Explaining absence of texture development in cold rolled two-phase Zr-2.5 wt% Nb alloy

摘要

In the present study, two distinct starting microstructures of Zr-2.5 wt% Nb have been used: (1) single-phase alpha hcp martensitic structure; and (2) two-phase, 10% bcc beta and rest hcp alpha, Widmanstatten structure. In the second case, two types of alpha were present-near grain boundary predominantly single-phase alpha (about 5% of the total alpha) and cc plates in an apparently continuous beta matrix. Both (1) and (2) had similar starting crystallographic texture of the hcp alpha phase and were deformed by unidirectional and cross rolling. In the two-phase structure the changes in the bulk texture on cold rolling was found to be insignificant, while in the single-phase material noticeable textural changes were observed. Taylor type deformation texture models predicted textural changes in single-phase structure but failed to predict the observed lack of textural development in the two-phase material. Microtexture observations showed that alpha plates remained approximately single crystalline after cold rolling, while the beta matrix underwent significant orientational changes. Relative hardening, estimated by X-ray peak broadening, was observed mainly in beta phase; while aspect ratio of alpha plates remained unchanged with cold rolling-indicating absence of effective macroscopic strain in the hcp alpha plates. Based on microstructural and microtextural observations, a simple model is proposed in which the plastic flow is mainly confined to the beta matrix within which the alpha plates are subjected to 'in-plane rigid body rotation'. The model explains the observed lack of textural developments in the two-phase structure. (C) 2002 Acta Materialia Inc. . .