Disconnections and other defects associated with twin interfaces

Abstract

The general topological model for interfacial defects is reviewed and expanded, and the role of these defects in the coupled shear-migration of interfaces is explored. We focus on twinning in hexagonal metals for many defect examples. The definition of shuffles within the topological model is presented. The concept of partitioning of the rotational component of elastic distortions at a grain boundary or interphase interface has recently been elucidated. This work shows that rotational coherency has an important role in twinning. The role of disconnections in type II twins is presented.

Interface structures and twinning mechanisms of twins in hexagonal metals

“…the total Burgers vector content normal to the interface, produces tilt but no long-range strain in a bi-crystal. However, for a twin domain embedded in the matrix, this cannot be accomplished by a single type of defect, but can be produced by TDs with several Burgers vectors [8], by a set of interface dislocations with different Burgers vectors [50,51], or by a combination of interface dislocations and TDs [33]. Finally, pure steps have no long-range strain field [18].…”

“…PB or BP facets that bond basal and prismatic planes (the first character, B or P, represents the twin plane and the second character represents the matrix plane) on a TB represent a pair of disclination dipoles with associated strain fields that can be large [8,19]. More subtly, domain defects at corners of boundaries often have some dislocation character and associated strains [8,[52][53][54]. In particular, this is true for coherent twins at or near the critical size for twin nucleation [32].…”

“…two planes, as shown in the coherent dichromatic pattern (CDP) in Figure 2 shows the shuffles that complete the transformation. The repeatable e-cell, also shown, contains all necessary information [8,41]. Of importance for later detailed analysis, the vector is the sum of four a-type vectors, two in the matrix and two in the twin (Figure 2(a) 4 of the a and a T type are shown in a reference dichromatic complex (Figure 2(c)) and the actual relaxed interface (Figure 2(d)).…”

“…Annealing twins form similarly during recrystallization and grain growth [6,7]. Both processes occur at relatively low driving forces and hence low growth rates,Ġ and the faulting probability is related to relative nucleation rates,Ṅ: henceṄ/Ġ is large [8]. They also typically occur where the thermally controlled relaxation of the interface structure, say by dislocation emission, is relatively easy and where final interfaces tend to be in near-equilibrium configurations mixture of TDs and interface dislocations [20][21][22][23][24].…”

“…See also the reviews in [35,36]: we refer to the models in [6,11,35,36] as the classical model. These are incorporated in the topological model (TM) as recently reviewed [8,17,27,37]. There are many observations of twinning that are consistent with this TD-shear-shuffle model [24,[38][39][40][41][42][43].…”