Activation Of Deformation Twinning Research Articles

Low cycle fatigue properties of CoCrFeNiMn high-entropy alloy with heterogeneous microstructure

Heterogeneous microstructure is a hot topic in materials science which facilitates tackling the dilemma of strength-ductility trade-off in engineering materials including high-entropy alloys. The present investigation was conducted on the impact of different microstructures including heterogeneous microstructure, constructed by thermomechanical treatment including cold rolling followed by annealing, on the monotonic and cyclic behavior of a CoCrFeNiMn high-entropy alloy. The tailored microstructure contained three features: non-recrystallized as a hard domain, ultrafine-recrystallized, and fine-recrystallized regions. A desirable combination of strength and ductility including UTS of ∼1.1 GPa together with an elongation of ∼15 % was achieved due to benefiting from hetero-deformation-induced hardening and activation of deformation twins in the fine recrystallized grains. Regarding low-cycle fatigue behavior, at the strain amplitude of 0.4 %, heterogeneous microstructure exhibits remarkable fatigue properties, including high maximum stress and extraordinary lifetime (more than 2 × 105). The compelling reasons for these desirable properties are triggering deformation twins in fine recrystallized grain, the absence of brittle sigma phase as a good location of crack initiation, and the non-recrystallized area as a hard domain for suppressing crack propagation. The results suggested that the heterogeneous microstructure has significant benefits during applying low strain amplitude, however, at high strain amplitude, grain refinement is not recommended, and coarse-grained microstructure provides better properties. The present investigation is a step forward to understand the significance of heterogeneous microstructures to improve fatigue properties of high- or medium-entropy alloys.

Optimizing the microstructure and mechanical properties of Fe-30.5Mn–8Al-1.0C lightweight austenitic steel through thermomechanical treatment

This study presents an application of thermomechanical treatment technique (cold rolling followed by aging treatment) to significantly enhance the mechanical properties of a novel lightweight austenitic steel (Fe-30.5Mn–8Al-1.0C), mainly focusing on the impacts of cold-rolling reduction and subsequent aging temperature on its microstructure, texture, and tensile properties. It has been found that the dislocation density increased with an increase in cold rolling reduction from 25 % to 45 % and reached a saturation at a reduction of 45 %, facilitating the activation of deformation twins and accelerating κ-carbide precipitation. The tensile strength of the steel exhibited significant improvement with increased cold-rolling reduction, owing to the combined effects of dislocation strengthening and the Hall–Petch mechanism. A tensile strength as high as 1.6 GPa was obtained in samples treated by 65 % cold rolling reduction. Aging treatment can generally enhance the ductility in most samples, owing to an improved work hardening capability induced by κ-carbide precipitation. Consequently, an optimum thermomechanical process, involving a 45 % cold rolling reduction followed by aging at 600 °C for 10 min, was established to achieve a better combination of tensile strength (1428 ± 35.8 MPa) and elongation (26.7 ± 1.3 %), showcasing its potential for applications demanding high strength and exceptional ductility.

Activation of deformation twinning in ultrafine-grained high-entropy alloys via tailoring stacking fault energy and critical twinning stress

In this study, we propose a new material design approach to activate deformation twinning in ultrafine-grained (UFG) AlCoCrCuFeNiTi-based high-entropy alloys (HEAs) by considering both thermodynamic and microstructural aspects. Stacking fault energy of the solid-solution phase is reduced by tailoring composition with increasing Co/Ni ratio and diminishing the concentration of Al and Ti through precipitation. Furthermore, grain size is adjusted to enable the flow stress to surpass the critical twinning stress. It results in the activation of nano-twinning in the UFG HEAs, making the alloys exhibit high strain-hardening after yielding from 1.6 to 2.3 GPa.

Enhancing the fatigue property of high entropy alloy by regulating characteristics of grains and precipitates

High entropy alloys (HEAs) are a new class of alloys with many excellent mechanical properties, having the potential as structural materials. One advantage of HEAs is the vast space for designing compositions and microstructures, which provides more possibilities to develop fatigue-resistant materials. In this study, a multi-phase fine-grained microstructure was engineered through cold rolling and annealing treatment in Al0.3CoCrFeNi HEA to obtain an enhanced fatigue property. This alloy consists of FCC matrix phase with an average grain size of ∼ 1.13 ± 0.84 μm, and B2 and σ precipitates with sub-micron scale. Their phase fractions are ∼ 90 %, ∼ 9 % and ∼ 1 %, respectively. Stress-life (S-N) data indicate that fatigue endurance limit and ratio in terms of stress amplitude are ∼ 281 MPa and ∼ 0.3, based on stress ratio of 0.1. Good fatigue property is related to high ultimate tensile strength derived from good work-hardening ability and fine microstructure. In addition, activation of deformation twins, blunting of microcracks and deflection of fatigue cracks associated with precipitates are beneficial for enhancement of fatigue resistance. The above results provide theoretical guidance on how to enhance fatigue resistance of HEAs by regulating microstructures.

On the strain delocalization mechanism of Cu/Nb nanolayered composites with amorphous interfacial layers

Nanostructured metals and alloys possess ultrahigh strength but suffer from severe shear instability (strain localization). Recent experiments have shown that the strength and strain delocalization capability of some novel nanostructured alloys and nanolayered composites can be enhanced simultaneously by introducing nanoscale amorphous interfacial layers. However, the study on the underlying mechanism is still in an embryonic stage due to the ignorance of the complicated elemental composition of the interfacial layers, especially the compositional gradient along the interface thickness. Here, the atomic mechanisms of the tensile deformation of Cu/Nb nanolayered composites with amorphous interfacial layers are systematically investigated by molecular dynamics simulations. Depending on whether the composition of the interfacial layers is invariable or has a gradient distribution along the interface thickness, these samples are classified as amorphous or gradient samples, respectively. The simulations of normal Cu/Nb nanolayered composites with ordinary incoherent Cu‒Nb interfaces are also included for comparison, the results of which show that strain localization occurs due to the inhomogeneous plastic deformation between soft and hard grains in the Cu and Nb layers. The strain localization is inhibited in the amorphous samples mainly through the activation of deformation twinning in the Cu and Nb layers that produces a co-deformation between grains. Intriguingly, the gradient arrangement of the elemental composition of the amorphous interfacial layers gives a further stronger strain delocalization by further promoting the co-deformation between grains through stimulating more nanotwins in Cu layers and hindering twin growth in Nb layers, and by producing a much more uniform von Mises strain distribution in the interfacial layers. In addition, a better strain delocalization capability can be obtained when the thickness of the interfacial layers is closer to that of the crystalline ones in the amorphous sample or the range of gradient composition is larger in the gradient one.

Effect of deformation temperature on strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C low-density steel

We have investigated the influence of the deformation temperature from 25 °C (RT) to -196 °C on the dislocation structures associated with strain localization phenomena in an austenitic Fe-30Mn-6.5Al-0.3C (wt.%) low-density steel by electron channeling contrast imaging (ECCI), electron backscatter diffraction (EBSD), and bright-field transmitted forescattered electron imaging ((BF) t-FSEI) techniques. The characteristics of the dislocation structures were evaluated on the main texture components, i.e. <111>//tensile axis, <112>//tensile axis, and <001>//tensile axis directions. The inhomogeneous character of the plastic behavior is promoted upon cryogenic deformation due to the formation of dislocation structures associated with strain localization, namely microbands (MBs) and deformation bands (DBs). ECCI analysis of the dislocation structure reveals that the deformation temperature has a strong influence on the thermal-assisted dislocation processes controlling the dislocation configurations and MB formation mechanisms. The MB nucleation mechanism evolves from a cross-slip-assisted mechanism at RT deformation conditions to a slip band-assisted mechanism at cryogenic deformation temperatures. This effect has a profound effect on the grain orientation dependence of the MB structure but not on its crystallographic alignment. On the other hand, cryogenic deformation temperatures (-196 °C) enhance the material's mechanical strength and ductility due to the activation of deformation twinning, which is associated with the reduction of the stacking fault energy. We find that MBs have a small contribution to strain-hardening and ductility due to the small mechanical resistance of these dislocation structures against the advance of deformation twins and dense dislocation layers, and the comparatively small plastic strain accommodated by them, respectively. These findings provide new insights into the microband-induced plasticity (MBIP) effect.

A comparative study on dynamic deformation and ballistic impact response of Ti–4Al–2.5V–1.5Fe–0.25O and Ti–6Al–4V alloys

Titanium alloys are most suitable for lightweight armour applications due to their superior strength to weight ratio, and α+β Ti alloy Ti–6Al–4V is widely used in armour applications. One of the critical factors limiting the widespread use of Ti alloys in armour application is their high cost. Recently, Allegheny Technologies Incorporated introduced ATI-425® Ti alloy with the chemical composition of Ti–4Al-2.5V-1.5Fe-0.25O as a low-cost alternative to Ti–6Al–4V. Though ATI-425 Ti alloy is primarily intended for armour applications, only limited ballistic results are available in the open literature. In the present work, we compare the quasi-static, dynamic deformation, ballistic impact response and post-deformation microstructure of a low-cost α+β Ti alloy ATI-425 with that of Ti–6Al–4V alloy. Analysis of the results showed that both alloys showed similar yield strength values during quasi-static tension and compression tests, while ATI-425 Ti alloy showed higher tensile ductility, Charpy-V notch impact energy and dynamic flow stress than Ti-6Al-4V. The ATI-425 Ti alloy also exhibited slightly better ballistic performance measured in terms of V50 ballistic limit velocity. Detailed microstructural analysis on quasi-static and dynamic compression-tested samples showed mainly {101–2} tensile twins, whereas {101–2} and {112–1} tensile twins are observed in ballistic-tested samples of both the alloys. Activation of deformation twinning in ATI-425 Ti alloy with higher oxygen is attributed to lower Al content which enhances the twinning activity. Adiabatic shear band (ASB)-induced plugging mechanism is ascertained as the perforation mode in both alloys during the ballistic impact. Analysis of propensity to ASB formation indicated that Ti-6Al-4V has a slightly better resistance to ASB formation than ATI-425 Ti alloy. The improved ballistic performance of ATI-425 Ti alloy in comparison to Ti-6Al-4V is attributed to the higher dynamic flow stress, Charpy-V notch impact energy absorption, and moderate resistance to adiabatic shear band formation.

Enhanced strength-ductility synergy in high-entropy alloys via architecting three-level gradient hierarchical nanostructure

Gradient microstructural design represents a new strategy for enhancing strength-ductility synergy of high/medium entropy alloys (H/MEAs). However, most of the reported gradient H/MEAs have only one type of gradient microstructure, which has limited ability to realize the full potential of gradient structure. Here, we report a three-level gradient hierarchical nanostructure (GHN) in a Al0.3CoCrFeNi HEA by ultrasonic surface rolling processing (USRP) followed by aging treatment that can result in a six-fold enhancement in yield strength with almost no loss in ductility compared to the coarse structured counterpart. The proposed three-level GHN is characterized with gradient variations in grain size, nano-twin and dislocation density, B2 and σ precipitates content. Systematic experiments indicate that strong hetero-deformation-induced (HDI) hardening associated with the activation of deformation twinning, are responsible for this exceptional strength-ductility synergy. This three-level GHN engineering strategy in this work can also be feasibly applied to many other H/MEAs, and offers an available pathway to develop high strong and ductile metallic materials.

Tuning mechanical behavior and deformation mechanisms in high-manganese steels via carbon content modification

In this paper, two high-manganese steels with comparable grain sizes and manganese contents, namely 34Mn0.1C and 32Mn0.6C, were investigated to explore the effect of carbon content on the mechanical behavior and deformation mechanisms at both room temperature (RT) and liquid nitrogen temperature (LNT). The results indicate that increasing the carbon content promotes the transition of deformation mechanism from dislocation slip to deformation twinning, which is similar to the effect of reducing the deformation temperature. At RT, the strength and elongation are effectively improved by increasing the carbon content, which is attributed to the transformation of deformation mode. Specifically, the yield strength, the ultimate tensile strength and the total elongation increase from 262 MPa, 595 MPa and 50.4% for the 34Mn0.1C steel to 360 MPa, 846 MPa and 87.4% for the 32Mn0.6C steel, respectively. However, at LNT, the 32Mn0.6C steel exhibites higher strength but lower elongation than the 34Mn0.1C steel. The total elongation decreases from 70.6% to 52.3%, which is because the high carbon content and cryogenic temperature induce the rapid activation of deformation twins, leading to premature fracture.

Microstructure and mechanical behavior of additively manufactured CoCrFeMnNi high-entropy alloys: Laser directed energy deposition versus powder bed fusion

CoCrFeMnNi high-entropy alloys (HEAs) are additively manufactured by laser directed energy deposition (L-DED) and laser powder bed fusion (L-PBF) processes. Comparative studies are conducted for the microstructures and deformation mechanisms of L-DED and L-PBF samples. In both types of samples, highly heterogeneous microstructures are formed, consisting of columnar grains, solidification cells, and dislocation networks. However, substantial differences are measured in the crystallographic texture, cell size, and elemental distribution. Deeper melt pools in the L-DED samples promote a mixed crystallographic texture of <101>/<111> as opposed to <001> along the build direction in the L-PBF samples. The <101>/<111> texture elevates the flow stresses and facilitates the activation of deformation twins in the L-DED samples. Moreover, their larger solidification cell sizes and associated chemical segregation across cell walls increase the dislocation storage capability and resistance to dislocation motion, leading to profuse planar slip bands and microbands during plastic deformation. The enhanced plastic deformation capabilities in the L-DED samples give rise to more sustained strain hardening and thus higher ductility compared to the L-PBF samples. Our work not only provides fundamental insights into the deformation mechanisms of additively manufactured HEAs, but also underscores the critical impact of processing conditions on the solidification microstructure and material design by additive manufacturing.

Microstructure and deformation behavior of MoReW in the temperature range from 25°C to 1500°C

Refractory alloys which retain high strength above 1000 °C and, at the same time, have excellent malleability, are in high demand for high temperature structural applications. Here we report the microstructure and mechanical properties of an equiatomic MoReW alloy, which has demanding combination of strength and ductility in the temperature range of 25 °C to 1500 °C. The alloy was prepared by arc melting followed by hot isostatic pressing at 1400 °C, 207 MPa for 5 h and it has a single-phase BCC structure with the average grain size of ∼170 μm. The alloy yield stress is relatively low (555 MPa) at 25 °C, but it has weak temperature dependence resulting in remarkably good yield stress values at high temperatures, e.g. 402 MPa at 1000 °C and 247 MPa at 1400 °C. The alloy shows strong strain hardening at 25–1000 °C, with the true flow stress tripled at 25 °C and doubled at 1000 °C after true strain of 0.4. The microstructural analysis of the deformed specimens reveals extensive deformation twinning activity at the beginning of deformation and activation of other deformation modes at later deformation stages in this temperature range. This is unusual behavior as generally activation of twinning occurs at high stresses, after extensive dislocation activity, and deformation twinning in refractory alloys has never been reported above 400 °C. To understand and explain the observed behaviors of the alloy, a detailed analysis of strengthening mechanisms associated with screw and edge dislocation glide, as well as with twin nucleation and propagation, has been conducted and reasonable qualitative models of yielding, strain hardening and ductility of the studied MoReW alloy have been proposed.

The dual effect of grain size on the strain hardening behaviors of Ni-Co-Cr-Fe high entropy alloys

• The double effects of grain size on strain hardening behaviors of fcc HEAs were obtained. • The grain size affects the stage II hardening through tuning dislocation glide mode. • The stage IV hardening was also influenced by grain size due to tuned twining activity. It has been well documented that grain size plays a critical role in the strain hardening behaviors of metals and alloys. However, the influence of grain size on the strain hardening of high entropy alloys (HEAs) was not fully understood. Here, we report that the grain size not only affects the twinning-induced plasticity (TWIP) effect but also changes the dislocation-based deformation behaviors of face-centered-cubic (fcc) HEAs significantly. The strain hardening and deformation micro-mechanisms of NiCoCrFe and Ni 2 CoCrFe were investigated using electron channeling contrast (ECCI) analysis. Our results showed that Ni 2 CoCrFe exhibits a typical three-stage strain hardening behavior and NiCoCrFe shows the fourth stage at high strains due to the TWIP effect. For both NiCoCrFe and Ni 2 CoCrFe, the increase of grain size leads to a transition of dislocation glide from wavy to planar mode, resulting in a low value and the recovery of strain hardening rate in stage II. The large-grain NiCoCrFe showed a higher strain hardening rate in stage IV due to the promoted deformation twinning. Combining the strain hardening behaviors of the TWIP-NiCoCrFe and the mechanically stable Ni 2 CoCrFe, we showed that the grain size influences the stage II hardening through tuning dislocation glide mode and the stage IV by tailoring deformation twinning activity of the Ni-Co-Cr-Fe HEAs. The grain size just affects stages I and III slightly in the current cases. These findings will also provide some insights into the understanding of strain hardening behaviors in other face-centered-cubic HEAs.

Deformation Rate and Temperature Sensitivity in TWIP/TRIP VCrFeCoNi Multi-Principal Element Alloy

High-entropy alloys (HEAs) and medium-entropy alloys (MEAs), also sometimes referred to as multi-principal element alloys (MPEAs), present opportunities to develop new materials with outstanding mechanical properties. Through the careful selection of constituent elements along with optimized thermal processing for proper control of structure, grain size, and deformation mechanisms, many of the newly developed HEA systems exhibit superior strength and ductility levels across a wide range of temperatures, particularly at cryogenic deformation temperatures. Such a remarkable response has been attributed to the hardening capacity of many MPEAs that is achieved through the activation of deformation twinning. More recent compositions have considered phase transforming systems, which have the potential for enhanced strengthening and therefore high strength and ductility levels. However, the strain rate sensitivity of such transforming MPEAs is not well understood and requires further investigation. In this study, the tensile properties of the non-equiatomic V10Cr10Fe45Co30Ni5 MPEA were investigated at different deformation rates and temperatures ranging from 77 K (−196 °C) to 573 K (300 °C). Depending on the deformation temperature, the considered MPEA exhibits plasticity through either crystallographic slip, deformation twinning, or solid-state phase transformation. At 300 °C, only slip-mediated plasticity was observed for all the considered deformation rates. Deformation twinning was detected in samples deformed at room temperature, while face-centered cubic to body-centered cubic phase transformation became more favorable at cryogenic deformation temperatures. The trends are nonlinear with twinning-induced plasticity (TWIP) favored at the intermediate deformation rate, while transformation-induced plasticity (TRIP) was observed, although limited, only at the slowest deformation rate. For all the considered deformation rates at cryogenic deformation temperature, a significant TRIP activity was always detected. The extent of TRIP, however, was dependent on the deformation rate. Increasing the deformation rate is not conducive to TRIP and thus hinders the hardening capacity.

Interface-Mediated Twinning-Induced Plasticity in a Fine Hexagonal Microstructure Generated by Additive Manufacturing.

The grain size is a determinant microstructural feature to enable the activation of deformation twinning in hexagonal close-packed (hcp) metals. Although deformation twinning is one of the most effective mechanisms for improving the strength-ductility trade-off of structural alloys, its activation is reduced with decreasing grain size. This work reports the discovery of the activation of deformation twinning in a fine-grained hcp microstructure by introducing ductile body-centered cubic (bcc) nano-layer interfaces. The fast solidification and cooling conditions of laser-based additive manufacturing are exploited to obtain a fine microstructure that, coupled with an intensified intrinsic heat treatment, permits to generate the bcc nano-layers. In situ high-energy synchrotron X-ray diffraction allows tracking the activation and evolution of mechanical twinning in real-time. The findings obtained show the potential of ductile nano-layering for the novel design of hcp damage tolerant materials with improved life spans.

Unique transition of yielding mechanism and unexpected activation of deformation twinning in ultrafine grained Fe-31Mn-3Al-3Si alloy

Tensile mechanical properties of fully recrystallized TWIP steel specimens having various grain sizes (d) ranging from 0.79 μm to 85.6 μm were investigated. It was confirmed that the UFG specimens having the mean grain sizes of 1.5 μm or smaller abnormally showed discontinuous yielding characterized by a clear yield-drop while the specimens having grain sizes larger than 2.4 μm showed normal continuous yielding. In-situ synchrotron radiation XRD showed dislocation density around yield-drop in the UFG specimen quickly increased. ECCI observations revealed the nucleation of deformation twins and stacking faults from grain boundaries in the UFG specimen around yielding. Although it had been conventionally reported that the grain refinement suppresses deformation twinning in FCC metals and alloys, the number density of deformation twins in the 0.79 μm grain-sized specimen was much higher than that in the specimens with grain sizes of 4.5 μm and 15.4 μm. The unusual change of yielding behavior from continuous to discontinuous manner by grain refinement could be understood on the basis of limited number of free dislocations in each ultrafine grain. The results indicated that the scarcity of free dislocations in the recrystallized UFG specimens changed the deformation and twinning mechanisms in the TWIP steel.

Mechanical properties and deformation mechanisms in CoCrFeMnNi high entropy alloys: A molecular dynamics study

We used molecular dynamics simulations to investigate the relationship between mechanical properties and deformation mechanisms in CoCrFeMnNi alloys having different compositions. According to deformation twinning activity, the deformation mechanisms in the different CoCrFeMnNi compositions under tensile loading can be classified into three major categories: easy-shear (ES), inter-locks (IL) and bulk hexagonal closed packed (hcp) transformation (BH). We found that the compositions with ES were more likely to rupture early because of the short and fragmented twins. The compositions following the IL pattern promoted the movement of interlocking stacking faults in the twin boundaries that could prolong tensile deformation. Moreover, BH could further increase ductility because the hcp transformation could absorb and dissipate the stacking faults. Experimental observations of immobile stacking-fault networks that impede dislocation motion and further provide preferred sites for the formation of the hcp phase were commonly found in the ES, IL and BH patterns. The calculated intrinsic stacking fault energies of CoCrFeMnNi and CoCrFeNi were consistent with the experimental measurements. The combination of high unstable stacking fault energy with the formation of the IL pattern results in high strength and high ductility in the CoCrFeMnNi HEA system.

Strength–ductility enhancement in multi-layered sheet with high-entropy alloy and high-Mn twinning-induced plasticity steel

In this study, a high-entropy-alloy-cored multi-layered sheet (MLS) clad with an austenitic high-Mn twinning-induced-plasticity steel (HEA/high-Mn MLS) is proposed and its room- and cryogenic-temperature tensile behaviors were investigated. The MLS was fabricated using a commercial roll-bonding procedure. Its interface was well bonded without critical defects such as pores or voids. During the MLS fabrication and subsequent annealing, the interdiffusion of alloying elements and interfacial friction resulted in the formation of ε-martensite and V-rich carbides in the high-Mn and HEA layers, respectively, as well as grain refinement. Nevertheless, the interface remained strongly bonded after the tensile deformation at room and cryogenic temperatures, thereby providing larger strength and elongation than the values calculated based on the rule of mixtures. The improvement in tensile properties beyond the calculated values was interpreted by the activation of deformation twins, transformation-induced plasticity, and generation of geometrically necessary dislocations in the HEA/high-Mn interfacial region. Deformation twins were populated at both high-Mn and HEA layers at room temperature. However, at cryogenic temperatures, ε-martensite and body-centered cubic martensite were additionally formed in the high-Mn and HEA layers. Thus, both strength and ductility were considerably improved. In addition, the consequent good strength–ductility balance was comparable to or better than those of other HEAs or medium-entropy alloys. This indicates that our MLS can be an attractive design strategy to tailor various properties by controlling the thickness fraction of each layer and develop strong alloys for cryogenic applications.

Stress-induced crystallographic and microstructural evolution for hierarchically twinned martensite in single- and dual-phase Ni-Mn-Ga alloys

Polycrystalline Ni54+xMn25Ga21-x high temperature shape memory alloys were developed to examine the Ni-content dependent crystallographic features, phase transformation and deformation behavior. The alloys were characterized by a hierarchically twinned martensite structure with the co-existence of ductile γ phase in Ni57 and Ni58 alloys. Two types of twinning relationships existed, i.e., (112) compound twin in the internal nano-lamellae and 11¯2 type-I twin in the adjacent micro-variants. The martensitic start temperature, compressive strength and ductility increased with increasing Ni content. However, the corresponding shape recovery capability was significantly deteriorated in the higher Ni-containing alloys. For the single-phase alloy, the detwinning/reorientation of internal nano-lamellae and activation of deformation twins contributed to the macroscopically recoverable strain in case of low pre-strains. In contrast, large pre-strains led to piles-up of dislocation, bending and kinking of twinning interfaces, formation of deformation bands and crossing of twin structures. These irreversible processes produced massive unrecoverable strain. Almost no detwinning and twining processes were observed in the dual-phase alloys due to the severe lattice distortion. Instead, most deformation occurred via dislocation motion in γ phase.

An Overview of High Yield Strength Twinning-Induced Plasticity Steels

Twinning-induced plasticity (TWIP) steel is a second-generation advanced high strength steel grade developed for automotive applications. TWIP steels exhibit an excellent combination of strength and ductility, mainly originating from the activation of deformation twinning. However, TWIP steels generally exhibit a relatively low yield strength (YS), which limits their practical applications. Thus, developing high YS TWIP steels without ductility loss is essential to increase their industrial applications. The present work summarizes and discusses the recent progress in improving the YS of TWIP steels, in terms of precipitation strengthening, solid solution strengthening, thermomechanical processing, and novel processes. Novel processes involving sub-boundary strengthening, multi-phase structure, and gradient structure as well as the control of thermomechanical processing (recovery annealing and warm rolling) and precipitation strengthening were found to result in an excellent combination of YS and total elongation.

Plastic deformation behavior of titanium alloy by warm laser shock peening: Microstructure evolution and mechanical properties

The plastic deformation behavior of Ti6Al4V titanium alloy subjected to 300 °C-warm laser shock peening (WLSP) was inferred from its microstructure evolution and mechanical properties. Through dynamic strain aging (DSA), WLSP achieved higher dense dislocations than room temperature laser shock peening (LSP). In addition, significant 101—2 deformation twinning activity was observed after WLSP. Twinning activity at such an ultrahigh strain rate and high temperature is a first-time observation in titanium alloy, and was attributed to an enhanced dislocation dissociation mechanism during WLSP. Moreover, an amorphization layer was generated on the top surface of the WLSP-processed sample, but not on the LSP-processed sample. This amorphization was effected by the increased free energy provided by the multiplying dislocations and deformation twins, combined with the reduced energy barrier of the crystal-to-amorphous transformation at high temperature. Relative to LSP, WLSP increased the width and depth of the compressive residual stress in the titanium alloy by 36.2% and 21.8% respectively, and improved the surface microhardness by 5%. The latter enhancement was conferred by the increased density of dislocations and deformation twins. Overall, WLSP improved the mechanical properties of the titanium alloy.