Strengthening at intermediate temperatures in iron aluminides

The strength peak found at intermediate temperatures for Fe3Al and FeAl alloys is of interest both from the fundamental point of view and also because such strengthening increases the temperature range where good strength can be obtained. Several theories have been proposed to explain the strengthening involved, but no clear agreement has been reached as to the true mechanisms involved. After reviewing likely strengthening mechanisms, some recent results on mechanical behaviour and dislocation microstructures will be presented. In particular the temperature and strain-rate dependencies of mechanical behaviour in a Fe3Al based alloy will be considered, as well as the influence of strain-rate jumps. Dislocations found in deformed samples, both in the as-deformed condition as well as after subsequent annealing treatments, will be discussed. Observations on deformed FeAl samples are shown to suggest somewhat different conclusions than for Fe3Al as to the important strengthening mechanisms. Processes such as APB relaxation, local climb, and superdislocation reactions and decompositions may all play roles in determining strength.