Low energy dislocation structures caused by martensitic transformations

Abstract

The low energy dislocation structures in microdomains, twin domains and self-accommodating interfaces, causes by martensitic transformations, are evaluated for several B2 alloys with 18R, 9R and 3R martensite structures by the modern theory of martensitic transformations. The transformations are accomplished by two lattice-variant shears and a volume change relaxation and are one dimensional in origin when the order parameter couples linearly with the spontaneous strain. The primary shear, on a system with degenerate stress sense, periodically alternates its direction to minimize long-range stress fields, while the habit plane shear minimizes long-range stress fields by “self-accomodation” between variants. The habit planes and shape strains, computed by this theory, are the same as those for the phenomenological theory; this is discussed. The lattice-variant transformation dislocations originate from the three-dimensional network, where screw dislocation segments dissociate to form fluctuating microdomains (localized soft modes), and these thicken when the transformation dislocations multiply by a cross-slip dissociation mechanism. The computed Schmid factors for variants in self-accommodating groups show opposite signs, indicative of a minimum long-range stress field. The resulting low energy dislocation structure is an interface of primary shear dislocations of alternating signs and habit plane shear dislocations in self-accomodating variants with almost opposite shear strains. The results indicate criteria for high pseudoelasticity and shape memory behavior with a mobile low energy dislocation structure which is highly susceptible to an applied stress.