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The preparation of H13 steel using wire-arc directed energy deposition has broad application prospects; however, the high hardenability of H13 steel makes it prone to forming cold cracks during deposition, thus limiting its further applications. Fortunately, the oxide metallurgy technology holds promise in addressing this issue. Specifically, nano oxides have the capability to refine the grain and facilitate the formation of intracrystalline ferrite, ultimately resulting in an improvement in steel's toughness and subsequently mitigating its propensity towards cold cracking. In this study, H13 steel wires, incorporating Al-Si-Ti composite oxides, were used to deposit part (W-O-H13) with excellent performance and no cold cracks. Owing to the special thermal cycling during wire-arc directed energy deposition, a periodic build unit was formed that could be roughly divided into three zones, namely, fine grain, columnar grain, and heat-affected zones. The microstructure of the parts contained lath martensite, high-alloy martensite, and ferrite. After arc exposure, the oxide formed a complex core-shell structure, with vanadium carbide (VC) encapsulating the amorphous oxide core. Moreover, the oxide particles refine the grain structure and induce the formation of ferrite, thereby enhancing the toughness of the part and further improving its strength and wear resistance. The low mismatch between the VC thin layer wrapping the oxide and ferrite was a crucial factor in the conductive ferrite phase transformation. Additionally, the elemental segregation caused by the rapid cooling rate ultimately led to the formation of high-alloy martensite having a maximum hardness of 5.32 GPa. The results reveal that the average tensile strength of W-O-H13 steel in the X, Y, and Z directions is improved by 21.0 %, 20.3 %, and 14.3 % respectively, compared to the parts fabricated solely from H13 steel wire without the addition of oxide (W-H13). Additionally, the impact toughness of W-O-H13 steel surpasses W-H13 steel in all three directions, achieving an average of 33, 27, and 28 J/cm2 respectively in the X, Y, and Z axes. In comparison to W-H13, the impact toughness of W-O-H13 has witnessed a substantial increase ranging from 10 % to 30 %. Furthermore, the wear resistance of W-O-H13 steel significantly surpasses that of W-H13 steel, particularly under high-temperature wear conditions. Specifically, the average wear volume of W-O-H13 steel at temperatures ranging from 400 to 600 °C is merely 0.163 and 0.224 mm³, representing only 30 % and 40 % of the wear volume exhibited by W-H13 steel under the same wear conditions. [ABSTRACT FROM AUTHOR]

The present study focuses on modifying the microstructures and mechanical properties of a recently developed metastable β Ti–5Al–5V–5Mo–3Cr (Ti-5553) alloy through cold rolling technique. Three different degrees of deformations (5 %, 10 %, and 20 % thickness reductions) are employed on the as-cast Ti-5553 alloy. Microstructural analysis reveals presence of single β-phase in the as-cast alloy, which is further confirmed through X-ray diffraction technique. Cold rolling however activates different deformation mechanisms with increasing degree of rolling and hence evolve the microstructure as well. While the alloy subjected to the lowest extent of thickness reduction, deforms mainly via slip, that with the highest degree of rolling deforms through formation of slip lines, zigzag slip lines, and deformation bands. Overall, activation of specific twins such as {332} <113> β along the closed packed direction of the β matrix is noted for the optimally cold rolled specimen. Most interestingly, occurrence of stress-induced martensitic phase is also confirmed for all the cold rolled specimens with its volume percentage being directly proportional to the degree of deformation. This cold rolling imparted microstructural modifications also influence the local and global mechanical performance of Ti-5553 alloy. Highest values of hardness and compressive yield strength are noted for the Ti-5553 alloy cold rolled up to the highest extent. This systematic study is the first of its kind to report the influence of cold rolling on the stress induced martensite formation and hence establishes the processing-structure-property correlation for Ti-5553 alloy. [Display omitted] • Presence of stress-induced martensite is noted in all the cold rolled Ti-5553 alloy. • {332} <113> β twin activates along closed packed direction of the β matrix. • Hardness and compressive yield strength increase with increase in degree of cold rolling. • A novel hardness-strength relationship is established for Ti-5553 alloy. • Localized strain concentration at 45° to compressive loading axis is observed for as-cast and cold rolled Ti-5553 alloys. [ABSTRACT FROM AUTHOR]

Solution-aging treatment is a conventional method for enhancing the strength of titanium alloys contributed by the decomposition of metastable phases, like martensite. The traditional viewpoint suggests that the martensite transformation occurring after water quenching does not significantly contribute to the strengthening of titanium alloy and this perspective actually overlooks the high strength resulting from the high-density defects formed in the martensite microstructure due to rapid cooling. So, previous studies have not fully taken advantages of the excellent strengthening capability of martensite. This study adopted a technique which combined solid solution and incomplete aging treatment to control the decomposition behavior of martensite and as a result, the strength of the as-forged Ti-6Al-4V (TC4) alloy had been obviously improved and good ductility had been maintained. Furthermore, the changes of phase structure and microstructural defects in TC4 titanium alloy during solution and aging process have been detailly studied by XRD refinement, EBSD, and TEM. Specifically, the highest tensile strength increased by 28.8 % which could reach 1224 MPa, and the elongation at fracture could still remain at 13 %. The reason is as follows. The formation of martensite after water quenching led to a substantial increase in internal microstructural defects, which were partly retained during the subsequent aging process because of lower temperature and short aging time, and the remaining martensite and microstructural defects can help maintain high strength. What's more, a small part of defects disappeared during this incomplete aging process due to martensite decomposition and recovery, which was beneficial to ductility. [ABSTRACT FROM AUTHOR]

This work presents the effects of spatially varying thermal history on local microstructure and properties in directed energy deposition additively manufactured AF9628 high-strength low-alloy steel walls. Hardness was shown to decrease with increasing height followed by an abrupt increase in hardness at the top of the wall. Austenite content was characterized with x-ray diffraction, martensite structures were characterized with electron backscatter diffraction, and carbide character was observed via electron microscopy. Carbide precipitation and the evolution of matrix composition during building were modeled using Thermo-Calc TC-PRISMA with ABAQUS temperature history simulations calibrated with thermocouple data during fabrication. A hardness model was used to describe the contributions of each microstructural feature measured or simulated to the local hardness. Toward the top of the martensitic structure, all subsequent layers reheated the area only above the austenitization temperature, resulting in an untempered microstructure. This study demonstrates how variations in thermal history result in differing extents of intrinsic tempering and hardness throughout the build. [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]

The effect of austenitization temperature on crystallographic aspects of martensite and mechanical properties for AISI 420 martensitic stainless steel with 0.33%C was investigated in this study. The samples were characterized by X-ray diffraction (XRD), electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD) techniques, and dilatometry, hardness, and tensile tests. The refining mechanism observed for the steel with the increase of austenitization temperature consisted of increased interfaces formed by variants that showed high misorientation. The formation of blocks with smaller widths and more irregular Bain groups refined the microstructure. The increment of carbon in solid solution due to increased carbide dissolution with increasing austenitization temperature is the main effect on microstructure refinement. The blocks also showed a misorientation gradient between two lower misorientation variants (V1/V4 variant pair). The refining mechanism and increased carbon in solid solution promoted the increase in YS, UTS, and hardness. The higher percentage of retained austenite reduced the yield strength and altered the mechanical behavior of the sample austenitized at 1100 °C. [ABSTRACT FROM AUTHOR]

Defect-free (single-pass multi-layer) maraging steel components were prepared by wire arc additive manufacturing (WAAM). The study on microstructure evolution and strengthening behavior of the components at different conditions was deeply investigated. It was found that the as-printed components consisted of lath martensite, residual austenite, and a few in-situ strengthening phases. There was significant element segregation among intragranular and intergranular, and the ultimate tensile strength (UTS) of the components reached 1184 MPa with an elongation of 16.8 %. After direct aging treatment at 480 °C for 4 h, unevenly distributed nanoprecipitates formed within the lath martensite, resulting in an increase of UTS about 1484 MPa. The presence of over 10 % reverted austenite in the microstructure led to elongation reached 14.8 %. 840 °C for 4 h solution treatment was applied to eliminate element segregation and reduce the content of retained austenite to below 3 %. Subsequent aging treatment at 480 °C for 4 h resulted in the dispersed precipitation of strengthening phases such as Ni 3 Ti and Ni 3 Mo, which lead to UTS increased to 1686 MPa while the elongation decreased to 9.2 %. Finally, the Orowan-modified model was used to calculate the theoretical yield strength (YS) of the aged components. The calculated values were found to be in good agreement with the actual results. • The ultimate tensile strength and elongation of maraging steel fabricated by wire arc additive manufacturing (WAAM) after heat treatment are reached to1684 MPa and 9 % respectively, which is the highest strength and elongation in currently researches on WAAM-processed maraging steel of the same grade. • The solution treatment at high temperature (840 °C for 1 h) was used to eliminate the element segregation in the intragranular and intergranular regions of the maraging steel components prepared by WAAM, making the element distribution tend to be uniform. • By conducting different heat treatments on the components and changing the size and distribution of strengthening phases, the performance control of WAAM-processed maraging steel components has been achieved. [ABSTRACT FROM AUTHOR]

Advanced (ultra)high-strength steels that utilise bcc-fcc microstructures are appealing solutions for producing a combination of high strength and deformability. However, they are also susceptible to hydrogen embrittlement (HE). As larger less stable retained austenite (RA) can impair mechanical performance, its size and morphology are critical factors for achieving and maintaining the desired properties. Here, we present a combined experimental–density functional theory (DFT) study on HE with medium-carbon direct-quenched and partitioned (DQ&P) martensitic steels with varying vol% and film-thickness of RA, showing significantly improved HE resistance as a function of bcc-enrichment and increasing RA film-thickness. DFT reveals low attraction of hydrogen in bcc-Fe with Al, implying a stronger push towards fcc. Furthermore, the solubility of hydrogen increases dramatically with the carbon-enrichment of fcc-Fe when hydrogen and carbon are second neighbours. The theoretical results explain the observed differences in hydrogen diffusion, trap density, and the consecutively lower HE in Al-DQ&P over Si-DQ&P. Our combined experimental and theoretical study thus highlights the important interplay of bcc and fcc phases and H-uptake within austenite-containing carbon steels. • Improving HE resistance as a function of RA film-thickness and bcc-alloying with Al. • Insights into the interplay of bcc/fcc and H-uptake for austenite-containing carbon-steels. • Outstanding performance of Al-DQ&P with thickest RA-films under in situ hydrogen-charged SSRT tests. • DFT reveals H-solubility in bcc-Fe lower with Al-alloying over Si-alloying. • H-solubility in fcc increases significantly when H and C are second neighbours. [ABSTRACT FROM AUTHOR]

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]

Benefiting from the transformation-induced plasticity effect has been recognized as a new approach to develop high-performance Si-containing high-entropy alloys with outstanding room-temperature strength-ductility balance. However, closely controlling the kinetics of deformation-induced α΄-martensite formation is essential to further boost ductility in addition to ultrahigh strength. In the present work, this aim was fulfilled by manipulating the chemical composition of the promising FeCoCrVMnSi system with the replacement of Mn with Ni and adjusting the Si content to fine-tune the stacking fault energy and the driving force of austenite to martensite transformation. The experimental results were obtained by EBSD, TEM, XRD, and tensile testing, and the results were supported by thermodynamic calculations, work-hardening analysis, and assessment of α΄-martensite formation kinetics. Accordingly, the single-phase Fe 47 Co 30 Cr 10 Ni 5 V 8- x Si x system was designed. In this system, the Fe 47 Co 30 Cr 10 Ni 5 V 2 Si 6 alloy exhibited exceptional tensile strength (∼1.1 GPa) and total elongation (72 %), outperforming the competitive dual-phase Fe 46 Co 30 Cr 10 Mn 5 V 2 Si 7 alloy. High strength was attributed to the high solid-solution strengthening effect as well as the volume fraction of α΄-martensite, while its ductility benefited from fine-tuned kinetics of martensitic phase transformation. [ABSTRACT FROM AUTHOR]

In this study, we introduce a novel approach to enhance the strength of face-centered cubic (FCC)-structured 316L austenitic stainless steel (ASS) while preserving its tensile ductility. The proposed method involves subjecting the 316L ASS to a pre-strain of 3.5 % at a cryogenic temperature of 4.2 K, resulting in the formation of multiple-fine-defects, including mechanical twins, stacking faults, hexagonal close-packed phases, and body-centered cubic (BCC) phases, which act as BCC martensite seeds. The formation of multiple-fine-defects in 316L ASS through pre-straining at 4.2 K leads to a substantial increase in the yield strength of approximately 45 %, while also maintaining tensile ductility during the subsequent tensile testing at room temperature. Moreover, these pre-existing defects can effectively suppress cross-slip, resulting in a high fraction of planar BCC martensite, which lead to a higher strain-hardening ability than that of the as-annealed 316L ASS. These findings suggest that the mechanical properties of metastable FCC alloys with low to medium stacking fault energies can be effectively enhanced by facilitating martensite seeds via pre-straining at cryogenic temperatures. • Pre-straining at deep cryogenic temperature of 4.2K was implemented on 316L austenitic stainless steel. • The effect of microstructural evolution by cryogenic pre-strain on subsequent room-temperature tensile properties was investigated. • Pre-strained 316L showed enhanced in both yield- and tensile-strengths while preserving ductility. • SIMT accelerated by pre-existed planar defects, resulting in excellent mechanical properties. [ABSTRACT FROM AUTHOR]

A dual phase lamellar heterostructured steel was designed and prepared by introducing soft phase lamellar δ-ferrite within hard phase lath martensite matrix. Comparing to the steel with fully martensitic, the dual phase lamellar heterostructured steel exhibits substantial enhancements in both strength and plasticity. The enhance of strength is attributed to the back stress strengthening and solid strengthening of Al. Additionally, the improvement of plasticity is largely due to the high strain hardening rate and presence of lamellar δ-ferrite which buffers microcrack propagation. This research offers a strategy to overcome the strength-plasticity trade-off in ultrahigh strength steels and shows potential for widespread industrial application. [ABSTRACT FROM AUTHOR]

In the face of increasingly demanding service conditions for high-end bearings, the need for bearing steels that demonstrate superior mechanical properties, specifically a robust balance of strength and toughness, is growing. The adoption of near-net-shaped cold rolling methods has emerged as a leading international technology to enhance bearing performance. In this study, the effect mechanisms of pre-cold deformation on the phase transformation kinetics, microstructure evolution and mechanical properties of nano-bainite bearing steels are systematically studied. Results reveal that pre-cold deformation treatment effectively reduces the A c1 temperature of the experimental steel, expedites nano-bainite transformation and promotes strong variant selection during bainite phase transformation. With the increase in pre-cold deformation from 0 % to 50 %, the size of the prior austenite grains decreases from 6.7 μm to 3.3 μm, and the size of carbides in specimens becomes more uniform, whilst the refinement of austenite grains can considerably reduce the thickness of bainitic ferrite plates. The decrease in grain size and the increase in V1–V2 variant pairs lead to an improvement in the toughness of the nano-bainite bearing steel from 64.9 J/cm2 to 114.3 J/cm2. A marginal increase in the hardness of the bearing steel is observed with the increase in pre-cold deformation. Furthermore, this study discusses the evolution mechanism of microstructure in the subsequent phase transformation by different microstructures in the early stage. Results show that pre-cold deformation leads to the transition of the martensite twin substructure in the specimens from {112} <111> twins to twinned variants. These insights offer critical understanding into the microstructural underpinning the enhanced performance of nano-bainite bearing steels subjected to pre-cold deformation treatment. • Finding pre-cold deformation leads a decrease in the degree of carbide dissolution during the austenitising process of high carbon bearing steel. Building models to prove that there are differences in the mechanism of carbide dissolution and precipitation in bearing steel with different grain sizes during austenitising. • Finding that grain refinement has both accelerating and delaying effects on bainite transformation kinetics. • Revealing the influence of the difference of crystallographic characteristics on the impact toughness of bearing steel. • Revealing a novel twinning mechanism caused by grain refinement in high carbon bearing steel exists. The increase of strain energy and mechanical stability of retained austenite lead to the transition of the martensite twin substructure in the specimens from {112} <111> twins to twinned variants. [ABSTRACT FROM AUTHOR]

The presence of δ-ferrite makes it challenging to achieve higher strength-ductility in low-Mn lightweight steels. In this study, a large deformation warm rolling process was used to construct a precursor enriched with Mn, nano-precipitations, ultra-fine grains, and high-density dislocations. The subsequent flash annealing process not only promotes the formation of multi-heterogeneous structures but also refines and reduces δ-ferrite while increasing the ultrafine-grained hard phase martensite. Additionally, this process enhances the retention of retained austenite, whose ultrafine grains contribute to improved mechanical stability. The synergistic effect of multiple strengthening mechanisms enables low-Mn lightweight steel to achieve outstanding comprehensive mechanical properties, with a tensile strength of 1423 MPa and a total elongation of 23.0 %. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (LPBF) offers the potential to modify the characteristics of NiTi alloys, allowing for the creation of bespoke solutions tailored to specific applications. In this study, four functionally graded NiTi alloys with varying martensite/austenite distributions were designed and manufactured. A comprehensive investigation was conducted into the local microstructure, phase transformation behaviour, and tensile mechanical behaviour. The findings illustrate that, in comparison to homogeneous NiTi, gradient NiTi displays a propensity for preferential deformation within the martensitic region, attributable to the presence of localised performance variations. Gradient NiTi exhibits evident strain hardening and a wider range of plateau stress due to its intricate step-by-step phase transformation behaviour. The strain gradient strengthening near the interface between martensite and austenite endows gradient NiTi with enhanced fracture strength. Furthermore, while ensuring a discernible gradient in strain, the design of the interface with narrow spacing and parallel to the tensile direction can effectively enhance the mechanical properties of gradient NiTi. This study validates that gradient design based on LPBF is an effective approach for controlling the macroscopic mechanical behavior of NiTi alloy. [ABSTRACT FROM AUTHOR]

Medium manganese steel has a ferritic matrix with retained austenite as the secondary phase at room temperature (RT). The effect of the transformation-induced plasticity (TRIP) of retained austenite on steel has earlier been found to result in an excellent combination of high strength and ductility, making it a promising candidate as a third-generation advanced high-strength steel (AHSS) for automotive applications. Therefore, extensive research has recently been conducted with the aim to increase the amount of retained austenite in steel, thereby enhancing the strength and ductility of the steel. However, an increase in the amount of retained austenite can be detrimental to the HER, which is due to its low mechanical stability and the transformation to martensite when forming holes by punching. However, there has been relatively little research on this topic. The impact of the mechanical stability of retained austenite on the tensile properties and hole expansion ratio (HER) of high-strength medium-Mn steel have been investigated in the present study. Samples were prepared using one-step austenite-reverted-transformation (ART) annealing, which resulted in steel with 24.7 % retained austenite and moderate mechanical stability. The steel showed also an excellent combination of strength and ductility. Samples were also prepared using two-step annealing, which showed a positive effect on the HER of the steel, regardless of the processing method used for the formation of a central hole on the samples. However, it slightly reduced the ductility of the steel. For all these samples, the impact of the retained austenite on the HER was found to be closely related to its mechanical stability and content in the steel. Retained austenite with poor stability transformed into hard martensite during the punching processing of holes. In contrast, the retained austenite with a relatively high stability showed a positive effect during the whole hole expansion deformation process. It decreased the stress concentration and, thereby, improved the HER. Thus, the present study has found that it is of the greatest importance to improve the stability of the retained austenite in high-strength medium-Mn steel, and to suppress its transformation to martensite during the punching of holes in the steel. [ABSTRACT FROM AUTHOR]

Microstructure and mechanical properties of the Fe 65 Co 12.5 Ni 12.5 Cr 9.5 C 0.5 (in at.%) medium-entropy alloy obtained by the laser-based powder bed fusion (PBF-LB) was investigated in the as-produced condition and after (i) annealing in the interval 800–1100 °C or (ii) cold rolling followed by annealing at 800 °C. The as-produced microstructure was found to be surprisingly stable; noticeable recrystallization occurred only after annealing at 1100 °C. Cold rolling resulted in the formation of typical deformed microstructure, while further annealing led to the formation of a fine-grained recrystallized microstructure. A considerable improvement in cryogenic mechanical properties was obtained after some thermo-mechanical treatments due to the transformation-induced plasticity (TRIP) effect. The cold-rolled alloy had the best cryogenic strength characteristics; yield strength and ultimate tensile strength were found to be 1740 MPa and 2235 MPa, respectively. High strength was combined with a pronounced value of elongation to fracture of 24 %. Detailed analysis of microstructure and corresponding mechanical behavior changes caused by various processing of the as-produced alloy are presented. [ABSTRACT FROM AUTHOR]

This work investigates the asymmetry under tension and compression for the homogenized Ti 50 Ni 45 Fe 5 B2 intermetallic. B2 matrix undergoes localized stress induced martensitic transformation (B2 → B19') under compression, resulting in a visible nonlinear elastic regime in the stress-strain curve. This was absent during deformation under tension; which could possibly be ascribed to lower resolved shear stress required for slip under tension than compression and a higher stress required for stress induced martensite transformation under tension than compression due to different martensite variants. The plastic deformation of B2 matrix under both tension and compression was marked by the presence and movement of stable elongated super intrinsic stacking faults, oriented along particular direction, which could be attributed to the activation of ⟨ 111 ⟩ screw dislocations. [ABSTRACT FROM AUTHOR]

Additively-manufactured metastable high entropy alloys are emerging as promising alternatives for energy absorption in protective equipment due to their exceptional ductility. In this study, an elongation of over 50 % was achieved in a directed energy deposition fabricated Fe 45 Mn 35 Co 10 Cr 10 metastable high entropy alloys (MHEAs). The inherent high cooling rate and repeated heat treatment in the directed energy deposition process were harnessed to deliberately induce an uneven distribution of Mn elements, forming a cellular structure with chemical boundary. The unique heterogeneity stacking fault energy results in a synergistic interplay of multiple deformation mechanisms such as, dislocation slip, transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP). In the initial stage of deformation, dislocation slip and TRIP contribute to the main plasticity. Stacking faults/martensite are confined within the cellular structure with low stacking fault energy (SFE). As strain increases, TWIP and TRIP are activated in the high SFE regions. Twins and martensite across the cell wall were observed. As strains exceed 40 %, primary martensite variant and twinning with a particular intersection angle take place to further enhance the deformation ability. The chemical boundary with stacking fault energy heterogeneity not only simultaneously activates both TRIP and TWIP, but also maintains the martensite and twins at the nanoscale. • Fe 45 Mn 35 Co 10 Cr 10 MHEAs were manufactured by DED with high ductility. • The uneven distribution of Mn elements results in stacking fault energy heterogeneity. • The stacking fault energy heterogeneity enables the simultaneous activation of TRIP and TWIP. • Chemical discontinuity maintains the width of martensite and twins consistently at the nanoscale. • Monoclinic structure MnO particles do not influence the TRIP/TWIP processing. [ABSTRACT FROM AUTHOR]

This work investigates the asymmetry under tension and compression for the homogenized Ti 50 Ni 45 Fe 5 B2 intermetallic. B2 matrix undergoes localized stress induced martensitic transformation (B2 → B19') under compression, resulting in a visible nonlinear elastic regime in the stress-strain curve. This was absent during deformation under tension; which could possibly be ascribed to lower resolved shear stress required for slip under tension than compression and a higher stress required for stress induced martensite transformation under tension than compression due to different martensite variants. The plastic deformation of B2 matrix under both tension and compression was marked by the presence and movement of stable elongated super intrinsic stacking faults, oriented along particular direction, which could be attributed to the activation of ⟨ 111 ⟩ screw dislocations. [ABSTRACT FROM AUTHOR]

Microstructure and mechanical properties of the Fe 65 Co 12.5 Ni 12.5 Cr 9.5 C 0.5 (in at.%) medium-entropy alloy obtained by the laser-based powder bed fusion (PBF-LB) was investigated in the as-produced condition and after (i) annealing in the interval 800–1100 °C or (ii) cold rolling followed by annealing at 800 °C. The as-produced microstructure was found to be surprisingly stable; noticeable recrystallization occurred only after annealing at 1100 °C. Cold rolling resulted in the formation of typical deformed microstructure, while further annealing led to the formation of a fine-grained recrystallized microstructure. A considerable improvement in cryogenic mechanical properties was obtained after some thermo-mechanical treatments due to the transformation-induced plasticity (TRIP) effect. The cold-rolled alloy had the best cryogenic strength characteristics; yield strength and ultimate tensile strength were found to be 1740 MPa and 2235 MPa, respectively. High strength was combined with a pronounced value of elongation to fracture of 24 %. Detailed analysis of microstructure and corresponding mechanical behavior changes caused by various processing of the as-produced alloy are presented. [ABSTRACT FROM AUTHOR]

Additively-manufactured metastable high entropy alloys are emerging as promising alternatives for energy absorption in protective equipment due to their exceptional ductility. In this study, an elongation of over 50 % was achieved in a directed energy deposition fabricated Fe 45 Mn 35 Co 10 Cr 10 metastable high entropy alloys (MHEAs). The inherent high cooling rate and repeated heat treatment in the directed energy deposition process were harnessed to deliberately induce an uneven distribution of Mn elements, forming a cellular structure with chemical boundary. The unique heterogeneity stacking fault energy results in a synergistic interplay of multiple deformation mechanisms such as, dislocation slip, transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP). In the initial stage of deformation, dislocation slip and TRIP contribute to the main plasticity. Stacking faults/martensite are confined within the cellular structure with low stacking fault energy (SFE). As strain increases, TWIP and TRIP are activated in the high SFE regions. Twins and martensite across the cell wall were observed. As strains exceed 40 %, primary martensite variant and twinning with a particular intersection angle take place to further enhance the deformation ability. The chemical boundary with stacking fault energy heterogeneity not only simultaneously activates both TRIP and TWIP, but also maintains the martensite and twins at the nanoscale. • Fe 45 Mn 35 Co 10 Cr 10 MHEAs were manufactured by DED with high ductility. • The uneven distribution of Mn elements results in stacking fault energy heterogeneity. • The stacking fault energy heterogeneity enables the simultaneous activation of TRIP and TWIP. • Chemical discontinuity maintains the width of martensite and twins consistently at the nanoscale. • Monoclinic structure MnO particles do not influence the TRIP/TWIP processing. [ABSTRACT FROM AUTHOR]

Thin-wall samples of 17-4 PH stainless steel were prepared by pulsed TIG wire arc additive manufacturing technology (P-TIG WAAM), and the microstructure and mechanical properties of the samples in different regions (bottom, middle, top) were studied. The results show that the microstructure of the thin-wall samples are mainly consisted of martensite matrix embedded by worm-like (bottom) and strip-shaped (top) δ-ferrite, with a small amount of austenite. The δ-ferrite content gradually decreases along the building direction, resulting in a gradual increase in microhardness from the bottom to the top. Along the building direction, the precipitation density and size distribution of nano-scale CRPs (Cu-rich precipitates) in δ-ferrite and martensite, as well as at the δ-ferrite/martensite interface are significantly different: the CRPs in δ-ferrite is precipitated with lower density, larger size, and inhomogeneous distribution at the bottom region, while that in martensite is precipitated with higher density, smaller size, and homogeneous distribution. At the bottom region, a chain of large size fcc-CRPs (∼65 nm) is precipitated along the δ-ferrite/martensite interface to coordinate the strain between them and improve the strength and ductility simultaneously, thus excellent mechanical properties (tensile strength = 1356 ± 24 MPa, elongation = 15.16 ± 2.6 %) are obtained. The relationship between the evolution of CRPs and the intrinsic heat treatment (IHT) during P-TIG WAAM is also discussed. [ABSTRACT FROM AUTHOR]

In this study, we investigated the effect of strain rate on the work hardening, strain-induced martensite transformation, strain partitioning, and crystallographic variant selection in a medium-Mn steel. Uniaxial compression tests were performed under quasi-static and dynamic loading conditions at the strain rates of 1 0 − 3 , 1 and 1 0 3 s − 1 . Besides, to determine the contribution of temperature rise due to adiabatic heating at a higher strain rate, compression tests were carried out at 409 K (estimated temperature rise) under quasi-static conditions. The work hardening rate was higher at high strain rates and it rapidly decreased at all strain rates up to a true plastic strain of ∼ 0.05. Beyond the true plastic strain of ∼ 0.05, the work hardening rate increased at a low strain rate up to a true plastic strain of ∼ 0.15 while at a high strain rate, it remained relatively constant and gradually decreased beyond a true plastic strain of ∼ 0.15. From the analysis of microstructures obtained from electron backscatter diffraction, it was observed that the strain-induced transformation of retained austenite to α ′ -martensite is significantly impeded at higher strain rates and in the sample tested at 409 K at low strain rates. The higher stability of retained austenite is attributed to the adiabatic heating which decreases the chemical driving force for retained austenite to martensite phase transformation and increases the stacking fault energy. Analysis of the misorientation axis and angle evolution revealed that while the Kurdjumov–Sachs relationship was followed under quasi-static conditions, it deviated at higher strain rates. Analysis of crystallographic variants related to strain-induced martensitic transformation at a true plastic strain of ∼ 0.15 revealed that the samples deformed at a lower strain rate did not show any noticeable variant selection. However, in samples deformed at a higher strain rate, a significant variant selection was observed. It is reasoned that the martensitic transformation nucleates initially on favorably oriented habit planes of retained austenite grains at all strain rates and other variants get activated with an increase in strain. As the transformation is impeded beyond a critical strain at a higher strain rate, the variants activated at lower strains remains predominant. [Display omitted] • Post-critical strain, lower work hardening is observed in high strain rate condition • The strain induced α ′ martensite transformation is suppressed at higher strain rates • High strain rates and the associated adiabatic heating stabilize retained austenite • Adiabatic heating lowers the driving force for transformation and enhances the SFE • At high strain rates, transformation is restricted to favorably oriented habit planes [ABSTRACT FROM AUTHOR]

The segregation-dependent mechanical stability of reverted austenite (RA) and resultant tensile properties was investigated on Co-free maraging steel fabricated by wire-arc directed energy deposition (DED). The results showed that the segregation in the interdendritic regions enabled the formation of gradient reverted austenite in Co-free maraging steel fabricated by wire-arc DED when subjected to aging heat treatment. The content and mechanical stability of reverted austenite and resultant tensile properties were strongly dependent on the degree and compositional gradient of segregation in the interdendritic regions as well as the post heat treatment process. The reverted austenite with compositional gradient inherited from the segregation, transformed into martensite successively from the edge to the interior within individual reverted austenite grain at different strain levels. Besides, the variations in the degree and compositional gradient of segregation in the interdendritic regions widened the range the mechanical stability of the reverted austenite and thereby activating martensite transformation over a broad strain regime. Annealing process prior to aging could modify the degree and compositional gradient of segregation in the interdendritic regions and accordingly tune the content and the range of mechanical stabilities of reverted austenite grains, thus attaining a gradual and consistent transformation induced plasticity effect over whole deformation regime and thereby increasing ductility without sacrificing strength of Co-free maraging steel fabricated by wire-arc DED. [ABSTRACT FROM AUTHOR]

Medium Mn steel is considered as a third-generation advanced high-strength steel. However, the bottle-neck problems of excessive strength and insufficient plasticity seriously limited its development. In this study, a warm rolling process was employed to refine grains and acquire high-density dislocations. Subsequently, a quenching-tempering process was employed to prepare a symbiosis microstructure containing multi-phase structures and double precipitates (κ- and V-carbides). On the premise of providing a gigapascal-level yield strength, the introduction of multi-phase structures activated various deformation mechanisms during the straining and contributed to a tensile strength of 1777 MPa and total elongation of 14.1 % in sample W500. In sample W700, the high-density dislocations in martensite gradually passed through the austenite/martensite interfaces and entered the neighboring austenite, enhancing the coordinated deformation capacity of martensite. Sample W700 exhibited a total elongation of 21.8 % and tensile strength of 1468 MPa. Combined with a microstructure analysis during the interrupted tensile deformation, the influences of martensite transformation, dislocation motion, double precipitates, and deformation twins on the work hardening and ductility are discussed in detail. [ABSTRACT FROM AUTHOR]

A self-developed high strength steel (14Ni3Cr3Mo1MnTiAl HSS) was used to manufacture a thin-walled structure by wire + arc additive manufacturing, the microstructural evolution and the anisotropy of mechanical properties were investigated. The tensile properties and impact toughness at room temperature, −20 °C and −40 °C were compared and discussed in detail. According to the results of OM and SEM, the main microstructure of WAAM-ed 14Ni3Cr3Mo1MnTiAl HSS was martensite and ferrite, and numerous Ni 3 Ti IMCs existed in the matrix. Reversed austenite only existed in the bottom and middle samples, and there was a K-S orientation relationship in reversed austenite and martensite/ferrite. The formation of reverted austenite could be attributed to repeated thermal cycles and high content of Ni, providing beneficial environment and condition for the formation of reverted austenite. Low temperature could increase the strength of vertical samples, while it had a negative effect on plastic and had little effect on the strength of horizontal samples. Differently, the impact toughness hardly decreased at −20 °C, while it had significant decrease at −40 °C. The anisotropy of tensile properties was attributed to the weak bonding of interlayer, phase content, the size of martensite packet/lath, and the size of Ni 3 Ti IMCs. Columnar crystals were one of the key factors leading to the low impact toughness of horizontal samples. The impact direction was parallel to the growth direction of columnar crystals, and the crack was prone to propagate along the boundaries of columnar crystals. [ABSTRACT FROM AUTHOR]

In this study the grain size effect on mechanical properties of a body-centered-cubic Ti–30Zr–5Mo alloy was investigated. Double yielding behavior in the stress-strain curves and four-stage behavior in the strain hardening rate curves can be seen in all Ti–30Zr–5Mo materials with different average grain sizes ranging from 6 to 475 μm, which is attributed to the occurrence of the stress-induced α′ transformation. The static Hall-Petch coefficient (k) for phase transformation was calculated to establish the relationship between grain size and trigger stress of the various materials. With the increase of strain, the hindrance of αʹ/β grain boundaries and αʹ/αʹ grain boundaries to dislocations gradually replaced β/β grain boundaries, thus the work hardening ability and k value changed. β grains were segmented by α′ martensite, resulting in a dynamic Hall‒Petch effect. Combined with a large stress field in the fine-grained materials with an average grain size of 6 μm, the highest work hardening rate with a value of 13 GPa was obtained. As the β grain size increased, the ultimate strength gradually decreased, while both trigger stress of the stress-induced αʹ transformation and elongation fluctuated. The trigger stress can be adjusted between 211 and 464 MPa by controlling the grain size. The grain size has little effect on the amount of the stress-induced αʹ phase. With a high trigger stress of 464 MPa in the material with the finest grains, the excellent ductility of 21% is obtained. The best comprehensive mechanical properties with a strength-ductility index value of 252 MPa is obtained in the material with an average grain size of 113 μm. • The grains in BCC allys can be refined via stress-induced phase transformation and its reverse transformation. • The grains are refined to 6 μm in metastable Ti-30Zr-5Mo alloy without severe plastic deformation. • The grain size has little effect on the amount of the stress-induced α′ phase. • The trigger stress can be adjusted between 211 and 464 MPa by controlling the grain size. • With a high trigger stress of 464 MPa in the material with the finest grains, the excellent ductility of 21% is obtained. [ABSTRACT FROM AUTHOR]

In this work, the reverse transformation of ε -martensite located inside nanotwin lamellae has been introduced to improve the ductility of a high-strength Fe–Mn–C steel with a high density of nanotwins. The steel was designed to have a low stacking-fault energy (SFE, 17.8 mJ/m2) to introduce high density of nanotwins through pre-deformation. Meanwhile, the fine ε -martensite was also obtained in nanotwin lamellae. The steel was strengthened significantly, with an increase in yield strength from 357 to 1275 MPa by the introduction of nanotwins. A subsequent intermediate annealing was applied to control the reverse phase transformation from ε -martensite to austenite to enhance the deformation ability of the nanotwinned steel (rising tensile elongation from 5 to 16%). The nanotwins with a width of 32 nm on average were stable during the heat treatment. The high strength was retained because the size of the austenite transformed from ε -martensite was restrained by the stable nanotwins. This strategy holds promise and can be widely applied to alloys with low SFE. [ABSTRACT FROM AUTHOR]

The present study systematically investigated the effect of Si additions from 0 to 4 wt % on the deformation mechanism of a Fe-0.2C-6.8Mn (wt. %) medium Mn steel. The duplex microstructure consisting of film-like retained austenite (RA) and lath-shaped annealed martensite (AM) was produced for different Si alloyed steels by annealing treatments. They have similar volume fraction of RA. The effect of Si content on the dynamic strain aging (DSA) of RA and the serration type of tensile curves, as well as the fine structure evolution within AM during deformation was studied in detail. For the different Si alloyed steels, their tensile curves all exhibit flow serrations, accompanied with the propagation of the Portevine-Le Châtelier (PLC) band caused by DSA in RA. The serration type was sensitive to Si content. For the high Si steel, only type B serrations appear. This is attributed to the high Si content in RA, promoting aged dislocations by C atoms due to the repulsive interaction between C and Si atoms. For the low Si and Si free steels, type A serrations occur after type B serrations disappear at a certain strain. The mechanical stability and Mn/Si contents of RA play an important role in the PLC band propagation. The Si content has also an important effect on the evolution of dislocation configuration within AM during deformation. In the low and Si free steels, the dislocation multiplication and tangles, as well as the formation of dislocation walls, dividing up the laths appear in the AM. While in the high Si steel, it is found that the dislocation cells appear within the AM due to Si addition promoting the formation of screw dislocations by neutron diffraction technique, and the cross slip of screw dislocations as a necessary mechanism for dislocation cell formation. The formation of dislocation configuration with low energy is believed to be another essential factor improving the mechanical properties, besides the transformation-induced plasticity effect. This work paves a new pathway towards tailoring dislocation configuration for designing a new generation advanced high-strength steel. • The effect of Si on deformation mechanism was investigated by neutron diffraction. • The stability and solute contents in RA significantly affect PLC band propagation. • The serration type of tensile curve was sensitive to Si content. • Si has an important effect on the evolution of dislocation configuration. • Si addition can promote formation of screw dislocations and thus dislocation cells. [ABSTRACT FROM AUTHOR]

This study explored the potential significance of controlled forging with finish forging temperature as low as ∼650 °C in improving the mechanical properties of a novel V + Nb-microalloyed medium-carbon steel 42CrNiMoVNb. The results show that compared with conventional quenching and tempering, the martensite microstructure of the controlled forged and direct quenched sample with high dislocation density was refined with accelerated precipitation of carbides when tempered either at 200 °C or 600 °C. After tempering at a low temperature of 200 °C, the controlled forged sample showed excellent tensile properties compared with conventional quenched sample, this is, a tensile strength of 2010 MPa, a yield strength of 1687 MPa and a considerably high ductility of total elongation of 10.5%. When the tempering temperature was increased to 600 °C, the strength of both the controlled forged and conventional quenched samples decreased significantly while the former still exhibited considerably higher tensile strength (1443 MPa) than the latter (1343 MPa). Therefore, controlled forging is more effective in improving the tensile properties of the tested steel tempered at a low temperature of 200 °C than at a high temperature of 600 °C, which is mainly due to significant dislocation strengthening for the 200 °C tempered sample. Enhanced precipitation strengthening and grain refining strengthening mainly account for the still considerably high strength for the 600 °C tempered sample. [ABSTRACT FROM AUTHOR]

The balance between strength and ductility in spring steel is a perennial issue. However, production using traditional quenching and tempering (Q-T) process is challenging because tempered martensite provides high tensile strength but significantly reduces ductility. To solve this problem, we propose a novel quenching and partitioning (Q-P) process to enhance the comprehensive properties of 60Si2CrVAT spring steel. The novel Q-P process introduced 31.56% excellent mechanical stability retained austenite, which existed mostly as a film-like between the martensite and bainite laths, and ensured a continuous and long-lasting transformation-induced plasticity (TRIP) effect. Moreover, Q-P process reduced the effective grain size due to the diffusion of carbon, and changed the fracture behavior due to the coordinated deformation of multiphase microstructure. Consequently, the total elongation of the spring steel increased by approximately 180%, from 7.8% to 21.7%, while the tensile strength of the spring steel was not affected and remained at a high of 1809 MPa. In conclusion, the novel Q-P process adjusted the volume fraction of the martensite, bainite, and retained austenite multiphase microstructure, and fully leveraged the synergistic effect of the multiphase microstructure to optimize the properties of spring steel. [ABSTRACT FROM AUTHOR]

Laser powder bed fusion (L-PBF) processed Ti–6Al–4V components present two substantial challenges: the presence of tensile residual stress and brittleness of the as-fabricated component due to the formation of the martensite (α′) phase. Industrially, post-heat treatment is performed to improve the quasi-static mechanical properties of the Ti–6Al–4V component through martensite (α′) decomposition and residual stress relaxation. However, detailed insights into the martensite decomposition and stress relaxation mechanisms are still lacking. To address this, in-situ high-energy synchrotron x-ray diffraction (HEXRD) was employed to simultaneously track: (I) martensite (α′) decomposition into a mixture of hexagonal α and body-centered cubic β phases, (II) residual stress relaxation process during heating. Rietveld refinement of 2D diffraction patterns recorded over a temperature range spanning from 25 to 1000°C revealed a three-stage evolution in the α′ unit cell parameters. Unit cell parameter analysis combined with the phase fraction analysis based on the diffraction patterns and the chemical composition profiles predicted by Thermocalc simulation gave detailed insight into the martensite (α′) decomposition process. In Stage 1 (until 375°C), the α′ phase unit cell exhibited linear expansion without indications of any phase transformation or stress relaxation. Upon commencement of stage II at 375°C, an increased expansion rate of the martensite (α′) unit cell was observed, attributed to the enhanced outward diffusion of vanadium (V), succeeded by β phase nucleation at 575°C. Simultaneously, during stage II, residual stress relaxation was observed at both macro and micro scale, culminating in a stress-free state achieved at approximately 750–800°C. Lastly, α-to-β phase transformation was observed in stage 3. The thermal evolution of the hexagonal unit cell dimensions was compared between martensitic and mill-annealed α+β microstructures. This comparison unraveled the link between the initial microstructure and the thermal expansion behavior of the hexagonal α′ and α phase. [Display omitted] [ABSTRACT FROM AUTHOR]

This study investigated the effect of grain size distribution on the deformation behavior in fine-grained (FG) austenitic stainless steels (γ-SSs). Commercial Cr–Mn–N γ-SS was selected as the coarse-grained (CG) specimen, which was cold rolled and then annealed at 1000 °C for 60 s and 700 °C for 3600 s to obtain FG specimens with similar grain sizes but different grain size distributions. The CG specimens annealed at 1000 °C for 60 s and 700 °C for 3600 s were referred to as unimodal-FG and bimodal-FG specimens, respectively. The tensile properties revealed that the yield strength was enhanced from 383 MPa to 685 MPa when the grain size was refined from CG (10.7 μm) to FG (∼1.8 μm). Moreover, the bimodal-FG specimen exhibited a higher total elongation of 48% than 36.5% for the unimodal-FG specimen. Transmission electron microscopy, electron backscatter diffraction and X-ray diffraction were used to identify the microstructural characteristics during tension. The results indicated that the main deformation mechanisms of γ-SSs specimens during tension were dislocation generation and accumulation at the initial stage, deformation twinning at the intermediate stage, and deformation-induced martensite (DIM) at the final stage. Compared with the unimodal-CG counterpart, grain refinement in the unimodal-FG specimens restricted the formation of DIM due to the increased austenite stability. A newly discovered phenomenon was that the bimodal-FG specimens promoted the formation of geometrically necessary dislocation (GND) and DIM compared to the unimodal-FG, which was due to the uneven deformation in grains located on hetero-boundaries (HBs). Moreover, the bimodal-FG specimen was preferentially formed DIM on HBs, especially in the smaller grains side rather than inside larger grains due to the higher stress concentration in smaller grains. [ABSTRACT FROM AUTHOR]

NiTiCu shape memory alloy is subjected to thermomechanical processing which deals with local canning compression followed by annealing at 300, 450 and 600 °C, respectively. Transformation temperatures of annealed NiTiCu specimens increase with elevating annealing temperature. Following annealing at 300 °C, NiTiCu specimen exhibits high yield stress of about 2000 MPa due to the existence of nanocrystalline grains. NiTiCu sample annealed at 600 °C possesses an extremely narrow phase transformation hysteresis width of about 1 °C. Nanotwins are found in NiTiCu sample annealed at 300 °C, while a large number of twin variants appear in NiTiCu samples annealed at 450 and 600 °C. Nanocrystalline grains contribute to suppressing the occurrence of twin variants. Furthermore, nanocrystalline grains are not beneficial to the formation of martensite variants. In the case of compressive loading, martensite reorientation and detwinning take place more readily in NiTiCu samples annealed at 450 and 600 °C. [ABSTRACT FROM AUTHOR]

We investigated the influence of hydrogen on a 1500 MPa hot-stamped steel 22MnB5 with different prior austenite grain size (PAGS, ranging from 14.0 μm to 22.2 μm), which were produced by precise control of the austenitizing temperature and time. All steel specimen exhibited a significant reduction of ductility (e.g., by 70%) and a small decrease in ultimate tensile strength (≤15%), measured using the linearly increasing stress test. The effect of PAGS on hydrogen embrittlement of the hot-stamped steel was not significant for these test conditions. Martensite with smaller PAGS exhibited a slightly lower hydrogen diffusion coefficient. Fracture for all steel variants after yielding occurred for hydrogen contents greater than those that actually occur during service. • The influence of hydrogen on the press-hardened steel (PHS) 22MnB5 was investigated. • The prior austenite grain size (PAGS) of the PHS increased with increasing austenitizing temperature and with increasing soaking duration. • Fracture for all steel variants after yielding occurred for hydrogen contents greater than those that actually occur during service. • Martensite with smaller PAGS showed slightly greater resistance to hydrogen embrittlement at low hydrogen fugacities. [ABSTRACT FROM AUTHOR]

The evolution of the deformation-induced martensite in the drawn 316L stainless steel and its effect on mechanical performance are examined. Both the yield strength and ultimate tensile strength of the 316L wire reach over 2.3 GPa at a drawing strain of 3.44. The deformation production of the wire during the drawing process is dislocation, twin and finally, deformation-induced α′-martensite. The neighboring misorientation between α′-martensite shows a preferential distribution around 60° due to variant selection limited by a double K–S orientation relationship. This preferential misorientation distribution around ∼60° is stable during the drawing process, and the α′-martensite has been refined to possess an average boundary spacing of ∼50 nm at a strain of 3.44. • The strength of the 316L reached 2340 MPa through cold drawing. • 316L with nanoscale lamellar structures of γ (52 nm) and α′ (53 nm) were prepared. • Misorientation between α′-martensite shows a preferential distribution around 60°. [ABSTRACT FROM AUTHOR]

In this work, a new equation to evaluate the yield strength change caused by grain refinement was proposed in low-carbon martensitic steel, based on the Hall-Petch formula. MATLAB analysis showed that martensite and the prior austenite grain (parent phase) were more consistent with the Kurdjumov and Sachs orientation relationship in the martensitic transformation process. The martensite variant selectivity became significant and was dominated by the CP group, among which the V1, V3, V4, V7, V11, and V12 variants were higher in content. Using OM, SEM, EBSD, and TEM characterized the sample at different quenching temperatures. Then, the relationship between the refined hierarchical microstructure and both yield strength (YS) and impact toughness (IT) was carefully analyzed. Researched results found that with grain refinement, the YS first increased rapidly and then slowly increased. It indicated the Hall-Petch relationship was a discontinuous formula. Utilizing the new equation, we concluded that martensite laths and prior austenite grains were the microscopic units that determined the YS of the steel. Martensite packets and blocks contributed nearly same to the strength of the steel, while the growth of martensite packets into square shapes would led to a decline in the strength of the steel. Additionally, the IT of the steel was determined by the overall grain boundary length of high strain gradient plastic zone and martensite laths width, in which high-angle grain boundaries contributed more to IT than low-angle grain boundaries. • A new equation had been proposed to evaluate the YS changes caused by grain refinement, based on the Hall-Petch formula. • Martensite laths and PAG were the microscopic units that determined the YS of low-carbon martensitic steel. • The shapes of martensite packets and blocks had a greater impact on the strength of steel. • Overall grain boundary length of high strain gradient plastic zone and martensite laths width determined the IT of steel. [ABSTRACT FROM AUTHOR]

Tempering behavior of a 0.53%C-1.6%Si-0.9%Mn-0.76%Cr-0.14%V-0.05%Nb steel was examined. Water quenching produced lath martensite structure with a 10.5% volume fraction of retained austenite (RA) with film-like shape. Low temperature tempering (LTT) leads to precipitation of transition carbides and carbon partitioning between martensite and RA. The following precipitation sequence was found: martensite →non-stoichiometric η-carbide→stoichiometric η-carbide→Fe 3 C. Highest density of non-stoichiometric Fe 2 C η−carbides was found after isochronal tempering at 280 °C that provided attractive combination of high yield stress (YS) (1890 MPa) with a ductility of ∼6% and a Charpy V-notch (CVN) impact energy of ∼10 J/cm2. At 400 °C, the replacement of non-stoichiometric η-carbide by the stoichiometric one decreases strength and increases ductility due to 30% decrease in its volume fraction. No tempered martensite embrittlement was found due to the fact that Si effectively suppresses the precipitation of cementite at T ≤ 400 °C. The replacement of transition η-carbide by boundary cementite after isochronal tempering at 500 °C leads to considerably decrease in YS down to 1360 MPa, while elongation to failure increases up to 9.3%. Effect of decomposition of RA on strength, ductility and fracture toughness is insignificant. • Low-temperature tempering of a ultra-high-strength steel with 1.6wt.% Si leads to precipitation of transition η-carbides. • Carbon content in transition η-carbides strongly affects volume fraction of carbides and mechanical properties of steel. • High volume fraction of non-stoichiometric η-carbides in steel promotes the best combination of yield stress and ductility. • Replacement of non-stoichiometric η-carbides by stoichiometric ones decreases strength and increases ductility. • Si effectively suppresses the precipitation of paraequilibrium cementite at T ≤ 400 °C. [ABSTRACT FROM AUTHOR]

The effect of ultrasonic surface rolling process on tensile properties of a novel Fe-0.6C-2.0Al-1.5Mn-0.04 Nb quenching-partitioning-tempering steel was investigated. Optical microscope, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were brought out to characterize the microstructure. Compared with quenching-partitioning-tempering steel, the strength and plasticity of the ultrasonic rolling specimen were increased simultaneously, reaching 1329.1 MPa and 23.6%, respectively, and the product of strength and elongation reached the range of the third generation Advanced High Strength Steels. The plasticity improvement mechanism of the USRP1specimen is mainly stemmed from two aspects: First, obvious strain gradients during the tensile process was found in the USRP specimens, which not only avoids the strain localization, but also generates a biaxial stress state. The geometrically necessary dislocations induced by the strain gradient and the dislocations induced by the biaxial stress state promoted extra work hardening, thus ensuring high plasticity of the material. Second, there was a moderate content and small size of twinned martensite in the USRP1 specimen. The improvement in yield strength of the USRP specimens primarily results from grain refinement strengthening, dislocation strengthening, phase transformation strengthening, and back stress strengthening. Among them, phase transformation strengthening contributed the most. [ABSTRACT FROM AUTHOR]

This work focused on the effects of martensite morphology on the mechanical properties of the dual phase (DP) steels produced via partial reversion from martensite, which is different from the traditional DP steels. A commercial DP780 steel was used, and the needle and blocky DP steels were produced by controlled heat-treatments. Combinations of SEM, EBSD and tensile tests were performed for understanding the property and damage mechanisms. The needle DP steel has higher ductility while lower strength than that of blocky DP steel. Much higher density micro-voids were uniformly formed in the needle DP steel than that of blocky case, and the coalescence of micro-voids into microcracks is strongly suppressed, due to the high-density ferrite/martensite interfaces and uniform local plastic strain. On the other hand, the blocky martensite islands distribution leads to the non-uniform local plastic strain, which caused the lower density micro-voids formation and the generation of micro-cracks, thus the resultant lower ductility. In addition, the higher strength of the blocky DP steel is due to strengthening of the sub-micron sized needle martensite. [ABSTRACT FROM AUTHOR]

The quasi-static superelastic responses and hierarchical martensite transformation from body-centered cubic (BCC) to face-centered cubic (FCC) under dynamic impact in Fe 42 Mn 34 Al 15 Ni 7.5 Ti 1.5 alloys were investigated. Polycrystalline and oligocrystalline alloys were produced through solution heat treatment and cyclic heat treatment processes, respectively. The results show the volume fraction of residual martensite for oligocrystalline alloys is lower, which exhibits better superelastic responses compared with polycrystalline alloys. Dynamic impact tests indicate that, despite the weakening of the grain boundary strengthening effect, the ultimate strength of the oligocrystalline alloys closely matches that of the polycrystalline alloys under dynamic impact. The martensite transformation of the FeMnAlNiTi alloy is characterized as hierarchical under dynamic impact, and increasing strain rates and grain sizes can enhance the BCC→FCC martensite transformation, resulting in higher martensite phase fractions for oligocrystalline alloys. The increase in ultimate strength is attributed to the dynamic Hall-Petch effect introduced by more martensite phase interfaces under dynamic impact. [ABSTRACT FROM AUTHOR]

For metastable β Ti alloys, it still remains a key challenge to enhance their limited yield strength (YS) while retaining good strain hardenability and ductility. Here we report a new strategy to evade this trade-off dilemma via introducing an uncommon hierarchical structure with the assistance of spinodal structures. The new Ti–11Mo–2V alloy exhibits high YS (693 MPa), high ultimate tensile strength (857 MPa) and large total elongation (31.8 %). Such excellent strength-ductility synergy originates from successive activation of multiple deformation mechanisms, including primary {332}<113> twins, secondary martensite and secondary {112}<111> twins. Importantly, the stress-induced secondary martensite is the desirable α′ martensite rather than the common α'' martensite. Spinodal decomposition in parent β grains is expected to facilitate the unusual hierarchical structure where primary {332}<113> twins ensure high YS while secondary α′ martensite and {112}<111> twins result in excellent strain hardening ability. • The new alloy exhibits high yield strength while retaining large ductility. • An uncommon hierarchical structure, including primary {332}<113> twins and secondary α′ martensite, is created. • The products of SIMT are the unusual α′ martensite due to the assistance of spinodal decomposition. [ABSTRACT FROM AUTHOR]

In this study, several variations of Ti–6Al–4V β-grains and α′-martensites were observed while changing the combinations of laser parameters and keeping the energy density (ED) constant in the laser powder bed fusion (LPBF) process. Several combinations of laser power, scanning speed, and hatch spacing were considered, resulting in high product density between 99.3% and 100.0%. Accordingly, tensile specimens were fabricated to observe the above strategic fabrication and microstructural effects on tensile properties. At the same time, microhardness was also measured to observe the similarities. However, the size and shape of the β-grains differed significantly, while the scanning speed gradually decreased along with the laser power, with the shape changing from irregular to classically hexagonal and the size increasing sharply. Using the results of differential thermal analysis (DTA), it can be said that most of the tertiary and quaternary α′-martensites formed after the following few thermal cycles below 370 °C. Similarly, the α′-particles decomposed and formed β-particles due to thermal treatment at 370 °C. Therefore, a denser and higher number of tertiary and quaternary α′-martensites and a lower number of primary and secondary α′-martensites occurred, while the cooling rate decreased and the number of thermal cycles increased due to a lower scanning speed. These phenomena increased the hardness 370–395 HV and deteriorated the yield strength 1250-840 MPa. Changes in track overlap (hatch spacing) of 30–50% affect mechanical and microstructural characteristics more than track overlap of 10–25%. The columnar β-grains became longer and wider as the lane overlap increased from 30 to 50%. At the same time, some β-grains also changed to contain finer and fewer α′-martensites. These factors reduced both hardness and tensile properties. • Relation among Ti64 β grain with α′ formation and laser factors in LPBF was studied. • Longer and less α′ formed below 900 °C but finer and majority of α′ formed at 370 °C. • Likewise, finer α′ formed while heat cycle number rose by decreasing scanning speed. • Size, shape and contents of β grain differed with scanning speed and hatch spacing. • Above microstructural features attributed to change mechanical properties widely. [ABSTRACT FROM AUTHOR]

The combination of repetitive melting–solidification cycles, directional laser scanning, and microstructure design using additive manufacturing (AM) is effective for enhancing the mechanical properties of metallic alloys. Because the ultrafine metastable phases in as-built metallic alloys are beneficial to their mechanical properties, there is a design window to enhance the strength–ductility combination by controlling the metastable phase distribution with AM. In this study, ultrafine lamellar-structured maraging steel consisting of martensite and austenite was manufactured using directed energy deposition. The stripe laser scan strategy induced linear (Mo, Ti, Ni)-rich segregations that formed lamellar-structured austenite by reducing the martensitic transformation starting temperature in the high-Ni-content maraging steel. The deformation-induced martensitic transformation and back-stress hardening near the austenite–martensite interfacial region enhanced the plasticity capability of the AM-processed maraging steel and achieved a better strength–ductility combination compared with conventional and recently developed AM-processed maraging steels. This result indicates that artificial microstructural design via AM is important for enhancing the performance of metallic alloys, which can also be achieved with conventional metallic alloys by distributing the metastable phase with processing variable control. [ABSTRACT FROM AUTHOR]

We present here a new pathway of combining hot rolling, coiling and tempering for multiphase heterostructure to achieve a balanced combination of strength and ductility in low alloy steels. The multiphase heterostructure was composed of bainite/martensite and retained austenite (RA), which was tuned by coiling and tempering. The steel coiled at 450 °C exhibited superior mechanical properties compared with the steels coiled at 350 and 550 °C. After tempering at 300 °C, the steel achieved excellent combination of strength and ductility, with twice the elongation and a corresponding product of strength and elongation (PSE) of 25.1 GPa·%. After coiling, 17 vol% of RA was retained. The RA fraction decreased slightly to 14 vol% by tempering, the steel still sustained considerable austenite. Tempering further enabled carbon partitioning and improved the mechanical stability of RA. The continuous transformation induced plasticity (TRIP) effect from stable RA over the entire strain range resulted in good ductility of the steel. [ABSTRACT FROM AUTHOR]

While work-hardening is typically considered in Ti as a prerogative of the β-metastable alloys, this paper introduces a novel perspective, presenting a set of alloy design rules to develop solute lean α + β titanium alloys exhibiting increased work-hardening capabilities. More specifically, reaching this goal is made possible through the development of α + α′ microstructures exhibiting Reorientation Induced Plasticity (RIP) within the α′ martensitic phase. The microstructural requirements for activating RIP and maximizing mechanical properties (i.e., combining high work-hardening, yield strength and ductility levels) are derived from an analysis of the microstructures/mechanical property relationships of various α + α′ samples. A set of design rules is provided. Emphasis is laid on the pivotal role of the chemistry of the α′ martensitic phase in RIP activation and a Molybdenum equivalent chemical criterion is proposed. The α phase is here suggested as a mean to reduce the prior β grain size and the resulting size of the martensite plates. This approach reveals that the versatile thermal treatments leading to α+α' structures broaden the mechanical property landscape, achieving large work-hardening capabilities (typically over 500 MPa) that can be combined with high yield strength (over 800 MPa). [Display omitted] [ABSTRACT FROM AUTHOR]

To improve the mechanical properties of the as-rolled composite under large rolling deformation, the effect of solid solution and aging on the microstructural characteristics and mechanical properties of the as-rolled TiB/(TA15-Si) composite was detailedly investigated. The solid solution temperatures of 1050 °C in the β-phase field and different aging temperatures of 550–750 °C were chosen to regulate the evolution of matrix and silicides. After solid solution treatment, the matrix transformed from elongated and kinkled lamella into acicular martensite microstructure. And the silicides completely redissolved into matrix and a uniform diffusion of Si atom was presented. Meanwhile, the aging temperature can obviously affect the evolution of martensite and silicides. As aging temperature increased, the martensite gradually decomposed into acicular α+β lamella (at 650–700 °C) and abnormal growth of α phase appeared (at 750 °C), which dominated the decreasing trend of microhardness and high-temperature strength. Besides, the silicides gradually precipitated along the prior β phase and TiB whiskers (at 600 °C) as well as α/β phase boundaries (at 650 °C). Eventually, 600 °C and 700 °C strengths of the as-rolled composite can be obviously improved by 39.7 % and 84.6 % after solid solution and aging processes. And the composite aged at 600 °C can obtain the highest 600 °C strength of 992 MPa, which is mainly attributed to martensite, precipitation of silicides and weak α 2 phase. [ABSTRACT FROM AUTHOR]

A bidirectional powder deposition strategy was employed to additively manufacture Ti–15Mo wt% using laser metal deposition. The as-built alloy was subsequently subjected to post-fabrication uniaxial thermomechanical processing at strain rates of 0.00055s−1, 0.0011s−1, 1s−1, and 4s−1, with strains of 20 % and 40 %. Experiments were conducted at room and elevated temperatures. Phase identification, elemental and microstructural characterisation were conducted using x-ray diffraction, energy dispersive spectroscopy and scanning electron microscopy. The three distinct zones, namely the fusion, remelted and heat affected zones, identified in each deposited layer of the as-built microstructure were retained after thermomechanical processing. After processing, electron backscatter diffraction was used to analyse deformation mechanisms. Deformation accommodation in β matrix was predominantly by a combination of slip and α′′ martensite which formed as a primary product at columnar and sub-columnar grain boundaries. However, the operation of { 332 } ⟨ 113 ⟩ and { 112 } ⟨ 111 ⟩ β-twinning was also determined, howbeit with a very small surface fraction. This implies a small surface fraction of secondary α′′ martensite forming within β-twins in the deformed microstructure. Compressive mechanical properties showed a strong dependence on strain rate as higher flow stress and compressive strength were obtained at higher strain rates. Grain structure homogenisation was not achieved after thermomechanical processing as there were α dominated regions as well as martensite/twin dominated regions which implies that an-isotropic tensile properties would emerge after tensile deformation on multiple pre-TMCPed samples. However, columnar β-grains were refined by a combination of precipitated α and deformation induced β-twins and martensite. [ABSTRACT FROM AUTHOR]

This study investigated the influence of parent austenite grain refinement on the intervariant boundary network (population and connectivity) in a lath martensitic steel. Parent austenite grain refinement revealed a progressive reduction in the fraction of the 60° misorientation boundaries in martensite, which was linked to a decrease in the 60 ° / [ 110 ] intervariant boundary population. The phenomenological theory of martensite crystallography demonstrated that the variant selection mechanism altered from the 3-variant clustering (V 1 V 3 V 5) in the coarse parent austenite towards the 4-variant clustering (V 1 V 2 V 3 V 4) in the fine parent austenite grain, due to the change in the lattice parameter of the parent and daughter phase in which the martensite transformation occurs, as measured using in situ neutron diffraction. The change in the variant clustering arrangement with the parent austenite grain refinement led to a progressive promotion of 60 ° / [ 111 ] and 10.5 ° / [ 011 ] intervariant boundaries at the expense of 60 ° / [ 110 ] martensite intervariant boundaries. Subsequently, the connectivity of low energy {110} tilt intervariant boundaries gradually increased through the refinement of parent austenite grain size, eventually reducing the high energy {110} twist boundary connectivity. This change improved the impact toughness of martensite produced from the fine-grained austenite as the weak connectivity of high energy boundaries delays the coalescence of voids, promoting ductile fracture. [ABSTRACT FROM AUTHOR]