Microstructural characteristics of cold-rolled Zr-2.5Nb alloy annealed near the monotectoid temperature

Abstract

A dual-phase Zr-2.5Nb alloy was rolled at room temperature to 50% reduction and then annealed at two temperatures (560 and 580°C) near the monotectoid temperature. X-ray diffraction, electron channeling contrast imaging and electron backscatter diffraction techniques were jointly used to characterize microstructural characteristics developed in the as-rolled and annealed specimens. Results show that plastic deformation occurs in both bulk α -Zr grains and thin β -Zr films during rolling, allowing large lattice strains to be accumulated in β -Zr and active dislocation slip (especially the prismatic a a n slip) to be initiated in α -Zr. During subsequent annealing at 580°C, the prior β -Zr films are transformed into submicron β -Zr particles, which lose coherency (the Burgers orientation relationship) with surrounding α grains. In the specimen annealed at 560°C, however, the prior β -Zr films are found to be decomposed into nanoscale β -Nb particles. In both the annealed specimens, the β -Zr and the β -Nb particles appeared to be linearly distributed along the rolling direction. Two types of α structures, i.e., small equiaxed crystallites formed by recovery of dislocation structures and coarse bamboo-like recrystallized grains, are revealed in the annealed specimens. Effective boundary pinning due to the dense β -phase particles is demonstrated to play a key role in forming such unusual bamboo-like grains.