Correlation of the hardness and vacancy concentration in FeAl

Microhardness measurements of FeAl containing 40-51 at.% Al were carried out at ambient temperature on specimens which had been subjected to different heat treatments. The hardness values for specimens heat-treated at 500°C and water-quenched to ambient temperature increase slowly with composition until at about 48 at.% Al. At higher Al concentrations the hardness values increase more rapidly. For specimens heat-treated at temperatures higher than 700°C and water quenched to ambient temperature, the hardness values increase with Al concentration with little change in slope through the entire composition range of 40-51 at.% Al. The compositional dependences of the hardness values are similar to those of the vacancy concentrations in FeAl. The vacancy concentrations as a function of Al composition were obtained from a thermodynamic model and experimental data at the stoichiometric composition. The close resemblance of the shapes of the hardness curve and the vacancy concentration curve suggests that the hardness and thus the yield strength may be related to the presence of vacancies in the lattice. This conclusion is further supported by the evidence that the hardness of FeAl increases with the square root of the vacancy concentration. This type of relationship agrees well with established point-defect strengthening models, based on the interaction of a moving dislocation with a point defect. © 1993.