Microstructural and textural differences between rolled Zr702 and Zr-2.5Nb alloys: The role played by preexisting β phase

Abstract

Microstructural and textural characteristics of Zr702 and Zr-2.5Nb alloys before and after rolling (50% reduction in thickness) were characterized and compared by employing electron channel contrast imaging and electron backscatter diffraction techniques, with underlying reasons for the differences between them explored. Results show that the starting microstructure of the Zr702 is mainly comprised of well-equiaxed α-Zr grains while the Zr-2.5Nb presents a typical dual-phase microstructure featured by bulk α grains enclosed by thin β-Zr films. After the 50% rolling, plastic deformation readily occurs through the active operation of prismatic and pyramidal slip in both the Zr alloys. Most grains are found to be elongated along rolling direction with a few less-deformed grains also present (more evident in the rolled Zr702). The β-Zr preexisting in Zr-2.5Nb alloy is softer than α-Zr and could thus be deformed more easily, allowing fewer plastic strains to be accommodated by the α grains. The starting Zr702 has a typical bimodal basal texture that is essentially unchanged after the 50% rolling. The majority of grains in the starting Zr-2.5Nb show scattered orientations with their c-axes away from normal directions (ND). After the same rolling, however, c-axes of most α-grains in the Zr-2.5Nb alloy are aligned to be close to transverse direction or ~30° from the ND (ring-like distribution), different from that of the Zr702. It is demonstrated that the formation of such textural features is related to the preexisting thin β films in the Zr-2.5Nb alloy, in addition to the slip deformation.