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 degrees C. The martensite-austenite transformation (reversible for the alloy with the lowest Ni content) takes place between 60 and 300 degrees C according to the nickel content of the alloy. In the 63.4 at.% Ni alloy, the Ni2Al phase originates above 350 degrees C and exists up to 580 degrees C. After an incubation period, the Ni5Al3 phase starts to nucleate from austenite above 640 degrees C. The Ni5Al3 phase then dissolves above 740 degrees 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 degrees C retains the possibility of martensitic transformation in these alloys. (C) 1998 Elsevier Science S.A. All rights reserved.