Effect of silicon additions on the high temperature oxidation behaviour of Cr–Cr2Ta alloys

The effect of Si additions between 0 and 15 at.% on the microstructure and oxidation resistance of Cr–Ta alloys has been studied. After annealing at 1300 °C for 500 h, the microstructures of the alloys were observed to consist predominantly of a Cr-rich solid solution and a Cr2Ta Laves phase, with the Si partitioning preferentially to the Laves phase. Si was found to be beneficial to high temperature oxidation resistance, with the ternary alloys showing a lower weight gain than binary Cr–Ta alloys after isothermal exposure at 1100 °C for 100 h. However, no further improvement in oxidation behaviour under these conditions was achieved with increasing the Si-content beyond 7 at.%. The oxide scales formed on the alloys were found to comprise typically of Cr2O3 and mixed (Cr,Ta)-based oxides. The scales formed at 1100 °C and 1300 °C spalled readily, whilst those formed at 1000 °C showed better adherence to the substrate. Importantly, the presence of Si was also found to eliminate the internal nitridation otherwise encountered during the oxidation of binary Cr–Ta alloys. The formation of SiO2 in the alloys was observed only at 1300 °C, where the alloys displayed a massive weight gain with rapid oxidation of the Cr-rich solid solution.