On the common topological conditions for shear-coupled twin boundary migration in bcc and hcp metals

The most abundant deformation twins in metals with hcp and bcc crystallographic structures, i.e. (10-12) and (112) respectively, proceed with the same mechanism of deformation under an applied shear stress, i.e. shear-coupled twin boundary migration. Despite the differences in the atomic structure and degrees of symmetry of bcc and hcp crystals, the topological aspects that rule the processes of growth and shrinkage of these deformation twins are equivalent. In this paper, we revisit the atomic level processes, occurring at the interfaces, through which both coherent twin boundaries displace in a conservative manner and accommodate deformation. These processes include the creation of a twin boundary dislocation that acts as a source of disconnections, namely, line defects at the interface that have both dislocation and step character. The glide of disconnections along the twin boundary is responsible for the displacement of the interface, implying the growth or shrink of the twin. We describe the interaction of these twins with crystal dislocations and show that the product of the interaction is a source of disconnections. We show that these interactions can be described in terms of the structure of the interfaces and the Burgers vector of dislocations. The study is based on the topological theory of interfacial defects and we show the usefulness of the dichromatic pattern associated to each interface in the prediction and analysis of the interactions.