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

Abstract The present study investigated the microstructural evolution in α + βZr regime of Zr-2.5 wt% Nb (a dual phase alloy with α (hcp) and β (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 α to β phase followed by Nb enrichment in the β phase. Widmanstatten and ω phase transformations were exhibited by β phase at certain heat treatment conditions. The critical temperature for the widmanstatten transformation was found to be 750 °C, below which the high temperature β 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 β phase morphology and the transformations associated with it. The presence of fine transformed products inside the β phase was observed to promote higher extent of deformation twinning.