Twin-twin Interactions Research Articles

Structural evolution on {10−12} co-zone twin-twin junction in magnesium single crystal

The local stress state induced by twin-twin interaction leads to the complex microstructural evolution. In this paper, we studied the local structural characteristics on the {10–12} co-zone twin-twin junction in the compressed Mg single crystal. For parent crystal, the primary lattice rotation through the low-angle tilt around the [1−210] axis facilitates the formation of the reorientated parent-crystal substructure on the acute-angle side of incoming twin. Experimental results reveal that local stress field is formed on the acute-angle side of incoming twin, and the level of stress concentration increase with further twin-twin interaction. On the twin-twin junction, the local stress caused by twin-twin interaction induces the local lattice rotation of incoming twin, leading to the orientation of the incoming-twin tip close to that of encountering twin, which indicates a tendency of the twin-twin merger interaction. And the structural evolutions of the interacted twins are consistent with twinning dislocation reaction. The size difference between the interacted twins may be an important reason for twin-twin merger interaction.

Phase transformation mediated anomalous plasticity of titanium under severe loading conditions

Titanium (Ti) with ω phase is always considered brittle and non-deformable. Therefore, little is known of the role of ω phase in the anomalously high plasticity of titanium alloys undergoing severe plastic deformation. The deformation mechanisms of shocked Ti samples in uniaxial compression and severe shear deformation was investigated by using molecular dynamics (MD) simulations. Results show that anomalous plasticity of Ti is accommodated by a forward and reverse α-ω martensitic transformation and subsequent twin-twin interactions, rather than conventional dislocation activity. The dislocation-free deformation mechanism in ω-Ti is due to an energy barrier that favors ω→α martensitic transformation compared to dislocation-based slip. Moreover, the α↔ω martensitic transformation process is accompanied by grain/domain refinement, which can be explained by the symmetry change during the phase transformation. This work advances the understanding of the interplay between phase transformation and plastic deformation in titanium under extreme loading 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.

Shock induced gradient microstructure with hierarchical nanotwins to enhance mechanical properties of Ti6Al4V alloy

In the present work, Ti6Al4V alloy was treated by one/two passes explosion hardening (EH) technique to enhance mechanical properties. The phase structures, grain size distribution, dislocation and twin structures of EH treated Ti6Al4V alloy were characterized in detail. Mechanical properties of EH treated Ti6Al4V alloy were measured by tensile and compressive tests. The mechanism of shock-induced strengthening was subsequently analyzed. Experimental results showed that gradient grain microstructure with nanotwins/hierarchical nanotwins and β-α phase transformation occurred near the surface of EH treated Ti6Al4V alloy. Furthermore, the tensile and compressive strengths of Ti6Al4V alloy exhibited significant enhancement after EH treatment. The compressive strength of the materials increased from 1008 MPa in untreated condition to 1121 MPa in one pass EH treatment and 1365 MPa in two passes EH treatment, while the fracture strains always remained more than 0.15. The significant strengthening of Ti6Al4V alloy after two passes EH treatment was mainly attributed to the formation of hierarchical nanotwins, which can effectively impede dislocation motion. In summary, EH is an effective technique to improve mechanical properties of Ti6Al4V alloy by producing gradient microstructure with hierarchical nanotwins. The present research may have the potential use for metallic materials in the field of load-carrying capacity, tribological property and fatigue property. • Gradient microstructure with hierarchical nanotwins was produced by explosion hardening. • After two passes explosion hardening, compressive strength of Ti6Al4V increased by 35.4%. • Both twin-dislocation wall interactions and twin-twin interactions contributed to grain refinement. • Shock strengthening was mainly due to the formation of hierarchical nanotwins, which effectively impedes dislocation motion.

Understanding orientation-dependent plasticity in laser beam powder bed fusion stainless steel through crystal plasticity modelling

Single-crystal-like stainless steel 316L (SS316L) was tailored by laser beam powder bed fusion (PBF-LB). Tensile responses along the <100>, <110> and <111> crystallographic directions show considerably different strength ratios in different orientations from a standard face-centered cubic (FCC) alloy. A multi-scale crystal plasticity modelling methodology is developed to simulate and understand such unique orientation-dependent plastic deformation behaviour. The polycrystal model, based on the visco-plastic self-consistent (VPSC) formulation, considers deformation by slip and twinning, adapting a dislocation-based approach with latent hardening at single crystal level, including slip-slip, slip-twin, twin-slip and twin-twin interactions, to account for the orientation-dependent strain hardening behaviour. Based on the proposed multi-scale model, the orientation-dependent superior strength-ductility behaviour of textured PBF-LB SS316L can be attributed to the beneficial effects of deformation twinning and the cellular sub-grain structure.

Effect of multiple laser peening on microstructural, fatigue and fretting-wear behaviour of austenitic stainless steel

The present work investigates the influence of laser peening without coating (LPwC) on the fatigue and fretting wear behaviour of AISI 304 alloy subjected to one and five peening passes peened at 9 GW cm−2. Laser peened samples were characterised for microstructure, residual stress, phase evolution and hardness. After LPwC, the samples induced maximum compressive residual stress of −380 and −581 MPa for one and five peening passes at 50 μm from the surface. Microhardness was increased to 31 % and 71 % for one and five pass samples at 50 μm depth. The microstructure of laser peening samples was characterised using a transmission electron microscope (TEM) and electron backscattered diffraction (EBSD). Dislocation density was increased after peening with twin-twin interaction observed in both the peening cases and martensite induced at the junction of the twin-twin in five-time peened samples. Low angle grain fraction increased from 29 % (unpeened) to 41 % (one pass) and 63 % (five passes), and grain refinement was observed, where the average grain size reduced from 22.28 μm (unpeened) to 18.36 μm (one pass) and 9.51 μm (five passes) for LPwC samples. Fretting wear behaviour at various loading (5, 10 and 20 N) displayed significant improvement in wear resistance of LPwC samples, as evident from lower wear volume loss. Fatigue testing performed at R = 0.1 exhibited an improvement in fatigue life of 1.4 and 2.4 times for one and five pass samples compared to unpeened samples at a stress amplitude of 450 MPa. Fatigue fractography revealed a decrease in striation spacing for LPwC samples (979 nm/cycle and 565 nm/cycle for one- and five-time peening) with a delay in crack-propagation rate.

Influence of bimodal non-basal texture on microstructure characteristics, texture evolution and deformation mechanisms of AZ31 magnesium alloy sheet rolled at liquid-nitrogen temperature

Cryogenic rolling experiments have been conducted on the AZ31 magnesium (Mg) alloy sheet with bimodal non-basal texture, which is fabricated via the newly developed equal channel angular rolling and continuous bending process with subsequent annealing (ECAR-CB-A) process. Results demonstrate that this sheet shows no edge cracks until the accumulated thickness reduction reaches about 18.5%, which is about 105.6% larger than that of the sheet with traditional basal texture. Characterization experiments including optical microstructure (OM), X-ray diffractometer (XRD), and electron backscatter diffraction (EBSD) measurements are then performed to explore the microstructure characteristics, texture evolution and deformation mechanisms during cryogenic rolling. Experimental observations confirm the occurrence of abundant {10–12} extension twins (ETs), twin-twin interactions among {10–12} ET variants and {10–12}-{10–12} double twins (DTs). The twinning behaviors as for {10–12} ETs are responsible for the concentration of c-axes of grains towards normal direction (ND) and the formation of transverse direction (TD)-component texture at the beginning of cryogenic rolling. The twinning behaviors with respect to {10–12}-{10–12} DTs are responsible for the disappearance of TD-component texture at the later stage of cryogenic rolling. The involved deformation mechanisms can be summarized as follows: Firstly {10–12} ETs dominate the plastic deformation. Subsequently, dislocation slip, especially basal <a> slip, starts to sustain more plastic strain, while {10–12} ETs occur more frequently and enlarge continuously, resulting in the formation of twin-twin interaction among {10–12} ET variants. With the increasing rolling passes, {10–12}-{10–12} DTs incorporate in the plastic deformation and dislocation slip serves as the major one to sustain plastic strain. The activities of basal <a> slip, {10–12} ETs and {10–12}-{10–12} DTs benefit in accommodating the plastic strain in sheet thickness, which contributes to the improved rolling formability in AZ31 Mg alloy sheet with bimodal non-basal texture during cryogenic rolling.

Gradient structure induced simultaneous enhancement of strength and ductility in AZ31 Mg alloy with twin-twin interactions

Gradient nanostructure was introduced to enhance the strength and ductility via deformation incompatibility accommodated by geometrical necessary dislocations for most metallic materials recently. However, few intensive researches were carried out to investigate the effect of gradient structure on the deformation twin evolution and resulting performance improvements. In the present paper, we produced gradient-structured AZ31 Mg alloy with fine-grain layers, parallel twin laminates and a coarse-grain core from two upmost surfaces to the center of plate. Surprisingly, this architected Mg alloy exhibited simultaneous enhancement of strength and ductility. Subsequent microstructural observations demonstrated that abundant twin-twin interactions resulting from higher strength and multi-axial stress state could make great contributions to the increase of work-hardening capability. This was further proved by the measurement of full-field strain evolution during the plastic deformation. Such a design strategy may provide a new path for producing advanced structure materials in which the deformation twinning works as one of the dominant plasticity mechanisms.

Effect of secondary twins on strain hardening behavior of a high manganese austenitic steel at 77 K by quasi in situ EBSD

Two high manganese austenitic steels with different grain size were prepared to comprehensively investigate the size effect on secondary twinning and resultant strain hardening at 77 K. The microstructure was characterised by means of quasi in situ electron back-scattered diffraction, transmission electron microscopy and X-ray diffraction. An abnormal phenomenon was found that both the tensile strength and total elongation showed a decreasing trend with the increase of grain size, especially the strain hardening ability decreased significantly in the later stage of deformation. The microstructure indicates that the difference of the fractions of secondary twins may be the reason for this change. Twin-twin interactions are an important strain hardening mechanism. In addition, the dynamic grain refinement caused by the primary twins is a “one-dimensional” refinement, therefore, with the increase of the density of primary twins, the activation of secondary twins becomes difficult. A high density of primary twins are formed in the CG steel in the early stage of deformation, but these high-density primary twins prevent the activation of secondary twins, leading to a significant decrease in strain hardening capacity. However, in the FG steel, the secondary twins are sufficiently activated, leading to a significantly increase in yield strength and strain hardening capacity.

Effect of grain size on twinning behavior of pure titanium at room temperature

The grain size effect on deformation twinning behavior of pure titanium during uniaxial compression was investigated using electron backscattered diffraction (EBSD) technology. In this work, EBSD statistical analyses on big data sets of twins in pure titanium specimens with different initial grain sizes were carried out. Results show that a greater deformation coordination ability caused by small initial grains results in higher yield strength. Moreover, upon increasing the grain size, more twin variants were activated and more twin lamellae developed. The types of activated twins in specimens with larger grain size were more diverse. The twin growth of different twin modes was affected by grain size differently. The volume of the {10–12} extension twinning was sensitive to the grain size due to the small shear force generated by it per unit volume. In addition to grain size, grain orientation and the amount of deformation also significantly affect twinning activity during deformation. The primary twin lamellae were more abundant in the normal direction (ND) specimens as the deformation increased, while more lamellar thickening occurred in the rolling direction (RD) specimens. Because of more diverse primary twin modes in larger grains, the secondary twinning was also various. The secondary twinning {10–12}→{11–22} later developed. It is speculated that the activation of the secondary twins may be related to the volume ratio of the primary twin to the matrix grain. Simultaneously, the twin-twin interactions were more common in larger grains, restricting twin growth and promoting the material's hardening. The types and structures of twin-twin interactions in specimens with large grain size were also varied.

Effect of strain rate and low temperature on mechanical behaviour and microstructure evolution in twinning-induced plasticity steel

The mechanical behaviour and microstructure evolution of twinning-induced plasticity (TWIP) steel subjected to tensile tests at high strain rate and low temperature were investigated. Under extreme conditions (3 × 103 s−1 and liquid nitrogen temperature), the TWIP steel exhibits high strain hardening capability and high strength, which are intimately related to deformation twinning. Here we investigate the plasticity and failure processes of a TWIP steel under extreme conditions, such as shock and/or cryogenic loading, and identify that deformation twinning no longer serves the fine plasticity. Obvious ductility loss has been found in TWIP steel even with strong strain hardening capability, which always leads to large uniform tensile ductility against local necking. In this study, the microstructure observations suggest that both of the second twin systems and nanotwin structures are highly active under extreme conditions. Twin-twin and dislocation-twin interactions accelerate the crack nucleation at and propagation along the twin boundaries. The conversion from twinning-induced plasticity to twinning-assisted cracking is controlled by twin structure and its spatio-temporal evolution, which are most susceptible to strain rate and temperature. These results provide an underlying micromechanism that controls the plasticity and failure in TWIP materials under extreme conditions and contributes to the understanding of twin structure-related processes to a broad community.

Spectral phase-field model of deformation twinning and plastic deformation

Deformation twinning is critical to understanding and predicting the heterogenous deformation of many important material systems, e.g., hexagonal metals. In this work, we report a Fast-Fourier-Transform (FFT) based spectral phase-field method (PFM) model of deformation twinning and plasticity by directly incorporating the spectral FFT elasto-viscoplastic formulation, which allows two-way coupling between morphological evolution of deformation twinning and elastoplastic deformation. We verify the model by studying the twinning morphology evolution and plastic deformation of Mg single crystals and comparing the results with existing experimental measurements. We then apply the model to investigate heterogeneous twinning nucleation, growth and propagation behaviors, including twin-twin interactions, double twinning, and twin-grain boundary interactions. This work not only provides useful insights into the underlying deformation twinning mechanisms in hexagonal crystals but also presents an efficient PFM model of deformation twinning and plasticity which is generally applicable to modelling and predicting morphological evolution of deformation twinning and its effects on elasto-plastic responses of polycrystalline materials.

The [formula omitted] non-cozone twin-twin interactions in Mg: A stability and mobility study using 3-D atomistic simulations

Due to easy activation of tensile twinning in Mg, multiple {101¯2} twin variants can interact with each other and form twin-twin boundaries over the course of plastic deformation. Previous studies using 2-D settings provide only a partial understanding of these interactions, especially the non-cozone ones. Here, atomistic simulations are used to study the 3-D structural characteristic and evolution of the non-cozone {101¯2} twin-twin junctions. The study reveals the existence of new twin-twin boundaries (TTBs) such as TTBBP and TTBK2, formed after the interaction between the basal prismatic and conjugate twin interfaces with the coherent twin boundary. For both non-cozone twin-twin interactions, the {1¯21¯2} TTB and its associated twin-twin junctions are found to play a major role in the {101¯2} twin's stability and mobility. Specifically, they promote the growth of the 3-D twin in both the normal and forward directions during the interaction and hinder the detwinning process upon unloading.

A comparative study of plastic deformation mechanisms in room-temperature and cryogenically deformed magnesium alloy AZ31

Abstract In this work, a commercial cast magnesium alloy AZ31 was compressed at room and cryogenic temperatures (RT and CT) to study how the twins and dislocations affect the flow stress, plastic strain and strain hardening. Cryogenic deformation suppresses the dislocation activities to enable the effects of dislocation slip and twinning on plasticity to be separately interrogated. A quasi-in-situ Electron Backscatter Diffraction (EBSD) method was used to trace the evolution and distribution of twins and geometrically necessary dislocation (GND) density manifested as kernel average misorientation (KAM) in the same group of grains before and after plastic deformation, at RT and CT respectively. Compared to the RT deformation, higher flow stress, higher strain hardening rate and moderately reduced strain to failure were observed in the CT deformation. These interesting findings were rationalised by the subsequent EBSD analysis. It is found that the increased fraction of finer twins and twin-twin interactions rather than dislocation density leads to higher stress and higher strain hardening at CT. This suggests that the hardening effect induced by twin boundary and twin-twin interactions is considerably more effective than dislocation density hardening.

Role of slip and {10-12} twin on the crystal plasticity in Mg-RE alloy during deformation process at room temperature

The deformation mechanism of slips and twins has a considerable influence on the plasticity of magnesium alloys. However, the roles of slips and twins in the room-temperature deformation of Mg-rare earth (Mg-RE) alloys with high contents of rare earth elements is rarely investigated. Here, the microstructural evolution and deformation mechanism of an aged Mg-5Y-2Nd-3Sm-0.5Zr alloy during uniaxial compression at room temperature were systematically investigated using in-situ electron-backscattered diffraction and transmission electron microscopy. The results indicated that in the early stage of deformation, the Mg-RE alloy was mainly controlled by the slip of <a> dislocations in the basal plane and the coordinated c-axis strain of the {10-12} twin. With an increase in the strain, the grain orientation became more suitable for the initiation of pyramidal II <c + a> dislocations in the later stage of deformation; these dominated the deformation mechanism. In the twin evolution of the Mg-RE alloy, there were three types of twin-twin interaction behaviors: (i) single twin variant ‘parallel’ structure, (ii) single twin variant ‘cross’ structure, and (iii) multi twin variant ‘cross’ structure. In addition, three types of twin-grain boundary interaction behaviors were summarized: (i) twin ‘refracting through’ grain boundary, (ii) twin ‘parallel through’ grain boundary, and (iii) twin ‘fusing through’ grain boundary, which are expected to act as new means and solutions for the twin strengthening of magnesium alloys.

A phenomenological model of deformation twinning kinetics

Abstract Inspired by the classical Olson and Cohen model for martensitic phase transformations, a simple phenomenological model of the deformation twinning kinetics is proposed with account for twin-twin interactions. The model was validated for different materials using experimental data abundantly available in the literature for the evolution of the twin volume fraction with strain.

Microstructure evolution and residual stress distribution of nanostructured Mg-8Gd-3Y alloy induced by severe shot peening

A gradient nanostructured layer was prepared on the surface of Mg-8Gd-3Y alloys by severe shot peening (SSP). The microstructure evolution, residual stress distribution and mechanical property of the gradient nanostructured layer were systematically investigated by Rietveld refinement, transmission electron microscopy, and X-ray stress and hardness analyses. The results show grain sizes of the peened samples were 80–100 nm at the top surface and increased gradually with depth. The refinement process of Mg-8Gd-3Y alloys to nano size during SSP mainly involved three steps: i) introduction of deformation twinning and dislocations in grains with a large size, ii) subdivision of large grains into substructures by twin-twin interaction and dislocation cells interacting twins, iii) evolution of subgrains to nanograins by the dislocation-assisted lattice rotation. Simultaneously, high-level compressive residual stresses were induced with a maximum magnitude of −206 MPa. Besides, the surface roughness values obviously increased and several surface cracks were detected at the near surface, which might have detrimental effects on surface behavior. Owing to the microstructural improvement, the top surface of the peened Mg-8Gd-3Y alloy attained the maximum microhardness of 155 ± 5 HV, exhibiting approximately 2 times higher than the pristine alloy.

{[formula omitted]} twinning mechanism during in situ micro-tensile loading of pure Mg: Role of basal slip and twin-twin interactions

An SEM in situ uniaxial tensile testing setup allowing HR-EBSD acquisition during deformation was used to study the extension twinning mechanism in magnesium (Mg) at the micron scale. Structures were fabricated with two different crystal orientations, respectively perfectly aligned with, and at 5° to, the [0001] axis. Limited {101¯2} twin formation was identified in the former case, while twinning was found to largely accommodate the plastic deformation in the latter case. These two different mechanisms are explained by the activation of basal slip when loading at 5° to the c-axis, which triggers {101¯2} twin nucleation and favors twin growth and propagation. The other orientation shows the activation of pyramidal slip together with only limited {101¯2} twin growth. The critical resolved shear stress for {101¯2} twinning has been determined to be ten times higher than in bulk material. 3D HR-EBSD mapping enabled reconstruction of the three dimensional twin structure after deformation. From this, the interaction between the dislocations located ahead of the incoming twin and a pre-existing twin boundary was investigated, where the GND distribution and the local shear stress were determined. The results show plastic accommodation up to ~11% of strain, revealing higher ductility than usually reported for bulk materials.

Nano-twinning enhanced room temperature fatigue crack growth in single crystalline CoCrFeMnNi high entropy alloy

The equiatomic high entropy alloy CoCrFeMnNi is known for its unique strain hardening behavior which extends to cryogenic temperatures and gives remarkable monotonic toughness. Studies show that activation of twinning at such low temperatures is the key factor for the exceptional mechanical properties of this alloy. However, room temperature deformation has been assumed to be slip dominated. If deformation twinning develops during room temperature fatigue crack growth (FCG) it could impart beneficial properties and the current paper focuses on single crystals exhibiting twinning at increasing crack lengths. The results are demonstrated for <001> single crystals and compared to the <111> single crystals. The various activated slip/twin systems influencing FCG were identified using high resolution digital image correlation (DIC) and electron backscattered diffraction (EBSD). Threshold stress intensity value for crack initiation and the Paris slope were unambiguously established. Kmax for <001> and <111> samples extracted from precise displacement measurements turn out to be as high as 230 MPa-m0.5 and 195 MPa-m0.5 respectively, which far exceeds most materials. Concurrently, high resolution EBSD revealed extensive nano-twinning at the crack tip for the <001> case. EBSD results also elucidate slip-twin interactions and dislocation induced lattice rotation inside twinned volumes. In conjunction, TEM images from the crack tip confirm the presence of deformation twins and additionally reveal twin-twin interactions. Moreover, the local crack tip R-ratio (Kmin/Kmax) turns out to be much higher (>0.6) than the global R-ratio (σmin/σmax) due to the accumulation of large crack tip strains (>20%). Synchronously, the crack tip displacement profiles from DIC point to the absence of crack closure. Thus, extensive strain hardening of the material ahead of the crack tip without the loss of ductility due to activation of twinning results in the remarkable intrinsic damage tolerance for CoCrFeMnNi high entropy alloys.

Role of intersecting grain boundary on the deformation of twin-twin intersection

Using in situ nanomechanical testing under high-resolution transmission electron microscope, we reveal that the grain boundary at the twin-twin interaction region (referred to as GBT) can either migrate via a shuffling process to contribute to the dislocation transmission or facilitate the dislocation cross-slip between the two intersected twins. Stress condition, energy barrier and geometric structure of the GBT can influence these two modes of dislocation activities at the intersection region. Besides, the dislocations inside the incoming twin can traverse the GBT by dislocation-GBT reactions. These findings should have general implications to the understanding of the deformation of nanotwinned materials.