Secondary Twins Research Articles

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.

Effect of Various Processes on Microstructure of CoCrFeNiAlx High-Entropy Alloy Shot Peening Layer

The change in microstructure caused by shot peening can strengthen the material and play an important role in improving the fatigue properties of the material. In order to investigate the related properties such as plastic strain anddislocation activity, the microstructure of CoCrFeNiAlx alloy shot peening layer under different processes was studied. The material exhibited a single austenitic phase, and the FCC crystal structure remained unchanged despite variations in shot peening intensity. Microstructure analysis indicates that with the increase in shot peening intensity, the grain size of the shot peening layer decreases obviously, and the content of microscopic distortion on the surface of the shot peening layer is the highest, and gradually decreases with the increase in depth. At the same time, the roughness of the sample surface is also reduced, which can enhance the fatigue strength and life of the sample. A TEM study revealed the microstructure of the shot peening layer. During the impact of shot peening, the twins produced gradually subdivided the initial grain into smaller slices. With the accumulation of plastic strain, dislocation activity begins to dominate the deformation process. The deformation-induced dislocations accumulate gradually in the small pieces and accumulate into dislocations perpendicular to the secondary twins. These results could be conducive to providing reference and theoretical basis for improving and strengthening the mechanical properties of a series of materials such as high-entropy alloy.

Investigation of dynamic tensile mechanical responses and deformation mechanism at high strain rates in a TWIP steel

In view of the potential demand for the materials with excellent dynamic mechanical behavior in protective construction, the dynamic tensile mechanical properities and deformation mechanism of twinning-induced plasticity (TWIP) steel at various high strain rates were investigated in this paper by combining test research and numerical simulation. In order to explore the impact of strain rates on mechanical behaviors within the tested range, a split Hopkinson tensile bar (SHTB) was utilized to measure the mechanical responses of TWIP steel ranging from 1000 s−1 to 6500 s−1. The Johnson–Cook (J-C) dynamic constitutive model was established based on the mechanical test results, and the mechanical properties were studied by using ANSYS/LS-DYNA. EBSD and TEM were employed to investigate the microstructure evolution of TWIP steel under dynamic loading. The results demonstrate that both strength and plasticity increase with increasing strain rate, demonstrating positive strain rate sensitivity (PSRS) which is primarily due to the activation of multiple twin systems at higher strain rates. As the strain rate increases, the mean free path of dislocation slip reduces, causing a higher strain hardening rate. The critical shear stress required for twinning is almost unchanged with an increase in strain rate, while the shear stress required for dislocation increases consistently. As the strain rate exceeds 3000 s−1, the critical shear stress required for dislocation becomes greater than that for twinning. With increasing strain rate, the proportion of secondary twins in TWIP steel rises and the deformation twins are bent when the strain rate is 6500 s−1 owing to the effect of dislocations and stacking faults. The production of nanoscale twins at higher strain rates is a beneficial phenomenon, as it enhances both strength and plasticity. The deformation twins become thinner with an increase in strain rate, leading to a rise in the stress needed for the motion of dislocations to pass the twins, which contributes to the improvement of strength in TWIP steel. The slip systems that deforms at a disadvange have new orientations because of defromation twins. The established J-C model is appropriate well for analyzing the mechanical properties of TWIP steel in the tested range of strain rates accoring to the fingdings of test and numerical simulation. Establishing the connection of microstructure and macroscopic behavior of TWIP steel could provide a guidance for the its further study and the application in the fields of impact resistance and anti-explosion.

{332} mechanical detwinning as a deformation mechanism in the β-metastable Ti-15Mo alloy during cyclic loading

The cyclic shear study of the Ti-15Mo (wt.%) alloy, with twinning induced plasticity deformation mechanism, shows that whereas conventional {332}<113> twinning is obtained during forward loading, room temperature backward loading in Bauschinger tests are able to cause twins reversion, suggesting that detwinning can act as a proper deformation mechanism. After reversion, the mechanical twins leave behind residual bands with low misorientation (∼2°) in the matrix, and secondary twins. When the backward deformation is continued, new twins of different {332}<113> systems favorable to backward stress are then produced in the microstructure. The formation of the various deformation products is rationalized by trace analysis and calculation of the resolved shear stress on the considered twinning systems.

Microstructural evolution and tensile property of gradient microstructure CoCrFeMnNi high-entropy alloy induced by pre-torsion

Gradient-structure was obtained in CoCrFeMnNi high-entropy alloys utilizing both the forward pre-torsion and forward-reverse pre-torsion preparation technique. The slope between hardness and r/R is high corresponding to increase in the pre-torsion angles attributable to the gradient structure after pre-torsion processing. Under 2% tensile strain, the change of dislocation and twin morphology takes place as pre-torsion angle increases, i.e., from discrete dislocations, to dislocation tangles and to dislocation cells, and from single twins to secondary twins. According to tensile test results, yield strength of samples increases but fracture strain linearly decreases with increasing pre-torsion angles as: Yield strength=905.47−11.67elongation. Yield strength of FRPTed specimens is slightly higher than that of FPTed specimens under the same pre-torsion angles with the maximum increment of 56 MPa. Strength mechanism indicates that the improvement of yield strength of the HEAs is mainly attributed to gradient dislocation and twin strengthening. In addition, a strength predictive model, which takes gradient dislocation and twin strengthening into consideration, is proposed, and predictive yield strength match well with the experimental data of FPTed- and FRPTed- samples.

Microstructural evolution and twins formation under tensile test of Ni-free Mn-N austenitic stainless steel using laser powder bed fusion method

At the initial uniaxial tensile stage, predominant planar dislocation slip occurs to generate dislocation pile-ups and dipole bundles. Low-energy Taylor lattice structure is then formed by co-planar slip bands along the primary slip system along [001]γ and [111]γ directions to form primary twins. Increasing the straining and generation of numerous micro-bands and crystallographic defects led to the strong restriction for dislocation motion, enhancing the formation of secondary twins with stacking fault near the deformation twin's energy. Decreasing the mean slip band and micoband spacing and configuration of complex dislocation arrangement would cause the dislocation slip, generating an extensive number of dislocation walls and cell blocks, resulting in forming a noncrystalline structure with multi-orientation. Molecular dynamics (MD) analyses revealed that deformation began by nucleation of Shockley partial dislocations with a burger vector of 1/6 〈112〉 from none-periodic boundaries. Afterwards, dislocations junctions created Hirth dislocations with a Burger vector of 1/3 〈100〉, which causes strain hardening in the system. At this point, Shockley dislocations nucleated from Hirth dislocations and strain hardening increases.

Triple and double twin interfaces in magnesium—the role of disconnections and facets

Twin boundaries have been shown to deviate from the twinning planes in hcp metals, and facets have often been observed in twin interfaces. This study presents a twinning disconnection-based model for faceting in single, double and triple twin boundaries in magnesium. Primary twinning disconnections predicted via symmetry arguments are shown to produce commensurate facets in single twin boundaries, which are then transformed into commensurate facets in double twin boundaries via the action of secondary twinning disconnections. In contrast, it is shown that for triple twin boundaries with tension-compression-tension twinning sequence, no commensurate facets can be produced by the action of tertiary twinning disconnections. The effect of facets on the macroscopic orientation of twin interfaces is discussed. Theoretical findings are validated by a transmission electron microscopy study of a hot rolled Mg-1.18wt%Al-1.77wt%Nd alloy. Single and double twins are observed, as well as rare triple twins, and the interface between the matrix and a triple twin is captured for the first time. Facets consistent with theoretical predictions are imaged via high-resolution TEM and macroscopic deviations of the boundaries from the primary twinning planes are measured.

The isotopic origin of Lord Howe Island reveals secondary mantle plume twinning in the Tasman Sea

Thermochemical convective instabilities in the mantle, often referred to as mantle plumes, cause mantle melting that give rise to ocean island basalts (OIB) in intraplate settings. The array of radiogenic isotope signatures in global intraplate OIB indicate that although plumes are individual entities, they share enriched components that resemble various parts of subducted crust and a hypothetical mantle matrix termed “focal zone” (FOZO). Each plume is expected to rise with an individual buoyancy flux, thereby producing variable volumes of melting with some OIB forming subaerial islands, whereas others produce submarine volcanoes.Here, we report the first radiogenic isotope data (Sr, Nd, Pb and Hf) for lavas of Lord Howe Island (LHI) in the Tasman Sea, the most prominent subaerial expression of the hypothesised Lord Howe mantle plume. Major element data are consistent with a moderate degree of partial melting, and heavy rare earth element depletion indicates melting occurred in the presence of garnet, consistent with other global plume lavas. Radiogenic SrNd isotopic data are similar to those defining the relatively primitive FOZO component with no clear enriched mantle affinity. The nearby Tasmantid Seamounts are also sourced from a mantle plume, and have similar SrNd character. However, combined 208Pb/204Pb and 207Pb/204Pb vs 206Pb/204Pb ratios of LHI lavas are inconsistent with a FOZO-type source. Instead, Pb isotopes overlap with typical enriched mantle 1 (EM1) lavas (e.g., Pitcairn, Tristan da Cunha), trending slightly higher than the Northern Hemisphere Reference Line (NHRL). Hafnium isotopes follow trends observed in lavas of the archetypal EM1 Pitcairn-Gambier islands, and are partially decoupled from 143Nd/144Nd; this decoupling is rare among OIB. The combined element-isotope data indicate that LHI forms the most recent expression of a mantle plume track in the Tasman Sea. Albeit unique in its 208Pb*/206Pb* isotope code among global mantle plumes, the resemblance of LHI lavas with the spatially related and contemporaneous Taupo seamount of the parallel Tasmantid plume track in the Tasman sea supports a genetic relation of both plumes. The proposed common origin of the two parallel plume tracks requires a larger plume underpinning the Tasman sea with individual plume fingers rising towards the surface, possibly in a triple plume network considering the on-shore Cosgrove track in eastern Australia. The subaerial expression of LHI contrasts the submarine Tasmantid Seamounts; this expression is only possible due to the continental ribbon that forms the island's foundation, suggesting secondary plume fingers display similar low buoyancy fluxes. Plain language summaryLord Howe Island is predominantly composed of basaltic rocks, which are created when mantle underneath the crust melts and the subsequent magma rises to the surface and erupts out of a volcano. The chemical composition of these volcanic rocks can provide us with valuable information about geologic processes. Moreover, we can compare the composition of islands like Lord Howe to better investigate and understand volcanic networks that exist deep within the Earth. The composition of Lord Howe Island basalts was investigated for the first time using specialised instruments that can separate individual elements into different isotopes. The ratios of these different isotopes reflects the composition of the mantle source which can be enriched by the presence of subducted crustal rocks, oceanic rocks and sediments. Our data indicate the distinctive reservoir that Lord Howe Island samples were derived from is the same reservoir as that of the nearby Taupo Seamount volcano. We infer from this that a piece of ancient crust is stuck in the mantle, and the two volcanoes are sampling it. This study supports a theory of a large, linked volcanic network under the Tasman Sea, and adds to the growing body of research investigating ‘double-chain’ volcanism in the Pacific and Atlantic oceans.

The twins of α2 phase in β-solidifying TiAl alloy during high strain rate rolling

High strain rate rolling (HSRR) was successfully used to produce TiAl alloy sheets at average strain rate of 7.23 s−1. In present study, β-solidifying Ti-42Al-9V alloys were rolled with a single pass reduction of 80 % within (α + γ) double phase region at 1250 °C. The microstructure of the Ti-42Al-9V alloy sheet was mainly composed of fine needle-like β/γ mixture structure and the block α2 phase which was divided into different contrast by mechanical twins. As investigated, {1 1 2¯ 2}〈1 1 2¯ 3¯〉 contraction twins are the main component. And few {1 0 1¯ 2}〈1 0 1¯ 1¯〉 extension twins are generated in {1 1 2¯ 2}〈1 1 2¯ 3¯〉 primary contraction twins, namely {1 1 2¯ 2}-{1 0 1¯ 2} secondary twinning. Besides, multiple parallel twins and intersectional twins were activated.

Twin and dislocation induced grain subdivision and strengthening in laser shock peened Ti

A significantly refined microstructure with high twin and dislocation densities is a common feature in the Ti surface layers subjected to laser shock peening (LSP). In this study, in order to develop a detailed understanding of the different twin variants and accurate estimation of twin and dislocation densities that result from LSP, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) were performed at different depths below the peened surfaces of single and triple LSP impacted Ti. To overcome challenges associated with characterization of the nano-sized LSP-induced twins during the conventional EBSD analysis, a novel data mining strategy is developed to identify the characteristic straight twin boundary traces from the band contrast maps. Results show five types of twins, whose total density initially increases and then monotonically decreases with increasing distance from the LSP treated surface. The intersection of the primary and secondary type twins subdivides the grains and leads to grain refinement. In contrast, the dislocation density is highest at the LSP treated surface but decreases monotonically with the increasing depth. A similar trend in the variation of microhardness as a function of the depth indicates that LSP-induced strengthening is dominated by the dislocations and grain refinement. Further analysis reveals that the former's contribution is more pronounced than that of the latter. The understanding developed in this work can be useful for the design of the alloys and the LSP process for improved mechanical performance.

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

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

Evolution of primary and secondary twins during tensile cyclic loading in magnesium alloy ZM21 by quasi in situ EBSD

Large-scale microscopy at consecutive strains using quasi in situ EBSD was used to monitor the texture evolution in specific areas of extruded magnesium alloy ZM21 samples under cyclic tensile loading-unloading. Twin formation throughout the various cycles is highly dependent on slip and twin activity in the neighboring grains, so slip-trace analysis was conducted to monitor the activity of various slip systems. Slip traces were observed on three different glide planes: prismatic plane, first-order pyramidal plane, and second-order pyramidal plane. Formation of primary (including extension and contraction) twins and secondary twins was investigated in detail by overlapping the EBSD maps after pre-compression and after subsequent tension. Parent grains and twins were classified based on their orientations with respect to the ED (loading direction). The inverse pole figure map for the twins was plotted for the ED to calculate the position of the c-axis for twins. All extension twins that formed under reverse loading were located within 45°–90° of the ED axis, whereas the twins that formed during pre-compression were located within 45° of the ED axis. Extension twins are formed during tension loading perpendicular to the pre-existing twins from pre-compression (which have undergone a detwinning process under load reversal). Twin residues from compression interact with grain boundaries or new twins that have formed within the grains during tensile loading. Such interactions between twins and dislocations can result in damage-accumulation mechanisms which lead to crack initiation.

Impact tension behavior of heavy-drawn nanocrystalline CoCrNi medium entropy alloy wire

High-strength metallic wire is a vital bearing structure used in many industrial fields. Impact loads often challenge the service safety of metal wire in engineering applications. However, few studies have been made on the dynamic mechanical behavior of metallic wires, especially for newly developed high-entropy alloy wires. By equipping split Hopkinson tension bar (SHTB) with specially designed test fixtures, we have carried out a systematic study on the dynamic deformation behavior of the heavily-drawn CoCrNi medium-entropy alloy (MEA) wire in impact tension at both room and cryogenic temperatures. We show that these millimeter-diameter MEA wires with nano-scale grains can achieve an excellent combination of impact tensile strength and ductility at 293 K and 77 K. More interestingly, we find that the strength and ductility of the MEA wire were enhanced simultaneously with decreasing temperature and increasing strain rate. Detailed microstructure characterizations and molecular dynamics simulations reveal that the increased strength and ductility at coupled high strain and low temperature resulted from the multiplication and thinning of nanoscale twins, which further caused additional strengthening and toughening mechanisms such as stack faulting net and secondary twin. This study highlights the advantage of CrCoNi MEA wire for cryogenic temperature and impact applications and provides an experimental reference for the design and evaluation of high-strength metal wires under such extreme conditions.

Formation of nanocrystalline grain structure in an Mg-Gd-Y-Zr alloy processed by high-pressure torsion

In this paper, the microstructural and hardness evolutions of an Mg-5.91Gd-3.29Y-0.54Zr (wt%) alloy during high-pressure torsion (HPT) were investigated. Deformation twinning played a crucial role in the HPT-induced grain subdivision process. In the 1/8-revolution disk, 101¯1 and 101¯2 twins with different twin variants, 101¯1−101¯2 secondary twins and twin-twin interactions were activated. Primary twins prevailed at the very central region of the disk, while much finer multiple twins were formed at the edge region of the disk corresponding to the area subjected to a relatively large plastic strain. Besides, dislocation cell substructures were also developed. Nanocrystalline structure was attained after 5-revolution HPT processing, and the maximum hardness reached ~120 Hv at the edge region of the disk, which is much higher than that achieved from other traditional plastic deformation methods.

Activation Behavior of {10-12}-{10-12} Secondary Twins by Different Strain Variables and Different Loading Directions during Fatigue Deformation of AZ31 Magnesium Alloy

The tensile and compression fatigue deformation tests of AZ31 magnesium alloy were carried out at room temperature, and the activation behavior of {10-12}-{10-12} secondary twins in different loading directions (ND normal direction, RD rolling direction) and different strain amplitude (0.5%, 1%) was studied. The results showed that {10-12}-{10-12} secondary twins were observed on the fracture surface of the fatigue samples in the ND at the strain of 1%. In contrast, no secondary twins were observed in the ND at the strain of 0.5% nor in the RD at the strain of 1% and 0.5%, and the fatigue life in the ND was greater than that in the RD. The analysis showed that the generation of {10-12}-{10-12} secondary twins was associated with the grain size and loading direction and that the fatigue deformation at 1% strain was more likely to generate {10-12}-{10-12} secondary twins due to the large lamellar thickness of the primary twins generated at relatively large strain conditions. The fatigue deformation along the ND tension was more favorable for the activation of multiple {10-12} primary twins, and the interaction of multiple twin variants was more favorable to activate {10-12}-{10-12} secondary twins.

Twinning susceptibility of CrCoNi medium entropy alloy to cryogenic temperature during asymmetric rolling

In this study, the CrCoNi medium-entropy alloys (MEA) were processed by asymmetric rolling (298 K) and asymmetric cryorolling (ACR) (173 K and 83 K), respectively. During the rolling process, as the strain increased (from 50% to 80% reduction), the dislocation distribution inhomogeneity intensified. Meanwhile, the variation of twinning in different systems was more prominent. The cryogenic temperature promoted the inhomogeneity of the structure in the CrCoNi MEA, allowing dislocation cells and dislocation tangling zones to generate at lower strains. And the twinning susceptibility to temperature was more notable than that to strain. Secondary twins, and high-density stacking faults were found in the 173 K ACR CrCoNi MEA at low strain. And tertiary twins were found in the 83 K ACR CrCoNi MEA. It can be found from the experimental results that the improvement effect of the cryogenic environment during rolling on the mechanical properties of the CrCoNi MEA was greater than that of the increase in reduction (50%–80%). Asymmetric cryorolling shows a more efficient strengthening influence on CrCoNi MEA subjected to smaller deformation.

Abnormal Twinning Behavior Induced by Local Stress in Magnesium

This study investigated the twinning behavior with increasing compressive strain in rolled AZ31 alloy. With that purpose, a polycrystalline structure with an average grain size of 30 μm was utilized to perform the uniaxial compression tests. Microstructure evolution was traced by in situ electron backscattered diffraction (EBSD). Multiple primary twin variants and extension double twins were observed in the same grain. A comprehensive analysis of kernel average misorientation (KAM) and Schmid factor (SF) revealed that the nucleation of twins in one special grain is not only based on the SF criterion, but that it is also strongly influenced by surrounding grains. Moreover, the existing primary twins modified the inner and outer strain distribution close to the twin boundaries. With continued compression, the strain inside the primary twins stimulated the nucleation of double twins, while the strain in the matrixes facilitated twin growth. Therefore, the primary twin growth and the new nucleation of secondary twins could take place simultaneously in the same twinning system to meet the requirements of strain accommodation. Twinning behaviors are controlled by the combined effect of the Schmid factor, strain accommodation between surrounding grains, and variation in the local stress state. The local stress exceeded the critical resolved shear stress (CRSS), implying that twin nucleation is possible. Hence, the twinning process tends to be a response of the local stress rather than the applied stress.

Slip transmutation between basal [formula omitted] dislocation and [formula omitted] twinning across [formula omitted] twin boundary in titanium

Molecular dynamics simulations reveal slip transmutation mechanisms between basal 〈a〉 dislocations and {112¯1} twinning in titanium. When a co-planar pileup of basal 〈a〉 dislocations approaches (112¯4) coherent twin boundary (CTB), the 1st dislocation transmutes into a four-layer (112¯1) twin in the (112¯4) twin, forming a (112¯4)−(112¯1) double twin, and the 2nd and 3rd dislocations climb along the (112¯4) CTB and then transmute into twinning dislocations, thickening the (112¯1) secondary twin. When the (112¯1) secondary twin propagates and encounters the other (112¯4) CTB, a reverse transmutation happens associated with the nucleation and emission of partial dislocations.