Disorder Trapping in Rapidly Solidified Intermetallic Compounds

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 β-Ni3Ge/ 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) recrystallised, and (vi) dendritic seaweed, are observed imbedded within a featureless matrix. Selected area diffraction (SAD) in the TEM analysis confirmed that it is only the spherulite microstructure that is partially ordered amongst the above listed microstructures, which are disordered. However, SAD analysis indicated that the featureless background material of all above microstructures is chemically ordered. While, in the examination of 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. SAD analysis in the TEM reveals that the partial plate and laths and plate and laths are partial ordered variant of έ-Ni5Ge3 and ε-Ni5Ge3 respectively, whilst the featureless matrix of both microstructures are the ordered variant of the same compound. However, isolated hexagonal crystallites are a disordered variant of ε-Ni5Ge3, although featureless matrix are the ordered variant of the same compound. SAD analysis in the TEM also indicated that, at the highest cooling rates, single crystal structure along with featureless matrix is the completely disordered 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. The micro-Vickers hardness results confirmed that the ε-Ni5Ge3 (1021 Hv0.01) is significantly harder than the β-Ni3Ge (526 Hv0.01) compound.