Microstructure evolution of directionally solidified Ni–43Ti–7Al alloy during heat treatment

The microstructure change of directionally solidified Ni–43Ti–7Al alloy after heat treatment has been investigated by transmission electron microscopy and back scattered electron imagings in this paper. After solution and aging treatment, the NiTi + Ti2Ni anomalous eutectic structure nearly cannot be observed and Ti2Ni phases become spheroidized. The β′-Ni2TiAl precipitates are nearly spherical at the early stage of aging at 800 °C (0.1–1 h); however, they become aligned along 〈100〉 directions and change to cubic shape after aging for 20 h. In the course of further aging, the coarsened semi-coherent β′ precipitates occur preferentially in intercellular regions. Then the coarsened β′ precipitates begin to occur in intracellular regions with the increasing aging time and aging temperature. These β′ precipitates change the shape to sphere and plate, accompanied with loss of their coherency by introducing interface dislocations surrounding them. Finally, the formation mechanism of β′ precipitates are discussed compared to other studies in the Ni–Ti–Al system.