High-Temperature Oxidation Behavior Of FeCoNiCu_xAl_yTi_0.5 High Entropy Alloys

High entropy alloys has received the widespread attention due to the high entropy effect,the lattice distortion effect,hysteresis diffusion effect and cocktail effect.The high entropy effect and hysteresis diffusion effect make high entropy alloys have good high temperature performance,so it has very vital significance both in the high temperature oxidation theory and application to study high temperature oxidation behavior.In this thesis,high-entropy alloys(HEAs),including FeCoNiCuAlxTi0.5(x=0?0.2?0.5?1.0?1.5)and FeCoNiAlTi0.5 were prepared by the arc melting method.High temperature oxidation resistance of HEAs were measured by discontinuous weighing method at 700?,800?,900? and 1000?.The morphology,composition and structures of the oxidized layers were characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM)and Energy Disperse Spectroscopy(EDS).The micro structure of the as-cast and oxidized alloys were compared,and the oxidation mechanism of the alloys and the effects of Al and Cu were discussed.The main results are as follows.The results show that Al enhances the resistance to oxidation of FeCoNiCuAlxTi0.5 HEA.When A1 content increased from 0.5 to 1.0,the oxidation resistance will greatly enhance.With the increase of temperature,the oxidation resistance of alloy performance degrades,that is,it deteriorates rapidly when x=0-0.5,but slowly when x=1.0-1.5.Al0 and Al0.2 alloys have similar oxidation behavior and oxidation morphology.The oxidation can be divided into the external oxidation and internal oxidation,and the oxidation layer distributes from outside to inside:CuO?CoO?Fe3O4?TiO2+Al2O3,where Fe3O4 layer is the thickest,and TiO2 and A12O3 co-exist in the internal oxidation layer.The surface of the oxidation has only CuO at 700?,but CuO and Co3O4 at 800?.The oxidation surface morphology of Al0.5 alloy consits of CuO dendrite region and Al2O3 interdendritic region,and the oxidation surface is covered by CuO with the temperature increasing.The oxidation layers from surface to inside are CuO?CoFe2O4?TiO2+Al2O3.On the oxidation surfces of the alloys with x is 1.0 and 1.5,the dendrite regions are covered by a granular Al2O3 layer and the interdendrite regions are the mixture of Al2O3 and CuO.Cross section of oxidation layer of Al1.0 alloy is multielement coexistence mixed oxide layer?Al2O3 and TiO2 alternate growth?Al2O3,cross section of oxidation layer of Al1.5 alloy is only Al2O3.Both alloys has very thin oxidation layer,no internal oxidation.The structure of FeCoNiCuAlxTi0.5 alloys varies from Fee into Fee+Bcc structures with Al increasing,and the microstructrue changes from two Fee phases into dendrit Fee phase+interdendrit Bcc phases,and finally is converted to dendrite Bcc phase+interdendrite Fee phase.Al promote the formation of Bcc phase.After high temperature oxidation,tiny rich Cu phase precipitates in the alloys.And with the rise of temperature,the size of Cu-rich phase increases gradually.When x is 0.2 and 0.5,the fine needle-like(Al,Ti)-rich phase can also be found in the alloys oxidized at high temeratures.FeCoNiAlTi0.5 alloy is complete anti-oxidation at 700? and anti-oxidation at 800-1000?.The as-cast FeCoNiAlTi0.5 alloy has a Bcc structure with Fe-rich in dendrite region and(Al,Ni)-rich in interdendritic region.After oxidation at 700?,the CoFe2O4 forms in the dendrite region and a thin layer of Al2O3 forms in the interdendritic region.CoFe2O4 covers gradually the entire surface of the alloys with the rise of temperature.The oxidation in dendrite region is divided into external and internal oxidation layer,the external oxdiation layer is CoFe2O4 and the internal oxdiation layer consists of TiO2 andAl2O3.Interdendritic oxidation zone is a thin layer of Al2O3 and has no internal oxidation.After high temperature heat treatment,interdendritic phase in the as-cast alloy disappears,and fine Fe-rch phase precipitate in the dendrite area.The sizes of the Fe-rich precipitations gradually increase with the rise of the temperature.The addition of Cu makes Al uniformly distributed in the alloy,which prompts the formation of Al2O3 oxide layer and improves the oxidation resistance of alloy.