Microstructural characteristics of directionally solidified Ni-43Ti-7Al alloys

Ni-43Ti-7Al (at-%) alloy was directionally solidified at different withdrawal rates (2, 20 and 100 mu m s(-1)) and a constant temperature of 1550 degrees C by liquid metal cooling method. Results show that as the withdrawal rate decreases from 100 to 2 mu m s(-1), the cellular arm spacing increases from 39.5 to 126 mu m, the size of Ti2Ni and the stability of the liquid/solid interface also increase, while the volume fraction of Ti2Ni decreases from 3.1 to 0.9%. Moreover, microstructural analysis reveals that a NiTi+Ti2Ni anomalous eutectic structure is formed in intercellular regions of directionally solidified samples withdrawn at 20 and 100 mu m s(-1). However, in the sample withdrawn at 2 mu m s(-1), Ti2Ni phases represent strip and liquid droplet morphologies in the intercellular region. Finally, the possible explanation to the change of microstructure is discussed.