From a combination of dilatometric and micrographic evidence, the precipitation of the β′ phase at temperatures above the metastable ordering transition, Ttm, was demonstrated for NiAlFe alloys containing at least 19at%Al. Ni-AlFe alloys which had been heated above Ttm and then slow-cooled were found to contain a network of disordered γ phase which outlined the boundaries of antiphase domains in the ordered γ′ phase. The implication of these observations was that the formation of equilibrium γ phase from off-stoichiometric γ′ requires antiphase domain boundaries as nucleation sites: the Ttm determined by dilatometry represents an upper absolute instability limit at which the ordered phase was converted into disordered phase of the same composition, whereas the equilibrium disordered phase would have a different composition. The microstructure of a series of binary Ni-Al alloys were examined. At 22 to 23at%Al, the γ′ phase consisted of grains which partly contained fine antiphase domains (with γ at the antiphase domain boundaries) and partly very coarse antiphase domains; microanalysis established that this was due to microsegregation, and that the coarse antiphase domains were in regions with excess Al so that Ttm was locally above the freezing temperature. The structural distinction between directly and sequentially ordered alloys was thereby made clear: fine antiphase domains formed only in alloys which freeze in disordered form.

The Order-Disorder Transformation in Ni3Al and Ni3AlFe Alloys-II. Phase Transformations and Microstructures. Cahn, R.W., Siemers, P.A., Hall, E.L.: Acta Metallurgica, 1987, 35[11], 2753-64