Effect of cooling process on the formation of duplex microstructure in Zr-2.3Nb alloy

The cooling process is important for the formation of duplex microstructure in Zr-Nb alloy. To further understand its effect on the microstructure, three cooling methods, furnace cooling, air cooling and water cooling, were carried out on the annealed specimens. The results show that the equiaxed microstructure can be obtained in furnace-cooled specimens with equiaxed α grains and residual β phase, and the duplex microstructure can be obtained via the air cooling and water cooling processes, in spite of the different transformation mechanisms and phase constitutions. The duplex microstructure for the air cooling process was formed via diffusion-type transformation and consisted of primary α phase, secondary α phase, ω phase and the residual β phase, while for the water cooling process it was formed via displacive-type transformation and consisted of primary α phase, α′ phase and ω phase. There were many dislocations within the α′ phase for the microstrain during the phase transformation. The strength increased and elongation decreased with increasing cooling rate. However, the air- and water-cooled specimens possessed a high value of product of strength and elongation, revealing a high security for the two duplex microstructures, in spite of their differences.