Stoichiometry Splitting in the Ni-Fe-Al Ternary β phase

Abstract

It is shown that the stoichiometry of the ternary β phase in the Ni-Fe-Al system splits into two through the measurement on hardness distribution and through careful examination on the available lattice parameter data. The first stoichiometry lies on the compositions connecting NiAl and FeAl, whereas the second on those connecting NiAl and the Fe corner. The former is attributed to the situation where all the Al atoms preferentially occupy one of the two sublattices so that the compositions are expressed strictly as (Ni, Fe)Al. The latter case, on the other hand, is the situation where Fe atoms are evenly distributed among the two sublattice sites. The lattice parameter of the β phase exhibits a negative deviation from the Vegard’s law in the vicinity of the first stoichiometry but it shifts into a positive deviation as the aluminum concentration decreases. The reason for the negative deviation would be due to the increasing stability of the β phase, while that for the positive deviation to the effect of miscibility gap exsisting in the Ni-Al-Fe ternary bcc phase. There found three types of concentration dependence of solid solution hardening or defect hardening in the ternary β phase, for which a rigorous explanation cannot be made by any of the conventional theoretical treatments.