Crystal defect associated selection of phase transformation orientation relationships (ORs)

Abstract

Phase transformation in solids always follows specific orientation relationships (ORs). The OR usually ensures a minimum lattice deformation for the structure change. However, in many cases different ORs are respected by the same transformation. The selection role and the link between the OR and the existing crystal defects needs further investigation. Thus, in this work, the α to β heating phase transformation induced by high density Electric Current Pulse (ECP) treatments in an annealed Cu–40%Zn alloy was investigated. Results show that the β phase obeys the K-S OR when formed along the α grain boundaries or in their vicinities, or the N-W OR when formed in the α grain interiors. In the former sites, the {111}α/<11¯0>α dislocation arrays were frequently observed, whereas in the latter, the {111}α/<112¯>α stacking faults were often found. Transformation strain analyses revealed that under the K-S OR the maximum lattice deformation required is a shear on the {111}α plane in the <11¯0>α direction, whereas under the N-W OR the maximum deformation is a shear on the {111}α plane in the <112¯>α direction. Thus the existing {111}α/<11¯0>α dislocation arrays along the α grain boundaries and in their vicinities provide pre-strain required by the transformation via the K-S path, whereas the {111}α/<112¯>α stacking faults boarded by {111}α/<112¯>α partial dislocations offer pre-strain facilitating the transformation via the N-W path. The present results provide new information on the role of crystal defects on phase transformation strain path and the selection of transformation ORs.