Evidence for dendritic fragmentation in as-solidified samples of deeply undercooled melts

The congruently melting, single phase intermetallic β-Ni3Ge has been subject to rapid solidification via drop-tube processing. We establish that the rapidly solidified material growing during the recalescence phase of solidification can be distinguished from the post-recalescence material in the as-solidified sample by the degree of chemical ordering displayed. This can in turn be used to visualize the material from the recalescence phase of solidification. At intermediate cooling rates this recalescence material consists of fragmented dendrites. The occurrence of fragmentation is compared against established theoretical models based on the growth of Rayleigh instabilities with excellent agreement being found. EBSD mapping is used to establish the relationship between these dendritic fragments and the final grain size distribution. The dendritic fragments are found to be poor nuclei for new grains and the fragmented dendrites do not consistently give rise to classical grain refined structures.