Effect of casting temperature on microstructure in a directionally solidified Ni-44Ti-5Al-2Nb-1Mo alloy

Abstract

Ni-44Ti-5Al-2Nb-1Mo (atomic fraction, %) alloy was directional solidified (DS) at different temperature (1350, 1450, 1550, 1650 °C) and a constant withdrawing rate of 100 μm/s, by liquid metal cooling method. The microstructural characteristics indicate that the DS temperature do not influence the phase composition, but rather to change the phase morphology. The phase composition of directional solidified samples is the same as the as-cast sample, which is composed of NiTi (B2), Ti2Ni and Nb-rich phases. After DS, [100] becomes the preferred orientation in NiTi matrix. With the casting temperature increasing, [100] orientation is significantly enhanced, the solid/liquid (S/L) interface morphology changes from cellular dendrite to cellular, and the interface shape changes from convex to plane. Meantime, the volume fraction of the Nb-rich phase decreases with the temperature increasing.