Internal Oxidation And Internal Nitridation Behavior Of Co-Re-Cr Based Alloys

As a high melting point metal（3186℃）,Re can form an insoluble solid solvent with Co,which gives a prospect that increases the service temperature of Co-Re based alloy to above 1200℃.Thus,the alloy attracts more attentions in recent years.However,Re is easy to form volatile Re O₃.In view of the service requirements of alloy in harsh environmen,and especially the volatility of Re elements,the oxidation behaviors of Co-Re-Cr-based alloy in different atmosphere were investigated by using Co-Re-Cr-Al and Co-Re-Cr-Si-Al as the model alloys,and then the mechanisms of internal oxidation and nitride were clarified.The results will provide theoretical and experimental reference for the development and application of Co-Re-based alloys,and further enrich the high-temperature oxidation theory of multiple alloys.High temperature oxidation behaviors（1000-1300℃）of Co-20Re-25Cr-xAl（x=5,7,10）alloy in air and 3.0×10^-5PaO₂/0.1 MPaN₂were studied,respectively.（1）Under the condition of air atmosphere,oxidation kinetic curves of three alloys were all not obvious,in which Co-20Re-25Cr-xAl（x=5,7）showed oxidation weightlessness.The external oxided layers were composed of Co O and（Co,Cr,Al）mixed oxides.Internal oxidation and nitride of Al elements were found to generate inside the alloy.After the addition of 10 at.%Al,the oxidation kinetic curve of the alloy at 1000-1200℃changed from massloss to mass gain,while the external oxidation of Al elements was still observed in the temperature of 1000℃and1100℃.（2）Under the condition of 3.0×10^-5PaO₂/0.1 MPaN₂,the kinetic curves of Co-20Re-25Cr-xAl（x=5,7）alloy both complied with the parabolic law,forming an Cr₂O₃ oxidation film inhibiting the of Re O₃ volatilization.But serious nitride corrosion formed inside the alloy.Similarly to the air atmosphere,external oxidations of Al were produced in 1000℃and 1100℃at 3.0×10^-5PaO₂/0.1 MPaN₂,but there was not external oxidation of Al at 1200℃and 1300℃.Overall,for the Co-Re-Cr-Al alloy,the reduction of oxygen pressure promoted the formation of Cr₂O₃ and thus inhibited the volatilization of Re O₃.But severe nitride corrosion was formed inside the alloy because Cr₂O₃ failed to prevent the diffusion of N elements.High temperature oxidation behavior（1000-1300℃）of Co-20Re-25Cr-2Si xAl（x=5,7,10）alloy in air and 3.0×10^-5PaO₂/0.1 MPaN₂was studied,respectively.（1）Under air atmosphere,three oxided layers were all mainly composed of（Co,Cr,Al）mixed oxides and a small amount of Co O,while the internal oxide region of Al and Si and the internal nitride region of Al were also found in the matrix below the oxide layer.Among them,the external oxidation of Al formed in the Co-20Re-25Cr-2Si-10Al alloy at the temperature 1000-1300℃.（2）Under 3.0×10^-5PaO₂/0.1 MPaN₂condition,the oxidation kinetic curves of Co-20Re-25Cr-2Si-xAl（x=5,7,10）alloy conformed to the parabolic law.Due to the reduction of oxygen pressure,the surface layer of Co-20Re-25Cr-2Si-xAl（x=5,7）inhibited Re O₃ volatilization,but severe internal nitride corrosion still formed inside the alloy.With Co-20Re-25Cr-2Si-10Al alloy,the oxidation of Al element effectively prevented the diffusion of N₂ to the alloy and therefor improved the nitride resistance of the alloy.Compared with Co-20Re-25Cr-xAl（x=5,7,10）alloy,the addition of 2 at.%Si promoted the internal oxidation transition of Co-20Re-25Cr-2Si-10Al alloy,thus greatly improved the oxidation resistance of Co-20Re-25Cr-2Si-xAl（x=5,7,10）alloy.