A combination of dilatometry and high-temperature diffractometry was used to determine the order-disorder transition temperatures for a series of nickel-aluminum-iron alloys. Extrapolation to zero iron content gave an estimate of the transition temperature for a Ni77.5Al22.5 binary alloy: this was 1375C, just 10C below the solidus. Further dilatometric runs with several binary Ni-Al alloys, exploiting a new technique of dilatometric curve-fitting, permitted the transition temperature to be plotted as a function of Ni Al ratio. At ~23at%Al, the transition temperature crossed over the liquidus temperature; alloys containing less aluminum than this freeze in the disordered form and order on further cooling (sequential ordering); alloys containing more than ~23at%Al froze directly into the ordered form and, unlike the former category, such directly ordered alloys contained no antiphase domains, or very few. Empirical evidence concerning ductility of binary and ternary alloys containing γ′ (Ni3Al) was reviewed, and some new experiments reported. The tentative conclusion reached was that sequential ordering favored polycrystalline ductility because of the role of antiphase domains in braking dislocations. In this way, the ductile-brittle transition between 24 and 25at%Al could be interpreted. The congruent (virtual ordering) temperature of stoichiometric Ni3Al was estimated to be 1450C, and from this value, the antiphase domain energy on a (111) plane was calculated to be 112mJ/m2.

The Order-Disorder Transformation in Ni3Al and Ni3AlFe Alloys-I. Determination of the Transition Temperatures and their Relation to Ductility. Cahn, R.W., Siemers, P.A., Geiger, J.E., Bardhan, P.: Acta Metallurgica, 1987, 35[11], 2737-51