The Effects Of Uncommon Silicides On The Oxidation Of Alloys From The Nb-Cr-Si System

Niobium based alloys are being tapped as potential successors to current nickel base superalloys in high temperature applications. In the aerospace industry, high temperature materials are constantly being pushed to higher temperature regimes in order to improve engine efficiency however current superalloys cannot be pushed further due to melting temperature limitations. Niobium, being a refractory metal, can withstand much higher temperatures than nickel however its oxidation properties are woefully inadequate. The purpose of this study is to ascertain whether increased silicon additions can improve niobium oxidation properties by: forming ternary silicides with better oxidation properties and by preventing the formation of Nb2O5. Chromium is shown to form CrNbO4 which protects niobium alloys better than Nb2O5. If the concentrations of chromium found in all phases can be increased, the likelihood for a complete CrNbO4 scale increases as well. Increased silicon concentrations could allow this to happen by forming ternary silicides. Four compositions of Nb-Cr-Si alloys where chosen: Nb-20Cr-10Si, Nb-20Cr-20Si, Nb-20Cr-30Si and Nb-20Cr-42Si. These alloys were tested in temperature ranges between 700° to 1400°C. Gravimetric data was taken and plotted in order to assess weight gain per unit area versus temperature. Oxidation properties were variable based on alloy except for the 42Si alloy which showed very little oxidation. Increased silicon content pushed the pesting regime toward higher temperatures. The highest concentrations of silicon eliminated bulky oxide formation. The alloys and oxides were characterized by XRD, SEM, BSE, and EDS