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 degrees 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 degrees 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 degrees C and 1300 degrees C spalled readily, whilst those formed at 1000 degrees 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 degrees C, where the alloys displayed a massive weight gain with rapid oxidation of the Cr-rich solid solution. (C) 2012 Elsevier Ltd. All rights reserved.