A Study on the Phase Decomposition and Precipitation Behavior of B2 type NiAl-NiTi Alloy System

摘要

A transmission electron microscope (TEM) investigation has been performed on the microstructural variations and hardness changes in B2-ordered NiAl-NiTi alloy system. 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 L2_1-type Ni_2AlTi precipitates keep a lattice coherency with the NiAl and NiTi matrices. After longer periods of aging Ni_2AlTi 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 decrease with increasing aging temperature in both the systems. The results of the present observations are compared with the previously reported phase diagrams.