5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

Spherical pre-alloyed Nb-20Si-24Ti-2Cr-2Al (at.%) powders were prepared by plasma rotating electrode processing (PREP). The diameters of the pre-alloyed powders ranged from 45 μm to 380 μm. The microstructure and surface oxides of Nb-Si based alloy powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectra (XPS) and Auger electron spectroscopy (AES). The main phases of Nb-Si based alloy powders were Nb solid solution (Nbss), βNb5Si3 and Nb3Si. Fine lamellar eutectic structure was formed during solidification, taking the central parts of eutectic branches. Anomalous eutectics were distributed at the edges of eutectic branches. The lamellar spacing (λ) of lamellar eutectics ranged from 0.12 μm to 0.38 μm, increasing with the increase of powder diameter. In addition to the fine lamellar eutectic colonies, large silicides (>15 μm) are occasionally observed in the sectional microstructures of some Nb-Si based alloy powders. The bulk oxygen content of the Nb-Si based alloy powders was as low as 0.06 wt.%. An oxygen-enriched layer with 5.14 nm in thickness, consisting of Nb2O5, SiO2, TiO2, Cr2O3 and Al2O3, was formed on the surface of pre-alloyed powders. The oxygen content was found to decrease exponentially with etch depth, and the amount in the oxygen-enriched layer accounts for 49.66 % of that in a whole single powder particle with a diameter of 120 μm. The outermost surface of Nb-Si based alloy powder was Nb-depleted and (Ti, Si)-enriched, resulting from the lower thermodynamic stability of Nb2O5 compared to the thermodynamic stability of TiO2 and SiO2.