The ordering transition in Ni 3Al alloys

Abstract

The ordering of nickel rich non-stoichiometrico Ni 3Al alloys has been examined by transmission electron microscopy. Utilizing superlattice dark-field microscopy and diffraction, it has been possible to observe an earlier stage of ordering than heretofore reported, as a result of very rapid helium quenching. Alloys of over about 17 at.% Al undergo ordering almost completely throughout the lattice if allowed to reside perhaps a few hundred ths of a sec in the region of most rapid reaction, which is roughly 1000°–1100°C. Subsequent phase separation and coarsening occur much more slowly. Alloys of less than about 16 at.% Al exhibit SRO domains on quenching which are unstable and disappear in the presence of quenched in vacancies at 270°C. These two compositions thus appear to be separated by a spinodal curve. The results substantiate the concept that SRO microdomain size increases with ordering energy, V( S). It would appear that ordering by a second order transition ( ∂ 2G ∂T 2 discontinuous through T c ) is possible in A 3 B-f.c.c. alloys, and that such a transition can be distinguished by its mechanism. The mechanism of such an ordering transition must involve diffusion, not down a chemical gradient, but rather that resulting from individual biased vacancy jumps. The vacancy-atom interchanges of greatest probability will be those which result in the formation of the greatest number of “right” bonds.