Influence of heterogeneous nuclei on the solidification of Pd 77.5Cu 6Si 16.5 glassy alloy

Abstract

Heterogeneous nuclei may be present in high concentration in the undercooled melt and could certainly require a higher cooling rate Ṫ than in the case of pure homogeneous nucleation process to avoid crystallization. The critical cooling rate T ̇ c required for glassy solidification of a Pd 77.5Cu 6Si 16.5 alloy was measured by monitoring the critical velocity v c for the formation of amorphous structure during the directional solidification. The measured T ̇ c = 221 K s −1 was higher than the Ṫ c value computed from a continuous-cooling-transformation (CCT) diagram using the Uhlmann treatment considering only homogeneous nucleation. The nucleation and growth during devitrification of a glassy sample were studied in comparison with another glassy sample produced in a drop tube. The existence of about 4 × 10 11 heterogeneous nuclei m −3 was estimated and could explain the high experimental T ̇ c value. Microstructures show typical heterogeneous nucleation observed in the samples, illustrating the effect of container walls and motes on the solidification process.