Single-phase -Ni3Ge has been rapidly solidified via drop-tube processing. At low cooling rates (850 -300 m 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 m diameter particles, 2800 -25000 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 (< 75 m diameter particles, > 25000 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 -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.

a Corresponding author: pmnh@leeds.ac.uk Keywords: Intermetallic; rapid solidification; phase transitions; dendrites; seaweed structure. AbstractThe single phase intermetallic β-Ni 3 Ge has been subject to rapid solidification via drop-tube processing. Droplets spanning the size range 212 -38 m, with corresponding cooling rates of 5800 -54500 K s -1 , have been subject to microstructural investigation using SEM. Three dominant solidification morphologies have been identified with increasing cooling rate, namely; (i) well-defined dendrites with orthogonal side-branching, (ii) dendrites with non-orthogonal side-branching and (iii) dendritic seaweed. Selected area diffraction analysis in the TEM reveals that both types of dendrites are the disordered form of β-Ni 3 Ge in a matrix of the ordered, L1 2 , form. However, the diffraction pattern from the dendritic seaweed cannot be mapped onto a cubic structure, indicating a change in the underlying crystallography coincident with the transition to the seaweed structure.

The congruently melting intermetallic b-Ni 3 Ge has been subject to rapid solidification via drop-tube processing. Droplets spanning the size range 75 to 53 lm, with corresponding cooling rates of 23,000 to 42,000 K s À1 , have been found to undergo spontaneous grain refinement by recrystallization and recovery. Outside of this relatively narrow size range, the primary solidification morphology is retained, either dendritic for larger particles or dendritic seaweed for smaller particles.

Abstract:The congruently melting, single phase, L1 2 intermetallic β-Ni 3 Ge has been subject to rapid solidification via drop-tube processing. Four different cooling rates are used in this process, at very low cooling rates (≥850 µm diameter particles, ≥700 K s −1 ) and slightly higher cooling rates (850-500 µm diameter particles, 700-1386 K s −1 ) the dominant solidification morphology, revealed after etching, is that of isolated spherulites in an otherwise featureless matrix. At higher cooling rates, (500-300 µm diameter particles, 1386-2790 K s −1 and (300-212 µm diameter particles, 2790-4600 K s −1 ) mixed spherulite and dendritic morphologies are observed. Indeed, at the higher cooling rate dendrites with side-branches composed of numerous small spherulites are observed. Selected area diffraction analysis in the TEM indicate that the formation of spherulites is due to an order-disorder transformation. Dark-field TEM imaging has confirmed that the spherulites appear to consist of lamellae of the ordered phase, with disordered material in the space between the lamellae. The lamellar width within a given spherulite is constant, but the width is a function of cooling rate, with higher cooling rates giving finer lamellae. As such, there are many parallels with spherulite growth in polymers.

Congruently melting, single-phase Ni 5 Ge 3 (T m = 1185°C) has been rapidly solidified via drop-tube processing wherein powders, with diameters between 850 -53 m, are produced. At low cooling rates (850 -150 m diameter particles, 700 -7800 K s -1 ) the dominant solidification morphology, revealed after etching, is that of isolated plate & lath microstructure in an otherwise featureless matrix. At higher cooling rates (150 -53 m diameter particles, 7800 -42000 K s -1 ) the dominant solidification morphology is that of isolated faceted hexagonal crystallites, again imbedded within a featureless matrix. Selected area diffraction analysis in the TEM reveals the plate & lath, and isolated hexagonal crystallites, are a disordered variant of -Ni 5 Ge 3 , whilst the featureless matrix is the ordered variant of the same compound. Thermal analysis and in situ heating in the TEM indicate a reversible solid-state order-disorder transformation between 470 -485°C.