Vacancies, vacancy aggregates and hardening in FeAl

It is well established that point defects can significantly affect the mech anical properties of intermetallic compounds. In particular, FeAl and NiAl with the B2 ordered structure have been confirmed to retain a high concentr ation of thermal vacancies during cooling from elevated temperatures leadin g to significant increases in hardness. The excess vacancies can be elimina ted during low-temperature anneals by various processes that appear to depe nd on alloy composition and ternary element additions. The kinetics of hard ening and softening of different FeAl alloys during high- and low-temperatu re anneals have been studied in detail. Low activation energies for hardeni ng deduced from high-temperature anneals have been related to the increase in vacancy concentration. TEM observations of quenched specimens and of spe cimens at different stages during low-temperature annealing have helped elu cidate the mechanisms responsible for vacancy elimination. There is often a n incubation period during annealing before softening, during which inhomog eneous distributions of various [111] and [100] dislocations can be observe d. The temperature dependence of softening observed depends therefore on bo th vacancy mobility and sink arrangements. (C) 1998 Elsevier Science S.A. A ll rights reserved.