Abstract

The congruently melting, single phase, intermetallic compounds β-Ni₃Ge and ε-Ni5Ge₃ were produced by arc melt. Each was subject to rapid solidification via drop-tube processing. Each compound remained fully single phase (either β-Ni₃Ge or ε-Ni5Ge₃) irrespective of the imposed cooling rate. In the investigation of β-Ni₃Ge compound, droplets spanning the size range ≥850 to ≤38 μm diameter particles, with corresponding cooling rates of <700 to >54500 K s-1, were subject to microstructural investigation using SEM. Six dominant solidification morphologies were identified with increasing cooling rate, namely; (i) spherulites, (ii) mixed spherulites and dendrites, (iii) dendrites-orthogonal, (iv) dendrites-non-orthogonal, (v) recrystallized, and (vi) dendritic seaweed, are observed imbedded within a featureless matrix. For the ε-Ni5Ge₃ compound, four dominant solidification morphologies were observed, namely; (i) partial plate and lath, (ii) plate and lath microstructure (iii) isolated hexagonal crystallites, and (iv) single crystal imbedded within a featureless matrix. Micro-Vickers hardness test result of both compounds showed a complex dependence of micro hardness upon cooling rate. At 700 K s-1 the hardness was significantly lower in both compounds than the reported equilibrium value, although in both cases this subsequently increased with further increases in cooling rate. Moreover, in both cases, microstructural transition, such as change in growth direction, led to abrupt drops in hardness. The micro-Vickers hardness results confirmed that the ε-Ni5Ge₃ is significantly harder (maximum 1021 Hv0.01) than the β-Ni₃Ge compound (maximum 526 Hv0.01).