Resistometric investigation of phase transformations in NiAl alloys

Abstract

The paper represents a contribution to the martensitic transformation and to the existence regions of Ni 2Al and Ni 5Al 3 phases in the Ni–Al phase diagram. Resistivity, optical microscopy and differential thermal analysis (DTA) investigations were performed on 63.4, 64.8 and 66.2 at.% Ni alloys in the temperature region between −190 and 850°C. The martensite–austenite transformation (reversible for the alloy with the lowest Ni content) takes place between 60 and 300°C according to the nickel content of the alloy. In the 63.4 at.% Ni alloy, the Ni 2Al phase originates above 350°C and exists up to 580°C. After an incubation period, the Ni 5Al 3 phase starts to nucleate from austenite above 640°C. The Ni 5Al 3 phase then dissolves above 740°C forming Ni 3Al+NiAl phases in a peritectoid reaction. In Ni-richer alloys, martensite rapidly changes into the Ni 5Al 3 phase which does not transform martensitically. Only very rapid heating to 600°C retains the possibility of martensitic transformation in these alloys.