Special interfaces: military transformations

Abstract

The objective of this paper is to review the mechanism of military transformations. Evidence is presented showing that these diffusionless transformations, which involve shear dominant deformation concomitant with interface motion in stiff engineering materials, occur by motion of disconnections along the interfaces. Disconnections are interfacial line-defects that exhibit both dislocation and step character. In the theoretical part of the review, topological arguments are used to identify the constraints on diffusionless motion of disconnections, and it is proposed that five criteria (one compositional, three crystallographic and one relating to the intrinsic mobility of disconnections) must be satisfied for feasible transformations. Subsequently, experimental observations in heterophase interfaces, namely, martensitic transformations, and homophase materials, i.e. deformation twinning and intervariant boundaries, are presented and interpreted in this framework. The generic nature of this mechanistic approach is further illustrated by a military transformation in a soft biological material, and a diffusive-displacive transformation in an intermetallic alloy where the compositional criterion is relaxed.