{10-12} Twinning Supremacy

El Kadiri, H., & Barrett, C. D. (2016). {10-12} Twinning Supremacy. International Materials Research Congress. IMRC - Mexico.

The invasive growth capability of {10 12} twinning in hexagonal close packed (HCP) metals is by far the most popular attribute of this twinning mode, but one which has profound consequences on the mechanical properties. However, no formal mechanism yet exists to explain this superior ease of propagation as compared to other twinning modes. For instance, in traditional wrought magnesium polycrystals, an adequate loading can induce {10 12} twins to consume the entire parent matrix in a fashion no other twin mode could achieve regardless of the c/a ratio of the HCP material in question [1–6]. There is however a difficulty in understanding this important issue from the perspective of classical literature. One may in fact confuse between two entirely different topics; that related to {10 12} universality and the one related to its fast and profuse growth, being the focus of this talk. The first issue is rather connected to its particular ease of nucleation regardless of the c/a ratio, in contrast to all other twin modes which can be made to vanish by choosing an existing c/a ratio material. This phenomenon could probably find reasonable grounds in pure crystallography. Twinning on {10 12} plans seems to offer an optimum combination of relatively low characteristic shear and very simple shuffles. This explanation is however somehow vague as no relationship could yet be identified between the mobility of twin boundary (TB) and the magnitudes of shear and shuffle operating at the disconnection core [7]. Atomistic simulations by Serra et al. [8], Serra. et al. [9] cast further doubts on this rationalization as the core width of {11 21} twinning disconnection (TD) was computed to be much wider than that of {10 12} twinning, but still causes very slow propagation rates of the former in Ti and Zr. With regard to either shear or shuffle alone, they could absolutely provide no consistent explanation, and ample counterexamples could be listed. For instance, the generally very low shear of {10 11} and {11 22} twinning and the minimum required shuffle in {11 21} twinning did not preclude their sluggish edgewise thickening. One must understand that the second problem essentially lies in the peculiar invulnerability of {10 12} twinning mode to concomitant slip and/or pre-existing dislocations. On one hand, slip has the potential to cause roughness of the twin boundary (TB), and thus provoke a severe loss of local coherency, both of which would impede twinning disconnection glide and thereby further twin propagation. Recently, Wang et al. [10] showed {11 21} twins quickly developing roughness on one side of the boundary, which obstructed their growth. On the other hand, inhomogeneous deformation that develops within both the twin and the parent should in principal induce the K1 plane to rotate away from either sides of the twin composition plane, implying a tendency of the boundary to lose perfect coherency. Another plausible argument put forward by Asgari et al. [11] is the development of lattice curvature due to the inhomogeneity of strain which has the effect of reducing the length scale over which deformation twins are formed Asgari et al. [11]. Thus, since smaller grains require higher twin nucleation stress, in-grains misorientation may significantly impede further twin nucleation and propagation. This talk identifies an intricate mechanism governing the well known supremacy of {10 12}twinning, which is still one of the most difficult conundrums for the experts dealing with the fundamentals of deformation in HCP metals. We show that deformation facets at TBs form as a rotational relaxation process of lattice slip dislocations merging onto the TB. The change in misorientation across the facet junction is mediated by an interfacial disclination which is able to move in a couple of ways. Any disconnection entering the facet from either side of the disclination always transforms to glissile disconnections across the disclination. This property bestows the twin with enhanced mobility. The twin can in fact punch through slip dislocations and destroy them by conversion to disclination dipoles able to migrate with TB. We introduce trichromatic patterns and complexes as an efficient way to describe the topology of interfacial disclination nucleation, motion, and its effect on transmuting cross-gliding disconnections. The trichromatics are a particular case of the Belov groups of three-colored symmetry point group extensions