We report here a study of the crystal phase nucleation in undercooled droplets of Pd-Si alloys with composition near the Pd84.5Si15.5 eutectic composition. Molten droplets are released at the top of a drop tube and solidify (to either a crystalline or glassy state) during descent. This provides a containerless (and nearly gravity free) environment so that nucleation due to container walls or vibrations is eliminated. We found that crystallization, due to homogeneous nucleation, is bypassed in droplets of 1 mm diameter when cooled at 760 K/sec. From this an upper limit of the homogeneous nucleation rate is estimated. Results are compared with our previously published study of nucleation in 0.06 mm to 0.33 mm diameter droplets, which indicated that nucleation results from heterogeneous surface nucleation and that the number of these nuclei is dependent on the atmosphere in the drop tube.
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The authors thank A.L. Greer, F. Spaepen and C.V. Thompson for many useful discussions. This work was supported, in part, by NASA under contract NAS8-32691.
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Drehman, A.J., Turnbull, D. Crystal Nucleation in Pd-Si Alloys. MRS Online Proceedings Library 9, 81–85 (1981). https://doi.org/10.1557/PROC-9-81