Study On High Temperature Oxidation Mechanism Of (FeCoNi)₈₆Al₇Ti₇ High Entropy Alloys By Electron Microscopy

As a new type of alloy material,high entropy alloys has excellent comprehensive properties,many of which have wear resistance,structural stability,oxidation resistance,thermal stability,which has been gradually used in nuclear industry,aerospace and other high temperature fields.It is inevitable for alloys to undergo high temperature oxidation reaction under high temperature environment.However,the current research on high entropy alloys mainly focuses on the mechanical properties,and there are few studies the research on high temperature oxidation behavior.Therefore,in this thesis,(FeCoNi)₈₆Al₇Ti₇high entropy alloys was prepared by arc melting method,and its oxidation behavior after isothermal oxidation with Ar-20%O₂at 600?for 4 h,12 h,24 h and 48 h were characterized by XRD,SEM,EDS and TEM.The main conclusions are as follows:(1)The primary phases of the prepared(FeCoNi)₈₆Al₇Ti₇high entropy alloy are mainly face-centered cubic structure(FCC)and L1₂ordered phase.After high temperature oxidation,a dense oxide scale is formed on the surface.With the extension of oxidation time,the oxide particles on the surface grow up,and the bulging oxide appears on the surface of the sample.The growth of oxide film thickness accords with the law of kinetic growth.In addition,there are many small pores at the interface between the oxide and the substrate due to the Kokendall effect.Large pores appear in the oxide scale under in the bulge,which caused by the combined action of thermal stress and growth stress between different oxide layers.(2)After high temperature oxidation of(FeCoNi)₈₆Al₇Ti₇high entropy alloy,the oxides show obvious multilayered structure,and the layered oxides mainly consist of Co O,Fe₃O₄,Al₂Ti O₅,Ni Fe₂O₄,Al₂O₃,Ti O₂and other spinel phases.The main mechanism of oxide formation is as follows:firstly,Al is dominated in accordance with the laws of thermodynamics and kinetics on the surface of the alloy to form Al₂O₃;with the development of diffusion process,Ti O₂,Fe₃O₄and Co O layers are formed from inside to outside according to thermodynamic conditions.Furthermore,the enrichment of Ni elements around the Fe₃O₄pores eventually forms a discontinuous layer of Ni Fe₂O₄.Part of Al and Ti diffused into the oxide layer with Fe plasma,and gathered at the upper edge of some holes in the Fe₃O₄layer to form Al₂Ti O₅phase,and at the bottom of the holes to form Co O layer.Subsequently,a multilayered structure with alternating distribution of Al₂O₃and spinel is formed outside the Al₂Ti O₅phase at the edge of the hole.Finally,due to the continuous growth of oxides around the hole,the hole is gradually enlarged and the surface is gradually raised due to the extrusion under various stresses.