Resistometric investigation of phase transformations in NiAl alloys

The paper represents a contribution to the martensitic transformation and to the existence regions of Ni2Al and Ni5Al3 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 Ni2Al phase originates above 350°C and exists up to 580°C. After an incubation period, the Ni5Al3 phase starts to nucleate from austenite above 640°C. The Ni5Al3 phase then dissolves above 740°C forming Ni3Al+NiAl phases in a peritectoid reaction. In Ni-richer alloys, martensite rapidly changes into the Ni5Al3 phase which does not transform martensitically. Only very rapid heating to 600°C retains the possibility of martensitic transformation in these alloys.