Vacancies, vacancy aggregates and hardening in FeAl

It is well established that point defects can significantly affect the mechanical properties of intermetallic compounds. In particular, FeAl and NiAl with the B2 ordered structure have been confirmed to retain a high concentration of thermal vacancies during cooling from elevated temperatures leading to significant increases in hardness. The excess vacancies can be eliminated during low-temperature anneals by various processes that appear to depend on alloy composition and ternary element additions. The kinetics of hardening and softening of different FeAl alloys during high- and low-temperature anneals have been studied in detail. Low activation energies for hardening deduced from high-temperature anneals have been related to the increase in vacancy concentration. TEM observations of quenched specimens and of specimens at different stages during low-temperature annealing have helped elucidate the mechanisms responsible for vacancy elimination. There is often an incubation period during annealing before softening, during which inhomogeneous distributions of various 〈111〉 and 〈100〉 dislocations can be observed. The temperature dependence of softening observed depends therefore on both vacancy mobility and sink arrangements.