Effect of cooling rate on microstructure evolution of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy during multi-step heat treatment

Microstructure evolution of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy (at.%) resulted from different cooling rates during the whole phase transformation process has been thoroughly investigated. The results show that the cooling rate during beta -> alpha transformation plays a crucial role in the modification of the crystallographic orientation relationship between the alpha grains and their parent beta phase. The decrease of cooling rate can reduce the volume fraction of Burgers alpha grains and avoid the preferential distribution of lamellar interface traces. This could be attributed to the different nucleation and growth kinetics of alpha phase affected by the cooling rate. Moreover, the lamellar structure characteristics are closely related to the cooling rate during alpha -> alpha(2) + gamma transformation and their responses to stabilization treatment shall vary accordingly. A fast cooling rate retains a high quantity of high temperature alpha/alpha(2) phase. These alpha/alpha(2) phase could decompose into ultrafine lamellar structure during subsequent stabilization treatment. While the average lamellar spacing of the slow cooled microstructure has a small increase by limited continuous growth of the pre-existing gamma lamellae during stabilization treatment.