On the strength of nickel-containing B2 iron aluminides

When some of the iron in the B2 iron aluminide Fe-45 at.% Al is replaced by nickel, several important changes occur. Nickel dramatically slows down the removal of quenched-in thermal vacancies during annealing. Also, nickel additions render the hardening due to thermal vacancies less effective than that in binary iron aluminides. Both of these effects may be linked to the formation of nickel-vacancy complexes. Nickel additions also cause substantial solid solution strengthening, and possible reasons for this result are discussed. Our findings are supported by vacancy concentration determinations, by yield strength measurements, and by transmission electron microscope observations.