CHROMIUM PRECIPITATION FROM BETA-NI(AL, TI) AND GAMMA'-NI3(AL, TI) IN THE ALLOY (NI70AL20TI10)0.9CR0.1

The crystallography and nucleation of the Cr phase (alpha) from both beta-Ni(Al, Ti) and gamma'-Ni3(Al, Ti) in an alpha-beta-gamma' three-phase alloy are investigated using analytical electron microscopy. The alpha-particles in beta are spherically shaped and oriented parallel with the parent phase. Those in gamma' are lath-shaped with a well-defined habit plane of (121BAR)gamma'/(312BAR)alpha, a growth direction close to [111BAR]alpha/[101BAR]gamma', and are related to gamma' by the Kurdjumov-Sachs (K-S) orientation relationship. It is shown that alpha particles can nucleate only from the disordered-gamma phase which was stabilized at higher temperatures. The heterogeneous nucleation and distribution of alpha-particles in gamma' are interpreted in terms of a transformation sequence. Modification of the microstructure by rapid solidification is also discussed.