Strengthening of iron aluminides by vacancies and/or nickel

At room temperature, iron aluminides may be strengthened by quenched-in thermal vacancies as well as by solid solution alloying additions. Since the separation of these two effects requires precise vacancy concentration measurements, a novel extrapolation procedure was developed to determine vacancy volume fractions with an accuracy approaching 1 part in 10 000. The rate of removal of quenched-in vacancies during annealing, as well as the hardening by vacancies, were found to be influenced by solid solution additions of Ni to Fe-45 at.% Al. First, yield strength and density measurements showed that nickel slows down the removal of vacancies, making it more difficult to obtain iron aluminides with low vacancy concentrations. Second, in the absence of vacancies, additions of nickel result in substantial strengthening. Third, for high vacancy concentrations (e.g. 0.2 vol.%), nickel additions can cause softening. These results are rationalized in terms of the interaction between the vacancies and the nickel atoms.