High temperature oxidation of the refractory alloy glass Nb_(35)Ni_(60)Sn_5

The newly discovered Ni–Nb–Sn glass is considered a promising refractory material due to its high glass transition temperature (T_g > 880 K). An adequate oxidation resistance at high temperatures is therefore necessary. In this paper the oxidation behavior of the Nb_(35)Ni_(60)Sn_5 refractory alloy glass has been studied in dry air in the temperature range of about 803–863 K. Isothermal thermogravimetric analyses reveal paralinear oxidation kinetics with similar oxidation rates that Zr-based glasses experience 200 K below these temperatures. X-ray diffraction and cross-sectional electron microscopy allows phase identification in the scales revealing their formation mechanism. The oxide scale resulting from oxidation consists of an outer layer of Ni oxide and a continuous inner layer of Nb oxide. This sequence of different oxide layers is repeated as the oxidation proceeds. The depletion of Nb in the alloy underneath the scale seems to induce the crystallization of Ni-rich intermetallics such as NbNi_3 and the ternary intermetallic NbNi_2Sn. Thermodynamics and kinetic aspects of the interaction between oxidation and crystallization are also discussed.