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The mechanisms associated with the interaction of {332} twins are investigated in metastable Ti 15Mo 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. [Display omitted] • The growth of the interacted {332} twins is preferred only on one side for each type. • Twin-twin interactions could stimulate sequential twinning with a specific variant promoted. • The general analysis method for interaction mechanisms of localized shear bands is proposed. [ABSTRACT FROM AUTHOR]

Abstract Shock loading causes complicated twinning processes in Ti and Ti alloys. In this work, two types of {10 1 ¯ 2} sequential twinning processes are for the first time reported according to electron backscatter diffraction (EBSD) characterization of titanium sheets that are subjected to shock loading. One is described as {11 2 ¯ 1} ⇒ P → {10 1 ¯ 2} ( T i I I ⇒ P → T j I ), i.e., {10 1 ¯ 2} sequential twinning is activated in the parent grain (P) along with {11 2 ¯ 1} twins. The other is described as {11 2 ¯ 2} ⇒ {11 2 ¯ 4} → {10 1 ¯ 2} ( C i I ⇒ C j I I → T k I ), i.e., {10 1 ¯ 2} secondary twinning is stimulated inside {11 2 ¯ 4} twin by co-zone {11 2 ¯ 2} twin. A statistical analysis according to EBSD characterization reveals the well-defined crystallographic relations between sequential {10 1 ¯ 2} twin variants and the primary/incoming twins, i.e., T i I I ⇒ P → T i I o r T i + 1 I and C i I ⇒ C i + 3 I I → T i I or T i + 1 I . We proposed and examined two sequential twinning mechanisms, (i) emissary twinning disconnections at steps along twin boundary for the first case and (ii) shear transformation into the primary twin for the second case, based on dislocation theory and the deformation accommodation ability of the sequential {10 1 ¯ 2} twinning to the shear deformation of the primary/incoming twin. Crystal plasticity modelling is performed to calculate the local stress field associated with the proposed twinning mechanisms. The results demonstrate that the preferred twin variant observed in experiments has the maximum resolved shear stress among the six twin variants. Our work suggests that complicated twinning processes under shock loading obey crystallographic relations according to deformation accommodation ability. The findings from this study can be implemented into meso-/macro-scale crystal plasticity models for predicting mechanical behaviors and texture evolution of polycrystalline aggregates of hexagonal materials. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

We experimentally studied sequential { 10 1 ¯ 2 } twinning in rolled pure titanium at room temperature and identified a new sequential twinning mechanism, i.e., { 10 1 ¯ 2 } ( T i I ) extension twin stimulated by the twin-twin junction of two primary {11 2 ¯ 1 } extension twins T i − 1 II and T i II , which holds a certain relation T i − 1 II T i II → T i I . Schmid factor analysis, as the local stress is taken into account, is able to determine the position of sequential twin variant, but cannot determine the twin variant. Displacement gradient accommodation is used to determine the twin variant. The well-known secondary {10 1 ¯ 2} ( T j I ) extension twins in primary {11 2 ¯ 2} ( C i I ) contraction twins (referred to as C i I → T j I double twinning) are also analyzed according to a generalized Schmid factor analysis. Displacement gradient accommodation and twin nucleation based on dislocations dissociation only work well for the most active twin variants, but cannot address other phenomena associated with C i I → T j I double twinning. For rarely activated twin variants, displacement gradient accommodation was not satisfied. [ABSTRACT FROM AUTHOR]

By compression along the normal direction of rolled pure titanium sheet, primary { 11 2 ¯ 2 } type compression twins were observed, followed by secondary { 10 1 ¯ 2 } extension twins. The latter can be classified into three groups according to their misorientation with respect to the parent matrix grains: 41.34° around < 5 143 ¯ > (Group I), 48.44° around < 5 ¯ 50 3 ¯ > (Group II), and 87.85° around < 7 43 ¯ 0 > (Group III). The experimental observations revealed the following activity frequency of these groups: Group II is the most frequent followed by Group I, and only a few secondary twins can be seen for Group III. When the classical Schmid factor (SF) analysis is applied, the smallest activity is predicted for Group III, in agreement with the experimental observations. However, the SF based criterion fails to distinguish between Group I and Group II variants. Similarly, the twin-shear accommodation based variant selection model proposed earlier for titanium [Qin and Jonas, Acta Mater. 75 (2014) 198–211] is not effective because the easiest accommodation by prismatic slips favors Group III variants which are nearly absent in the experiment. A possible explanation can be based on the smallest inclination angle of the secondary twinning habit plane with respect to the primary one, proposed by Barnett et al. [Acta Mater. 56 (2008) 5–15] which clearly favors Group II. It cannot make distinction between variants of the same group but can predict the right variants if complemented with the SF criterion. In the present work a new approach is proposed in order to disclose the preference for Group II variant over Group I. It is based on the special twinning geometry that applies to Group II; the intersection line of the primary and secondary twin planes lies in an active prismatic plane in the primary twin. Consequently, dislocation reactions are possible for the selection of secondary twin variant in Group II. Namely, prismatic < a > type dislocations in the primary twin can produce partial dislocations that activate the corresponding secondary twin variant. Nevertheless, a further selection rule has to be applied to choose one out of the two possible Group II variants; this is based on the higher Schmid factor of the two secondary variants. By contrast, pyramidal dislocations would be required for the formation of Group I twins, which is less likely due to their relatively high critical resolved shear stress. [ABSTRACT FROM AUTHOR]

The intersection of two 10 1 ̅ 2 twins has been extensively reported in hexagonal metals and appears at grain boundaries with low misorientation angles. In the current work, compression was applied along the normal direction of rolled pure titanium. Primary 11 2 ̅ 2 contraction twins accommodated by adjacent 10 1 ̅ 2 extension twins were found at high angle grain boundaries with misorientation angles close to 90°. The mechanism accounting for the variant selection of the adjoined 10 1 ̅ 2 twins is examined in terms of Schmid factor and accommodation capacity of potential twin variants. [ABSTRACT FROM AUTHOR]

Deformation twins are likely nucleated at and impeded by grain boundaries (GBs), developing shear transformation bands. Knowing the accommodated deformation in adjacent grains to the twinning shear is essential for understanding mechanical properties and the evolution of microstructure and texture of polycrystalline aggregates. Based on EBSD analysis, we characterize and statistically group accommodated deformation modes in the neighbor to {11 2 ¯ 2} compression twins (CTWs) in titanium with respect to the misorientation axis/angle. Two types of CTWs are preferably activated in adjacent grains with low misorientation angles, two types of extension twins (ETWs) are favorably activated in adjacent grains with high misorientation angles, and some of {11 2 ¯ 2} CTWs terminate at GBs with high misorientation angles. We discuss the preference of accommodated deformation modes and the selection of twin variants based on the SDGA criterion which couples the displacement gradient accommodation (DGA) and stress driven accommodation (SDA) criteria. Although misorientation angle of GBs is widely used to predict the preferred deformation modes, we demonstrate that misorientation axis that was often ignored, must be considered in determining the preferred deformation modes especially for large misorientation angles. Finally, we predict the misorientation angle/axis pairs with respect to the activity of four twinning modes and two slip systems as the best accommodation mechanism in adjacent grains. The results are useful for the implementation of intergranular deformation mechanisms in multiscale modeling and the design of HCP materials with specific GB characteristics. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: In the current paper, pre-compression is performed along the rolling direction (RD). Subsequently, the samples are deformed in plane-strain compression along the transverse direction (TD) and not allowed to expand in the normal direction (ND). The effect of plane-strain compression on the texture and anisotropy modification of a pre-twinned hot-rolled Mg–3Al–1Zn alloy is investigated by EBSD observation. The results reveal that the generation of double twins in the previous twins dominates the plane-strain pre-compression along TD while the twinning is difficult to occur in the matrix due to the constraint along ND. Additionally, the double twins generated in the previously formed twins effectively modify the compression behavior and enhance the compressive yield strength of the pre-compressed samples along ND. The anisotropy along RD, TD and ND is decreased. [Copyright &y& Elsevier]

Abstract: The current paper investigates the effect of extension twins identified by using electron backscattered diffraction on the interrupted properties of an extruded Mg–3Al–1Zn (AZ31) alloy. Compressive and recompressive tests are conducted along extrusion direction (ED). It is discovered that the yield strength of recompression is enhanced due to grain refinement by extension twins. The reinforcement effect of extension twins on the yield stress of recompression is mainly reflected at the stage of small deformation while the improvement of yield strength is mainly attributed to the pile-up and intersection of dislocations as large deformation occurs. Furthermore, the yield stress of recompression is identical to the interrupted stress of precompression. In situ observation reveals that some twins generated in precompression could also become thicker in the following recompression. [Copyright &y& Elsevier]

Abstract: The effects of compressive pre-deformation along extrusion direction (ED) on yield asymmetry between tension and compression in hot-extruded Mg–3Al–1Zn (AZ31) alloy was investigated in this paper. {10–12}-type twinning occurred when samples were subjected to compressive pre-deformation. The compressive yield strength of sample with compressive pre-deformation increased gradually with the increase in compressive prestrain. The tensile yield strength of the sample decreased rapidly with the increase in compressive prestrain from zero to ~1.7%, but it was nearly unaltered when compressive prestrain was higher than ~1.7%. Detwinning occurred in the twinned regions played roles in the subsequent tensile process so that the yield stress was lower than sample without prestrain. Yield asymmetry between tension and compression decreased gradually when compressive prestrain was lower than ~1.1%, but it began to increase after that. It suggests that yield asymmetry can be effectively controlled by appropriate pre-deformation. [Copyright &y& Elsevier]