Composition Dependence of the Ms Temperature in the β′NiAl Compound

Martensitic transformation start (Ms) temperatures have been measured in binary NiAl sintered alloys (36-38 at%Al) using an electrical resistivity method. The composition dependence of the Ms temperature has been investigated by a statistical approach employing the Bragg-Williams approximation. The results obtained are as follows: (1) In these alloys, the linear relationship between Ms tempearture and aluminum content, having a slope of about-160 K/at%Al, was observed. The Ms tempertaure is about 30 K below the corresponding Af temperature. (2) There are no differences in the composition dependence of the Ms temperature between the sintered and cast alloys. (3) The Composition dependence of the Ms temperature is found to have a good correlation with that of To. (4) The interaction parameter VNiAl′ between Ni and Al atoms in the martensite phase could be estimated at about −12.7 kJ/mol. (5) The composition dependence of the Ms temperature is closely related to the interaction parameters in the parent and martensite phases and the temperature dependence of the difference in the Gibbs free energy of constituent elements between the Al (fcc) and the A2 (bcc) crystal structure.