Effects of Yttrium on High Temperature Oxidation Resistance of W-Si-Y Self-Passivating Alloys

Abstract

Self-passivating W alloys have excellent high temperature oxidation resistance and are expected to be used as a key component in high temperature environment. In this study, a series of W-Si-xY self-passivating alloys were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). Effects of Y content on phase compositions, microstructures, and oxidation resistance at high temperatures were investigated systematically. The results show that the oxidation resistance of the alloys increases with the increase of Y content (0.0~5.0 wt.%), but the oxidation resistance of the alloys deteriorates when the Y content reaches 9.6 wt.%. The alloy with 3.8 wt.% Y shows the best oxidation resistance. The thickness of its oxide layer is ~249.1 μm when the oxidation time reaches 80 h, which is thinner than that of other alloys. Effects of Y content on the oxidation resistance are revealed. During the oxidation process, Y can react with other elements to form molten-like W-Y-O and Y2Si2O7 particles. It is found that these two phases play a key role in the oxidation resistance of the alloys. When the Y content is in the range of 0~5.0 wt.%, a W-Y-O covering layer gradually forms with the increase of Y content, which can prevent further oxidation of the alloy. In the meantime, small Y2Si2O7 particles were formed and the microcracks formed around these particles were isolated, which have little effect on the oxidation resistance of the alloys. However, when Y content reaches 9.6 wt.%, large Y2Si2O7 particles and extensive cracks around them are formed. These cracks are interconnected to form penetrating channels for O2 from the environment to the alloy interior, which deteriorates the oxidation resistance of the alloys.