Study On High Temperature Oxidation Behavior Of Co-Re-Cr-Si-Al Alloys

Materials used in high temperature environments must have good mechanical properties and oxidation resistance simultaneously.The use of Ni-based superalloys is limited due to the relatively lower operating temperatures which are no more than1100°C.Therefore,it’s necessary to develop new materials whose operating temperature is higher than that of nickel-based superalloys.As a metal of high melting point（3186°C）,the addition of Re in cobalt-based alloys increases the melting point and the strength of the material with formation of a solid solution with Co.Co-Re-based superalloys are promising candidates for the future choice due to their potential application over 1200°C.A new Co-Re-Cr-Si-Al system has been designed for investigation at 1000-1300°C under 0.1 MPa pure O₂ and 3.0?10^-5 Pa O₂.Special attention is focus on the effect of the addition of Al and low oxygen pressure on the formation of a protective chromia layer.The result of this research can help to further clarify and develop the theory of high-temperature oxidation of multi-component two-phase alloy,thereby providing a theoretical basis for design and develop the commercial high-temperature alloy.（1）The isothermal oxidation behavior of Co-20Re-25Cr-1Si-x Al（x=1,3,5）has been studied at 1000-1300°C under 0.1 MPa pure O₂ and 3.0?10^-5 Pa O₂.The oxidation kinetics under 0.1 MPa pure O₂ are irregular while those under 3.0?10^-5 Pa O₂ conform to parabolic law completely.The scales formed at 1000-1200°C are very similar for the three alloys:the outer layer is composed of a mixture of（Co,Cr）oxides and the inner layer is a Cr₂O₃ layer,which provides some protection for the alloys.There is an internal oxidation zone of Si and Al beneath the Cr₂O₃ layer.With the increase of Al content,the Cr₂O₃ layer becomes more continuous and denser and thus provides a better protection for the alloys.Compared with the corresponding Co-Re-Cr-Si alloy with a similar composition,the addition of a small amount of Al promotes the rapid formation of continuous Cr₂O₃ layer.Furthermore,the reduction of oxygen pressure makes the Cr₂O₃ layer more continuous and denser.The combination of the addition of Al and low oxygen pressure has shown a synergistic effect on the formation of a protective Cr₂O₃ layer.（2）The isothermal oxidation behavior of Co-20Re-25Cr-2Si-x Al（x=1,3,5）has been studied at 1000-1300°C under 0.1 MPa pure O₂ and 3.0?10^-5 Pa O₂.Similar to the Co-20Re-25Cr-1Si-x Al alloys,the oxidation kinetics of Co-20Re-25Cr-2Si-x Al are rather irregular under 0.1 MPa pure O₂.With the increase of Al content,the oxidation process changes from mass loss to mass gain gradually.By contrast,the oxidation kinetics conform to parabolic law under 3.0?10^-5 Pa O₂,and the oxidation process presents continuous mass gain.Similar to the Co-20Re-25Cr-1Si-x Al alloys,the microstructure of the scales formed on Co-20Re-25Cr-2Si-x Al is similar under two different oxygen pressures.With the increase of Al content,the Cr₂O₃ layer becomes more continuous and denser and thus provides a better protection for the alloys.The increase of Si content from 1%to 2%has no obvious effect on the oxidation resistance of the alloys.