Rapid solidification morphologies in Ni3Ge: Spherulites, dendrites and dense-branched fractal structures

Single-phase β-Ni3Ge has been rapidly solidified via drop-tube processing. At low cooling rates (850–300 μm diameter particles, 700–2800 K s−1) the dominant solidification morphology, revealed after etching, is that of isolated spherulites in an otherwise featureless matrix. At higher cooling rates (300–75 μm diameter particles, 2800–25,000 K s−1) the dominant solidification morphology is that of dendrites, again imbedded within a featureless matrix. As the cooling rate increases towards the higher end of this range the dendrites display non-orthogonal side-branching and tip splitting. At the highest cooling rates studied (25,000 K s−1), dense-branched fractal structures are observed. Selected area diffraction analysis in the TEM reveals the spherulites and dendrites are a disordered variant of β-Ni3Ge, whilst the featureless matrix is the ordered variant of the same compound. We postulate that the spherulites and dendrites are the rapid solidification morphology and that the ordered, featureless matrix grew more slowly post-recalescence. Spherulites are most likely the result of kinetically limited growth, switching to thermal dendrites as the growth velocity increases. It is extremely uncommon to observe such a wide range of morphologies as a function of cooling rate in a single material

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