CRYSTALLIZATION OF AMORPHOUS METAL MATRIX COMPOSITES

Crystallization characteristics of WC-amorphous-Pd77.5Cu6Si16.5 metal matrix composites are studied by differential scanning calorimetry and transmission electron microscopy. The activation energy Ec of crystallization of non-preannealed amorphous Pd77.5Cu6Si16.5 is 3.4 eV. This is greater than those of preannealed amorphous alloys, but less than those of WC-amorphous-metal matrix composites which have been preannealed at 330°C for various annealing times. All non-preannealed amorphous alloys yield an Avrami exponent n = 1.7, however, alternative values of n were obtained for WC-containing composites preannealed at 330°C for various annealing periods. In these composites n increases with increasing volume fraction of WC, approaching n = 3.6. It is found that the crystallization progresses faster in WC-dispersed composite than in non-dispersed amorphous Pd77.5Cu6Si16.5 alloy. This information strongly suggests that the effects of particle dispersion and increasing short-range ordering in an amorphous structure lead to different nucleation and growth mechanisms.