Primary Twins Research Articles

Synergistic interaction behavior of deformation mechanisms in a new metastable β-Ti alloy with high strain hardening rate

The present study successfully designed a novel metastable β titanium alloy, Ti–8.8Cr–1.9Sn, exhibiting exceptionally high strain-hardening rate (∼2.4 GPa) as well as significant total elongation (∼48.2 %). Detailed analysis of the microstructure revealed that the successive activation of primary mechanical twins and primary SIM α" within β phases, along with secondary mechanical twins and secondary SIM α" within twinning zones during deformation, which results in a highly intricate network of deformation microstructures. Meanwhile, during deformation the microstructure undergoes dynamic refinement through the continuous occurrence of mechanical twinning and phase transformations, leading to a reduction in dislocation mean free path, thereby enhancing the strain-hardening rate. Importantly, the microstructural heterogeneity is formed due to generation of numerous heterogeneous interfaces, including β/α'', β/twin, α''/twin as well as twin/twinnano, thereby inducing additional HDI hardening resulting from the formation of GNDs for accommodating strain gradients near these interfaces. Furthermore, the relative mobile dislocation density ρm/ρm0 is significantly high during deformation, due to a reduced exhaustion rate and a high nucleation rate of mobile dislocations. The values of activation volume Va and V∗ typically range from 1b3–100b3, indicating the occurrence of cross-slip and dislocation nucleation. The interaction and accumulation between GNDs and mobile dislocations further contribute to the improvement of strain hardening rate. Therefore, the synergistic interaction activities of multiple deformation mechanisms lead to enhanced strain-hardening rate along with the large plasticity.

Influence of martensite and grain size on the mechanical behavior of austenitic Fe–C–Mn–Ni–Al medium Mn steels with TWIP mechanism under uniaxial tension/compression and micromechanical creep

Medium Mn steels with induced plasticity mechanisms have effectively addressed the high-strength and ductility requirements for structural applications. However, there is still uncertainty about the impact of residual martensite on medium Mn steels with an austenitic matrix under uniaxial tension and compression. Furthermore, research on the influence of grain size through incomplete recrystallization in medium Mn alloys is scarce. In this context, our study focused on design and manufacturing a medium Mn Fe–C–Mn–Ni–Al alloy through controlled atmosphere casting incorporating martensite, aiming to analyze its effects under uniaxial stress conditions. Thermomechanical and annealing heat treatments were applied to control grain growth and nucleation of new grains, allowing us to explore how grain size affects the twinning process and mechanical reinforcement in the alloy. Microstructural characterization techniques, including optical and scanning electron microscopy, were employed to assess the mechanical response and damage mechanisms. Mechanical tests included tension, compression, and nanoindentation. The results indicated that residual martensite generates an accelerated damage mechanism under tension due to co-deformation at the interface with austenite, resulting in crack nucleation and rapid propagation through mode I fracture. Consequently, ductility in tension significantly decreases, while no apparent failure is observed during the compression deformation process. Furthermore, incomplete recrystallization was observed to enhance the creep response in both tension and compression. The strain hardening rate results revealed the activation of primary twinning in tension and both primary and secondary twinning in compression for annealed samples. Nanoindentation tests confirmed the presence of twinning and revealed that diffusion creep and grain boundary sliding are the predominant creep mechanisms.

Deformation Behavior of AZ31 Magnesium Alloy with Pre-Twins under Biaxial Tension.

In the present study, the mechanical response and deformation behavior of a Mg AZ31 plate with different types of pre-twins was systematically investigated under biaxial tension along the normal direction (ND) and transverse direction (TD) with different stress ratios. The results show that significant hardening was observed under biaxial tension. The yield values in the direction of larger stress values were higher than those under uniaxial loading conditions, and the solute atom segregation at twin boundaries generates more obvious strengthening effect. Noting that, for TRH (with cross compression along the rolling direction (RD) and TD and annealing at 180 °C for about 0.5 h) sample, the strength effect of the RD yield stress σRD:σND = 2:1 was higher than that of the ND yield stress under stress ratio σRD:σND = 1:2. There is a complex competition between twinning and detwinning under biaxal tension along the ND and TD of the pre-twinned samples with the variation in the stress ratio along the TD and RD. The variation in the twin volume fractions for all samples under biaxial firstly decreases and then increases with a higher stress ratio along the ND. As for the TDH sample (precompression along the TD and annealing), the changes of the twin volume fraction were lower than that of the TR sample (cross compression along the TD and RD). However, the amplitude of variation in twin volume fraction of the TRH sample is higher than that of the TR sample. This is because the relative activity of detwinning decreases and that of twinning increases, as the ND stress mainly leads to the growth of pre-twins and the TD stress often promotes detwinning of primary twins. With a higher stress ratio along the ND, the activity of twinning deformation increases and that of detwinning decreases.

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

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

Omphalopagus parasitic twin: A case report and literature review

Parasitic conjoined twins arise from the embryonic death of one twin, whose body parts remain vascularized and attached to the primary twin. Parasitic twins are rare and present unique challenges regarding the location of their attachment to the autosite and shared organs. In this case report, we describe the presentation and treatment of a parasitic omphalopagus twin that shared its liver with the autosite left liver lobe. The condition was not recognized before delivery, most likely due to the lack of prenatal care in the remote rural area to which the patient belonged. After a vaginal delivery, the twins were referred to our tertiary center, where a successful separation of the parasitic omphalopagus twin from the autosite was achieved.

Continuous precipitation behavior of Mg17Al12 phase during multiple steps of twinning-aging procedure in Mg-Al alloys

Twinning was regarded as an effective way to modify the precipitation in Mg alloys. Two procedures were designed to illustrate twinning-aging consequence on Mg17Al12 phase evolution. The effect of multiple steps of twinning-precipitation on the morphology and density of the continuous precipitates inside the twins and the matrix were comprehensively analyzed through phase-orientation related observation. The results show the shape of precipitates in twins was similar to that in the matrix as the CPs mostly lay on the basal planes with an asymmetric-lozenges shape. The CPs were remarkably refined and its density became much higher inside twins. Moreover, the aspect ratio of the CP plates in twins was around 45% of that in the matrix, which means the CPs inside twins became more globular. The precipitation behavior of Mg17Al12 phase inside double extension twins was different from the primary twin or the matrix. The Mg17Al12 phases lay on both basal and prismatic planes can be observed in double twins of two-step precipitation. The growth behavior of pre-precipitates in twins is different from those in the matrix. The growth rate along its thickness became higher than that in the matrix, which is probably related to dislocation-assisted diffusion at the basal plane.

Novel role of thermomechanical grain boundary engineering in the microstructure evolution of austenitic stainless steel

The effect of special grain boundary development via the grain boundary engineering process (GBE) was explored in a bimodal microstructured 316L austenitic stainless steel with an average grain size of 1.92 μm. The GBE process was carried out by the rolling-annealing method via two routes of low and medium applied strain, followed by a short annealing period in a single-step and iterative manner. The grain boundary character distribution was obtained by Electron Backscatter Diffraction (EBSD) analysis, and the intergranular corrosion resistance and the tensile behavior of the samples were examined. Microstructural characterization showed that applying low strain repetitively increased the coincidence site lattice (CSL) and Σ3 boundaries percentage and created large twin-related domains (TRD). The reduced sensitization was attributed to a high proportion of CSLs and large-sized TRDs by applying a low-strain route. In the early stages of the low-strain route, the grain boundary character distribution showed a high number of primary twin nuclei in coarse grains and a continuous network of single twin nuclei in smaller grains, creating a high percentage of Σ3-grain boundaries. The low applied strain and the absence of the necessary driving force for the recrystallization and GBs migration led to the formation of twin nuclei, which would assist with the reduction of microstructure energy. A high percentage of Σ3 boundaries and an increase in the percentage of triple points consisting of low energy boundaries were found to be influential factors in increasing elongation.

Cross-slip of extended dislocations and secondary deformation twinning in a high-Mn TWIP steel

Twinning-induced plasticity (TWIP) steels have become important materials in industry owing to the good combination of strength and ductility and high strain hardening rate. The excellent mechanical properties are highly related to the glide of dislocation and deformation twinning. However, the cross-slip behavior of the extended dislocation and the mechanism of deformation twinning are still controversial. Here, the partial dislocation motion and austenite twinning of a high-Mn steel at the early stage of deformation were investigated using in-situ tensile transmission electron microscope (TEM) technique. Results show that a large number of plane glide and cross-slip of extended dislocations can occur at the early stage of deformation. Extended dislocation nodes can be formed as a result of the reaction between adjacent extended dislocations on the same glide plane. In-situ tensile TEM experiments confirm two cross-slip models of partial dislocation: (1) the Friedel-Escaig model, cross-slip based on constriction of extend dislocation and re-dissociation; (2) the Fleischer model, cross-slip involving Lomer-Cottrell dislocation. Based on experimental results and energy calculations, it can be confirmed that the formation mechanism of austenite primary deformation twin induced by partial dislocations is different from that of secondary deformation twin. Grain boundary emits partial dislocation into the grain to form stable stacking fault which induces austenite primary deformation twin. The formation of secondary deformation twin is related to cross-slip of extended dislocation. Only the cross-slip of an extended dislocation containing a 90° partial dislocation can induce the formation of secondary deformation twin by introducing the Frank partial dislocation.

Effects of cryogenic pre-deformation on the microstructure and mechanical properties of Ti–Mo alloys

In the present study, pre-deformation at liquid nitrogen temperature (77 K) was applied to introduce twins into Ti–Mo alloys with different contents of molybdenum. The microstructural evolution and mechanical properties with different cryogenic pre-deformation amounts (from 0 to 9 %) were characterized and analyzed. The results show that the twinning activities and the evolution of twinning systems are mainly influenced by β phase stability and cryogenic pre-deformation amount. After cryogenic pre-deformation, the primary and secondary twinning of {332}<113>twins are observed in Ti–12Mo and Ti–15Mo alloys but absent in Ti–20Mo alloy. With the cryogenic pre-deformation amounts increasing, the width and quantity of twins increase constantly. Moreover, {112}<111>twinning is activated in Ti–15Mo alloy pre-deformed by 6 % and 9 %. Twins and high-density dislocations induced by cryogenic pre-deformation cause a significant enhancement in strength of all these three alloys. However, the effect of cryogenic pre-deformation on the elongation of alloys is complicated. For Ti–12Mo alloys, the yield strength increases monotonically (from 527 to 778 MPa) with the increase of cryogenic pre-deformation reduction, while the elongation is completely opposite (from 47 to 20 %). For Ti–15Mo alloys, the optimal matching of strength and ductility is achieved when the amount of cryogenic pre-deformation is 6 %, with a tensile strength of 815 MPa and an elongation of 24 %. Compared with the other two alloys, Ti–20Mo alloy gets a smaller increase in strength and a slight decrease in elongation with the amount of cryogenic pre-deformation increasing.

Delving into the intrinsic co-relation between microstructure and mechanical behaviour of fine-/ultrafine-grained TWIP steels via TEM and in-situ EBSD observation

To illustrate the microstructural factors of grain refinement for enhancing mechanical properties, the fine-/ultrafine-grained TWIP steels with a product of strength and elongation of ∼71 GPa·% were first prepared by combining rolling and stress relief annealing. Subsequently, the evolution of dislocations, stacking faults, and associated substructures of the fine-/ultrafine-grained TWIP steels was analysed by using in-situ EBSD tensile tests and TEM characterisation of the interrupted strain experiments. The results reveal that the excellent mechanical properties of the TWIP steels are attributed to dislocations and associated dislocation cells, dislocation walls, dislocation tangles, stacking faults and associated Lomer-Cottrell locks (LCs), nano-twins, primary and secondary twins and their interactions during plastic deformation. The density of geometrically necessary dislocations (GNDs) was evaluated based on the modified Ashby's model and compared with experimental results, indicating that grain size heterogeneity can promote the accumulation of GNDs, which facilitates the generation of subgrains and new boundaries to reduce the mean free path (MFP) of dislocations, thus enhancing strain hardening. Meanwhile, the interaction of lamellar primary and secondary twins in fine grains and the generation of stacking faults and nano-twins in ultrafine grains at higher strains can further promote strain hardening to elevate strength. Furthermore, the effects of grain orientation and grain size on the activation and evolution of dislocations and twins were elucidated. In ultrafine grains, twinning is strongly inhibited due to the elevated critical shear stress for twinning, resulting in more stacking faults and nano-twins, but fewer dislocation cells. The present work contributes to an in-depth understanding of the mechanical properties of fine-/ultrafine-grained materials to exploit their potential for industrial applications.

Insight into double twinning behavior of Mg–Gd rolled alloy at the nano scale: Role of micro-texture entropy

101¯1}− {101¯2} double twinning is a classical twinning mode with diverse variants in magnesium and its alloys. However, its variant selection criterion and its influence on the mechanical behaviors are still in debate. In this study, we proposed a view of micro-texture entropy to elucidate the double twinning behavior during large strain rolling of Mg–1.5Gd (wt%) alloy. Combined with the high spatial resolution scanning electron microscopy-transmission Kikuchi diffraction (SEM-TKD) technique, the detailed microstructure and micro-texture of the double twins in early stage of evolution were detected, and new insight into the variant selection and evolution of double twins was obtained. The observations suggest that the nucleation events of different double twin variants are influenced by their micro-texture entropy, the variants with lower micro-texture entropy are preferred. While the growth stage of double twins is affected both by their micro-texture entropy and the geometric factors between the primary and secondary twin systems; the former will influence their growth potential and the later can affect their growth space. A novel {101¯1}− {101¯3} double twin was also triggered to release the high local stress concentration during large strain rolling process.

Fine grains with high-density annealing twins and precipitates inducing favorable strength and excellent plasticity in laser powder bed fusion-fabricated Inconel 718 via deep cryogenic and heat treatments

Tailoring high-density annealing twins in laser powder bed fusion (LPBF)-fabricated alloys based on their intrinsic residual stress requires high annealing temperatures and/or long-term annealing, resulting in the abnormal growth of large recrystallized grains, which is detrimental to mechanical properties. This work proposes a new strategy for achieving a favorable strength–plasticity synergy of the LPBF-fabricated Inconel 718 superalloy by performing a deep cryogenic treatment (DCT) with the subsequent heat treatment (including annealing and double aging) to tailor fine grains with “high-density annealing twins + precipitates” architectures and compares the obtained material with an alloy subjected to a direct heat treatment without a prior DCT. The obtained results reveal that the additional internal stress generated during DCT increases the stored energy and dislocation density, which provide a sufficient driving force for activating high-density annealing twin boundaries (63.2 %) with fine grains (31.6 μm) within a short annealing time. The more homogeneous tailored microstructure with the “finer grains + high-density twins + precipitates” architectures decreases the mean free path of slipping dislocations, promoting intensive interactions with dislocations and inducing a strong strain hardening effect. The multiple deformation modes of stacking faults coupled with Lomer–Cottrell locks, thin primary deformation twins, and secondary twins activated during tensile loading, sustaining a strong work hardening ability and delaying the plastic instability, which exhibits a high strength (yield strength of 1088 MPa and tensile strength of 1369 MPa) and excellent plasticity (elongation of 30 %). This work not only describes a feasible method for simultaneously enhancing the strength and plasticity in additively manufactured (AM) alloys but also provides new insights into increasing the fraction of twins at a small grain size to improve the grain boundary-related properties without destroying the AM alloy shape.

Subsequent growth and sequential twinning induced by the interaction of {332} twins in Ti[sbnd]15Mo alloy

The mechanisms associated with the interaction of {332} twins are investigated in metastable Ti15Mo alloy. There are 12 possible twin-twin junctions (TTJs) induced by {332} twins that can be classified into 5 types according to the crystallographic relation. Twin-twin interactions are accompanied with the formation of twin-twin boundaries (TTBs) on the acute side and obtuse side. Experimental results based on transmission electron microscopy (TEM) and procession electron diffraction (PED) show that subsequent growth of the TTBs is asymmetric with the twin thickening preferred only on one side for each type. Besides, it is observed that the twin-twin interaction could stimulate sequential {332} twinning inside the primary twin with a specific twin variant promoted. To account for the subsequent growth of the TTBs and the preferred selection of the sequential twinning variant associated with twin-twin interactions, the elastic energy analysis of the resultant dislocations at TTBs indicates that the growth of the TTBs is energetically favorable only on one side. Displacement gradient based analysis shows that the active sequential twin variant could maximumly accommodate the accumulated strain induced by the twin-twin interaction. This work is useful for predicting the preferential growth of the interfaces and the associated accommodation mechanisms induced by the interactions of localized shear bands.

Strength-ductility synergy in twinned titanium fabricated via dynamic equal channel angular pressing and heat treatment

Dynamic equal channel angular pressing (DECAP) and heat treatment are applied to pure Ti to achieve a strength-ductility synergy. DECAP induces a complex multiscale twin structure in Ti including four primary twin types, secondary twins and tertiary twins with multiple variants, and an increased yield strength (58% higher than the as-received Ti). Subsequent annealing at 200 ℃ and 400 ℃ leads to improved ductility and strength. The yield strength (σy) and total elongation (ɛt) for the Ti sample processed with DECAP and 400 ℃ annealing are 555 MPa and 30%, respectively, and its σyɛt product is ∼45% higher than the as-received Ti. DECAP and subsequent annealing lead to a superior strength-ductility synergy in twinned Ti, providing a novel approach to produce high strength and ductility materials via dynamic deformation and subsequent heat treatment.

Birth outcomes of twins after multifetal pregnancy reduction compared with primary twins

BackgroundThe introduction of assisted reproductive technology and the trend of increasing maternal age at conception have been responsible for a significant rise in the incidence of multiple pregnancies. Multiple pregnancies bear several inherent risks for both mother and child. These risks increase with plurality and type of chorionicity. Multifetal pregnancy reduction is the selective abortion of one or more fetuses to improve the outcome of the remaining fetus(es) by decreasing the risk of premature birth and other complications. ObjectiveThis study compares the birth outcomes of trichorionic triplets reduced to twins with trichorionic triplets and primary dichorionic twins. The added value of this study is the comparison with an additional control group, namely primary dichorionic twins. Study designThis is a retrospective cohort study. Data from January 1990 till November 2016 were collected from the East Flanders Prospective Twin Survey, one of the largest European multiple birth registries. Eigthy-five trichorionic triplet pregnancies (170 neonates) undergoing multifetal pregnancy reduction (MPR) to twins were compared to 5,093 primary dichorionic twin pregnancies (10,186 neonates) and 104 expectantly managed trichorionic triplet pregnancies (309 neonates). The outcomes are gestational age at delivery, birth weight, and small for gestational age. ResultsPregnancy reduction from triplets to twins is associated with a higher birth weight (+365.44 g, 95%CI [222.75, 508.14 g], p < .0001) and longer gestational age (1.7 weeks, 95%CI [0.93, 2.46], p< .0001) compared to ongoing trichorionic triplets, adjusted for sex, parity, method of conception, birth year, and maternal age. A trend towards a lower risk of SGA is observed. Reduced triplets have, on average, a lower birth weight (-263.12 g, 95%CI [-371.80, -154.44 g], p < .0001), and a shorter gestational age (-1.13 weeks, 95%CI [-1.70, -0.56], p=0.0001) compared to primary twins. No statistically significant difference between primary twins and reduced triplets that reach 32 weeks of gestation is observed. ConclusionMultifetal pregnancy reduction from trichorionic triplets to twins significantly improves birth outcomes. This suggests that MPR of trichorionic triplets to twins is medically justifiable. However, the birth outcomes of primary twins before 32 weeks of gestation are still better in comparison with reduced triplets. The process of MPR includes at least one fetal death by definition, therefore prevention of higher-order pregnancies is preferred.

Induction of deformation twinning by dynamic strain aging effect: Rate-temperature coupling effect and constitutive modeling

This study focuses on the rate-temperature dependence analysis and constitutive modeling of the DSA-induced deformation twinning, which is a new plastic deformation mechanism discovered in several low-stacking-fault-energy metallic materials. Two types of deformation twinning, namely primary deformation twinning and DSA-induced deformation twinning, were noticed during the plastic deformation of the commercially pure titanium (CP–Ti). The strain, temperature, and strain rate ranges of the DSA-induced deformation twinning, as well as its rate-temperature coupling dependence, were systematically analyzed. The DSA-induced deformation twinning becomes more pronounced with the increasing strain rate and thereby plays a more significant role in enhancing the strain hardening rate than the primary deformation twinning. According to the dynamic Hall-Petch effect of the deformation twinning, a thermo-viscoplastic constitutive model based on the microstructural evolution and the thermal activation theory was developed. The volume fraction of the deformation twins during plastic deformation was expressed as a function of strain, temperature, and strain rate. The developed model was shown to be able to describe the S-shaped true stress vs. true strain curves over the wide ranges of temperature (77–998 K) and strain rate (0.001–8000/s). Finally, considering the interaction between the DSA and deformation twinning, the authors proposed the synergistic strengthening & toughening effect of DSA and deformation twinning, which was believed as a promising mechanism to achieve the simultaneous strengthening and toughing of the metallic materials.

Dynamic deformation behaviors and mechanisms of CoCrFeNi high-entropy alloys

Due to their unique microstructures and chemical compositions, high-entropy alloys (HEAs) exhibit remarkable mechanical properties, such as high strength, excellent ductility and good thermal stability. However, to date, there have been limited studies on the dynamic behaviors of HEAs. Here, we systemically investigated the mechanical behaviors and deformation mechanisms of CoCrFeNi HEAs and their connections under dynamic loading by combining mechanical tests and molecular dynamics simulations. Compressive testing on polycrystalline CoCrFeNi HEA samples at different strain rates showed that both yield and flow stresses increase with strain rate and exhibit a significant strain rate sensitivity due to solid solution strengthening and lattice distortion of the HEA. The strain rate sensitivity of the CoCrFeNi HEA transitions from 0.010 at low strain rates (5.0×10−5–2.5×103 s−1) to 0.333 at high strain rates (2.5×103–6.5×103 s−1). Large-scale molecular dynamics simulations further revealed that this transition is related to the plastic deformation mechanism transition from dislocation nucleation and slip at low strain rates to massive dislocation nucleation and drag at high strain rates. Furthermore, the CoCrFeNi HEA exhibited a remarkable strain-hardening capability at high strain rates, which originated from the formation of primary and secondary nanoscale twins and twin-twin and twin-dislocation interactions. Our current study sheds light on the plastic deformation mechanisms of HEAs under dynamic loading, providing guidance for the design and fabrication of HEAs with excellent dynamic mechanical properties.

Unraveling size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti by non-local crystal plasticity modeling

Titanium alloy is prominent in manufacturing of high-performance miniature devices, while size-affected plastic heterogeneity and asymmetry in micro-scaled deformation are not well understood due to its complex deformation mechanism. How the interplay among dislocation slip, deformation twinning, and strain gradient affects the size-dependent plastic heterogeneity and asymmetry is an intractable and non-eluded issue that needs to be unraveled. In this research, a non-local crystal plasticity model (NLCPM) considering discrete twins was established and implemented into a finite element framework. The proposed NLCPM successfully represented the size-dependent asymmetric mechanical response, primary and secondary twinning, and texture evolution. The size-dependent fracture behavior, plastic heterogeneity, and T-C asymmetry during micro-scaled deformation of commercial pure titanium (CP-Ti) were explored by coupling micro-tension/compression test and full-field simulation. Results showed that T-C asymmetry in flow stress was enhanced in the coarse-grained and small-diameter samples. For the sample with D≥0.6 mm, the ductility of CP-Ti was improved with the increase of grain size as there are more prevailing twins and the concentrating stress could be relieved by the abundant slip rather than cracking. Grain-level strain is more homogeneous in compression when d was increased from 60 to 110 μm since more non-prismatic slips were activated that accommodated strain along various directions. Meanwhile, plastic strain was allocated within discrete twin bands and thus alleviated strain concentration. Furthermore, the hardening effect of twin boundaries cancelled out the softening effect near the free surface, resulting in a narrower softening layer in the coarse-grained samples compared to its grain size. With the decrease of the sample size, the proportion of the softening layer increased, while the volume fraction of twinning decreased, leading to a reduction in macroscopic stress and fracture strain. This work enhances the in-depth understanding of size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti and supports the forming of high-quality micro components under complex deformation path.

Direct observation of the ledge-typed twin boundary in a martensitic steel

The structure at the {112} twin boundaries in martensitic steels has been extensively debated recently. On the one hand, it was claimed to be the omega phase, similar to Ti alloys. On the other hand, it was considered artifacts induced by the double diffraction effect from the overlaps of twin-matrix crystals at their boundaries. In this study, we clarify the twin structure in a martensitic steel providing a detailed investigation of the α′ martensite twin boundaries using experiment and simulation. The fundamental for rejecting the existence of the omega phase in steels is based on the atomic resolution imaging from the <111> direction of twin structure, which was so far challenged to be acquired for such nano-sized twinned systems. The obtained result confirms the ledge-typed twin boundary that causes the visually omega-like forms when observed from other zone axes. The ledges occurred on {110} planes, making them secondary twin planes with coherent characteristics. Furthermore, these ledges were found to transit in a gradual manner from the primary (112) twin plane to (011) and then to (1¯10) secondary twin planes; a perpendicular transition from (112) to (1¯10) was not observed. Such a transitional behavior was explained in view of lowering total twin boundary energy, evaluated by the theoretical prediction of system energy and the geometric phase analysis of the twin planes. Double diffraction and rotational Moiré effects clarified the extra spots in the diffraction pattern obtained at the twin-matrix overlapped regions. This study gives light of understanding twinning in the bcc crystals.

Superior low temperature mechanical properties and microstructure of (NiCoCr)95V5 medium entropy alloy

NiCoCr, as a model medium entropy alloy has been investigated extensively in the past decades. To further improve the mechanical properties of NiCoCr, there are mainly two methods. One is thermal-mechanical treatment. The other is to add some proper alloying elements to introduce second phase to fabricate the precipitate strengthening medium entropy alloys. Inspired by this, a superior medium entropy alloy (NiCoCr)95V5 was successfully fabricated by cold rolling and subsequent two-step annealing in this study. The strength and plasticity product exceeds 86 GPa·% that is comparable to the currently reported high-performance medium and high entropy alloys. The microstructure characterizations indicate that the dislocation slip and interacting with high density nanoscale precipitates, the initiation of primary and secondary deformation twinning which further segmenting the grains are the main reasons for such high properties. This work enriches the design and development of medium and high entropy alloys with high-strength and high plasticity.