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This study investigates the tensile and impact property of TRIP (transformation-induced plasticity)-aided steel with different retained austenite (RA) stability at different temperatures. The RA stability of experimental steels processed by hot rolling-coiling at 450 °C, short tempering and long duration tempering followed the sequence: un-temper (C450)

Laser direct energy deposition (LDED) exhibits great potential for application in rail repairing due to its low cost and high process automation. However, the martensite transformation accompanied by rapid solidification leads to the decrease in the mechanical properties and threatens the service safety. In this study, the reinforcement strategy of combining cobalt (Co) addition with preheating treatment was investigated systematically by combining thermodynamic calculations with experimental characterization to optimize the microstructure and mechanical properties of U75V steel by LDED. The results indicate that cobalt addition can promote the pearlite percentage by facilitating the leftward shift of the continuous cooling transformation (CCT) curve, and refining the interlamellar spacing. But the martensite start (Ms) temperature will also increase from 206.4 °C to 324.1 °C after 7 wt% Co addition. 350 °C preheating treatment on the substrate can also promote the pearlite transformation by elevating the valley temperature of thermal cycling over Ms, as well as the subsequent decrease in cooling rate, but it is not sufficient to prevent martensite formation. A synergistic reinforcement strategy of 7 wt% Co addition and 350 °C preheating was established, which can effectively suppress the occurrence of martensite in the bottom area of deposition layers and expand the pearlite range in the top area. The bonding strength between the substrate and deposition layers has been elevated by ∼20%, accompanied by the increase in elongation and wear resistance. A mixed mode of adhesive and abrasive wear has transformed into a mode dominated by adhesive wear after optimization.

Porous β-Ti composites hold large promise for lightweight components due to their exceptional strength-ductility synergy. However, their microstructural behavior under both impact and compressive conditions have received limited attention. This study investigates the compressive and impact behavior of porous β-type Ti–25Nb (at.%) composites fabricated using laser powder bed fusion (LPBF). The composites feature a rhombic dodecahedron (RD) lattice structure and incorporate a central solid cylinder with varying diameters. Microstructural characterizations and finite element simulations were employed to analyze mechanical properties and stress distribution. Incorporation of 3 mm radius cylinders (TIII ∼45% porosity) significantly enhances compressive yield stress (88 ± 10 MPa), impact yield stress (153 ± 10 MPa), and impact energy absorption (41.2 J/cm³) while maintaining material plasticity. Compressive and impact loading initiate a martensitic transformation from β to α″ phase, strengthening the alloy through refined α″ formation with a finer grain size than β phase. Notably, a β→ α″ transformation and grain refinement occur in the node region that connects the solid cylinder and lattices, which undergoes significant deformation during impact. This observation suggests the impact resistance of porous β-Ti composites can be enhanced through tailored microstructures and strategic structural design.

High-manganese steel (high-Mn) is valuable for its excellent mechanical properties in cryogenic environments, making it essential to understand its deformation behavior at extremely low temperatures. The deformation behavior of high-Mn steels at extremely low temperatures depends on the stacking fault energy (SFE) that can lead to the formation of deformation twins or transform to ε-martensite or α′-martensite as the temperature decreases. In this study, submerged arc welding (SAW) was applied to fabricate thick pipes for cryogenic industry applications, but it may cause problems such as an uneven distribution of manganese (Mn) and a large weldment. To address these issues, post-weld heat treatment (PWHT) is performed to achieve a homogeneous microstructure, enhance mechanical properties, and reduce residual stress. It was found that the difference in Mn content between the dendrite and interdendritic regions was reduced after PWHT, and the SFE was calculated. At cryogenic temperatures, the SFE decreased below 20 mJ/m2, indicating the martensitic transformation region. Furthermore, an examination of the deformation behavior of welded high-Mn steels was conducted. This study revealed that the tensile deformed, as-welded specimens exhibited ε and α′-martensite transformations at cryogenic temperatures. However, the heat-treated specimens did not undergo α′-martensite transformations. Moreover, regardless of whether the specimens were subjected to Charpy impact deformation before or after heat treatment, ε and α′-martensite transformations did not occur. [ABSTRACT FROM AUTHOR]

The effect of Ti content on the phase composition, microstructure manipulation and work-hardening behavior of ZrCu alloys during compression was investigated in this work. The results show that the phase composition, microstructure, mechanical properties and work-hardening ability of ZrCu alloys are very sensitive to Ti content. The introduction of Ti enhances the stability of the austenite phase and the amorphous forming ability of ZrCu alloys. The Zr45Cu50Ti5 alloy exhibited best comprehensive mechanical properties, its yield strength, ultimate compressive strength and fracture strain are 981 MPa, 1922 MPa and 16.9%, respectively. Zr49Cu50Ti1 and Zr47Cu50Ti3 alloys have the best work hardening ability under low strain, and the Zr43Cu50Ti7 alloy has the best work hardening ability under high strain. Finally, the austenite stabilization mechanism, strengthening mechanism and work hardening mechanism are revealed.

The effect of quenching and tempering (QT) process on the mechanical properties of the experimental high-strength low-alloy (HSLA) steel was analyzed by Grey-Taguchi method, thus achieving the optimum combination of parameters based on the appropriate nine sets of experiments on an orthogonal array. The grey relational analysis (GRA) reveals that the quenching temperature T1 has the greatest effect on each response variable, followed by the tempering temperature T2, while the quenching time t1 and tempering time t2 have similar and the least effect. Experimental assessment of microstructure evolution was performed by multi-scale characterizations and in-situ investigation combined with modeling, focusing on martensite transfomation kinetics that controlled by the quenching process. Results indicate that the prior austenite grain (PAG) size and the substructure of martensite are significantly refined, as the quenching temperature decreased from 950 to 850 °C. The PAG refinement leads to an increase in driving force for martensite transformation initiation, thus shifting the martensite transformation temperature to lower levels for a higher degree of undercooling, consistent with the experimental and modeling results. The refined microstructure obtained at low quenching temperature contributes to strength improvement, and the various carbides precipitated during tempering process offset the tempering softening of the steels. In general, the value increase of process parameters (T1, T2, t1 and t2) leads to a decrease of strength property, but an increase of ductility and toughness. Based on the theoretical and experimental basis, a 1 GPa grade HSLA steel with improved comprehensive mechanical properties can be produced.

The most commonly quenching process for carburizing gears is the oil-quenching (OQ) and salt-quenching (SQ), and finite analysis and comparison of OQ and SQ on the carburizing gear ring were performed. Wherein, the accurate simulation of gear carburization was obtained by the alloying element coefficient for diffusion coefficient and experiment validation. The heat transfer coefficients measured by the inverse heat transfer method was used to the temperature simulation, and the gear distortion mechanism was analyzed by the simulated results. By the comparison of OQ, SQ had higher cooling capacity in the high temperature region and slow cooling rate in the temperature range where martensite transformation occurs. The martensite transformation was more sufficient, and the compressive stress of the tooth was greater in the SQ. The tooth showed a drum-shaped and slight saddle-shaped distortion in the OQ and SQ, respectively. The simulated distortion results have good consistency with the measured results, and the SQ distortion was more uniform and stable based on the measured results.

We researched the influence of damage accumulation on the Poisson's ratio measured by echo-pulse acoustic method during plastic deformation of 12Kh18N10T steel. On the basis of the obtained experimental data we calculated the partial contributions to the change in the Poisson's ratio of damage accumulation and formation of the strain induced martensite phase. The characteristics of stable cracks forming near strain-induced martensite particles at small degrees of plastic strain have been analyzed by computer simulation. The theoretical dependence of the change in the Poisson's ratio due to crack formation during plastic deformation has been constructed. A good agreement between the experimental data and theoretical calculations has been obtained. [ABSTRACT FROM AUTHOR]

High-manganese steel (high-Mn) is valuable for its excellent mechanical properties in cryogenic environments, making it essential to understand its deformation behavior at extremely low temperatures. The deformation behavior of high-Mn steels at extremely low temperatures depends on the stacking fault energy (SFE) that can lead to the formation of deformation twins or transform to ε-martensite or α′-martensite as the temperature decreases. In this study, submerged arc welding (SAW) was applied to fabricate thick pipes for cryogenic industry applications, but it may cause problems such as an uneven distribution of manganese (Mn) and a large weldment. To address these issues, post-weld heat treatment (PWHT) is performed to achieve a homogeneous microstructure, enhance mechanical properties, and reduce residual stress. It was found that the difference in Mn content between the dendrite and interdendritic regions was reduced after PWHT, and the SFE was calculated. At cryogenic temperatures, the SFE decreased below 20 mJ/m2, indicating the martensitic transformation region. Furthermore, an examination of the deformation behavior of welded high-Mn steels was conducted. This study revealed that the tensile deformed, as-welded specimens exhibited ε and α′-martensite transformations at cryogenic temperatures. However, the heat-treated specimens did not undergo α′-martensite transformations. Moreover, regardless of whether the specimens were subjected to Charpy impact deformation before or after heat treatment, ε and α′-martensite transformations did not occur.

The most commonly quenching process for carburizing gears is the oil-quenching (OQ) and salt-quenching (SQ), and finite analysis and comparison of OQ and SQ on the carburizing gear ring were performed. Wherein, the accurate simulation of gear carburization was obtained by the alloying element coefficient for diffusion coefficient and experiment validation. The heat transfer coefficients measured by the inverse heat transfer method was used to the temperature simulation, and the gear distortion mechanism was analyzed by the simulated results. By the comparison of OQ, SQ had higher cooling capacity in the high temperature region and slow cooling rate in the temperature range where martensite transformation occurs. The martensite transformation was more sufficient, and the compressive stress of the tooth was greater in the SQ. The tooth showed a drum-shaped and slight saddle-shaped distortion in the OQ and SQ, respectively. The simulated distortion results have good consistency with the measured results, and the SQ distortion was more uniform and stable based on the measured results. [ABSTRACT FROM AUTHOR]

In the present study, we investigated a finite element simulation considering the martensite transformation and transformation-induced plasticity (TRIP) to predict the quench distortion of a cut-cylinder 4340 steel. The distortion predicted by the finite element simulation considering both the martensite transformation and TRIP showed good quantitative agreement with the measured results. We analyzed the effects of the martensite transformation and TRIP on the quench distortion separately and concluded that TRIP has a very critical impact on the quench distortion of cut-cylinder 4340 steel. In addition, the chemical composition and quenching condition were confirmed to be important factors influencing the quench distortion even though the same cut-cylinder sample was used. [ABSTRACT FROM AUTHOR]

The effect of plastic deformations on the hydrogen embrittlement (HE) of transformation-induced plasticity (TRIP) steel was studied. In situ tensile tests showed that with increasing hydrogen current density, total elongation loss was raised to 36.8% as compared to an uncharged specimen. The electron backscatter diffraction (EBSD) observation indicated that hydrogen charging decreased stacking fault energy (SFE), resulting in the formation of more α′- martensite by both indirect and direct transformation. The α′- martensite volume fraction at the same degree of deformation in uncharged and charged samples was 31% and 39%, respectively. With plastic deformation, reversible trap sites were raised because of the increased dislocation density and the formation of α′- martensite, which was obtained from EBSD characterization and had a good correlation with the results of the thermal desorption spectroscopy (TDS) analysis.

The phase transformation and coordinated deformation behaviors of Cu-10wt.%FeC alloys in the different thermo-mechanical treatment conditions were systematically studied. The results reveal that the γ-Fe (FCC FeC-rich phase), and α-Fe (BCC FeC-rich phase) are simultaneously formed in the rapidly solidified alloy. Although the martensite transformation can be further induced in the alloy by a solution and quenching treatment, the strength and elongation are all reduced compared with the rapidly solidified alloy. Additionally, the martensite transformation of γ-Fe (FCC) →α-Fe (BCC) can be induced in the two alloys during the 80% cold rolling, and resulting in the significantly increased strength. But the alloy without a solution and quenching treatment possesses the higher strength. Particularly, under the same strength conditions, the alloys in the final cold rolling state can still have a higher elongation than those of traditional Cu-based composites strengthened by ceramic particles (no phase transformation). The mechanisms of phase transformation and coordinated deformation of alloys were also deeply discussed in the paper.

DP980 is widely used in automotive bodies. In the present research, a numerical model was developed via the in-house finite element (FE) code JWRIAN-RSW for the multi-physics coupled process of resistance spot welding (RSW) of DP980 sheets and JWRIAN-Hybrid for residual stress. The high-temperature material properties and metallurgical behaviors of DP980 were clarified via experiment to ensure simulation accuracy. The measured thermal expansion curve indicated the austenite and martensite transformation in DP980 and clarified the critical temperatures. Material properties such as thermal expansion and stress-strain curves in heating and cooling processes were measured. The double-pulse electric current mode in RSW was employed and it was found that the molten zone disappeared temporarily between two current pulses. The final dimension of welding nugget was determined by the second pulse. The volume expansion of martensite transformation led to a clear stress drop (about 400MPa) in the welding nugget and heat-affected zone. The final value of X-stress and Y-stress on the center of surface was about 300MPa and 240MPa, respectively. Except for the solid-state phase transformation, the springback process also had a significant influence on residual stress, which cannot be ignored in numerical modeling. The predicted temperature field and residual stress distribution showed good agreement with the molten zone morphology and XRD measurements, which demonstrated the effectiveness of developed FE model. [ABSTRACT FROM AUTHOR]

The H-cathodic charging applied to a superaustenitic stainless steel produces a metastable H-expanded phase by interstitial solid solution, which discloses parallels with the austenite N-expanded (S-phase). The ∼1 μm modified layer comprises a high expansion region γ H followed by a low expansion γ e domain, the former presenting a lattice parameter 3% higher than the austenite γ. The complete decay γ H → γ e → γ occurs around 1-day time. The mechanical properties change accordingly: hardness increases twofold and elastic modulus 17% with H-inlet, returning progressively to the pristine values after hydrogen diffuses out from the modified layer, mainly towards the outer surface. This is consistent with a competitive effect, where the solid solution strengthening overcomes the H-enhanced plasticity in the initial period of decomposition. Moreover, the prevalence of one of these phenomena in plastic deformations is strain-rate dependent: high strain rates result in hindered dislocation mobility, whereas the material discloses embrittlement under low strains. • Cathodic hydrogenation produces H-expanded phase in superaustenitic stainless steel. • The H-expanded phase comprises low (first order) and high (second order) expansions. • Hardness and elastic modulus, higher than austenite, decay with the metastable phase. • Solid solution strengthening dominates over enhanced plasticity in indentation tests. • The mechanical response of the H-expanded austenite is strain rate-dependent. [ABSTRACT FROM AUTHOR]

Ti and V were bonded together and subjected to high-temperature treatment at 1000 or 1100 °C for 16 h to study the microstructural evolution and interfacial behavior of Ti–V diffusion interfaces. The samples were prepared by electro-polishing and analyzed using scanning electron microscopy, electron probe microanalysis, electron back-scattered diffraction, and nano-indentation. The results indicated that Ti–V diffusion bonding interfaces comprises a martensite Ti zone, a body-center-cubic Ti (β-Ti) zone, and a V-based alloy zone. They are divided by two composition interfaces with V contents of ~13.5% and ~46%. The original interface between the pure Ti and the V alloy substrate falls within the β-Ti zone. The observation of acicular-martensite rather than lath-martensite is due to the distortion caused by the β-to-α phase transformation in the adjacent pure Ti. The recrystallization of β-Ti is distributed along the interface direction. The hardness varies across the Ti–V interface bonding zones with the maximum value of 7.9 GPa. [ABSTRACT FROM AUTHOR]

• Reforming boosts achievable martensite by increasing accumulated strain level. • Reforming reduces the martensite gradient in two surfaces by load reversal. • Tool deflection is important for accurate geometry and thickness profile prediction. • Deformation in reforming case involves plane strain tension, compression, and shear. Incremental sheet metal forming is a die-less process known for its high flexibility, making it a suitable choice for fabricating low-batch, highly customized complex parts. In this paper, the deformation mechanism of double-sided incremental forming (DSIF) with single pass or reforming toolpaths and its effect on the deformation-induced martensite transformation kinetics are investigated by experiments and finite element (FE) simulations for a truncated pyramid part geometry for the first time. An FE model accounting for tool deflection is developed, considering the change in tools' position caused by the forces applied at the contact with the blank. This is done to enhance the simulation accuracy in comparison to experimental results. A comprehensive analysis is conducted with respect to the stress state change and strain accumulation during DSIF with the single pass and reforming toolpaths. The findings reveal that material points along the pyramid wall primarily deform by a combination of plane strain tension, plane strain compression, and shear, depending on the tools' locations on the sheet surfaces in relation to the material point of interest. The reversal of the forming direction in the reforming toolpath causes material points to experience different stress states and more strain accumulation than the single pass case. Under the same forming temperature, this phenomenon results in a higher and closer martensite transformation on both forming and supporting surfaces of the final part when employing the reforming toolpath as opposed to the single pass case. [Display omitted] [ABSTRACT FROM AUTHOR]

The martensite transformation (MT), magnetocaloric effect (MCE), and mechanical properties of the polycrystalline Ni 46 Mn 36 In 14 Co 4 (Co4) alloys with doping of 4 at.% Cu were examined. A large magnetization change (ΔM , 92 emu/g), large magnetic entropy change (ΔS M , 37.3 J kg−1K−1), high refrigerant capacity (RC , 261 J kg−1), relatively high sensitivity to magnetic fields (2.8 K/T), and two-way magnetic-field-induced MT were attained at approximately room temperature. Moreover, the compressive fracture strength and strain of the alloy reached 335 MPa and 7.4 %, respectively. Finally, the mechanism of the excellent combination properties of the Ni 42 Mn 36 In 14 Co 4 Cu 4 (Co4Cu4) alloy was discussed. This study can aid in the development of high-performance magnetic refrigeration and intelligent actuator materials. • A large magnetization change, a wide working temperature range, a large magnetic entropy change and large MCE were obtained. • The magnetic-field-induced transformation changed from one-way to two-way. • A kind of Ni-Mn-In based magnetic refrigerant material with excellent combination properties was obtained by co-doping. • The study has great purpose for the development of high-performance magnetic refrigerant and intelligent actuator materials. [ABSTRACT FROM AUTHOR]

The microstructure and deformation mechanism of a full orthorhombic αʺ martensite Ti-9V-1Fe-4Al (wt.%) alloy were investigated by XRD, TEM, EBSD analyses and tensile tests. The results show that the quenched alloy has a twinned-martensite microstructure and {111} αʺ type I internal twins are formed as the lattice invariant shear for martensite transformation. The alloy plastically deforms by a complex reorientation of αʺ martensite variants via the movement of the inter-variant boundaries of <211> α′′ type Ⅱ twin, and deformation twinning of {111} α′′ type I internal twin and of newly generated {011} αʺ compound twin in primary twinned martensites. {130} α′′ < 3 ¯ 10> α′′ and {110} α′′ <1 1 ‾ 0> α′′ plastic twins were not observed in the alloy. Such a kind of deformation induced martensite reorientation results in a good combination of strength and plasticity with yielding strength of 480 MPa, tensile strength of 1030 MPa and elongation of 23% in the alloy. The calculated results further show that the shears of {111} α′′ type I, <211> α′′ type II and {011} α′′ compound twins and the shuffle of {111} α′′ type I twin increase, while the shears of {130} α′′ < 3 ‾ 10> α′′ and {110} α′′ <1 1 ‾ 0> α′′ twins and the shuffle of {130} α′′ < 3 ‾ 10> α′′ twin decrease with an increase of the lattice parameter ratio b α′′ / a α′′. The shears of these activated twinning systems are larger than those of {130} α′′ < 3 ‾ 10> α′′ and {110} α′′ <1 1 ‾ 0> α′′ twins, and the shuffle of {111} α′′ type I twin is also bigger than that of {130} α′′ < 3 ‾ 10> α′′ twin in the present alloy, which implies that the operative twining modes cannot be derived simply in terms of the shear and shuffle magnitude in full αʺ martensite Ti based alloys. [ABSTRACT FROM AUTHOR]

Steels manufactured via Laser powder bed fusion (LPBF) usually exhibit a good synergy of strength and ductility due to their ultrafine microstructure. Yet, their toughness, in particular cryogenic toughness is intrinsically inferior as the formation of micro-voids and oxide inclusions can hardly be fully prevented during LPBF. In this study, a toughening strategy based on chemically heterogenous metastable austenite was proposed to improve the impact toughness of LPBF manufactured high strength steels. As demonstrated in a maraging stainless steel, cryogenic (-196 °C) impact toughness can be enhanced by three times without a sacrifice of strength via tailoring chemically heterogenous austenite in the strong martensitic matrix. Both experiments and molecular dynamic simulations demonstrate that upon impact deformation chemically heterogenous austenite could transform into martensite in a stepwise manner, which could not only absorb massive energy via deformation induced martensite transformation but also make a contribution to local stress mitigation, crack passivation and deflection. The chemically heterogenous austenite strategy has the potential to be utilized for improving the toughness of other high-strength steels. [Display omitted] [ABSTRACT FROM AUTHOR]

Reinforced concrete (RC) bridges often face great demands of strengthening or repair during their service life. Fe-based shape memory alloys (Fe-SMAs) as a kind of low-cost smart materials have great potential to enhance civil engineering structures. The stable shape memory effect of Fe-SMAs is generated by, taking Fe-Mn-Si alloys as an example, the martensite transformation of fcc(γ) → hcp(ε) and its reverse transformation which produces considerable recovery stress (400~500 MPa) that can be used as prestress for reinforcement of RC bridges. In this work, the mechanism, techniques, and applications of Fe-SMAs in the reinforcement of RC beams in the past two decades are classified and introduced in detail. Finally, some new perspectives on Fe-SMAs application in civil engineering and their expected evolution are proposed. This paper offers an effective active rehabilitation alternative for the traditional passive strengthening method of RC bridges. [ABSTRACT FROM AUTHOR]

The impact of cooling rate during high-pressure gas quenching on the fatigue performance of low-pressure carburized spur gears was studied for steel grades 20MnCr5 and 17NiCrMo6-4. The results show an increased fatigue limit by 10 to 11% when applying a slower cooling rate for both steel grades. Moreover, for 20MnCr5 the slower cooled gears show an increase in compressive residual stresses by 130 MPa compared to the faster cooling, although no significant difference was observed for 17NiCrMo6-4. It is also seen that the cooling rate affects the core hardness for both steel grades, while other properties like surface hardness, case-hardness depth and martensite variant pairing were unaffected. The results for the retained austenite content and average martensite unit size show no clear effect of the cooling rate. The possible influence of different carbon distributions after quenching for the two used cooling rates on the carbide precipitation and fatigue limit is discussed. [ABSTRACT FROM AUTHOR]

A novel approach for the correlation of local martensite tetragonality determined by electron backscatter diffraction and carbon distribution by atom probe tomography (APT) is presented. The two methods are correlated by site-specific sample preparation for APT based on the local tetragonality. This approach is used to investigate the local carbon distribution in high carbon steel with varying local martensite tetragonality. Regions with low tetragonality show clear agglomeration of carbon based on statistical nearest neighbour (NN) analysis, while regions with high tetragonality show only small elongated agglomerations of carbon and no significant clustering using NN analysis. The APT average bulk carbon content shows no quantitative difference between regions with low and high tetragonality, indicating that no significant long-range diffusion of carbon has taken place. [ABSTRACT FROM AUTHOR]

The preparation of Ni-Mn-Ga and Ni-Mn-Ga-Fe master alloy ingots and microwires was completed by high vacuum electric furnace melt melting furnace and melt drawing liquid forming equipment, and the lattice dislocations and defects formed inside the microwires during the preparation process were corrected by stepwise ordered heat treatment. The micro-structure and phase structure were characterized using a SEM field emission scanning electron microscopy and an XRD diffractometer combined with an EDS energy spectrum analyzer; the martensitic phase transformation process of the microwires was analyzed using a DSC differential scanning calorimeter; and the superelasticity of the microwires was tested by a Q800 dynamic mechanical analyzer. The results indicate that Fe doping can refine the grain, transform the phase structure from parent phase to single 7M martensite, reduce the number of martensitic variants, and increase the mobility of the twin grain boundary interface. The MT phase transition temperature (MS) is substantially increased in the martensite transition (MT) process by the increase of the number of free electrons in its lattice. During the superelasticity (SE) test, both microwires displayed superior recover-ability of SE curves, and the Fe doping curves showed similar characteristics of "linear superelasticity", showing higher critical stress values and complete SE in the experiment. The critical stress satisfies the Clausius-Clapeyron equation and exhibits higher temperature sensitivity than Ni-Mn-Ga microwires. [ABSTRACT FROM AUTHOR]

This review summarizes the current knowledge on the α″ martensite transformation in metastable β Ti alloys under different stress state conditions gained using various in situ techniques that range from the meso‐ to the nanoscale. Compared to ex situ observations, in situ observation can be conducted on a single region while changing external stimuli, and are therefore capable of elucidating the interim microstructural evolution. The strengths and shortcomings of different techniques to give insight into the martensitic transformation are identified. Both the formation of α″ martensite and its reversion on load removal are addressed. The phenomena of α″ martensite formation including preferential nucleation sites, its growth, interactions, and variant selection are described. The review also discusses the details of the macro‐ and micro‐level mechanical behavior caused by the phase transformation and highlights the research questions for further investigation. [ABSTRACT FROM AUTHOR]

This paper focuses on the effect of gear steel on distortion due to phase transformation in carburizing and quenching. The carburizing and quenching process of C-rings under suspension was studied by using the finite element method based on the thermo-mechanical theory, considering phase transformation. The phase transformation kinetics parameters, depending on different carbon contents, were measured by Gleeble-3500. The distortion behavior of the carburized C-ring during the cooling stage was analyzed, as well as the carbon concentration distribution and martensite volume fractions. The accuracy of the simulation was also verified by comparing the experimental data with the simulated result of the distortion and microstructure. A reliable basis is provided for predicting the distortion mechanism of gear steels in carburizing and quenching. [ABSTRACT FROM AUTHOR]

In the present work, the influence of β grain size on stress-induced martensite (SIM) was investigated in a metastable β-Ti alloy (Ti-4Mo-3Cr-1Fe-1Al). Samples with varying grain sizes were prepared by cold rolling and annealing. The triggering stress for SIM decreases with the grain size increase from 44 μm to 180 μm. Meanwhile, the yield strength increases with decreasing grain size, consistent with the Hall-Petch effect. Fine-grained samples formed a greater number of martensitic bands during deformation compared to coarse-grained samples, resulting in an ultra-high strain-hardening rate ∼4320 MPa. The deformation mechanism of the Ti-4Mo-3Cr-1Fe-1Al alloy consists of the ω to β transformation, SIM, martensite deformation twinning ({110}cc < 1–10> α " and {130}<-310> α " twins), reorientation of martensite and dislocation slip. Both the 44 μm and 59 μm samples exhibit ultra-high true tensile strengths (>1200 MPa), with a large work-hardening interval of nearly 600 MPa relative to the yield strength. This significant work-hardening capability is attributed to interfacial and dislocation strengthening arising from the dynamic formation of martensitic bands during deformation. • Trigger stress for Martensite transformation decreases with increasing grain size. • An ultra-high strain hardening rate was achieved in the fine-grained sample. • Stress-induced martensite plays a critical role in achieving the ultrahigh strength. • Deformation twins and martensite reorientation contribute significantly to plastic deformation. [ABSTRACT FROM AUTHOR]

Polycrystalline Ni-Mn-Ga-Fe fibers were prepared by melt-extraction technique. The internal stress and defects introduced by rapid solidification were modified by step-wise chemical ordering annealing. The microstructure and phase microstructure of fibers were characterized using Field Emission Scanning Electron Microscope (FESEM) and Transmission Electron Microscopy (TEM) and XRD diffraction technique. The phase transition behavior and two-way shape memory effect (TWSME) were tested using Dynamic Mechanical Analyzer (DMA). The results show that after heat treatment, the chemical ordering degree is increased, and the martensite twin boundaries are straight. Complete recoverable strains of parent and martensite phases during thermal cycles are achieved. During the two-way shape memory cycles, two characteristics of thermoelastic martensite transition are displayed in heat-treated fibers: reversibility and thermal hysteresis. In the thermal cycle test, the total strain reaches 1.32% when the fiber is loaded to 198 MPa. The thermomechanical tensile measurement shows that the phase transition temperature follows the Clausius-Clapeyron equation. Compared with other alloys such as Ti-Ni and Cu-Al-Ni, Fe-doped fiber exhibits a smaller strain-stress dependence, which is beneficial to the constant strain output.

The integration of laser direct energy deposition (L-DED) for manufacturing high-hardness D2 tool steel components presents a promising avenue for addressing the increasing complexity of product geometries demanded by modern industry. However, the inherent variability in microstructure and mechanical properties induced by L-DED across the build height necessitates the employment of post-processing techniques to achieve material homogeneity. This study explores the synergistic application of homogenization and deep cryogenic treatment (DCT) as a novel post-processing strategy aimed at mitigating microstructural inhomogeneities and enhancing the mechanical properties of L-DED-fabricated D2 tool steel. Initial analysis of the as-built samples revealed a microstructure characterized by a fine network of eutectic carbides enveloping an FCC matrix, which imposed constraints on martensite transformation during DCT. Homogenization was found to effectively alleviate these constraints, promoting compositional and microstructural uniformity and enabling a more consistent transformation to martensite across the samples upon subsequent DCT. The strategic combination of homogenization and DCT resulted in achieving uniformly distributed high hardness levels exceeding 750 HV throughout the entire sample, regardless of their initial positions within the build. This study demonstrates the potential of combining homogenization with DCT as an effective approach to enhance the structural integrity and mechanical performance of L-DED-fabricated D2 tool steel, offering valuable insights for the development of advanced manufacturing processes for alloyed materials. • D2 tool was fabricated via laser-direct energy deposition (L-DED) process. • The effect of homogenization and deep cryogenic treatment (DCT) was investigated. • The as-built microstructure constrained martensite transformation during DCT. • The homogenization induced the relaxation of martensite transformation constraints. • Applying DCT combining homogenization led to uniform hardness exceeding 750 HV 0.5. [ABSTRACT FROM AUTHOR]

Twin intersections have been reported to significantly impact on macroscopic properties by relaxing localized stress concentration and accommodating strain. Here the atomic-scale structural characterization of the twin intersection region in a deformed high Mn twinning-induced plasticity (TWIP) steel was conducted using high-resolution transmission electron microscopy. Detailed microstructural features were revealed at the twin intersection region, including twin intersection rotation, low angle grain boundary and ε-martensite. The twin intersections were observed to preserve the face-centered cubic structure, which exhibits deviation angles of ∼0–15o with respect to the barrier twin depending on the localized stress concentration. Interestingly, the ε-martensite was observed in the vicinity of the twin intersection, which shows a wedge shape, distinct from the usual plate-like deformation-induced martensite. Therefore, we identified here a novel stress relaxation mechanism at the twin intersection region. Based on the detailed microstructural characterization of the twin interaction region at the atomic scale, the fundamental dislocation mechanisms of twin transmission and interaction were discussed. The research thus advances the understanding of twin intersection rotation and ε-martensite transformation by twin-twin interactions. [Display omitted] [ABSTRACT FROM AUTHOR]

The austenitic transformation kinetics of quenched and tempered 22CrMnNiMo steel was investigated by dilatometric measurement at different cooling rates of 0.1∼20 K/s. It was found according to the dilatometric curves that the bainite transformation process of super-cooled austenite exhibited incompleteness during continuous cooling. The incomplete characteristics under different cooling rates were analyzed by determining both changes of ferrite fraction and formation rates with decreasing temperature. It indicated that the three-stage transformation during austenite decomposition was gradually transferred to a distinct two-stage process with increasing the cooling rate. On the basis of microstructural observation and transformation kinetic models, it was further demonstrated that the incompleteness of bainite transformation occurring at low cooling rate was mainly ascribed to the formation of displacive-controlled acicular ferrite, while that during intermediate or fast cooling was attributed to martensite transformation.

Ti and V were bonded together and subjected to high-temperature treatment at 1000 or 1100 °C for 16 h to study the microstructural evolution and interfacial behavior of Ti–V diffusion interfaces. The samples were prepared by electro-polishing and analyzed using scanning electron microscopy, electron probe microanalysis, electron back-scattered diffraction, and nano-indentation. The results indicated that Ti–V diffusion bonding interfaces comprises a martensite Ti zone, a body-center-cubic Ti (β-Ti) zone, and a V-based alloy zone. They are divided by two composition interfaces with V contents of ~13.5% and ~46%. The original interface between the pure Ti and the V alloy substrate falls within the β-Ti zone. The observation of acicular-martensite rather than lath-martensite is due to the distortion caused by the β-to-α phase transformation in the adjacent pure Ti. The recrystallization of β-Ti is distributed along the interface direction. The hardness varies across the Ti–V interface bonding zones with the maximum value of 7.9 GPa.

Reinforced concrete (RC) bridges often face great demands of strengthening or repair during their service life. Fe-based shape memory alloys (Fe-SMAs) as a kind of low-cost smart materials have great potential to enhance civil engineering structures. The stable shape memory effect of Fe-SMAs is generated by, taking Fe-Mn-Si alloys as an example, the martensite transformation of fcc(γ) → hcp(ε) and its reverse transformation which produces considerable recovery stress (400~500 MPa) that can be used as prestress for reinforcement of RC bridges. In this work, the mechanism, techniques, and applications of Fe-SMAs in the reinforcement of RC beams in the past two decades are classified and introduced in detail. Finally, some new perspectives on Fe-SMAs application in civil engineering and their expected evolution are proposed. This paper offers an effective active rehabilitation alternative for the traditional passive strengthening method of RC bridges.

The impact of cooling rate during high-pressure gas quenching on the fatigue performance of low-pressure carburized spur gears was studied for steel grades 20MnCr5 and 17NiCrMo6-4. The results show an increased fatigue limit by 10 to 11% when applying a slower cooling rate for both steel grades. Moreover, for 20MnCr5 the slower cooled gears show an increase in compressive residual stresses by 130 MPa compared to the faster cooling, although no significant difference was observed for 17NiCrMo6-4. It is also seen that the cooling rate affects the core hardness for both steel grades, while other properties like surface hardness, case-hardness depth and martensite variant pairing were unaffected. The results for the retained austenite content and average martensite unit size show no clear effect of the cooling rate. The possible influence of different carbon distributions after quenching for the two used cooling rates on the carbide precipitation and fatigue limit is discussed.