Effect Of High Temperarure Corrosion Resistance Of Ni-Cr-Al-Sc Superalloy

Ni-Cr-Al alloys are used for high-temperature components due to their excellent high-temperature mechanical properties and high-temperature oxidation resistance,such as new aero-engines and gas turbines.High temperature oxidation of superalloys is one of the important reasons for alloy failure,so it is very important to study the resistant oxidation properties of alloys.The effect of rare earth elements on the properties of alloys has been extensively studied experimentally.Rare earth element Sc have excellent properties and prospects in alloys.The addition of rare earth elements in the alloy can reduce the oxidation rate of the alloy,improving the high temperature oxidation resistance of the alloy.Adding rare earth elements to nickel-based superalloys is one of the research directions to improve the properties of superalloys,which has received widespread attention.Therefore,it is necessary to systematically study the effect of rare earth element Sc on the oxidation process of Ni-Al-Cr alloy,so that its high temperature resistance can reach a higher standard and meet the constantly harsh application environment.Therefore,Ni-x Cr-z Sc with different Cr contents（x=5,15,25,35;z=1,0;wt.%）was prepared by powder metallurgy in this paper;Ni-y Al-Sc with different Al contents（y=5,10,15,20;z=1,0;wt.%）;Ni-x Cr-y Al-z Sc with different Al and Cr contents（x=5,10,15;y=5,10,15;z=1,0;wt.%）alloy;the effect of adding Sc element on the high temperature oxidation behavior and mechanism of the alloy after cyclic oxidation at 1000℃for 100h was studied.From the aspects of oxidation rate,phase composition and surface morphology,the high temperature oxidation resistance,surface structure changes and alloy oxidation kinetics of alloys with rare earth element Sc added under 1000℃cyclic oxidation were analyzed.The relationship between the oxidation kinetics of alloys and the surface morphology and cross-section of oxide films was discussed.The high temperature oxidation mechanism of Ni-Cr-Al-Sc alloy and the action mechanism of rare earth elements are analyzed,which can provide reference for the research of this kind of alloy.（1）Ni-10Al-Sc,Ni-15Al-Sc,Ni-10Al,Ni-15Al alloy samples,theα-Al₂O₃diffraction peaks in the alloy with rare earth element Sc are higher than those without the addition of rare earth element Sc with the same Al content The diffraction peak of the alloy is stronger,and the diffraction peak ofθ-Al₂O₃is weaker.The addition of rare earth Sc promotes the formation of stableα-Al₂O₃in Al₂O₃.This reduces the stress of the alloy during the transition from unstable oxides to stable oxides,improves the adhesion between the oxide film and the substrate,and further improves the high-temperature oxidation resistance of the alloy.（2）Rare earth element Sc promotes the reaction of Ni O with Cr₂O₃and Al₂O₃to form spinel structure.Due to the high density of Ni Al₂O₄and Ni Cr₂O₄structures,the speed of ion diffusion is slowed down.The formation of Ni Al₂O₄inhibits the diffusion rate of elements in the alloy and slows down the oxidation rate.（3）The oxide films with low Al and Cr contents（5wt%）are all peeled off from the alloy substrate in the form of flakes.Sc promotes the formation of Al₂O₃and Cr₂O₃oxide films and reduces the critical concentration of Al₂O₃and Cr₂O₃.The formation of Al₂O₃and Cr₂O₃oxide films mainly consumes Cr and Al elements,resulting in Ni-rich in the rest of the surface layer of the alloy matrix.This will lead to the oxide film on the bottom layer of Al₂O₃and C_r2O₃mainly composed of Ni O,and Ni O is poorly shaped,resulting in the overall peeling of the oxide film.（4）When the mass fraction of Cr is higher than 20%,the stress increases due to the poor shaping of the oxide film;the volatile Cr O₃generated by Cr₂O₃forms voids on the surface of the alloy,resulting in the thinning of the oxide film;excessive internal oxides will lead to The alloy is cracked along the grain boundary and severely exfoliated,and the oxidation rate is high.Therefore,too high Cr content cannot effectively improve the high temperature oxidation resistance of Ni-Cr alloy.（5）The alloy containing Sc element refines the crystal of the Cr₂O₃oxide film,so that the grain is refined,the grain boundary is increased,the shape is improved,the integrity of the oxide film is improved,and the high temperature oxidation resistance of the alloy is improved.（6）After the addition of rare earth Sc,the alloy containing Al undergoes internal oxidation along the grain boundary.The rare earth element Sc forms"interfacial oxide pegs"and irregular oxide film/alloy interface in the alloy matrix,which is formed in the grain boundary of the alloy matrix."interfacial oxide pegs","interfacial oxide pegs"are integrated with the oxide film and are embedded into the alloy collective to form a"pinning effect",which increases the contact area between the oxide film and the alloy substrate,and improves the adhesion of the oxide layer and the substrate.The oxide film is not easy to peel off,which is beneficial to the protection of the alloy by the complete oxide layer and improves the oxidation resistance of the alloy.（7）The alloy of rare earth element Sc firstly oxidizes in the rare earth rich phase to form oxides and provide nucleation sites for Al and Cr elements.It is the first to form internal oxides at the grain boundaries.The distribution of Sc elements and Al elements is consistent.The segregation of element Sc hinders the outdiffusion rate of Al³⁺and may promote the formation of Al₂O₃The formation of a protective inner Al₂O₃layer helps to promote selective oxidation of Al.