THE MICROSTRUCTURE AND MARTENSITIC-TRANSFORMATION IN NI-AL ALLOYS WITH YTTRIUM ADDITION

The microstructure and martensitic transformation in tile Ni-36.3Al-0. 4Y (in at.%) alloy were investigated after homogenisation and plastic deformation by hot upsetting at 1100 degrees C. The alloy exhibited la rge, recrystallised grains of the Ni-Al beta phase with spherical prec ipitates inside tile grains and a thin layer of other phases located a t the grain boundaries. The SEM and EDX analysis showed that both type s of precipitates were composed of two intermetallic phases (NixAl1-x) Y-5 with different Ni to Al ratio. No yttrium content has been detecte d in the matrix beta phase, probably because of the low solubility lim it which is 0.1 at.%. The martensitic transformation was studied by TE M and DSC calorimetry. In large grains of the recrystallised material the M-s temperature was measured to be -4 degrees C and the transforma tion proceeded in the range of 60 deg. The M-s temperature was lower t han expected from the ratio of Ni to Al. Aging 1 hour at 240 degrees C did not influence the range of the transformation. The TEM study of t he beta phase revealed strong and regular contrast modulations, probab ly being premartensitic effects. Also a regular spaced network of disl ocations lying in the same directions as tile contrast modulations, bo th in matrix and martensite, was noticed. In foils 7R structure of mar tensite, internally twinned, was predominantly found. The alloy was ve ry brittle.