Vacancy formation and effects in FeAl

Iron aluminide, FeAl, has been widely studied because of its excellent high temperature oxidation and corrosion properties. At high temperatures, FeAl generates a large number of thermal vacancies, and the vacancy concentration increases with increasing aluminum content. The mechanical properties depend on the vacancy concentration, so a thorough understanding of the generation and annihilation of vacancies is necessary. Heat treatment, temperature, and time control the vacancy concentration. Increasing vacancy concentration will increase the hardness and decrease the ductility of FeAl. The yield strength anomaly of FeAl, increasing yield strength with increasing temperature is believed to be due to vacancy hardening, and the decrease in strength above the peak temperature is attributed to the creep of FeAl. Ternary alloying elements, except boron, have little effect on the hardening at high temperature and can increase the hardness after long time anneals at low temperature. This review and analysis presents a summary of the current literature available on FeAl vacancies and their affects, including hardness, the yield strength anomaly, and the effect of ternary alloying elements.