Order-disorder morphologies in rapidly solidified ε/ε′-Ni₅Ge₃ intermetallic

Congruently melting, single-phase Ni₅Ge₃ (Tm = 1185 °C) has been rapidly solidified via drop-tube processing wherein powders, with diameters between 850 - 53 μm, are produced. At low cooling rates (850–150 μm 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 μm 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 ε-Ni₅Ge₃, 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.