KINETICS OF THE CRYSTALLIZATION OF AMORPHOUS TI-NI AND TI-NI-SI ALLOYS

Three binary Ti-Ni alloys and three ternary Ti-Ni-Si alloys were rapid ly quenched by the splat-cooling method and found to be completely amo rphous. Their crystallization behaviour was studied by dynamic differe ntial scanning calorimetry and by isothermal heat treatment followed b y X-ray diffraction and transmission electron microscopy. Two of the a lloys with compositions close to Ti2Ni (one with 4 at.% Si) crystalliz ed in a single step to Ti2Ni. The others showed a two-stage reaction: primary crystallization of either Ti2Ni (cubic) or Ti5Si3 (hexagonal), depending on the composition, followed by the formation of a eutectic of the first phase with alpha-Ti or Ti2Ni respectively. Electron micr oscopy measurements of the growth kinetics of the first reaction step showed that it is diffusion controlled, except for the binary alloy wi th the composition Ti2Ni, in which a polymorphic reaction occurs. Anal ysis of the particle size distributions determined by electron microsc opy was used to determine the nucleation kinetics. This part of the in vestigation has not yet been completed, but the first results indicate that the first reaction step takes place by transient homogeneous nuc leation.