Research On High Temperature Oxidation And Corrosion Behavior Of High Entropy Alloys

Metallic materials are inevitable to cause loss due to various forms of failure during service process.The common modes of failure include fracture,deformation,wear,corrosion,etc.High entropy alloys have recently received widespread attention as they broke the traditional mode in alloy design.Some High entropy alloys exhibit great properties such as high strength,high plasticity,high hardness and good wear resistance,which make them new research focus of condensed matter physics and material science.Now researches about high entropy alloys are mainly focued on their mechanical properties,phase compositions and prediction,few studies about other failure behaviors are reported.This dissertation studied the high temperature oxidation,thermal cycling stability and corrosion behavior of MoTaTi-（Cr,Al）series alloys which is proved to exhibits great high temperature oxidation resistance,and the Cantor alloy（FeCoNiCrMn）.The main content of this study is as follows:In this dissertation we prepared MoTaTi-（Cr,Al）series refractory medium or high entropy alloys including MoTaTi,MoTaTi Al,MoTaTi Cr,MoTaTi Cr Al,Mo Ta Cr and Ta Ti Cr with powder compacting and arc melting.The characterization of their phases and microstructures indicates good ability of forming solid solution and good phase stability.5 of the 6 alloys produced in this dissertation form single solid solution phases,and keep single phase after being annealed at high temperature.This dissertation studied the oxidation behavior of the 6 alloys and found that Cr plays an important role in resisting high temperature oxidation,and the pivotal oxide is Cr Ta O₄.Measurement of the element distribution in the oxide layer proved that Cr Ta O₄can effectively prevent gas diffusion.Further study discovered that the evaporation of Mo oxides reduced the density of the oxide layer,which makes Cr Ta O₄ the appropriate oxide to form a dense protective layer because it has bigger PBR.Thermal cycling on FeCoNiCrMn was performed to find out its effect on mechanical properties.The results show that the mechanical properties of the alloy did not change during 200 cycling between liquid nitrogen temperature and room tenperature,indicating good stability of FeCoNiCrMn alloy during thermal cycling.Anode treatment was performed on FeCoNiCrMn alloy in this dissertation to improve the corrosion resistance of the alloy.The polarization curve shows that the anode treatment could efficiently reduce the self-corrosion current density of the FeCoNiCrMn alloy and improve its corrosion resistance.XPS result proved that a passivation layer that consisted of Cr-compound was formed on its surface.Internal oxidation was found in FeCoNiCrMn during high temperature oxidation,which shows that FeCoNiCrMn has poor oxidation resistance at high temperature,and it is not suitable for high temperature applications.We attempted to improve its oxidation resistance by adding Al.On the surface of the FeCoNiCrMn Al alloy formed a thinner oxide layer during high temperature oxidation and no internal oxide was observed,indicating that adding Al to FeCoNiCrMn could improve its oxidation resistance.