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

Abstract

Ni–43Ti–7Al (at-%) alloy was directionally solidified at different withdrawal rates (2, 20 and 100 μm s⁻¹) and a constant temperature of 1550°C by liquid metal cooling method. Results show that as the withdrawal rate decreases from 100 to 2 μm s⁻¹, the cellular arm spacing increases from 39·5 to 126 μm, 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that a NiTi+Ti₂Ni anomalous eutectic structure is formed in intercellular regions of directionally solidified samples withdrawn at 20 and 100 μm s⁻¹. However, in the sample withdrawn at 2 μm s⁻¹, Ti₂Ni phases represent strip and liquid droplet morphologies in the intercellular region.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.

Keywords:

• NiTi–Al alloy
• Microstructure
• Ti₂Ni phase
• Oxygen