Microstructural evolution in (α + βZr) region of Zr-2.5 wt% Nb annealed at different temperatures: Effect on mechanical properties

The present study investigated the microstructural evolution in alpha + beta(Zr) regime of Zr-2.5 wt% Nb (a dual phase alloy with alpha (hcp) and beta (bcc)) as a function of heat treatment temperature and holding time. The microstructures were studied in terms of composition and phase fraction evolution and the morphological changes of the constituent phases. The composition and quantification of phases revealed that, they significantly deviated from the corresponding equilibrium values for all the temperatures and holding times employed in the present study. In addition, an anomalous behavior was noticed in the time evolution of the phase fraction. The observed anomaly was rationalized by suggesting a possible sequence in attaining the equilibrium as: the crystal structure transformation of alpha to beta phase followed by Nb enrichment in the beta phase. Widmanstatten and (i) phase transformations were exhibited by beta phase at certain heat treatment conditions. The critical temperature for the widmanstatten transformation was found to be 750 degrees C, below which the high temperature beta phase retained to room temperature. Despite wide variations in the microstructures, flow curves were characteristically similar for all the heat treatment conditions. However, yield strength was found to be more sensitive to the beta phase morphology and the transformations associated with it. The presence of fine transformed products inside the beta phase was observed to promote higher extent of deformation twinning. (C) 2020 Elsevier B.V. All rights reserved.