The order-disorder transformation in Ni 3Al and Ni 3AlFe alloys—I. Determination of the transition temperatures and their relation to ductility

Abstract

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 77.5Al 22.5 binary alloy: this was 1375°C, just 10°C 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 ~23 at.% 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 ~23 at.% Al freeze directly into the ordered form and, unlike the former category, such directly ordered alloys contain no antiphase domains, or very few. Empirical evidence concerning ductility of binary and ternary alloys containing γ′ (Ni 3Al) is reviewed, and some new experiments reported. The tentative conclusion is reached that sequential ordering favors polycrystalline ductility because of the role of antiphase domains in braking dislocations. In this way, the ductile-brittle transition between 24 and 25 at.% Al can be interpreted. The congruent (virtual ordering) temperature of stoichiometric Ni 3Al is estimated to be 1450 ± 10°C, and from this value, the antiphase domain energy on a (111) plane is calculated to be 112 mJ/m 2.