Phase separation and lattice misfit in NiAl(β₁)-Ni₂AlTi(H)-NiTi(β₂)system

The lattice misfits at interphase interfaces, NiAl(B2)-Ni₂AlTi(L2₁) and Ni₂AlTi(L2₁)-NiTi(B2), in a pseudobinary NiAl(B2)-NiTi(B2) system have been estimated by transmission electron microscopy. When the size of Ni₂AlTi precipitates is smaller than a critical value, the precipitates keep their lattice coherency with the NiAl and NiTi matrices. After longer periods of aging the Ni₂AlTi precipitates lose their coherency by introducing interface dislocations surrounding them. The coherency loss occurs catastrophically and the shape of the precipitates changes to a globular one after the loss of coherency, while keeping the cubic-cubic orientation relationship. Contrast experiments have revealed that the Burgers vectors of these misfit dislocations are of a〈100〉 type with the pure edge component lying nearby on the surface of the Ni₂AlTi phase. The magnitudes of the lattice misfits at NiAl-Ni₂AlTi and NiTi-Ni₂AlTi interfaces have been estimated from the spacings of the misfit dislocations and found to increase with decreasing aging temperature in both the systems. The present results are compared with the previously reported theoretical and experimental values. A NiAl-NiTi pseudobinary phase diagram is proposed based upon microstructure observation, lattice misfit measurements and analytical electron microscopy.