Phase decomposition and hardening in NiAl-NiTi pseudo-binary alloy system

Abstract

Microstructural variations and correlated hardness changes in a pseudobinary NiAl(B2)-Ni2AlTi(L21)-NiTi (B2) system have been investigated by means of transmission electron microscopy (TEM) and hardness tests. The hardness after solution treatment is high in NiAl- and NiTi-base alloys suggesting the large contribution of solid solution strengthening in this alloy system. However, the amount of age hardening is not large as compared to the large microstructural variations during aging. TEM observations have revealed that spinodal and binodal types of decomposition occur depending on the chemical compositions and aging temperatures. At the beginning of aging, the L21-type Ni2AlTi precipitates keep a lattice coherency with the NiAl and NiTi matrices. By longer periods of aging Ni2AlTi precipitates lose their coherency and change their morphology to the globular ones surrounded by misfit dislocations. Misfit dislocations, which are observed on {100} planes of H-precipitates have the Burgers vector of a〈100〉 with a pure edge type. The lattice misfits of NiAl-Ni2AlTi and NiTi-Ni2AlTi systems are estimated from the spacings of misfit dislocations to be 1.1% and 1.3% respectively at 1273 K. The lattice misfits decrease with increasing aging temperature in both the systems. The results of the present observations are compared with the previously reported phase diagrams.