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 Ni-22.5Al binary alloy: 10C below the solidus. Dilatometry of binary Ni-Al alloys permitted the transition temperature to be plotted as a function of Ni/Al ratio. At ~23at%Al, the transition temperature crossed the liquidus temperature. Alloys containing less aluminum than this froze in the disordered form and ordered upon further cooling (sequential ordering). Alloys containing more than ~23at%Al froze directly into the ordered form and, unlike directly ordered alloys, contained few or no antiphase domains. The tentative conclusion 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. 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. R.W.Cahn, P.A.Siemers, J.E.Geiger, P.Bardhan: Acta Metallurgica, 1987, 35[11], 2737-51