The Microstructure and Martensitic Transformation in Ni-Al Alloys with Yttrium Addition

Abstract
The microstructure and martensitic transformation in the Ni-36.3Al-0.4Y (in at.%) alloy were investigated after homogenisation and plastic deformation by hot upsetting at 1100°C. The alloy exhibited large, recrystallised grains of the Ni-Al β phase with spherical precipitates inside the grains and a thin layer of other phases located at the grain boundaries. The SEM and EDX analysis showed that both types of precipitates were composed of two intermetallic phases (Ni_xAl_1-x)Y₅ with different Ni to Al ratio. No yttrium content has been detected in the matrix β phase, probably because of the low solubility limit which is 0.1 at.%. The martensitic transformation was studied by TEM and DSC calorimetry. In large grains of the recrystallised material the M₈ temperature was measured to be -4°C and the transformation proceeded in the range of 60 deg. The M₈ temperature was lower than expected from the ratio of Ni to Al. Aging 1 hour at 240°C did not influence the range of the transformation. The TEM study of the β phase revealed strong and regular contrast modulations, probably being premartensitic effects. Also a regular spaced network of dislocations lying in the same directions as the contrast modulations, both in matrix and martensite, was noticed. In foils 7R structure of martensite, internally twinned, was predominantly found. The alloy was very brittle.