Secondary Twins Research Articles

Structural characteristics of [formula omitted] non-cozone twin-twin interactions in magnesium

Twin-twin interactions form twin-twin boundaries (TTBs) which can prevent twin propagation, inhibit direct twin transmission, retard detwinning, and facilitate secondary twins. The current work studies the microstructure and interaction mechanisms of non-cozone twin-twin junctions by combining electron back-scatter diffraction observations and atomistic simulations. Non-cozone twin-twin interactions are defined as the intersecting line of the two twins isn't parallel to one <12¯10> zone axis (a-axis) and include two types, Type II(a) (T2→T1) and Type II(b) (T3→T1), according to the crystallography of two interacting twins. For Type II(a) interaction, both statistical results of experimental observations and interfacial energy calculation confirm TTB formation on the obtuse side of the incoming twin instead of the acute side. However, for Type II(b) interaction, the growth of twins on both sides is impeded, although the TTB on the acute side possesses the lowest interfacial energy. Atomistic simulation demonstrates that, for Type II(a) twin-twin interactions, positive resolved shear stresses on the obtuse side favor T1 and T2 twinning, while negative resolved shear stresses on the acute side impede T1 and T2 twinning. For Type II(b), negative resolved shear stresses on both the acute and obtuse sides result in impediment of twinning on both sides. These results can be used in developing micro/macro-scale predictive models that deal with the role of multiple twins and twin variants during mechanical processing. The analytical and simulation methods can be generalized and applied to atomistic analysis in different material systems to further explain the hardening mechanisms associated with twin-twin interactions.

Effect of temperature on the selection of deformation modes in Zircaloy-4

The effect of deformation temperature (77, 298 and 573 K) on the evolution of slip and twinning modes in Zircaloy-4 was studied. Samples of Zircaloy-4 were uniaxially compressed to different strains at a nominal strain rate of 10−3 s−1. The compression direction was chosen so as to activate twinning. Microstructural examination was carried out using electron backscatter diffraction to relate the evolution of microstructure and texture to the micro-mechanics and work-hardening behaviour. In conjunction with experiments a visco-plastic self-consistent crystal plasticity model was used to determine the evolution of deformation modes. Extensive twinning was observed at all the test temperatures. Twinning activity at 298 and 573 K was comparable, but was lower than that at 77 K. A strong basal texture parallel to the compressive direction was found at 298 and 573 K. The VPSC model predicted well the texture evolution at 298 and 573 K. The basal texture developed at 77 K was found to have partially reoriented back to the initial texture due to secondary twinning. Prism slip was the dominant deformation mode with secondary slip becoming more active at higher temperatures.

Transition of dynamic recrystallization mechanisms of as-cast AZ31 Mg alloys during hot compression

This study investigates the transition of dynamic recrystallization (DRX) mechanisms of as-cast AZ31 Mg alloys during hot compression. It is experimentally revealed that the transition between the twin-induced dynamic recrystallization (TDRX) and the grain boundary bulging dynamic recrystallization (GBBDRX) depends on the local strain accumulation and grain size. Based on the secondary twinning growth controlled by the grain size and stresses, a theoretical DRX transition criterion is built. Subsequently, two cooperating DRX schemes are implemented into the viscoplastic self-consistent crystal plasticity model to simulate the evolution of slip and twinning activity, textures and stress-strain relationships influenced by the double DRX mechanisms and the transition. The results show: (1) For as-cast Mg alloys with coarse grains, the TDRX is common at low-strain stage, and is the key to continuing plastic deformation. The smaller twin-walled grains that inherit the c-axis orientation of the parent twins can weaken the deformation basal texture, enhancing basal slip activity and reducing the extension twinning activity; (2) The GBBDRX that can decrease local dislocation density replaces the TDRX if the post-saturation state of secondary twinning is achieved by the applied stress together with local stress within refined grains. The dominant GBBDRX mechanism strengthens the basal texture by limiting pyramidal slip. The activities of basal and prismatic slip are reduced and enhanced by the GBBDRX, respectively; (3) The TDRX leads to a dramatic decrease of strain-hardening rate, and the transition between the two successive DRX mechanisms switches the decrease of the hardening rate to flow softening. Furthermore, some control schemes of DRX transition are proposed according to the transition criterion.

Explicit modeling of double twinning in AZ31 using crystal plasticity finite elements for predicting the mechanical fields for twin variant selection and fracture analyses

A number of studies have found that the formation of double twins in low symmetry metals can lead to the onset of strain localizations, leading further to void nucleation and ultimately fracture. This work extends a recently developed three-dimensional crystal plasticity finite element framework [1] to explicitly model kinematics and kinetics of nucleation/formation, propagation, and thickening/growth of a discrete double twin lamella ({101¯1}−{101¯2}) in a magnesium alloy AZ31. With this approach, morphological and crystallographic reorientations as well as the shear transformation strains associated with strain accommodation by the double twinning sequence are modeled during simple compression and tension. The simulations predict that the distribution of local stress-strain fields during formation and growth of primary contraction twin creates the driving force for the formation of a secondary extension twin variant, which is consistent with experimental observations in both compression and tension. In particular, the contraction twin variant (01¯11)[01¯12¯] is predicted to form an internal extension twin variant (011¯2)[01¯11]. Furthermore, the prediction of the underlining crystallographic slip deformation mechanism reveals a substantial activity of basal slip within the contraction portion of the double twin, causing strain localization in its vicinity. Finally, the simulations reveal a gradient in the traction force field across twin-parent interface, suggesting that contraction twin-parent boundaries are weak links in the microstructure where voids can nucleate.

Crystallographic analysis on the primary and secondary extension twin variants in Mg alloys

{10-12}-{10-12} double twins (DTs) are observed in AZ31 Mg alloys after one path uniaxial tension. Secondary twins (STs) are produced mainly from the intersecting points of two primary twins (PTs), forming the PTing-PTed-ST grouping. The relative orientations among these variants are analyzed. Statistical analysis reveals that the two primary variants have a specific orientation relationship with the geometric compatibility factor (m’) of 0.562. The detwinning of PTed and ST also have such type of orientation. Moreover, the PTing and the selected ST have almost opposite shear directions with the angle of 151.1°. Tensor analysis conforms that these geometric orientation relationships are beneficial for minimizing the local strain incompatibility.

Modeling the Effect of Primary and Secondary Twinning on Texture Evolution during Severe Plastic Deformation of a Twinning-Induced Plasticity Steel.

Modeling the effect of deformation twinning and the ensuing twin-twin- and slip-twin-induced hardening is a long-standing problem in computational mechanical metallurgy of materials that deform by both slip and twinning. In this work, we address this effect using the twin volume transfer method, which obviates the need of any cumbersome criterion for twin variant selection. Additionally, this method is capable of capturing, at the same time, secondary or double twinning, which is particularly important for modeling in large strain regimes. We validate our modeling methodology by simulating the behavior of an Fe-23Mn-1.5Al-0.3C twinning-induced plasticity (TWIP) steel under large strain conditions, experimentally achieved in this work through equal-channel angular pressing (ECAP) for up to two passes in a 90° die following route BC at 300 °C. Each possible twin variant, whether nucleating inside the parent grain or inside a potential primary twin variant was predefined in the initial list of orientations as possible grain of the polycrystal with zero initial volume fraction. A novelty of our approach is to take into account the loss of coherency of the twins with their parent matrix under large strains, obstructing progressively their further growth. This effect has been captured by attenuating growth rates of twins as a function of their rotation away from their perfect twin orientation, dubbed here as “disorientation” with respect to the mother grain’s lattice. The simulated textures and the hardening under tensile strain showed very good agreement with experimental characterization and mechanical testing results. Furthermore, upper-bound Taylor deformation was found to be operational for the TWIP steel deformation when all the above ingredients of twinning are captured, indicating that self-consistent schemes can be bypassed.

An algorithm for automatic identification of double twin variants in Mg alloys

101¯1−101¯2 double twins (DTs) are profuse in Mg alloys after rolling at a large reduction (>10%). In many cases, the secondary twins completely occupy the primary ones, causing difficulty in variant analysis. Here, we propose an algorithm for automatic identification of twin variants. This is applicable for the 101¯1−101¯2 DTs without knowing the primary twins. In addition, two kinds of stereographic projections are constructed to assist such variant selection. By this method, the variants of 67 DTs observed in a rolled AZ31 sheet are determined. Based on 14 sets of DTs with known primary twin orientations, the high efficiency and reliability of this proposed algorithm is verified. The reason for unreliable variant identification in two special cases is also discussed.

High speed dynamic deformation of polysynthetic twinned titanium aluminide intermetallic compound

Impact deformed TiAl polysynthetic twinned crystal was investigated by scanning and transmission electron microscopy including aberration-corrected scanning transmission electron microscopy. Shear bands, deformation twins and dislocations were observed in γ-TiAl phase. Deformation twins were identified on both primary (111¯) and secondary (11¯1¯) twin planes, and dislocation structures were resolved on the coherent {111} twin boundaries. Among them, 1/6 < 211] pseudo-twin dislocations were observed on the coherent twin boundaries co-existing with 1/2 < 101] super-partial dislocations. Two types of shear bands were identified, one was composed of (111¯) twin lamellae inclined to the propagation direction of shear bands, while the other was composed of curved (11¯1¯) twin lamellae and its propagation is nearly parallel to the (11¯1¯) twin plane.

Secondary twin variant selection in four types of double twins in titanium

We experimentally characterized four types of double twins in pure titanium and discussed the selection of secondary twin variants based on the apparent Schmid factor (a-SF), the displacement gradient accommodation (DGA), the modified DGA (m-DGA), and the nucleation via dislocation dissociation (NDD). The four types of double twins are classified into two families according to their zone axes: co-family double twins and non-family double twins. Co-family double twins share the zone axis <101¯0>. Non-family double twins have different zone axes with one along <112¯0> and the other along <101¯0>. Experimental observations reveal that the co-family double twins have one secondary twin variant prevailed over others and the non-family double twins have two secondary twin variants preferred than others. The a-SFs associated with secondary twins are always positive once the primary twin is activated, thus predicting no obvious preference for the selection of secondary twin variants. The m-DGA and NDD correctly predict the selection of secondary twin variants, implying that the preferred secondary twin variant would, to the greatest extent, relax plastic deformation associated with the primary twinning and the nucleation of the secondary twin variant is facilitated by the accumulation of available gliding dislocations at the primary twin boundary.

Intragranular cross-level twin pairs in AZ31 Mg alloy after sequential biaxial compressions

Sequential double extension twinning in AZ31 Mg alloy happens easily during sequential biaxial compressions. Widespread twin-twin interaction leads to the formation of intragranular cross-level twin pairs, each of which can be defined by a primary twin variant PVi (i = 1–6) and its nearby secondary twin variant PVj-SVk (j, k = 1–6, j ≠ i, k) across the twin-twin boundary between them. Theoretically, 9 possible misorientations exist between the two variants. Misorientation between two crystals can be expressed by a pair of rotation axis (r) and the minimum rotation angle (θ). The crystallography of a cross-level twin pair can be described unanimously by the misorientations between PVi and PVj-SVk (i.e., r1-θ1), PVi and PVj (i.e., r2-θ2), and the host grain and PVj-SVk (i.e., r3-θ3), resulting in 13 possible orientation relationships (ORs) that can be expressed explicitly by the three misorientation angles (θ1, θ2, θ3). A cross-level twin pair is a key element of a crossing or crossed twin structure, which is common in the deformed microstructure. Statistical electron backscatter diffraction analysis reveals that OR2 of (49.7°, 60°, 60°) appears the most frequently. Twinning shear transmission is almost irrelevant to the formation of a cross-level twin pair. Although the majority of the identified twins are high Schmid factor (SF) ones with SF > 0.3 and SF ratio > 0.8, SF analysis fails to explain the highly preference of OR2 and the abnormal absence of OR8 of (44.0°, 60°, 60.4°), probably due to the complexity of the formation of a cross-level twin pair.

The relationships of microstructure-mechanical properties in quenching and partitioning (Q&P) steel accompanied with microalloyed carbide precipitation

A low carbon steel microalloyed with Nb-V-Ti were heat treated by means of two-step quenching and partitioning (Q&P) process after partial austenitization. Combined use of SEM equipped with EBSD, XRD and TEM showed that the microstructure was composed of intercritical polygonal ferrite, tempered lath martensite, secondary twin martensite and carbon-enriched retained austenite, as well as microalloyed carbides (mainly VC) precipitated inside ferrite and martensite. When increasing partitioning time from 0 s to 3600 s, the volume fraction of retained austenite increased up to 6.5% with mean carbon content in austenite keeping a constant level. The retained austenite kept K-S orientation relationship with adjacent tempered lath martensite and neighboring secondary twin martensite in different martensite packet. As partitioning time increased up to 3600 s, the initial lath martensite tempered gradually to ferrite without any cementite formation.

Microstructural evolution of a nanotwinned steel under extremely high-strain-rate deformation

Nanotwinned metals are promising structural materials for resisting impact due to their excellent combination of strength and ductility. In this study, the microstructural evolution of a nanotwinned steel under extremely high-strain-rate ballistic impact was systematically investigated by nanoindentation as well as detailed electron microscopy characterization. It is found that the nanotwin structure remains similar after ballistic impact, while secondary twinning activates in a limited portion of grains. In contrast, dislocation gliding is the main plasticity mechanism in the nanotwinned steel during ballistic impact, which leads to substantial increase of hardness in the severely-deformed region close to the fracture surface. Dislocation multiplication is promoted during ballistic impact due to the phonon drag effect, resulting in a hardness increment that exceeds the maximum value achieved in quasi-static tension. In addition, recrystallization occurs in the nanotwinned steel during ballistic impact due to the significant temperature increase when the hot bullet contacted and transferred sufficient heat to the nanotwinned steel.

Ultrafine-grained CoCrFeMnNi high-entropy alloy produced by cryogenic multi-pass caliber rolling

The ultrafine-grained (UFG) structure (∼100 nm) of CoCrFeMnNi high-entropy alloy was achieved using cryogenic multi-pass caliber rolling at 77 K to impose less deformation (ε = 1) compared to severe plastic deformation (SPD, ε > 4). Highly increased twinning activity at 77 K caused the grain interiors to fragment significantly, so UFG structure could be induced without imposing SPD. Intersected twin morphology and secondary twins played a key role in twinning-induced grain refinement to achieve UFG structure. The processed material showed an exceptional high yield strength of ∼1.5 GPa with a reasonable fracture elongation of ∼10% at room temperature.

Examination of the twinning activity in additively manufactured Ti-6Al-4V

This present paper investigates the dominant twinning mode and secondary twin via transmission electron microscopy at different positions of additively manufactured (AM) Ti-6Al-4V deposition wall fabricated by laser metal deposition (LMD). {10−11} 〈−1012〉 twinning system is observed as the dominant twinning mode. Secondary twins, including the inserting secondary twin and neighboring secondary twin, are characterized at position with sufficient thermal cycle history. These twinning morphologies can be rationalized to some degree from the standpoint of the twinning effect and thermal condition in additive manufacturing. This work gets insight into the twinning evolution in additively manufactured Ti alloys.

The effect of surface step and twin boundary on deformation twinning in nanoscale metallic systems

The effect of surface step and twin boundary on the mechanical twinning process in nanoscale face-centred cubic metallic films was studied using atomistic simulations. Aluminium was considered as a model material but comparisons were made with silver and copper. Surface steps were identified as privileged sites for twin nucleation at lower stresses, leading to the formation of only one large twin in defect-free films. In presence of a coherent twin boundary which acts as a strong barrier to the propagation of dislocations, the extension of nucleated twins is much more limited but the density of secondary twin boundaries is found higher. The key role played by Lomer dislocations, resulting from the interaction between incipient twins and the coherent twin boundary, on the nucleation of new twins was demonstrated. These findings shed light on some elementary mechanisms that can be involved in the elaboration of nanotwinned materials with interesting mechanical properties.

Dislocation and twinning behaviors in high manganese steels in respect to hydrogen and aluminum alloying

The dislocation and twinning evolution behaviors in high manganese steels Fe-22Mn-0.6C and Fe-17Mn-1.5Al-0.6C have been investigated under tensile deformation with and without diffusive hydrogen. The notched tensile tests were interrupted once primary cracks were detected using the applied direct current potential drop measurement. In parallel, the strain distribution in the vicinity of the crack was characterized by digital image correlation using GOM optical system. The microstructure surrounding the crack was investigated by electron backscatter diffraction. Electron channeling contrast imaging was applied to reveal the evolution of dislocations, stacking faults and deformation twins with respect to the developed strain gradient and amount of hydrogen. The results show that the diffusive hydrogen at the level of 26 ppm has a conspicuous effect on initiating stacking faults, twin bundles and activating multiple deformation twinning systems in Fe-22Mn-0.6C. Eventually, the interactions between deformation twins and grain boundaries lead to grain boundary decohesion in this material. In comparison, hydrogen does not obviously affect the microstructure evolution, namely, the twinning thickness and the amount of activated twinning systems in Fe-17Mn-1.5Al-0.6C. The Al-alloyed grade reveals a postponed nucleation of deformation twins, delayed onset of the secondary twinning system and develops finer twinning lamellae in comparison to the Al-free material. These observations explain the improved resistance to hydrogen-induced cracking in Al-alloyed TWIP steels.

Influence of rolling asymmetry on the microstructure, texture and mechanical behavior of high-manganese twinning-induced plasticity steel

In this work, a Fe-23Mn-1.5Al-0.3C twinning-induced plasticity steel was subjected to both symmetric (SR) and asymmetric rolling (ASR) to investigate the influence of an additional shear strain component introduced during asymmetric rolling on the microstructure and texture evolution as well as on the mechanical properties. ASR was performed using a roll diameter ratio of 2:1. Electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction were utilized for careful characterization of the microstructures and textures in the top, middle, and bottom layers of the sheets. The overall refinement of the microstructure was found to be more developed in the asymmetrically rolled specimens as compared to the symmetrically rolled ones. Furthermore, the top layer (at the smaller roll) revealed a stronger deformation with higher density of deformation twins compared to middle and bottom layer. Additionally, secondary twinning was activated during ASR, whereas only primary twinning was observed during SR. With increasing rolling degree (up to 40% thickness reduction) a weak Brass-type texture dominated by α-fiber (〈110〉//ND) texture components and the {123}〈634〉 S texture component developed during rolling using both processing techniques. As a result of the additional shear strain introduced during ASR, the complete texture rotated around the transverse direction by ~ 6°. It was shown that additional simple shear introduced by ASR can affect the mechanical properties, microstructure and texture development.

Pre-compression effect on microstructure evolution of extruded pure polycrystalline magnesium during reversed tension load

The microstructure evolution of extruded pure polycrystalline magnesium under compression followed by subsequent tension in ambient air was investigated. Solid round dog-bone shaped specimens were compressed along the extrusion direction (ED) to −7.3% and −12.8% true strains, respectively, and then tensioned along the ED to failure. Electron backscatter diffraction (EBSD) was used to examine the microstructure evolution using companion specimens unloaded at multiple points along the deformation curves. Pre-compression along the ED resulted in 101¯2 tension twinning which increased in volume with increasing strain until exhaustion at approximately −10.5% true strain. Tensile reloading of the pre-compressed specimens led to a combination of low Schmid factor twinning, detwinning, and secondary 101¯2 tension twinning. Detwinning was more significant than secondary twinning in the −7.3% pre-compressed specimens, where only one secondary twin variant was observed. 101¯2 secondary twinning was more significant in the −12.8% pre-compressed specimens, where up to three secondary twin variants were observed. After complete detwinning of primary twins, the secondary twins were retained and appeared as sub-grains. The residual sub-grains resulted from secondary twinning may provide a strengthening mechanism for wrought Mg alloys.

Tension-compression asymmetry in yielding and strain hardening behavior of CP-Ti at room temperature

The tension-compression asymmetry including the yielding and strain hardening asymmetry of a commercially pure titanium (CP-Ti) at room temperature was studied using uniaxial tensile and compressive tests. The deformation twinning modes responsible for the tension-compression asymmetry were analyzed using the electron back scattered diffraction technology. The CP-Ti exhibits a strong tension-compression asymmetry in yielding and strain hardening. The prismatic 〈a〉 slip is the most easiest to activate either for tension or compression. The deformation twinning mode can be characterized by the secondary {101¯2} twin variants in primary {112¯2} twins for tension and the secondary {112¯2} twin variants in primary {101¯2} twins for compression. The secondary twin variants are the dominated twinning modes whether for tension or compression, determining the final yielding behavior of the titanium. The stress required to activate the secondary extension twins in tension is larger than that to activate the secondary contraction twins in compression, leading to the tension-compression asymmetry in yielding. The strain hardening enhancement of the CP-Ti is a result of the combination of the Hall-Petch hardening by grain refinement and texture hardening by lattice re-orientation both resulting from the deformation twinning. The stronger effect of these two hardening mechanisms on compression than on tension leads to the tension-compression asymmetry in strain hardening.

Deformation twinning in fatigue crack tip plastic zone of Ti-6Al-4V alloy with Widmanstatten microstructure

Titanium alloys with the Widmanstatten microstructure usually exhibit high resistance to crack propagation and high fracture toughness. However, this has rarely been connected to mechanical twins. In this study, deformation twinning, which is popular in the fatigue crack tip plastic zone of α/β titanium alloys with the Widmanstatten microstructure, was systematically investigated with the electron backscatter diffraction technique. With the propagation of the fatigue crack, large-scale twins were generated due to the periodic loading and the crystallographic feature of the Widmanstatten microstructure, and they ultimately consumed the major volume of the parent α colony. Twin development with various ranks of Schmid factors (SF) was characterized, while ordinary deformation twins with relatively high rank SF grew most, extraordinary twins with very low rank SF were also activated, although restricted by the ordinary twins. The occurrence of extraordinary twins (including primary and secondary twins) was mainly attributed to the non-uniform local stress distribution induced by the plastic deformation compatibility between the neighboring units. Lastly, the influence of deformation twinning on the mechanical properties concerning crack propagation in hexagonal α-Ti alloys was discussed.