Twinning-induced Plasticity Research Articles

Hydrogen embrittlement of twinning-induced plasticity steels: Contribution of segregation to twin boundaries

Metallic materials, especially steel, underpin transportation technologies. High-manganese twinning induced plasticity (TWIP) austenitic steels exhibit exceptional strength and ductility from twins, low-energy microstructural defects that form during plastic loading. Their high-strength could help light-weighting vehicles, and hence cut carbon emissions. TWIP steels are however very sensitive to hydrogen embrittlement that causes dramatic losses of ductility and toughness leading to catastrophic failure of engineering parts. Here, we examine the atomic-scale chemistry and interaction of hydrogen with twin boundaries in a model TWIP steel by using isotope-labelled atom probe tomography, using tritium to avoid overlap with residual hydrogen. We reveal co-segregation of tritium and, unexpectedly, oxygen to coherent twin boundaries, and discuss their combined role in the embrittlement of these promising steels.

Improvement of hot deformability of an optimized Nb-Ti-Si based alloy: Twining induced plasticity

The dynamic softening behavior of silicides in the optimized Nb–24Ti–16Si–2Al–4Fe–5V alloy during hot deformation was investigated. Except for the constituent phases Nbss and β-(Nb, X)5Si3, the Nb4FeSi silicide is obtained in the optimized microstructure. Under present deformation conditions, a new softening mechanism is observed for the alloy: the stress concentration around the small-sized β-(Nb, X)5Si3 grain excites the twinning in Nb4FeSi grain to generate twinning-induced plasticity (TWIP). The appearance of the TWIP mechanism significantly ameliorates the hot deformability of the alloy. Meanwhile, the contribution of large-sized β-(Nb, X)5Si3 to softening relies on its own continuous dynamic recrystallization, while the TWIP activity of adjacent Nb4FeSi grain is weakened.

Enhancing strength and ductility in a near medium Mn austenitic steel via multiple deformation mechanisms through nanoprecipitation

High-Mn austenitic steels, usually deformed via the TWIP (twinning induced plasticity) mechanism, suffer high cost and other technical issues (e.g., hot-dip galvanizing, welding, etc.) from the high Mn content. Nevertheless, decreasing the Mn content would result in a shift of deformation mechanisms from TWIP to TRIP (transformation induced plasticity), usually causing quasi-cleavage brittle fracture. Herein, we report that by massive nanoprecipitation of coherent disordered particles, the grain sizes of a near medium Mn austenitic steel are successfully refined to 0.9 ± 0.4 μm, leading to a significant stacking fault energy increment of 6.4–7.9 mJ m–2 and accordingly, the transition of deformation mechanism from TRIP to multiple deformation mechanisms, namely, stacking faults, dislocation slip, nanotwinning and ԑ-martensite transformation. Moreover, the high-density of coherent nanoprecipitates effectively refines ԑ martensite and nanotwins from 20–500 nm and 10–50 nm to a few atomic columns and 1–15 nm, respectively. More importantly, these deformation mechanisms were sequentially activated at different deformation stages, resulting in a consistently high work hardening rate. Based on the synergistic refinement effects of grains, twins and martensite and the transition of deformation mechanism, a near medium Mn ultrafine-grained (UFG) austenitic steel with 15 wt.% Mn is developed, which shows a unique combination of high tensile strength (1210 ± 19 MPa) and large elongation (72 ± 6 %). These findings provide a new route to addressing the trade-off between the Mn content and mechanical performance of high-Mn austenitic steels which could facilitate their widespread applications.

Thermodynamics, microstructure evolution and mechanical properties of Al- and C-added CoFeMnNi multi-principal element alloys

Navigating through the phase space of multi-principal element alloys (mpeas) with desired microstructures and outstanding properties is challenging due to extremely high degrees of freedom in alloy composition and processing conditions. Separate Al and C additions and the resulting precipitation of hard phases are commonly used to strengthen CoCrFeMnNi-type alloys. However, the combined alloying effect and especially the high hardening potential of C in solid solution of mpeas are rarely investigated, but the few existing investigations reveal a high potential. Therefore, the properties of the Al0−22.1C0−2.3(CoFeMnNi)100−75.6 (at.%) system were comprehensively investigated, where Cr was intentionally removed to keep C in solid solution. For theoretical and experimental screening of the system, a custom calphad database was extended, samples were manufactured by additive manufacturing (am) and annealed between temperatures of 1150 and 550 °C. Detailed microstructural and mechanical analysis on 40 different revealed the properties of the equiatomic CoFeMnNi alloy and the effect of the separate and combined additions of Al and C, including comprehensive precipitate characterization. The system was especially potent in its room temperature tensile properties in the Al- and C-added fcc-based region, where it caused the activation of twinning-induced plasticity (twip) and combined high strength and ductility increases.

Inhomogeneity of microstructure and mechanical properties in compression-type bulk formed specimens

The effect of elongation and spreading in a material on strain and microstructure inhomogeneity during compression-type bulk forming processes was investigated to improve the strain homogeneity of the materials. For systematic investigation, four different compression-type forming processes, namely wire flat rolling, plate flat rolling, wire caliber rolling, and uniaxial compression, were compared. Strain inhomogeneity and macroscopic shear bands (MSBs) were clearly observed during the compression-type bulk forming processes based on a comparative study on the shape change, effective strain and hardness distributions, and microstructural evolution in twinning-induced plasticity steel. The flat-rolled wire had the largest strain inhomogeneity, followed by the caliber-rolled wire, compressed specimen, and flat-rolled plate in the listed order. The inhomogeneity of the strain distribution along the horizontal direction tended to increase with the pure spreading during the forming process. Meanwhile, the inhomogeneity of the strain distribution along the vertical direction increased with decreasing elongation during the process. Therefore, a small pure spreading and large elongation of the specimen result in more homogeneous and higher quality products in the compression-type bulk forming process. For instance, the flat-rolled plate exhibited weak MSBs and low strain inhomogeneity because of the small pure spreading and large elongation during the forming process.

Effect of Si content and annealing temperatures on microstructure, tensile properties of FeCoCrNiMn high entropy alloys

In the present study, FeCoCrNiMnSix (x = 0, 0.1, and 0.2, molar ratio) high-entropy alloys (HEAs) were prepared via arc melting, and subsequently deformed via cold rolling and annealing (CRA). The effects of Si content and annealing temperature on the microstructural evolution and tensile properties of the alloys were systematically investigated. The results indicate that a single-phase FCC structure of coarse-grained FeCoCrNiMnSix HEAs was obtained after homogenized annealing (HOA), and the distribution of each element was uniform. When the molar ratio of Si was increased from 0 to 0.2, the strength of the HOA HEAs increased by nearly 50%, and the elongation did not decrease, largely due to solid solution strengthening and the reduction in stacking fault energy. Conversely, as Si content increased, the recrystallization ratio decreased during annealing. A heterogeneous grain structure composed of recrystallized and residual deformation regions (high density of dislocations and deformation twinning) was obtained by changing the CRA temperature and Si content. A heterogeneous grain structure of CRA HEAs with an annealing temperature of 700 ℃ and Si content of 0.2 exhibits approximately fourfold higher yield strength (733 MPa) compared to that of HOA FeCoCrNiMn HEAs (185.27 MPa). The structure’s tensile strength and ductility were measured at 959 MPa and 29.5%, respectively. The high strength was mainly attributed to the combined effect of multiple strengthening mechanisms. Heterogeneous deformation-induced strain hardening and twinning-induced plasticity provide good ductility.

Effect of manganese content on the hydrogen embrittlement of twinning-induced plasticity (TWIP) steels under hydrogen charging and hydrogen environment

The hydrogen embrittlement behavior of two stabilized austenitic twinning-induced plasticity (TWIP) steels under high-pressure thermal hydrogen charging and hydrogen environment were investigated by slow strain rate tensile and fatigue crack growth tests. The results show that the two TWIP steels with Mn content (16% and 25%) do not produce strain-induced martensite after deformation, but both show a certain susceptibility to hydrogen embrittlement after hydrogen charging, in which the TWIP steel with higher Mn content shows weaker hydrogen embrittlement. This is mainly due to the high hydrogen embrittlement susceptibility of deformation twins. In the H2 environment, compared with slow strain rate tensile tests, fatigue crack growth tests is more likely to reflect the hydrogen embrittlement susceptibility of the TWIP steel, and the hydrogen environment is more likely to promote the crack growth of the TWIP steel with lower Mn content. This is related to the inter-granular cracking of TWIP steel with low Mn content under H2 environment. The increase of Mn content reduces the hydrogen diffusion coefficient and hydrogen solubility of TWIP steel, which is also one of the reasons for the weak hydrogen embrittlement of TWIP steel with high Mn content in hydrogen charging and hydrogen environment.

Effect of intermediate annealing on microstructure and mechanical property of a Fe−19Mn−0.6C TWIP steel

Annealing is an effective strategy to improve the properties of high-strength twin-induced plasticity (TWIP) steels, however, the adaption of intermediate annealing during cold rolling (CR) is scarcely studied. Here, the Fe-19Mn-0.6C TWIP steel was subjected to CR-annealing and CR-intermediate annealing-CR-annealing processes at room temperature to determine the role of intermediate annealing in the improvement of microstructure and mechanical properties. The total cold-rolled reduction in both processes is 75%. The morphological and phase characterizations of the TWIP steel annealed for 1 h showed that uneven element distribution had occurred as the annealing temperature was greater than the recrystallization start temperature, causing the presence of minor carbides. Moreover, the carbides vanished at the recrystallization end temperature and were quantitatively analyzed content via the refined XRD. Finally, the recrystallized austenite grains completely replaced the cold-deformed microstructures. At the same total CR reduction of 75%, the TWIP steel exerted intermediate annealing facilitates the formation of twins, endowing the tensile strength vast increase. This would provide a significant reference to improve the mechanical properties of steels via annealing.

Influence of annealing on the microstructure and Charpy impact toughness of wire arc additive manufactured Ti5111 alloy

The microstructure of wire arc additive manufactured (WAAMed) titanium alloy was characterized by brittle martensitic, which was significantly different from that of wrought alloy. It was generally believed that brittle martensitic was detrimental to the toughness of titanium alloys. In order to obtain WAAMed titanium alloy with excellent combination of strength and toughness, Ti5111 alloy, which is a newly developed deformation twinning induced plasticity(TWIP) alloy with an excellent combination of impact toughness and strength, was prepared by arc additive manufacturing(WAAM) technology and then annealed at 900 °C + air cooling(AC) and 950 °C+ air cooling. Microstructure, tensile properties and impact toughness of WAAMed Ti5111 alloy were investigated. Electron backscatter diffraction (EBSD) observation was adopted to identify the deformation twins activated in impact samples. Results showed that as-built WAAMed Ti5111 with excellent toughness was obtained, which was 42.8% higher than that of WAAMed Ti–6Al–4V alloy. After annealing, more deformation twins and secondary twins were activated during impact loading, which could effectively improve the toughness of WAAMed Ti5111.

Effects of heterogeneous plastic strain on hydrogen-induced cracking of twinning-induced plasticity steel: A quantitative approach

Heterogeneous plastic strain is essential for understanding hydrogen-induced crack (HIC) initiation in twining-induced plasticity (TWIP) steel with relatively no microtexture. In this study, electron backscattered diffraction technology was used to quantitatively characterize the heterogeneous plastic strain after a slow strain rate test with hydrogen charging, and the deformation mode and HIC initiation mechanism were clarified. From a statistical analysis of more than 100 grain boundaries, the cracked grain boundary and its vicinity have high heterogeneous strain; that is, the geometrically necessary dislocation density is high, and vice versa. Therefore, high strain incompatibility is the key factor for HIC initiation.

Finite strain crystal plasticity-phase field modeling of twin, dislocation, and grain boundary interaction in hexagonal materials

Twin, dislocation, and grain boundary interaction in hexagonal materials, such as Mg, Ti, and Zr, has critical influence on the materials’ mechanical properties. The development of a microstructure-sensitive constitutive model for these deformation mechanisms is the key to the design of high-strength and ductile alloys. In this work, we have developed a mechanical formulation within the finite strain framework for modeling dislocation slip- and deformation twinning-induced plasticity. A dislocation density-based crystal plasticity model was employed to describe the dislocation activities, and the stress and strain distributions. The model was coupled with a multi-phase-field model to predict twin formation and twin-twin interactions. The coupled model was then employed to study twin, dislocation, and grain boundary interactions in Mg single- and polycrystals during monotonic and cyclic deformation. The results show that twin-twin interactions can enhance the strength by impeding twin propagation and growth. The role of dislocation accommodation on twin-twin interactions was twofold. Dislocation slip diminished twin-twin hardening by relieving the development of back-stresses, while it effectively relaxed the stress concentration near twin-twin intersections and thus may alleviate crack nucleation. The plastic anisotropy in each grain and the constraints imposed by the local boundary conditions resulted in stress variations among grains. This stress heterogeneity was responsible for the observed anomalous twinning behaviour. That is, low Schmid factor twins were activated to relax local stresses and accommodate the strain incompatibility, whereas the absence of high Schmid factor twins was associated with slip band-induced stress relaxation.

Origin, parameters, and underlying deformation mechanisms of propagating deformation bands in irradiated 316L stainless steel

Lüders-type propagating deformation bands were observed in specimens of irradiated 316L stainless steel samples removed from Spallation Neutron Source target vessels after service. Mechanical testing with digital image correlation (DIC) and in-situ tensile testing with scanning electron microscopy electron backscatter diffraction showed that the observed Lüders-type behavior was not related to the known transformation-induced plasticity or twinning-induced plasticity behavior. Instead, the phenomenon occurs at small local strain values before a significant amount of martensite or deformation twins appear in the microstructure. Microstructural analysis and in-situ mechanical test results suggest Lüders-type band formation and propagation were related to the appearance and evolution of defect-free channels—analogous to slip bands. A modified Swift equation with a Ludwigson-like component was offered to rationalize the phenomenon and model the strain-softening processes at small strain values. The results indicate complex microstructural processes and deformation mechanisms were active at small strain values and underline the benefits of advanced mechanical test approaches such as DIC.

Computational simulation of thermo-mechanical field and fluid flow and their effect on the solidification process in TWIP steel welds

The primary austenitic solidification mode of Twinning Induced Plasticity (TWIP) steel weld joints is one of the most important issues limiting its weldability. In order to gain knowledge on this topic, a comprehensive study was carried out through numerical simulation of diffusive-convective phenomena of fluid flow and heat transfer of the melted material in the weld pool of TWIP steel weld joints. Weldments were performed using different heat inputs and constraint conditions. The predictions of convective motion, thermal history and cooling rates estimated by finite element (FE) and finite volume (FV) simulations were correlated with chemical species segregation (Mn, C, Al and Si) and the solidification mode in the fusion zone. The microstructural characterization of weld joints and the numerical results provided a full analysis of the weld pool solidification process. Results revealed a relationship between both the dendrite size and Mn segregation with the estimated velocity field of liquid material in the melted zone. Moreover, the presence of δ-ferrite was experimentally detected in some weld joints and associated with the convective movement of melted material and chemical segregation. The cellular dendritic growth mode found in some weld joints contributed to the formation of δ-ferrite and promoted the transition of the solidification mode from austenitic to ferritic-austenitic. As well δ-ferrite allowed to mitigate hot-cracking and interrupted the Mn-enriched liquid film migration towards the heat-affected zone, avoiding the formation of liquation cracks. The presence of δ-ferrite phase maintained a high mechanical strength providing improved elongation regarding the welded material mechanical properties.

Active screen plasma nitriding of Fe-24Mn-2Al-0.45C TWIP steel: Microstructure evolution and a synergistic selective oxidation mechanism

In this study, Fe-24Mn-2Al-0.45C twinning-induced plasticity (TWIP) steel was investigated after active screen plasma nitriding (ASPN) at 350–480 °C. Up to 450 °C, the ASPN-treated TWIP steel showed a double-layered structure, with a topmost Fe4N-type γ’ layer and an underlying interstitial solid solution. The underlying nitrogen interstitial solid solution obtained on ASPN-treated TWIP steel can be identified as γN(i), being expanded austenite with small lattice expansions at low nitrogen (and carbon) absorption levels. However, the 480 °C ASPN-treated TWIP steel surface is composed of an unexpected MnO-containing ferritic topmost layer and a thick underlying γN(i) layer, which corresponds to a hardness-depth profile different to those obtained at 350–480 °C. Hence, we propose and elucidate a selective oxidation mechanism (that occurred alongside nitrogen inward diffusion) on TWIP steel during ASPN treatment, which can be attributed to (a) an elevated treatment temperature that allowed Mn migration, (b) a processing atmosphere with low oxygen partial pressure, and (c) a high-Mn substrate chemical composition with low Al/Mn (or Si/Mn) ratio.

In situ investigation on plastic deformation behaviors in austenite-ferrite heterostructured stainless steel

Heterostructured (HS) materials have attracted extensive attention due to their superior mechanical properties. However, there are still many fundamental issues to be solved, like the microstructural evolution process of the hetero-zones during deformation. Here we reported a HS 316L stainless steel generated by exploiting the chemical heterogeneity strategy via thermomechanical processing. The plastic deformation behaviors of the hetero-zones were investigated using three-point bending tests and interrupted tensile tests characterized by electron channeling contrast imaging (ECCI) combined with electron backscattered diffraction (EBSD) techniques. These in situ observations demonstrate the evolution of hetero deformation-induced (HDI) stress field near the austenite/ferrite interfaces in terms of geometrically necessary dislocation (GND) pileups, which can modulate the plastic deformation behaviors of both austenite and ferrite phases. At the early deformation regime, the imposed plastic deformation can be well maintained by the dense and homogeneous activation of slip systems in austenite phases and the stress concentration near austenite/ferrite interfaces can be alleviated by the gradual activation of plastic deformation in the ferrite phases. At the large deformation stage, a progressive twinning-induced plasticity (TWIP) effect caused by the chemical heterogeneity can provide a stable work hardening ability in a broad strain range, which can significantly enhance the ductility. At the late stage of deformation, strain-induced α′-martensite is also triggered, which provides an additional work-hardening ability. The strategy of micro-tuning deformation mechanisms by chemical heterogeneity is versatile and can be applied to many alloys.

A comparison of the mechanical and corrosion behavior of Fe49.5Mn25Cr15Ni10C0.5 medium-entropy alloy with its subsystems steels

We systematically compared the mechanical and corrosion behavior of Fe49.5Mn25Cr15Ni10C0.5, a face-centered-cubic (FCC) medium-entropy alloy (MEA) with low-cost, with its subsystems steels (Fe74.5Cr15Ni10C0.5 and Fe74.5Mn25C0.5). It was found that all three alloys exhibited similar ductility at 298 K. The MEA has the highest yield strength and lowest ultimate tensile strength (UTS) at 298 K and 77 K, and the Fe74.5Cr15Ni10C0.5 alloy has the highest UTS, followed by Fe74.5Mn25C0.5 alloy. The MEA's ductility increase when temperatures decrease from 298 K to 77 K, in contrast to its subsystems steels, which had decreasing ductility. As a result, the MEA has the highest product of UTS and ductility (∼75 GPa·%) at 77 K. The twinning-induced plasticity was dominant in the MEA, while deformation-induced martensitic transformation from FCC to body-centered-cubic (BCC), and from FCC to hexagonal-close-packed (HCP) was dominant in Fe74.5Cr15Ni10C0.5, and Fe74.5Mn25C0.5 alloys, respectively. In addition, Fe74.5Cr15Ni10C0.5 alloy has the best corrosion resistance, followed by the MEA and Fe74.5Mn25C0.5.

Martensitic twinning transformation mechanism in a metastable IVB element-based body-centered cubic high-entropy alloy with high strength and high work hardening rate

Realizing high work hardening and thus elevated strength–ductility synergy are prerequisites for the practical usage of body-centered-cubic high entropy alloys (BCC-HEAs). In this study, we report a novel dynamic strengthening mechanism, martensitic twinning transformation mechanism in a metastable refractory element-based BCC-HEA (TiZrHf)87Ta13 (at.%) that can profoundly enhance the work hardening capability, leading to a large uniform ductility and high strength simultaneously. Different from conventional transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) strengthening mechanisms, the martensitic twinning transformation strengthening mechanism combines the best characteristics of both TRIP and TWIP strengthening mechanisms, which greatly alleviates the strength-ductility trade-off that ubiquitously observed in BCC structural alloys. Microstructure characterization, carried out using X-ray diffraction (XRD) and electron back-scatter diffraction (EBSD) shows that, upon straining, α” (orthorhombic) martensite transformation, self-accommodation (SA) α” twinning and mechanical α” twinning were activated sequentially. Transmission electron microscopy (TEM) analyses reveal that continuous twinning activation is inherited from nucleating mechanical {351}α” type I twins within SA ‘‘{351}’’<2¯11>α” type II twinned α” variants on {351}α” twinning plane by twinning transformation through simple shear, thereby accommodating the excessive plastic strain through the twinning shear while concurrently refining the grain structure. Consequently, consistent high work hardening rates of 2–12.5 GPa were achieved during the entire plastic deformation, leading to a high tensile strength of 1.3 GPa and uniform elongation of 24%. Alloy development guidelines for activating such martensitic twinning transformation strengthening mechanism were proposed, which could be important in developing new BCC-HEAs with optimal mechanical performance.

Microstructure evolution in high-pressure phase transformations of CrFeNi and CoCrFeMnNi alloys

The extraordinary dynamics of microstructures, such as various dislocations, stacking faults, twinning, and polymorphism transformations, dominate the mechanical performance of high-entropy alloys. To reveal the phase transformation kinetics, we precisely detect the microstructure evolution in CoCrFeMnNi high-entropy alloy (HEA) and CrFeNi medium-entropy alloy (MEA) subjected to high-pressure compression using in situ angular-dispersive synchrotron X-ray diffraction. We find that controlling the initial microstructural state using cold-rolling can significantly reduce the stacking fault energy of CoCrFeMnNi and CrFeNi alloys, which results in lower onset pressures for phase transformations. The microstructure-induced stacking-fault energy reduction facilitates the formation of twins, which can act as nucleation sites for hexagonal close-packed (HCP) phase formation and recrystallization. The microstructure evolution-driven deviatoric deformation mechanism of CoCrFeMnNi HEA and CrFeNi MEA under quasi-hydrostatic compression is explored. Beyond the FCC phase, twinning in the HCP phase of CoCrFeMnNi alloy under high pressure is observed for the first time. This implies that the deformation mechanism of CoCrFeMnNi HEAs in the HCP phase is dominated by twinning-induced plasticity, which is verified using transmission electron microscopy.

Ultra-flash annealing constructed heterogeneous austenitic stainless steel with excellent strength-ductility

The application of austenitic stainless steel is seriously restricted due to its relatively low yield strength. To significantly improve the strength of austenitic stainless steel with an acceptable decrease in ductility, the as-received austenitic stainless steel is cold-rolled with 90% thickness reduction to obtain the hybrid structures including retained austenite, deformation-induced martensite, and subgrains. Subsequently, an ultra-flash annealing strategy is proposed to produce the heterostructured austenitic stainless steel consisting of recrystallized austenite grains and non-recrystallized austenite zones (diffusionless-reversion formation), and the hybrid microstructure formed by ultra-flash annealing shows superior strength-ductility synergy. The thorough investigation reveals that the origin of the high yield strength is not only grain boundary strengthening, but also the back-stress hardening induced by the constraint of hard domains to soft domains. The consequent high uniform elongation is provided by the synergistic effect of dislocation hardening, back-stress hardening and twinning, which is different from the coarse austenitic stainless steel with twinning-induced plasticity and transformation-induced plasticity.

Dependence of tensile deformation of twinning-induced plasticity steel on temperature

• Investigating the mechanical behaviors and underlying micromechanics in TWIP steels. • Profuse deformation twins are introduced in TWIP steel at cryogenic temperature. • The cross GBs and TBs provide an easy mean for cracking in TWIP steel. We study the tensile deformation behaviour and microstructure evolution of twinning-induced plasticity (TWIP) steel at room and cryogenic temperatures. The TWIP steel shows the higher strain hardening capability, excellent strength and obvious ductility loss at cryogenic temperature. The TWIP steel has excellent strength at cryogenic temperature due to the synergistic effect of dislocation slip and deformation twinning. High-density nanotwin structure can cause strong strain hardening capability. In general, strong strain hardening leads to uniform plastic deformation, which provides high ductility. According to the microstructure characterization, the intersecting position of high-density twin-grain and twin-twin boundaries act as the nucleation sites for nanovoids, which were formed by dislocation pile up. Therefore, its ductility reduced at cryogenic temperature even with continuous steady strain hardening.