced martensite. Shielding effect produced by neighboring harder ferrite decreases martensite in weld center, as confirmed by nanoidentation and Atomic Force Microscope (AFM). Additionally, martensite phase is found to have a Kurdjumov-Sachs (K-S) orientation relationship with austenite phase according to Inverse Pole Figure (IPF) and Pole Figures (PF) derived by EBSD. [ABSTRACT FROM AUTHOR]

Corrosion behavior of alloy 316L stainless steel capsule was studied by exposure to the supercritical water (SCW) at 500 °C and 25 MPa for 20,000 h. The microstructural observations have been conducted on the cross section of the exposed surfaces to the SCW to perceive the internal oxidation of the grains and/or grain boundaries. Transmission electron microscopy (TEM) observations as well as elemental analyses such as energy dispersive spectroscopy (EDS) and Electron energy loss spectroscopy (EELS) were used to study the internal oxidation and micro-crack initiation on the surface. Elemental analyses indicated that long-term exposure to the SCW resulted in formation of scales identified as Fe 3 O 4 (outer layer), Fe-Cr spinel/(Fe,Ni)Cr 2 O 4 /(Mn,Cr) 2 O 3 /SiO 2 (inner layer) on the substrate, and Ni-enrichment (chrome depleted region) in the alloy 316L. Micro-crack initiation was observed ahead of the oxidized grain boundaries in which elemental enrichments happened ahead of the crack tip. The relevance of the observed oxidation phenomena on the crack susceptibility of Alloy 316L was discussed. Finally, prolonging the exposure time up to 20,000 h has shown that the alloy 316L might be susceptible to micro-crack initiation in the supercritical water. [ABSTRACT FROM AUTHOR]

Metastable austenitic stainless steels are materials that can undergo austenite to martensite phase transformation when subjected to deformation and thus they represent a multiphase material with interesting mechanical properties. Different electron microscopy techniques are widely applied for the characterization of their deformation mechanisms at micrometric length scale. In doing so, Scanning Ion Microscopy (SIM) imaging, performed with a Focused Ion Beam (FIB), can be useful to evaluate microstructural features induced by different stress fields and, in certain cases, may substitute the conventional Transmission Electron Microscopy (TEM) technique. In this work, nanoindentation experiments (both monotonic and cyclic) were carried out on AISI 301LN metastable steel in order to induce localized deformation of individual austenitic grains. The activated plastic deformation mechanisms were evaluated by using different advanced characterization techniques (Electron BackScattered Diffraction (EBSD) and TEM), but mainly by FIB/SIM. FIB/SIM 3D-tomography was also conducted to reconstruct the deformation structure under the residual imprint. These observations, surprisingly, showed the existence of a good correlation between SIM and TEM images, concerning phase transformation and plastic zone development. [ABSTRACT FROM AUTHOR]

Electron backscatter diffraction (EBSD) was used to examine the microstructural evolution in a one-step thermomechanically processed 304 austenitic stainless steel specimen during the thermomechanical process of grain boundary engineering. Solution-treated materials were cold-rolled to 3% reduction and subsequently annealed at 1220 K for different annealing times. The EBSD observation of the specimen showed an increase in the frequency of coincident site lattice (CSL) boundaries and a decrease in the percolation probability of random boundaries. Additionally, the specimen exhibited heterogeneous growth of clusters of grains that contained a high frequency of CSL boundaries. These clusters of grains were developed in the entire observed area by strain-induced grain growth according to the results of grain orientation spread analysis. The details of the growth of the clusters and the disconnection of random boundaries were successfully observed in situ using EBSD and a heating stage. The frequency of CSL boundaries increased with the growth of the clusters. Disconnection of random boundaries between the clusters was achieved by the formation of annealing twins through the impingement of the growing clusters during the thermomechanical process. Twin variant selection to introduce CSL boundaries into a random boundary network was observed by the in situ EBSD observation. [ABSTRACT FROM AUTHOR]

This study has been aimed to find out the proper welding procedure/conditions for a newly developed Fe-16Cr-1Ni-9Mn-0.12N austenitic stainless steel (ASS), which is uniquely modified from 200 series alloy, in order to control the microstructural metastability of the weld metals. The welded joints of 4 mm thick sheet were prepared in a square butt configuration by using three modes of metal transfer, i.e., short-circuit (SC), spray (S), and pulse (P) mode of metal transfer and three austenitic filler metals (304L, 308L and 316L) under two shielding gas environments using gas metal arc welding (GMAW) process. It is observed that the variation in modes of metal transfer and filler metals can effectively manipulate the metastability of γ-phase, formation of δ-phase and grain size of the weld metals. Among the modes of metal transfer, pulse mode of metal transfer produces more metastable γ-phase and higher δ-phase fraction responsible for finer grain structure in weld metals irrespective of filler metals used. Again, higher microstructural metastability was obtained with the 304L filler metal irrespective of modes of metal transfer used. It is evident that the weld metals having higher microstructural metastability also improved mechanical properties (i.e. hardness, strength and toughness). However, weak fusion boundary (FB) region of welded joint limits the beneficial effect of metastability and creates a weakest link in the joint. On the other hand, high temperature heat affected zone (HTHAZ) having a coarse austenite grain surrounded by grain boundary precipitates is governed only by the modes of metal transfer. [ABSTRACT FROM AUTHOR]

Pseudo-3D orientation microscopy based on electron backscatter diffraction (EBSD) is a simple way to characterize all the 5 rotational parameters of grain boundaries. It is based on the observation of grain boundaries across a sharp edge of a sample using EBSD to determine the crystal orientation and orientation contrast from backscattered electrons to observe the grain boundary traces on the surface. In this paper we discuss the possible errors and the accuracy of these kind of measurements. It is found that the boundary misorientation can be measured better than 0.5°, while the boundary plane can be determined to better than 1°. A coherent twin grain boundary with well-known crystallographic parameters in an austenitic stainless steel with fcc crystal structure was taken as reference. [ABSTRACT FROM AUTHOR]

The aim of this work was to analyse the effect of the γ′-Ni 3 (Al,Ta) phase on the strengthening of the Ni-Ta-Al-Cr-C coating layer, deposited on austenitic stainless steel AISI 310. The hardfacing alloy was deposited using the Gas Tungsten Arc Welding process. The specimens were analysed in the as-welded, solution treated and aged states. The investigations involved a wide range of experimental techniques, that included scanning electron microcopy, electron probe microanalysis, microhardness measurements and single direction wear testing using the nanoindentation technique to characterize the effect of the γ′-Ni 3 (Al,Ta) phase on the strengthening of the investigated coating layer. Additionally, the block-on-ring wear test was performed in order to determine the macroscopic wear mechanism of the deposit during sliding wear under harsh conditions. The obtained results show that heat treatment (solution treatment + aging) did not significantly affect the chemical distribution along the first layer, but clearly reduced local intradendritic segregation and brought about a more uniform distribution and size of the γ′ phase in comparison to the as-welded state. An increase of Fe within the coating layer reduced the γ′ phase's nucleation driving force and its final volume fraction. It was found that the MC carbides exhibited a greater tendency to fracture during the single direction wear test in comparison to M 7 C 3 carbides. The heat treatment procedure used in this work allowed to increase the volume fraction of the γ′ phase in the coating layer. This manifested itself in a significantly enhanced microhardness and increased values of displacement into the surface vs the scratch distance made by the indenter during the wear test, which is associated with the improvement of abrasive wear resistance. [ABSTRACT FROM AUTHOR]