Phase selection for undercooled Ti-46Al-7Nb hypo-peritectic alloy

The rapid solidification of undercooled Ti-46Al-7Nb alloy was investigated by in situ observation of recalescence events during electromagnetic levitation. A maximum melt undercooling up to 240 K has been achieved. Levitated drops of controlled undercooling were quenched onto chill copper substrate and subjected to phase and microstructure analysis. Beyond a critical undercooling of 205 K, solidification mode changes from peritectic transformation into suppression of peritectic transformation. When the intial undercooling (δT) is lower than a critical value (δT^*), the solidification follows a typical manner of peritectic alloys, i.e. β phase solidifies as primary phase, after which, peritectic α phase forms through peritectic growth which is controlled by peritectic reaction, peritectic transformation. If δT≤δT^*, the formation of peritectic α phase will be suppressed, and the solidification of single β phase will prevail.This phenomenon can be explained by comparing the relationship between incubation time for peritectic reaction (τ_P) and post-recalescence time corresponding to pure β phase solidification (t_β). When the undercooling is no different, microstructure can obtain the β/α″ martensitic transformation by changing the cooling rate.