Spatial Localization of the Nucleation Rate in Deeply Undercooled Liquids
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SPATIAL LOCALIZATION OF THE NUCLEATION RATE IN DEEPLY UNDERCOOLED LIQUIDS S. Bossuyt, W. L. Johnson W.M. Keck Laboratory, California Institute of Technology, Pasadena, CA ABSTRACT In bulk metallic glass forming alloys cooled close to the critical cooling rate for glass formation, the nucleation density is observed to be spatially inhomogeneous; in an amorphous matrix there are spherical clusters with a high density of nanocrystals. This is attributed to the combined effect of recalescence due to the heat of crystallization and the fact that in deeply undercooled liquids the nucleation rate increases with increasing temperature. A linear stability analysis of the non-linear differential equations describing nucleation and growth reveals that in the early stages of crystallization, stability is determined by the temperature dependence of the nucleation rate. When the nucleation rate increases with increasing temperature, fluctuations are amplified, resulting in “hot spots” where both the nucleation rate and the growth rate are higher. EXPERIMENTAL Ingots of a variety of alloys in the Cu-Ni-Ti-Zr system, with compositions close to the bulk glass forming alloy Cu47Ni8Ti34Zr11 (Vit101) reported by Lin et al. [1], were prepared by induction melting the constituent elements (purity ≥99.9%) in a titanium gettered argon atmosphere. These ingots were subsequently remelted under vacuum in a fused quartz tube and injection cast into a copper mold, resulting in a wedge-shaped sample 40 mm long, 8 mm wide, and with thickness varying linearly from 0.2 mm at the tip to 8 mm at the end. RESULTS Figure 1 shows an optical micrograph of part of the cross section of such a sample. Near the tip of the wedge and at the surface, where the cooling rate was highest, the sample is completely amorphous as determined by X-ray and transmission electron microscopy (TEM) analysis. Near the base of the wedge, where the cooling rate was lowest, the sample is crystallized. In the region between the crystallized part and the fully amorphous part of the sample, spherical features are observed in the optical micrograph, with diameters ranging from
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