Twinning-induced Plasticity Effect Research Articles

Heterogeneous distribution of isothermal ω precipitates prevents brittle fracture in aged β-Ti alloys

Isothermal ω (ωiso) precipitates in β Ti-alloys can increase the yield strength of the material, but unfortunately at a dramatic cost of the ductility. The embrittlement is mainly attributed to the suppression of twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects, as well as the formation of dislocation channels associated with localized deformation. To address this issue, we propose a heterogenous microstructure in Ti-15Mo-3Al alloy using conventional rolling method, in which the heterogeneous distributions of both Mo element and ωisoprecipitates are realized after aging. Both the nano-hardness and elastic modulus of ωisorich (Mo-lean) regions are larger than that of ωisodepleted (Mo-rich) regions. A four-time increase in tensile ductility is achieved in the heterogeneous microstructure without a loss of tensile strength, compared with the homogeneous counterpart. The ductility improvement can be ascribed to the blocking effect of ωisodepleted regions against the propagation of dislocation channels which later evolve into micro-cracks.

Simultaneous improvement in strength and ductility of CT20 titanium alloy at cryogenic temperature

Quasi-static tensile deformation behaviors at room temperature (RT) and 77 K of the CT20 alloy with a <-12-11>//ND texture were investigated. The CT20 alloy with equiaxed microstructure exhibits excellent mechanical properties with simultaneous increase in strength and elongation at cryogenic temperature. In current study, the initial <-12-11>//ND texture promotes basal slip actuation effectively and continuously increases Schmid factor (SF) of other slip systems to promote multiple slip. Cryogenic temperature weakens the deformation texture due to the promotion of multiple slip. Since the decrease of the stacking fault energy (SFE), more twins in number and variety nucleate at 77 K and promote multiple slip. It causes strong cryogenic twinning-induced plasticity (TWIP) effect. Grain refinement induces strong cryogenic dynamic Hall-Petch effect resulting in high strain-hardening rate (SHR) and strength. The yield strength (YS, ∼1052.67 MPa), ultimate tensile strength (UTS, ∼1102.51 MPa) and elongation (EL, ∼21.1 %) of the 77 K specimen are about 84.1 %, 69.1 % and 38.8 % higher than its counterpart at RT, respectively. In addition, cryogenic temperature changes the initiation location of cracks and promotes cracks initiation at grain boundaries homogeneously.

Cryogenic impact fracture behavior of a high-Mn austenitic steel using electron backscatter diffraction and neutron Bragg-edge transmission imaging

A unique impact fracture behavior is found in a high-Mn austenitic steel (24Mn–4Cr-0.4C-0.3Cu) in this work. The steel exhibits concurrent twinning-induced plasticity (TWIP) effect and the transformation-induced plasticity (TRIP) effect. By analyzing the load-deflection curves recorded during Charpy impact testing, the resistance to crack initiation and propagation is quantified from the absorbed energy. The high-Mn steel demonstrates good resistance to crack initiation at 273 K and 77 K. However, as the temperature decreases from 273 K to 77 K, there is an accelerated transition from stable crack growth to unstable crack growth during impact, resulting in the deterioration of resistance to crack propagation. The plastic deformation of the impact-tested samples, especially in the region close to the crack-path profile was quantitatively analyzed using neutron Bragg-edge transmission (BET) imaging. The deformation zones, divided by using the width of the 200 Bragg edge, exhibit good agreement with the impact absorbed energy characteristics obtained from dynamic load-deflection curves. Moreover, the unstable growth transition point was roughly determined on the impact-tested sample. Then, the electron backscatter diffraction (EBSD) technique is employed to examine the deformation microstructure along the crack-path in the impact-tested samples. The results revealed the dual roles of TRIP effect in impact toughness of the high-Mn steel. On one hand, the TRIP effect plays a positive role in improving resistance to crack initiation and propagation. On the other hand, the excessive accumulation of brittle ε/α՛-martensite caused by the enhanced TRIP effect at 77 K leads to quasi-cleavage fracture, thereby playing a negative role. Finally, we discussed the prominent toughening mechanisms associated with the TWIP and TRIP effects, which greatly impact the impact fracture behavior.

MECHANICAL BEHAVIOR OF AN HIGH STRENGHT STEEL (AHSS) WITH MEDIUM MN CONTENT IN TWO ROLLING CONDITIONS: HOT AND WARM

The microstructure before intercritical annealing of an AHSS (Advanced High Strength Steel) with medium Mn content plays an important role in the final mechanical properties, since the transformations occurring during annealing modify phases, composition, and morphology. The microstructural changes that occur during intercritical annealing treatment of a medium Mn steel were examined. Two starting material comes from different conditions, hot rolling at 1200°C and warm rolling (initial rolling at 1200°C and subsequent at 680°C). The mechanical properties were related to the transformation phenomena that occur in these steels, mainly TRIP (Transformation Induced Plasticity) and TWIP (Twinning Induced Plasticity) effects. The transformations were verified by SEM (Scanning Electron Microscopy) and X-RD (X-Ray Diffraction). Tensile strength values of 1111 MPa and 17% elongation were obtained by hot rolling route. For the warm rolling route, 35% deformation and a tensile strength of 1357 MPa were obtained. The strain hardening curve was analyzed, showing the presence of the TWIP effect subsequently "saw" behavior related to the discontinuous TRIP effect. The mechanic properties values are related to the difference in morphology phases present. An acicular morphology of a/? (ferrite/austenite) provides a higher value of tensile strength, but low elongation percentage, and a mixture of lamellar and globular morphologies, provides an optimized combination of strength and ductility. Key words: AHSS, medium manganese steel, rolling, heat treatment, discontinuous TRIP effect, TWIP.

Graphene enables equiatomic FeNiCrCoCu high-entropy alloy with improved TWIP and TRIP effects under shock compression

Graphene (Gr) reinforced high-entropy alloy (HEA) matrix composites are expected as potential candidates for next-generation structural applications in light of outstanding mechanical properties. A deep comprehension of the underlying deformation mechanisms under extreme shock loading is of paramount importance, however, remains lacking due to experimentally technical limitations in existence. In the present study, by means of nonequilibrium molecular dynamics simulations, dynamic deformation behaviors and corresponding mechanisms in equiatomic FeNiCrCoCu HEA/Gr composite systems were investigated in terms of various shock velocities. The resistance to dislocation propagation imparted by Gr was corroborated to encourage the elevated local stress level by increasing the likelihood of dislocation interplays, which facilitated the onset of twins and hexagonal close-packed (HCP) martensite laths. Meanwhile, the advent of Gr was demonstrated to endow the HEA with an additional twinning pathway that induced a structural conversion from HCP to parent face-centered cubic (FCC) inside HCP martensite laths, different from the classical one that necessitated undergoing the intermediate procedure of extrinsic stacking fault (ESF) evolution. More than that, by virtue of an increase in flow stress, the transformation-induced plasticity (TRIP) effect was validated to be additionally evoked as the predominant strain accommodation mechanism at higher strains on the one hand, but which only assisted plasticity in pure systems, and on the other hand, can also act as an auxiliary regulation mode together with the twinning-induced plasticity (TWIP) effect under intermediate strains, but with enhanced contributions relative to pure systems. One may expect that TRIP and TWIP effects promoted by introducing Gr would considerably inspire a synergistic effect between strength and ductility, contributing to the exceptional shock-resistant performance of FeNiCrCoCu HEAs under extreme regimes.

Microstructure evolution and strain hardening behavior of thermomechanically processed low-C high-manganese steels: an effect of deformation temperature

Effects of reduced (– 40 °C), ambient (20 °C), and elevated (200 °C) deformation temperatures on the microstructure evolution and strain hardening behavior of two low-C thermomechanically processed high-manganese steels were studied. The microstructure was characterized by means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM) techniques. The temperature-dependent tendency of austenite to strain-induced ε/α′-martensitic transformation and mechanical twinning was qualitatively and quantitatively assessed using the EBSD technique. The steel containing 26 wt% of Mn showed the beneficial strength–ductility balance at reduced deformation temperature -40 °C due to the intense Transformation-Induced Plasticity (TRIP) effect which resulted in the formation of significant ε- and α′-martensite fractions during tensile deformation. The mechanical properties of steel containing 27 wt% of Mn were more beneficial at elevated deformation temperature 200 °C due to the occurrence of intense Twinning-Induced Plasticity (TWIP) effect expressed by the presence of significant fraction of mechanical twins. Moreover, at the highest deformation temperature 200 °C, the evidence of thermally activated processes affecting the mechanical behavior of the higher Mn steel was identified and described.

ULTRA-HIGH STRENGTH TWIP STEEL WITH HIGH CHROMIUM CONTENT

A new ultra-high strength steel with a fully austenitic microstructure and twinning-induced plasticity (TWIP) effect has been developed. TWIP effect gives this steel a good combination of high strength of over 1000 MPa and ductility of over 35%. This new steel has a high chromium content, which increases its corrosion resistance. By combining cold rolling and annealing, the steel has achieved a very fine austenitic microstructure with an average grain diameter of 2.85 µm. This steel could be used for structural applications or components that need to withstand high levels of stress, deformation and corrosion environment.

Enhancing strength and ductility combination via tailoring the dislocation density in medium Mn steel

A novel strengthening strategy including warm rolling and intercritical annealing was proposed to optimize the strength and ductility of a Fe–10.2 Mn–2.2Al–0.41C-0.6V (wt.%) steel. Numerous microbands and nano-size V-carbide (VC) particles were generated in the warm-rolled samples with high stacking fault energy (SFE) and reasonable dislocation density. During tensile deformation, the dislocation density actively influenced microband evolution, deformation twins, and martensitic transformation. The strong interactions between the microbands, as well as between the microbands and dislocations, activated the transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects. Finally, the VC precipitates, microbands, dislocations, and TRIP and TWIP effects synergistically enhanced the ductility, tensile strength, and work hardening. The best tensile properties exhibited a yield strength of 1286 MPa, tensile strength of 1569 MPa, and total elongation of 21%. Meanwhile, dislocation hardening played an essential role in yield strength according to the calculation of individual contributions of grain size, VC precipitates, and dislocations toward yield strength. This work provided valuable insight into enhancing the combination of strength and ductility in medium Mn steels.

Cryogenic mechanical behavior of a FeCrNi medium-entropy alloy fabricated by selected laser melting

A single-phase FeCrNi medium-entropy alloy (MEA) with a face-centered cubic structure was successfully fabricated by selected laser melting (SLM). The SLMed FeCrNi MEA exhibits hierarchical microstructures, including molten pools, coarse columnar grains, submicron cellular structures and high-density dislocations, and shows an excellent strength-ductility combination. Notably, the SLMed FeCrNi MEA has ultra-high yield strength and ultimate tensile strength of 1.1 GPa and 1.5 GPa, respectively, at 77 K, which are almost 1.5 times higher than those at room temperature, while still maintaining a high fracture elongation of 49%. It was also found that the SLMed FeCrNi MEA deformed at 77 K could produce more nanotwins compared with that deformed at room temperature. The twinning-prone matrix can produce more significant twinning-induced plasticity effects, which contributes to a high plasticity. Additionally, the high-density cellular substructure can effectively hinder dislocation motion, which is the main reason for the high strength.

Unveiling the strengthening mechanisms of as-cast micro-alloyed CrMnFeCoNi high-entropy alloys

The strengthening effects introduced by the addition of 2 at% titanium, vanadium, and niobium, as the well-known micro-alloying elements, to the model CrMnFeCoNi high-entropy alloy (HEA) were studied in the present work. Accordingly, the microstructure, mechanical properties, and strengthening mechanisms of the as-cast CrMnFeCoNi, (CrMnFeCoNi)98Ti2, (CrMnFeCoNi)98V2, and (CrMnFeCoNi)98Nb2 HEAs were investigated by electron-backscattered diffraction (EBSD), tensile testing, differential scanning calorimetry (DSC) thermal analysis, and theoretical calculations and measurements. Depending on the nature of the added elements and their segregation tendency during solidification, different degrees of microstructural refinement were observed in the as-cast ingots. The segregation tendency of Ti was found to be more pronounced compared to that of V (as predicted by the Scheil-Gulliver model), leading to a more refined secondary dendrite arm spacing (SDAS) and grains (resulting from the growth restriction factor and constitutional undercooling). Moreover, Nb addition led to the formation of the (Cr,Fe,Ni)2(Nb) Laves phase at the last stages via the eutectic solidification. The effect of the Laves intermetallic compound (type C14) and twinning-induced plasticity (TWIP) effect on the strength-ductility synergy was discussed. Moreover, a detailed modeling of the strengthening mechanisms revealed that the grain boundary strengthening (represented by the Hall-Petch relationship) and solid solution hardening (due to the lattice distortion) were the primary contributors to the increase in yield strength of V- and Ti-containing HEAs. On the other hand, the formation of the Laves phase, besides solid solution and grain boundary strengthening mechanisms, could lead to a considerable increase in the yield strength of the Nb-containing sample; although it would deteriorate the ductility of the alloy, as also discussed based on its brittle fracture surface appearance and the presence of micro-cracks. Accordingly, the present study is applicable to the design of modified Cantor-based HEAs.

Solidification microstructure evolution and its correlations with mechanical properties and damping capacities of Mg-Al-based alloy fabricated using wire and arc additive manufacturing

Magnesium (Mg) alloys, as the lightest metal structural material with good damping capacities, have important application prospects in realizing structural lightweight and vibration reduction. However, their engineering application is greatly limited by poor plastic formability. Wire and arc additive manufacturing (WAAM) provides a potential approach for fabricating large-scale Mg alloy components with high manufacturing flexibility. In this study, the evolution of the solidification microstructure of a WAAM-processed Mg-Al-based alloy was quantitatively analyzed based on the analytical models; then, the correlations between the solidification microstructure and mechanical properties/damping capacities were investigated. The results revealed that the WAAM-processed Mg-Al-based alloy with an equiaxed-grain-dominated microstructure displayed a simultaneous enhancement in mechanical properties and damping capacities compared to those of the cast Mg-Al-based alloy. The good combination of mechanical properties and damping capacities are mainly attributed to the weakened basal texture with a relatively high Schmid factor for basal <a> slip, the twinning-induced plasticity (TWIP) effect associated with the profuse {10-12} tensile twinning, and the relatively high dislocation density caused by the thermal stress during the WAAM process.

Trace B doping Fe50Mn30Co10Cr10 high entropy alloy: Mechanical response and multi-microstructure evolution under TWIP and TRIP effects

Face-centered cubic (FCC) high entropy alloy has attracted extensive attention because of its excellent mechanical response, while its poor strength limits its application in practical engineering. In this work, the Fe50Mn30Co10Cr10 alloy doped with 30 ppm B atoms and proper thermo-mechanical processing (cold rolling and annealing) has achieved excellent synthetic strength and plasticity. It was found that the grain refinement strengthening introduced by B doping and the strong dislocation strengthening of inhomogeneity structure produced by the annealing treatment contribute to enhanced yield strength. The multiple structure evolution during the deformation process contribute to the excellent ductility. With the increase of tensile strain, dislocations, FCC deformation twins, strain-induced close packed hexagonal structure (HCP) phase, HCP deformation twins and reverse FCC phase appear successively, implying an interesting mechanism that the FCC and HCP phases bear plastic deformation by twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP) effects successively. FCC twinning, FCC→HCP phase transformation, HCP twinning, HCP→FCC reverse transformation occurred successively, which simultaneously leads to the high plasticity of the alloy. Interstitial atoms doping combined with the TWIP and TRIP effects can lead to the overall improvement of the yield strength and plasticity of the alloy, which provides a feasible way for the subsequent design of high-strength plastic alloys.

Microstructure and mechanical properties of a novel twinning induced plasticity steel with two different grain morphologies

A novel heterogeneous-structure (HS) twinning-induced plasticity (TWIP) steel containing alternating columnar grain (CG) and equiaxed grain (EG) domains was successfully fabricated via prestraining, partial recrystallization, and directional solidification. Compared with the traditional EG sample, the HS sample exhibited a better tensile strength–elongation combination (yield strength ≈ 263 MPa, ultimate tensile strength ≈ 573 MPa, total elongation ≈ 101.5%). The mutually constrained structure could effectively control the unique deformation modes of the EG and CG domains at different deformation stages. The HS sample exhibited stable and continuous plastic deformation owing to the timely release of localized high strain or stress, postponed plastic instability, and restrained crack propagation owing to the constrained structure. The stress relaxation was due to twinning and the effects of geometrically necessary dislocations, which accommodated deformation incompatibility. The improved mechanical properties of the HS sample were due to the persistent occurrence of the TWIP effect under very low to high strain levels during the entire deformation process, the effective grain boundary hardening in the EG domain, and the additional work hardening provided by the GNDs.

In-situ investigation of strengthening and strain hardening mechanisms of Cu-added medium-Mn steels by synchrotron-based high-energy X-ray diffraction

A novel Cu-added medium-Mn steel with a chemical composition of Fe–0.27C–9.1Mn–1.86Al–3.3Cu (wt.%) was designed and subjected to intercritical annealing (IA) temperature range from 620 °C to 680 °C for 1 h. The ultimate tensile strength (UTS) increases and the yielding strength (YS) decreases with the IA temperature increasing. The YS of 824 MPa, UTS of 1222 MPa, total elongation (TE) of 55%, and product of strength and elongation (PSE) of 67.2 GPa·% are achieved after IA at 660 °C. Transmission electron microscopy confirmed that Cu-rich nanoparticles precipitate in the ferrite. The in-situ high-energy X-ray diffraction (HE-XRD) experiments show that at the beginning of plastic deformation, both austenite and ferrite bear the applied load. The load is mainly undertaken by martensite with effective transformation-induced plasticity (TRIP) effect triggered. The YS of ferrite is significantly higher than that of austenite. The individual contribution of solid solution strengthening, grain refinement strengthening, dislocation strengthening, and precipitation strengthening in ferrite and austenite is analyzed. The discrepancy between the YS of ferrite and austenite is mainly attributed to the precipitation strengthening due to the Cu-rich nanoparticles precipitation. The moderate mechanical stability and the collaboration of TRIP and twinning-induced plasticity (TWIP) effects of austenite contributed to the enhanced strain hardening capability and resulted in large ductility.

Temperature dependence of mechanical properties and deformation mechanism of Fe–25Mn–3Al–3Si alloy at high strain rate

High-Mn steels exhibit an excellent combination of strength and ductility owing to the twinning induced plasticity (TWIP) effect or transformation induced plasticity (TRIP) effect, and have become an important material under service conditions of high strain rate and low temperature. However, the mechanical properties and deformation mechanism have not been fully understood during this extreme service conditions. Using impact test on Split Hopkinson Pressure Bar apparatus and microstructure characterization, the relationship among deformation temperature, mechanical properties and the underlying deformation mechanisms of high-Mn TRIP/TWIP steels during high strain rate (HSR) deformation was investigated. Our study shows that no matter the high temperature impact or low temperature impact, the Fe–25Mn–3Al–3Si alloy has good mechanical properties under large impact load and HSR deformation. Moreover, the strain hardening rate of the HSR impact samples at 300 °C are significantly higher than that at −80 °C and room temperature (RT). Detailed transmission electron microscopy results revealed that the strain rate has an important influence on the deformation mechanism of high-Mn TRIP/TWIP steel. Thus, TWIP effect also become the main deformation mechanism of Fe–25Mn–3Al–3Si alloy with a high SFE at 300 °C during HSR deformation. Higher strain rate and lower temperature can promote the formation of ε-martensite twinning (εT) and the occurrence of reversible transformation of ε-martensite ⇆ γ-austenite, which improve great the toughness of the alloy at low temperature (such as −80 °C). The models of reversible transformation are proposed based on the detailed microstructural characterization.

New findings on the cryogenic TWIP effect in Ti-3Al-2Zr-1.5Mo alloy with lamellar microstructure

Quasi-static tensile deformation behaviors at room temperature (RT) and 77 K of Ti-3Al-2Zr-1.5Mo alloy with lamellar microstructure were systematically investigated in this study. The experimental results indicated the excellent cryogenic mechanical properties of Ti-3Al-2Zr-1.5Mo alloy with lamellar microstructure. The strength and elongation of the Ti-3Al-2Zr-1.5Mo alloy at 77 K are much larger than those of RT specimen. It demonstrated that the strength, plasticity and strain-hardening rate were improved simultaneously at cryogenic temperature. Microstructural analysis indicated that strong cryogenic twinning induced plasticity (TWIP) effect occurs in the Ti-3Al-2Zr-1.5Mo alloy with lamellar microstructure. The intersection between the substructure and the α/β interface is one of factors to the nucleation of twins at cryogenic temperature. The optimization of the properties of Ti-3Al-2Zr-1.5Mo alloy should be attributed to the significant TWIP effect at cryogenic temperature. In addition, the cryogenic environment weakens the hindering effect of the interfaces on crack propagation.

Twinning-induced plasticity (TWIP) effect in multi-phase metastable beta Zr-Nb alloys

• Twinning-induced plasticity (TWIP) strategy was first applied in Zr alloys for strain-hardening and ductility enhancements. • Zr-13.8Nb-0.7Hf and Zr-15.5Nb-0.7Hf alloys designed by electron-to-atom ratio exhibit multi-phase microstructures consisting of acicular body-centered tetragonal phase and monoclinic phase in beta grains. • TWIP-enhanced Zr-13.8Nb-0.7Hf alloy resulted in superior strain-hardening rate and ductility when comparing to Zr-15.5Nb-0.7Hf without mechanical twinning. • In-situ traction tests in TEM and SEM-EBSD were conducted to clarify the {332}<113> twinning operation and its interaction with body-centered tetragonal phase. The deformation mechanisms were studied in metastable Zr-13.8Nb-0.7Hf and Zr-15.5Nb-0.7Hf (wt.%) alloys exhibiting complex microstructures due to quenched-in precipitates (acicular body-centered tetragonal phase and globular monoclinic phase) in beta grains. Twinning-induced plasticity (TWIP) effect was observed in Zr-13.8Nb-0.7Hf via {332}<113> twinning operation, resulting in higher strain-hardening rate and uniform elongation than those of Zr-15.5Nb-0.7Hf deforming via only dislocation glide. In-situ characterizations, by applying tensile deformation during electron backscattered diffraction (EBSD) mapping and transmission electron microscopy (TEM) observation, were conducted to clarify the twinning process and the role of quenched-in precipitates when interacting with deformation twins.

Charpy impact behaviors of metastable β-Ti alloys: Transformation induce plasticity versus twining induce plasticity

TRansformation Induced Plasticity (TRIP) and TWinning Induced Plasticity (TWIP) have been successfully proposed to overcome the trade-off between strength and ductility of metastable β Ti alloys under static tensile deformation. However, whether they can be directly extended to impact deformation is still controversial due to the high strain rate and the existence of notch. In this work, Ti-4.7Al-6.26Mo-2.85Cr (TRIP Ti alloy) and Ti-8.5Cr-1.5Sn alloy (TWIP Ti alloy) were selected to compare the Charpy impact deformation behaviors and the corresponding deformation mechanisms. It was found that, under Charpy impact deformation, the primary deformation mechanism of TRIP Ti alloy was dislocation slip due to the massive stress-induced martensite (SIM) were inhibited at high strain rate; whereas, in the case of TWIP Ti alloy, the activity of stress-induced deformation twins (SIDTs) was promoted at high strain rate, which resulted in the intersection of different DTs and the formation of secondary DTs. The formation of the hierarchical DTs structures in TWIP Ti alloy not only effectively released the stress concentration to delay the crack initiation, but also remarkably increased the crack propagation path to enhance crack propagation resistance. Specifically, compared with TRIP effect, it seems as though TWIP effect was more efficient at accommodating the imposed high strain rate deformation. These findings enrich the understanding of TRIP and TWIP under impact deformation, and provide references for the design of metastable β titanium alloy with high impact toughness.

High strength and deformation stability achieved in CrCoNi alloy containing deformable oxides

• The spreading lattice distortion in CrCoNi-O high entropy solution locally relieved the severe interfacial mismatch and led to nanoscale variation of interfacial strain at the matrix-oxide interface. • Dislocations slip transferred from solid solution phase to ceramic phase, enabling the ceramic phase with significant deformability. • The yield strength of the dual-phase samples was twice of the single-phase CrCoNi-O alloy; strong strain hardening was obtained with ultra-high deformation stability, whereas the single-phase CrCoNi-O sample exhibited catastrophic shear localization. Hard secondary phases usually strengthen alloys at the expense of ductility. In this work, we made a dual-phase CrCoNi-O alloy containing a face centered cubic matrix and chromium oxide. On one side, the dispersed chromium oxide nano-particles impeded dislocation movement and increased the strength of the alloy. On another side, the spreading lattice distortion in CrCoNi-O high entropy solution locally relieved the severe interfacial mismatch and led to nanoscale variation of interfacial strain at the matrix-oxide interface, which facilitated dislocations’ transmission from one phase to another. Consequently, unlike the strong but brittle oxide nanoparticles used before, the oxide phase here can afford significant dislocation activities during material's plastic deformation. Comparing the mechanical properties of CrCoNi-O alloys with and without chromium oxide particles, it was found that the yield strength of the dual-phase samples was twice of the single phase CrCoNi-O alloy and strong strain hardening was obtained with ultra-high deformation stability. High density of nanotwins formed in dual-phase samples under high stress, resulting in significant strain hardening according to the well-known twinning-induced plasticity (TWIP) effect. Our results shed light on optimizing the combination of strength and plasticity of compounds by modulating the variation of interfacial strain field based on the spreading lattice distortion.

Quantitative characterization of mechanical twins based on new digital image processing method

To measure the volume fraction of mechanical twins in transmission electron microscopy (TEM) images, a new image processing method named “Oriented-Linear Object Segmentation (OLOS) method” was proposed. The processing results showed that the twin volume fractions in TEM images could be measured more directly, rapidly, and effectively. This OLOS method is potentially developed for quantifying the twinning-induced plasticity (TWIP) effect in metals, e.g. TWIP steels, high entropy alloys, and copper. • A new image processing method to characterize mechanical twins was proposed. • Mechanical twins in TEM images could be measured quantitatively by this method. • This new method is effective and fast to quantify mechanical twins automatically.