Scaling Behavior And Mechanism Of Multicomponenet Co-base Alloys Under High Oxidant Pressure And High Temperature

The development of modern advanced high temperature materials tends to use multielement alloys to obtain comprehensive properties such as high temperature strength,room temperature toughness and chemical stability that meet the requirements of service conditions.Co-based superalloys have good high temperature strength,oxidation resistance and thermal corrosion resistance,and are mainly used in aerospace,shipbuilding,thermal energy and petrochemical industries.The Co-Ni alloy formed by adding Ni to the Co-based alloy can further enhance the strength of the alloy and the resistance to high temperature oxidation.The current research on cobalt-based model alloys is extremely lacking,especially the oxidation of new Co-Ni alloys above 1000?.In addition,the minimum chromium-aluminum content required for the formation of external Al2O3 and the influence of temperature on the formation of Al2O3 film are also lacking.So,this paper studies the high temperature oxidation behavior of the ternary cobalt-based alloys Co-20Ni-(3,5)Al,Co-20Ni-(8,15)Cr and quaternary Co20Ni-(8,15)Cr-(3,5)Al alloys at 800-1000?,800-900? and 1000-1100?,respectively.This research is helpful to clarify the oxidation mechanism of multi-model superalloys,explore the influence of different Al and Cr contents on the oxidation resistance of alloys and the differences in different oxidation temperatures.These oxidation mechanisms provide a reliable theoretical basis for the development of Co-Ni-based alloys and the design of subsequent hightemperature corrosion-resistant alloys.At the same time,through Wagner theoretical calculation,the critical Cr/Al content for the transition from internal oxidation to external oxidation is obtained.By comparison,it is found that the Cr content is lower than the actual value.The increase of Al content in the ternary Co-20Ni-(3,5)Al alloy reduces the oxidation parabolic rate of the alloy,and inhibits the oxidation of the alloy to a certain extent.The increase in temperature increases the weight gain of the alloy,increases the parabolic oxidation rate of the alloy,and also intensifies the influence of the Al content on the oxidation of the alloy.In Co-20Ni-(8,15)Cr alloy,the increase of Cr content has little effect on the oxidation of the alloy at 800?.When the temperature reaches 900? and the Cr content reaches 15wt.%,Cr2O3 is formed near the interface of the substrate and the oxidation interface is very irregular.None of the four alloys can form a protective oxide layer at the research temperature.At the same time,through Wagner theoretical calculation,the critical Cr/Al content for the transition from internal oxidation to external oxidation was obtained.The comparison found that except for the difference between Co-20Ni-15Cr and the theoretical value,other conditions were in line with the calculated results.The oxidation performance of the quaternary Co-20Ni-(8,15)Cr-(3,5)Al alloy increases with the increase of Cr and Al content.Compared with the Cr content,the change of the Al content in the alloy has a greater impact on the oxidation rate.And with the increase of Al content,Al2O3 particles are also increasing,and a more continuous thin layer of Al2O3 can be formed near the interface of the matrix at 5Al.The increase in Cr content also further promotes the formation of Al2O3.However,in an alloy containing 15wt.%Cr,even if the Al content in the alloy is as high as 5wt.%,a complete outer layer of Al2O3 protective film cannot be formed.It is necessary to further increase Al The selective external oxidation of Al is possible only when the content or the refined structure of the alloy is used.In addition,according to the oxidation kinetics and the morphology and structure of the oxidation products,this paper also established four oxidation modes corresponding to different oxidation kinetic mechanisms under this system and the formation mechanism of the corresponding oxide film structure.Among them,Co at 1100? The-20Ni-8Cr-5Al and Co-20Ni-15Cr-5Al alloys have been able to achieve the preferential oxidation of aluminum in some areas and agglomerate to form a film.In addition,through comparison,it is found that the introduction of Ni into the ternary Co-CrAl system is more conducive to the selective external oxidation of the protective component Al,and the increase in temperature also effectively promotes the formation of Al2O3,which is the increase in temperature The result of increasing the diffusion rate of Al element in the matrix.