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 rapidly quenched by the splat-cooling method and found to be completely amorphous. Their crystallization behaviour was studied by dynamic differential scanning calorimetry and by isothermal heat treatment followed by X-ray diffraction and transmission electron microscopy. Two of the alloys with compositions close to Ti2Ni (one with 4 at.% Si) crystallized 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 microscopy measurements of the growth kinetics of the first reaction step showed that it is diffusion controlled, except for the binary alloy with the composition Ti2Ni, in which a polymorphic reaction occurs. Analysis of the particle size distributions determined by electron microscopy was used to determine the nucleation kinetics. This part of the investigation has not yet been completed, but the first results indicate that the first reaction step takes place by transient homogeneous nucleation.