Abstract
From a combination of dilatometric and micrographie evidence, the precipitation of the β′ phase at temperatures above the metastable ordering transition, T tm , is demonstrated for those NiAlFe alloys containing at least 19 at.% Al. Ni-AlFe alloys which have been heated above T tm and then slow-cooled are found to contain a network of disordered γ phase which outlines the boundaries of antiphase domains in the ordered γ′ phase. The implication of these observations is that the formation of equilibrium γ phase from off-stoichiometric γ′ requires antiphase domain boundaries as nucleation sites: the T tm determined by dilatometry represents an upper absolute instability limit at which the ordered phase is 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 are examined: at 22–23 at.%Al, the γ′ phase consists of grains which partly contain fine antiphase domains (with γ at the APD boundaries) and partly very coarse APD's; microanalysis established that this is due to microségregation, and that the coarse APD's are in regions with excess Al so that T tm is locally above the freezing temperature. The structural distinction between directly and sequentially ordered alloys is thereby made clear: fine APD's form only in alloys which freeze in disordered form.
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