Diffusive fluxes associated with interfacial defeet motion and interaction

Abstract

Abstract. The diffusional flux associated with the motion of interfacial defects is described by an equation expressed in terms of the topological parameters which characterise defects, namely their Burgers vectors and step heights, the defect velocity and the concentration of each atomic species in the two adjacent crystals. This expression demonstrates that glide/climb behaviour of grain boundary defects is analogous to motion of dislocations in single crystals; climb motion results if a component of b is perpendicular to the interface plane. However, the situation is more complex in the case of interphase interface defects, but the present approach, which considers the step and dislocation portions of defects separately, enables a straightforward analysis. Several examples are illustrated to show the various possibilities, such as climb motion even when b is parallel to the interface, and glide motion when b is not. The latter case arises in martensitic transformation where the existence of an invariant-plane-strain relation at the interface leads to equal and opposite fluxes to the step and dislocation portions of transformation defects so that overall the motion is diffusionless.Interfacial processes involve the motion and interaction of defects. The present analysis facilitates the consideration of diffusive fluxes associated with defect interaction since the step and dislocation portions can be treated independently. A general expression is derived for the total flux arising, and a particular case, the interaction of transformation dislocations with crystal dislocations which have reached the interface during lattice-invariant deformation in martensite formation, is considered.

Transformation defects on θ′ precipitates in Al–Cu alloys

“…No long-range diffusion occurs within the disconnection. This provides an example of one of the conditions for glissile motion given by Pond and Sarrazit [16]. The dislocation component b z of the TD is in this case a coherency dislocation.…”

“…The permissible unit step height is determined by the symmetry of the dichromatic complex [16]. In some cases, for example Ti-Mo [13], the minimum permissible height is two atomic layers.…”

Static and in-situ high-resolution transmission electron microscopy investigations of the atomic structure and dynamics of massive transformation interfaces in a Ti-Al alloy

“…This is not meant to imply that massive transformation interfaces that are partly coherent and possess defect structures do not exist. It means that such interfaces will, on average, have significantly deeper cusps in a ␥ plot and be constrained to move due to conservation of atoms and defects at the interface, [45] so that they are much more likely to retain their faceted morphology during growth of the massive phase. It is also possible that during the massive transformation, the product shape is largely determined by the mobility of the various interfaces during growth rather than by the interfacial energy, as described previously.…”

Disconnections and other defects associated with twin interfaces

“…There is a transition of crystal structure across the interface, but not of composition. The TM demonstrates that such terraced interfaces, with coherency strains accommodated by arrays of dislocations and disconnections, can migrate without long-range diffusion [49]. An example is illustrated in Figure 1(b) [50].…”