Twinning-induced Plasticity Effect Research Articles

A novel high-Mn duplex twinning-induced plasticity lightweight steel with high yield strength and large ductility

In this study, we report a novel high-Mn Fe–21Mn–6Al–4Si–1C (wt.%) duplex lightweight steel with concurrent chemical ordering and twinning-induced plasticity effects. This steel is capable of achieving an exceptional combination of strength and ductility following either cold-rolling and annealing at 1000 °C or subsequent short-term aging at 550 °C. In its as-annealed state, this steel primarily consists of γ-austenite and α-ferrite, with α-ferrite appearing as dispersed particles within fully recrystallized γ grains. Furthermore, L′12- and D03-type ordered nanodomains exist within these two phases, respectively. The aging treatment negligibly affects the size and volume fraction of both the γ-austenite and α-ferrite, yet it enhances the degree of ordering as well as the size and volume fraction of their ordered nanodomains, leading to a rise in yield strength from ∼800 to ∼1062 MPa and a decline in total elongation from ∼60.4% to ∼44.4%. The high yield strength of this steel originates from multiple strengthening mechanisms involving dislocation interactions with solute atoms, ordered nanodomains, γ grain boundaries and γ/α phase boundaries. This steel plastically deforms via planar slip during the initial stages. The rather small γ grain size, coupled with the presence of hard α-ferrite particles, fosters the dynamic slip band refinement (DSBR) effect, thereby enhancing the flow stress sufficiently to trigger deformation twinning in the steel during the later stages. The DSBR effect, combined with the progressive formation of deformation twins, stacking faults and Lomer-Cottrell locks, imparts pronounced strain hardenability to this steel, leading to its outstanding ductility.

Effect of the strain rate on the deformation mechanisms of TWIP-type steel

This paper describes the influence of the strain rate on the microstructure of X55MnAl25-5 steel from the group of high-manganese steels exhibiting the twinning-induced plasticity effect. The tests were carried out at a wide range of strain rates, starting with static tensile tests where the appropriate strain rates were 0.005 s−1, and ending with tests using a flywheel machine at strain rates of 1830 s−1 and 4650 s−1. The increase in the applied strain rate on the example of the tested steel results in hardening. Evolutionary changes leading to the predominance of dislocation slipping over the twinning are observed in the microstructure under static deformation conditions. The increase in the intensity of twinning with the simultaneous decrease in the mobility of dislocations progresses as the strain rate increases. The microstructure evolution was characterized using light optical microscopy and scanning electron microscopy, including electron backscatter diffraction. The qualitative analysis of the deformation twins was performed. For the lowest strain rate, 5·10−4 s−1, deformation twins appear in a single twinning system, while increasing the strain rate leads to the activation of multiple twinning systems. Proceeding to the range of dynamic tests reveals an increase in the participation of the twinning mechanism in the deformation of the examined steel. With an increasing strain rate, the effect of texture on the twinning process activity decreases. Based on the obtained results, a diagram was elaborated that presents the microstructural changes which take place in X55MnAl25-5 steel after deformation under static and dynamic tensile conditions in graphical form.

Microstructural Dependence of the Impact Toughness of TP316H Stainless Steel Exposed to Thermal Aging and Room-Temperature Electrolytic Hydrogenation.

This work deals with the effects of two individual isothermal aging experiments (450 °C/5000 h and 700 °C/2500 h) and the subsequent room-temperature electrolytic hydrogen charging of TP316H stainless steel on its Charpy V-notch (CVN) impact toughness and fracture behavior at room temperature. Microstructural analyses revealed that aging at 700 °C resulted in the abundant precipitation of intermediary phases, namely, the Cr23C6-based carbide phase and Fe2Mo-based Laves phase, whereas aging at 450 °C resulted in much less pronounced precipitation of mostly intergranular Cr23C6-based carbides. The matrix phase of 700 °C-aged material was completely formed of austenitic solid solution with a face-centered cubic (FCC) crystal structure, whereas an additional formation of ferritic phase with a base-centered cubic (BCC) structure was detected in 450 °C-aged material. The performed microstructure observations correlated well with the obtained values of CVN impact toughness, i.e., a sharp drop in the impact toughness was observed in the material aged at 700 °C, whereas negligible property changes were observed in the material aged at 450 °C. The initial, solution-annealed (precipitation-free) TP316H material exhibited a notable hydrogen toughening effect after hydrogen charging, which has been attributed to the hydrogen-enhanced twinning-induced plasticity (TWIP) deformation mechanism of the austenitic solid solution. In contrast, both aging expositions resulted in significantly lowered hydrogen embrittlement resistance, which was likely caused by hydrogen trapping effects at the precipitate/matrix interfaces in thermally aged materials, leading to a reduced TWIP effect in the austenitic phase.

Role of carbon on the enhanced strength-ductility synergy in a high-entropy alloy by multiple synergistic strategies

A new Fe47Mn31Co10Cr10C2 high-entropy alloy (HEA) was developed to investigate the role of carbon in enhancing the strength-ductility synergy by utilizing multiple synergistic strengthening strategies with mitigating the adverse effect of carbon. Compared with the Fe40Mn40Co10Cr10 twinning-induced plasticity (TWIP) HEA, the carbon-doped Fe47Mn31Co10Cr10C2 HEA after annealing at 1173 K for 10 min exhibited a higher strain-hardening rate, and significantly improved yield strength (YS) and ultimate tensile strength (UTS), with maintaining high elongation. The remarkable improvements in YS were mainly attributed to the multiple synergistic strengthening mechanisms, including enhanced grain boundary strengthening, hetero-deformation-induced (HDI) strengthening, and Cr23C6 precipitation strengthening. The higher strain-hardening rate could be ascribed to the enhanced TWIP effect and HDI strain-hardening. The effective stacking fault energy (SFE) of the Fe40Mn40Co10Cr10 and Fe47Mn31Co10Cr10C2 HEAs were calculated by thermodynamics to be 23.4 mJ/m2 and 22.1 mJ/m2, respectively. This revealed that reducing the Mn content effectively counteracted the increasing effect of carbon on the SFE for the carbon-doped HEA. The promotion of deformation twinning in the Fe47Mn31Co10Cr10C2 HEA could be ascribed to the comprehensive effect of the SFE and the stress levels of the alloys. On one hand, compared with the Fe40Mn40Co10Cr10 TWIP HEA, the lower SFE in the Fe47Mn31Co10Cr10C2 HEA led to a drop in the critical stress for deformation twinning. On the other hand, due to the strong multiple synergistic strengthening with carbon doping, the stress levels of the alloys were significantly enhanced, thus making the activation stress of twinning much easier to achieve. This study contributes to the alloy design guidance of properly utilizing carbon to achieve high strength-ductility synergy in alloys by multiple synergistic strategies.

Microstructure evolution and twinning-induced plasticity (TWIP) in hcp rare-earth high- and medium-entropy alloys (HEAs and MEAs) due to tensile deformation

The microstructure evolution due to the tensile deformation of the equiatomic quinary high-entropy alloy Ho-Dy-Y-Gd-Tb (HEA-Fb) is assessed. HEA-Fb has extraordinarily similar alloying elements. It is one of the few hexagonal-close-packed single-phase representatives of HEA. HEA-Fb is compared to the equiatomic quaternary medium-entropy alloy (MEA) Ho-Dy-Gd-Tb with no Y (4-Y). For a hexagonal HEA, in contrast to the cubic HEA, little information on plastic deformation and underlying mechanisms is available. A detailed study using electron microscopy-based multi-scale characterization (SEM, S/TEM, and STEM-EDS) explains significant differences between the ductile behavior of the quaternary MEA 4-Y and the brittle behavior of the quinary HEA-Fb at room temperature. Twinning during plastic deformation is decisive for ductility, which challenges the widely discussed high-entropy effect on the mechanical behavior of the HEA. For the quaternary MEA 4-Y, a twinning-induced plasticity effect is found. In the latter, oxidized twins are present in the undeformed state. In both alloys, the twin orientations are indexed as [2¯201], while the matrices have the perpendicular [112¯0] orientation. Additionally, the analysis of twin structures confirms the importance of twin boundaries as obstacles for dislocations and stacking fault mobilities. The results are discussed in the context of the existing knowledge gaps in the field of hexagonal MEAs and HEAs.

TiZrHfNb refractory high-entropy alloys with twinning-induced plasticity

The twinning-induced plasticity (TWIP) effect has been widely utilized in austenite steels, β-Ti alloys, and recently developed face-centered-cubic (FCC) high-entropy alloys (HEAs) to simultaneously enhance the tensile strength and ductility. In this study, for the first time, the TWIP effect through hierarchical {332<113> mechanical twinning has been successfully engineered into the TiZrHfNb refractory HEAs by decreasing temperature and tailoring the content of Nb to destabilize the BCC phase. At cryogenic temperature, the comprehensive hierarchical {332}<113> mechanical twins are activated in TiZrHfNb0.5 alloy from micro- to nanoscale and progressively segment the BCC grains into nano-scale islands during deformation, ensuring sustainable strain hardening capability and eventually achieving a substantial 35% uniform tensile elongation. In addition, at 77 K, the tensile strength and uniform elongation of TiZrHfNbx alloys can further be adjusted by a moderate increase of Nb. The results above shed new light on the development of high-performance refractory HEAs with a high and adjustable strength and ductility combination down to the cryogenic temperature.

Research on deformation behaviors at different temperatures of lean duplex stainless steel S32101 produced by direct cold rolling process

In this paper, the mechanical properties, work hardening characteristics and deformation mechanism of lean duplex stainless steel (LDSS) S32101 produced by direct cold rolling were studied under different temperature conditions. The yield strength (YS) and tensile strength (TS) of S32101 decreased with the increase of deformation temperature, while the elongation increased first and then decreased. The TS of S32101 at −70 °C was 1052.6 MPa, the elongation remained 57.2%, and the product of strength and elongation (PSE) was as high as 60.2 GPa·%, which was mainly due to the synergistic strengthening of transformation induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects in austenite at cryogenic temperatures. The efficiency of grain refinement at cryogenic temperature was improved by TRIP effect and deformation twins (DTs), indicating that it had the potential to be applied at low temperature (−70 ∼ −30°C). Moreover, the high density nanotwins and the strong interaction between the dislocation and nanotwin bundles also contributed to strain hardening at low temperatures and thus better comprehensive mechanical properties could be achieved.

ω-Strengthened Ti-23Nb alloy with twinning-induced plasticity developed via reverse martensitic transformation

The twinning-induced plasticity (TWIP) effect contributes to high strength and ductility synergy in metastable β Ti alloys. Moreover, stress-induced α″ martensites (SIMs) are usually concurrent with TWIP, resulting in low yield strength. As a new pathway, the production of ω-strengthened metastable β Ti alloys from full α″ Ti alloys can be accomplished via reverse martensitic transformation. However, the deformation mechanisms of the metastable β Ti alloys acquired by this pathway have not been adequately reported. Accordingly, in this study, the full α″-type solution-treated (ST) samples of Ti-23Nb at.% alloy were annealed at 573 K followed by slow cooling to acquire the air-cooled (AC) and furnace-cooled (FC) samples with β phase and ω precipitates. Mechanical properties and deformation behaviors of these samples were systematically investigated. The AC and FC samples exhibited higher yield strengths than the ST samples. Although SIMs still existed in the AC samples, they were completely suppressed in the FC samples. The annealed samples demonstrated high ductility due to the TWIP effects stemming from {332}<113>β twins. Furthermore, a distorted α″ phase exhibiting the abnormal orientation relationships (ORs) <11¯0>β//<001>α″, {332}β//{100}α″, and {110}β//{110}α″ with both β matrix and twin was revealed by high-resolution transmission electron microscopy and crystallographic analyses. These ORs were governed by the lattice distortion associated with a unique β matrix → distorted α″ → {332}β twin pathway. This study suggests that metastable β Ti alloys with enhanced mechanical properties can be achieved via the α″ → β reverse martensitic transformation and ω precipitation.

Effect of rolling on the microstructure and mechanical properties of (FeCrNi)94Ti3Al3 medium entropy alloy fabricated via selective laser melting

The microstructure evolution and mechanical properties of the (FeCrNi)94Ti3Al3 medium entropy alloy, fabricated through selective laser melting, were investigated after warm-rolling. The as-built (FeCrNi)94Ti3Al3 alloy exhibited a heterogeneous microstructure with alternating coarse face-centered cubic (FCC) phase grains (∼20 μm) and fine body-centered cubic (BCC) phase grains (∼1 μm). However, after warm-rolling and subsequent recrystallization, the alloy transformed into a dual-phase equiaxed grains structure consisting of FCC and BCC phases with an average grain size of approximately 2.8 μm. Additionally, the previously banded distribution of the BCC phase changed to a random distribution. Notably, during the deformation process, a twinning-induced plasticity (TWIP) effect was observed in the warm-rolled and recrystallized (FeCrNi)94Ti3Al3 alloy, resulting in excellent ductility and a wide work-hardening platform.

Enhanced yield stress and reduced elastic modulus of metastable β Ti–Nb alloys deformed by equal channel angular pressing attributed to the Synergistic effect of deformation mechanisms

This work aims to obtain Ti–Nb alloys with a high yield stress, good plasticity, and low elastic modulus. To this end, metastable β Ti–Nb alloys were subjected to multipass equal channel angular pressing (ECAP) deformation at room temperature, and a thorough investigation of their microstructure evolution, deformation mechanisms, and strengthening/toughening mechanisms was conducted through X-ray diffraction, transmission electron microscopy, electron backscatter diffraction, and tensile tests. In the initial deformation stages, multiple deformation mechanisms are observed, including dislocation slip, {332} <113> twinning, stress-induced martensite (SIM) α″-phase, {112} <111> twinning, and stress-induced ω-phase. However, as the strain accumulates, the ω-phase disappears, the volume fraction of the SIM α″-phase gradually decreases, and the dominant deformation mechanism changes to dislocation slip and {332} <113> twinning. The ECAP deformation results in a complex-network microstructure and the precipitation of the nanoscale SIM α″-phase, which leads to the twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects. As a consequence, the Ti–Nb alloy simultaneously exhibits a high yield stress and a good plasticity. Furthermore, the high-density dislocations and interfaces generated by the ECAP deformation reduce the stability of the β-phase, and the introduced nanopores cause a decrease in density, resulting in a decrease in the elastic modulus of the alloy.

Effect of grain size and segregation on the cryogenic toughness mechanism in heat-affected zone of high manganese steel

High manganese steels have been nominated as desirable materials used for various cryogenic applications due to their excellent cryogenic toughness caused by twinning. High manganese steel welded joints were obtained through gas metal arc welding, and the microstructural differences at three subzones of heat-affected zone (HAZ) were utilized. The aim was to investigate the effect of grain size and Mn/C/Cu segregation on the cryogenic toughness mechanism of high manganese steel welded joints. The results indicate that the average impact absorbed energies at −196 °C for 1.0, 3.0, and 5.0 mm from the fusion line (denoted by FL + 1, FL + 3, and FL + 5) is 118.6 J, 102.4 J, and 117.2 J, respectively. Compared with base metal, the embrittlement occurred in HAZ. Large grains improve toughness, while small grain boundaries, segregation, and precipitation in HAZ deteriorate toughness. Larger grain size facilitates the primary twins with thinner inter-twin spacing and twin thickness, and the high density of these primary twins inhibits the activation of secondary twins. This, in turn, enhances cryogenic impact toughness. Although the stacking fault energy (SFE) throughout the welding joint lays within the range for the twinning-induced plasticity effect, the enrichment of Mn/C/Cu slightly increases the local SFE and critical resolved shear stress for twining within the segregation zone. This phenomenon makes the activation of primary twins more challenging. Furthermore, the C segregation at grain boundaries leads to the precipitation of fine Cr23C6 carbides. These carbides serve as crack initiation points during impact tests, contributing to degradation in cryogenic impact toughness.

Cooperative roles of twinning, transformation and strain partitioning on the excellent cryogenic strength-toughness synergy of dual-heterogeneous metastable high entropy alloys

In a metastable Fe50Mn30Co10Cr10 high entropy alloy (HEA), a dual-heterogeneous structure, encompassing both grain size and dislocation heterogeneity, has been constructed to obtain excellent cryogenic mechanical performance by varying the annealing temperature (630 and 740 °C) following severe deformation. The HS-630 sample, consisting of recovered coarse-grains (CG), recrystallized fine-grains (FGs) and un-recrystallized ultrafine-grains (UFGs), possesses higher yield strength (YS) of ∼965 MPa, ultimate tensile strength (UTS) of ∼1680 MPa and uniform elongation (UEL) of ∼45 % when compared to the fully-recrystallized HS-740 sample at liquid nitrogen temperature (LNT). The difference in YS is primarily caused by the grain boundary and dislocation strengthening of UFGs, whereas the higher ductility of the HS-630 sample is associated with the hierarchical activation of transformation- and twinning-induced plasticity (TRIP and TWIP) effects, and compatible strain partitioning between heterogeneous domains. The dual-heterogeneity results in different critical stresses of ε-phase nucleation in heterogeneous domains, and promotes sustainable TRIP effect in CGs and the emergence of TWIP effect in UFGs with strain progressing. Moreover, the strain of the ductile transformed ε-phase is mainly accommodated by non-basal slip, which contributes to the strain hardening in the later deformation stage. Furthermore, the higher cryogenic impact toughness of the HS-630 sample can be attributed to the synergistic effects of the fracture mode, the transformation characteristics, and the heterogeneous strain partitioning.

Impact Toughness Dependent on Annealing Temperatures in 0.16C-6.5Mn Forged Steel for Flywheel Rotors

For the application of forged medium-Mn steels on flywheel rotors, the effect of annealing temperatures from 300 °C to 650 °C on the impact toughness of 0.16C-6.5Mn forged steel was investigated to demonstrate the microstructural characteristics and austenite reverse transformation determining the impact toughness. The results obtained through standard Charpy V-notch impact tests at ambient temperature show that the impact absorbed energy holds at lower than 10 J almost constantly at annealing temperatures of 300 °C to 500 °C, and a representative intergranular fracture is presented. At an annealing temperature of 600 °C, the impact absorbed energy increases to 147 J, with the ductile fracture characteristics showing plenty of fine dimples, and the high impact toughness is attributed to the high volume fraction above 30% and the moderate stability of reverted austenite. Subsequently, the annealing temperature rises higher than 600 °C, the impact absorbed energy decreases, and the fracture morphology shows brittleness characterized by more flat facets of intergranular fractures and small quasi-cleavage facets, presumably corresponding to the insufficient transformation and twinning-induced plasticity effect due to weakening the Mn partitioning from quenched martensite to reverted austenite, which results in lower austenitic stability. Furthermore, the ductile-to-brittle transition temperature (DBTT) of the 0.16C-6.5Mn forged steel annealed at 600 °C, which holds the highest impact absorbed energy, and is explored for the possibility of flywheel rotor application in a service environment. The DBTT reaches −21 °C, obtained through the Boltzmann function, and the impact absorbed energy is approximately 72 J.

Discovery of microsegregation-aided transformation and twinning-induced plasticity in low Mn steel through directed energy deposition of functionally graded materials

Functionally graded materials (FGMs) combining two dissimilar steels, stainless steel 316 L and high-strength low-alloy steel, were additively manufactured using directed energy deposition. High-throughput characterization of the dissimilar steel FGM led to the discovery of a low Manganese (Mn<1 wt.%) TRIP (transformation-induced plasticity) and TWIP (twinning-induced plasticity) steel with a metastable microstructure enabled by additive manufacturing. Microsegregation from non-equilibrium solidification caused phase stability and stacking fault energy heterogeneity, leading to TRIP and TWIP effects in the as-built condition without heat treatment. Tensile testing of the new as-built TRIP and TWIP steel resulted in ultimate tensile strength of 960 MPa, yield strength of 415 MPa, total elongation of 26 %, and a unique strain hardening rate that increases after yielding. We compare experimental measurements of microsegregation with thermodynamic modeling to discuss the impact of microsegregation on phase stability, stacking fault energy, and solidification cracking susceptibility in additively manufactured FGMs. The highlights of this work include the discovery of a novel pathway for achieving TRIP and TWIP effects in additively manufactured steels without heat treatment. This work also shows that the TWIP effect can be introduced without high Manganese content playing a critical role in adjusting stacking fault energy.

Designing the composition and optimizing the mechanical properties of non-equiatomic FeCoNiTi high-entropy alloys

Research into new high-entropy alloys (HEAs) holds significant promise for advancing aerospace materials. Nevertheless, the complexity of their composition presents formidable challenges in designing HEAs with both high strength and plasticity. In this study, molecular dynamics (MD) simulation was used to explore the optimal composition combination of FeCoNiTi high-entropy alloys with non-equiatomic ratios. Shear modulus (G) characterizes strength, while stacking fault energy (SFE) characterizes plasticity and ductility. Through molecular dynamics (MD) simulations, elastic constants (C11, C12, and C44) and generalized stacking fault energies (GSFEs) for 18 alloy variants were computed. Additionally, the elastic modulus (G, E, and B) for all components was estimated using the Voigt–Reuss–Hill (VRH) averaging method. Following standard guidelines, the composition for the FeCoNiTi alloy was predicted as Ni: 30%–60 %, Co: 30%–50 %, Fe: 5%–10 %, and Ti: 5%–10 %. Subsequently, five optimized variants underwent tensile calculations to identify the most suitable composition. The results indicate that the Fe0.05Co0.4Ni0.5Ti0.05 HEA exhibits the best combination of strength and plasticity. Microscopically, its enhanced plasticity is attributed to the twinning-induced plasticity (TWIP) effect. While Fe0.1Co0.4Ni0.4Ti0.1 HEA does not outperform the other variants, it displays a martensitic transformation and deformation twins with increased strain, which are mechanisms not observed in other components. The study of non-equiatomic FeCoNiTi HEAs offers a theoretical foundation for future alloy development. This innovative alloy design method fosters the rapid development of high-performance HEAs, facilitating their rapid development and application.

A Review of Deformation Mechanisms, Compositional Design, and Development of Titanium Alloys with Transformation-Induced Plasticity and Twinning-Induced Plasticity Effects

Metastable β-type Ti alloys that undergo stress-induced martensitic transformation and/or deformation twinning mechanisms have the potential to simultaneously enhance strength and ductility through the transformation-induced plasticity effect (TRIP) and twinning-induced plasticity (TWIP) effect. These TRIP/TWIP Ti alloys represent a new generation of strain hardenable Ti alloys, holding great promise for structural applications. Nonetheless, the relatively low yield strength is the main factor limiting the practical applications of TRIP/TWIP Ti alloys. The intricate interplay among chemical compositions, deformation mechanisms, and mechanical properties in TRIP/TWIP Ti alloys poses a challenge for the development of new TRIP/TWIP Ti alloys. This review delves into the understanding of deformation mechanisms and strain hardening behavior of TRIP/TWIP Ti alloys and summarizes the role of β phase stability, α″ martensite, α′ martensite, and ω phase on the TRIP/TWIP effects. This is followed by the introduction of compositional design strategies that empower the precise design of new TRIP/TWIP Ti alloys through multi-element alloying. Then, the recent development of TRIP/TWIP Ti alloys and the strengthening strategies to enhance their yield strength while preserving high-strain hardening capability are summarized. Finally, future prospects and suggestions for the continued design and development of high-performance TRIP/TWIP Ti alloys are highlighted.

Laser-directed energy deposition of a high performance additively manufactured (CoCrNi)94(TiAl)6 medium-entropy alloy with a novel core-shell structured strengthening phase

Additively manufactured (CoCrNi)94(TiAl)6 medium-entropy alloys (MEAs) were fabricated by laser-directed energy deposition. The strengthening induction mechanism in the additively manufactured (CoCrNi)94(TiAl)6 MEAs was investigated using electron backscatter diffraction and transmission electron microscopy. The results showed that the addition of TiAl powder led to increases of 29.6% and 43.5% in the ultimate tensile strength (UTS) and yield strength (YS), respectively, at 298 K, and a decrease of 17.1% in the elongation. For the samples tested at 77 k after the addition of the TiAl powder, the UTS and YS improved by 26.6% and 28.0%, respectively, and the elongation increased by 26.3%. The microstructural observation results indicated that the matrix grains changed from initial columnar grains with a uniform growth direction to fine dendrites. Numerous TiO clad Al2O3 strengthening nano-precipitates with a novel core–shell structure were found in the additively manufactured (CoCrNi)94(TiAl)6 MEAs, and both featured a face-centered cubic structure. No crystallographic orientation was observed between the nano-precipitates and the matrix. At 298 K, the dislocation line bypassed the nano-precipitates during the plastic deformation period and Lomer–Cottrell locks appeared near the nano-precipitates, thereby strengthening the additively manufactured (CoCrNi)94(TiAl)6 MEAs. At 77 K, plastic deformation resulted in the synergistic deformation of the nano-precipitates and matrix, the formation of a large number of dislocations, and a twinning induced plasticity effect, which further increased the strength and plasticity of the additively manufactured (CoCrNi)94(TiAl)6 MEAs.

Enhancing strength and ductility in the nugget zone of friction stir welded 7Mn steel via tailoring austenitic stability

The martensite often appears in the nugget zone (NZ) of friction stir welding (FSW) 7 wt.% Mn steel due to low austenite stability, deteriorating ductility and toughness. In this work, a 7 wt.% Mn steel was subjected to FSW, and preheating was used to tailor the austenitic stability to greatly improve the strength–ductility combination of the NZ. The austenitic deformation behavior and strain hardening mechanism in the NZ were systematically investigated. The microstructure of the as-welded NZ was composed of ultrafine blocky ferrite, austenite, and small amounts of martensite, whereas the as-preheated NZ contained ultrafine blocky ferrite and austenite, and the concentration of Mn in austenite was increased from 8.4 wt.% to 10.7 wt.%. This enhanced the austenitic stability, resulting in a significant increase in the volume fraction of austenite in the as-preheated NZ from 37.3% to 66.4%. The product of strength and elongation (PSE) in the as-preheated NZ increased dramatically from 42.6 GPa% to 67.1 GPa%, depending on a persistent high strain hardening rate (SHR). Multiple strain-hardening mechanisms were revealed. The austenite with enhanced stability can provoke sustained transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects, and massive dislocation multiplication occurs during tension, resulting in strong strain hardening.

TRIP-TWIP effect in deformation mechanisms of Co–Cr–Ni medium entropy via molecular dynamics simulations

Recently, Co–Cr–Ni medium-entropy alloys (MEAs) have garnered significant attention in the materials field owing to exceptional properties. However, the various mechanisms of transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effects with variation of chemical compositions, strain rates, temperature and grain size in polycrystalline Co–Cr–Ni MEAs remains challenging. By utilizing molecular dynamics simulations, the uniaxial tensile deformation of polycrystalline Co–Cr–Ni MEAs, including equiatomic CoCrNi, and non-equiatomic Co10(CrNi)90, Cr10(CoNi)90 and Ni10(CoCr)90, has been investigated. The influence of strain rates and deformation temperatures on the mechanical properties and deformation mechanisms of CoCrNi MEA has also been explored under uniaxial tensile loading. Further, dislocation multiplication, slip motion, microstructure evolution, and their interplay with tensile deformation under different conditions have also been studied. Additionally, the relationship between different grain sizes and the flow stress of polycrystalline CoCrNi MEA has also been discussed. The findings reveal that Co10(CrNi)90 MEA exhibits highest yield stress, Young's modulus, and flow stress. While CoCrNi MEA and Cr10(CoNi)90 MEA with equiatomic ratios display similar mechanical properties, Ni10(CoCr)90 MEA demonstrates superior ductility owing to the strong TWIP and TRIP effects. Besides, strain rate, deformation temperature, and grain size all have significant impacts on the deformation mechanisms of polycrystalline CoCrNi MEA.

TWIP-assisted Zr alloys for medical applications: Design strategy, mechanical properties and first biocompatibility assessment

This study proposes a novel strategy for the design of a new family of metastable Zr alloys. These alloys offer improved mechanical properties for implants, particularly in applications where conventional stainless steels and Co-Cr alloys are currently used but lack suitability. The design approach is based on the controlled twinning-induced plasticity (TWIP) effect, significantly enhancing the ductility and strain-hardenability of the Zr alloys. In order to draw a “blueprint” for the compositional design of biomedical TWIP (Bio-TWIP) Zr alloys—using only non-toxic elements, the study combines d-electron phase stability calculations (specifically bond order (Bo) and mean d-orbital energy (Md)) with a systematic experimental screening of active deformation mechanisms within the Zr-Nb-Sn alloy system. This research aids in accurately identifying the TWIP line, which signifies the mechanism shift between TWIP and classic slip as the primary deformation mechanism. To demonstrate the efficacy of the TWIP mechanism in enhancing mechanical properties, Zr-12Nb-2Sn, Zr-13Nb-1Sn, and Zr-14Nb-3Sn alloys are selected. Results indicate that the TWIP mechanism leads to a significant improvement of strain-hardening rate and a uniform elongation of ∼20% in Zr-12Nb-2Sn, which displays both {332}<113> mechanical twinning and dislocation slip as the primary deformation mechanisms. Conversely, Zr-14Nb-3Sn exhibits the typical mechanical properties found in stable body-centered cubic (BCC) alloys, characterized by the sole occurrence of dislocation slip. Cell viability tests confirm the superior biocompatibility of Zr-Nb-based alloys with deformation twins on the surface, in line with existing literature. Based on the whole set of results, a comprehensive design diagram is proposed.