Structure and stability of nickel-based refractory amorphous metal alloys

Ni-based refractory Bulk Metallic Glasses (BMGs) represent an exciting new class of materials intended for high temperature applications. Proper selection of alloy components allow critical cooling rates for glass formation ∼1K/s which should enable manufacturing methods capable of readily producing near net-shaped parts.; A series of ternary and quaternary Ni-based refractory alloys were studied in order to assess such qualities as atomic scale structure analysis, processing variability, and high temperature stability of the metastable glass phase. Sophisticated x-ray scattering techniques including standard laboratory reflection mode experiments, high-resolution synchrotron scattering, white-beam transmission x-ray methods, and anomalous x-ray scattering revealed a narrow glass-forming composition range for Ni-Nb-Sn ternary alloys as evidenced by the persistent existence of 1--2% of nanocrystalline residuals in all but one composition studied. Scanning Electron Microscopy and transmission mode x-ray experiments confirmed the heterogeneous distribution of these second phase particles throughout the thickness.; By comparison, quaternary Ni-Nb-Ta-Sn alloys showed large crystalline portions and high sample-to-sample variability. Also, the inclusion of the fourth component, Ta, with a high melting point (Tm = 3020°C) was not found to impart greater thermal stability at temperatures >450°C, as compared to the original Ni-Nb-Sn ternaries. Thermal stability experiments using an in-situ x-ray environmental thermal stage coupled with subsequent Arrhenius data analyses revealed local devitrification of ternary Ni-Nb-Sn materials at temperatures below Tg. This devitrification was consistent with the growth (or reduction) of impurities present in "as-received" samples via a diffusion process. Activation energies for the principle residual components, Nb2O5 and Ni3Sn, were determined to be 1.2 +/- 0.2 eV/atom and ∼1.6 eV/atom, respectively.; Radial distribution analyses from standard x-ray scattering experiments indicated local atomic structure that deviated from a random hard-sphere model. An extension of standard x-ray characterization to a more sophisticated technique, i.e. anomalous x-ray scattering, was used to probe the bonding characteristics of specific species by performing experiments near an absorption edge of one of the constituents. Specifically, differences in Radial Distribution Functions (RDFs) at minus 5eV and minus 100eV from the absorption edges of Ni and Nb have confirmed a non-random clustering effect.