Stability Of The ? Phase And Diffusion Behavior In The Al-Co-Cr-Fe-Ni High Entropy Alloys

AlCoCrFeNi alloy is a typical BCC-strengthen and a widely studied high-entropy alloys.In the present work,three series of high-entropy alloys,AlCoCrFeNi_2.1,Al_xCr_2-xCo Fe Ni and Co_xCr_2-xAl Fe Ni,were prepared by arc melting method.The alloys were also annealed in evaculatd tubes at 800?or 1000?for 72?720 h,respectively.The microstructure,phase com-position and mechanical properties of the as-cast or annealed alloys were examined by XRD,SEM,EDS,TEM,DSC and other experimental testing methods.Moreover,the experiment results were also compared with thermodynamic calculation results.At the same time,the Al/CoCrFeNi and Al/AlCoCrFeNi diffusion couples were also studied.The experimental results show that,with the increase of Al content,the FCC phase in the Al_xCr_2-xCo Fe Ni(x=0.25?1.75)high-entropy alloys decreases,and the phase constituent of the alloys transform from the FCC+BCC phases to the BCC+B2 phases.The as-cast Co_yCr_2-yAl Fe Ni(x=0.25?1.75)high-entropy alloys are composed of BCC+B2 phases.Annealing treatment has a greater impact on the microstructure of the alloys of these alloys.After annealing at 800?for 72?720 h,the?phase precipitated in the Al_xCr_2-xCo Fe Ni(x=0.25?1.25)high-entropy alloys.These alloy are composed of FCC+BCC+?three phases.However,after annealing at 1000?,the?phase is just stable in Al_0.25Cr_1.75Co Fe Ni and Al_0.5Cr_1.75Co Fe Ni alloys,while does not exist in other alloys.Among them,the Al_0.25Cr_1.75Co Fe Ni alloy is only composed of FCC+?phase.Experiment results indicated that the?phase also precipitated in the long time 800?annealed Co_0.5Cr_1.5Al Fe Ni and Co_0.75Cr_1.25Al Fe Ni alloys.It was found that the FCC phase precipitated in the 800 or 1000?annealed Co_xCr_2-xAl Fe Ni(x?1.25)alloys.Moreover,DSC test shows that the decomposition temperature of the?phase in the Al_xCr_2-xCo Fe Ni alloys is1033?912?,which decreases with the increase of Al content;while that in the Co_yCr_2-yAl Fe Ni alloys is 819?830?.Through the microhardness test,it was found that,when the?phase precipitated in the alloy annealed at 800?,the microhardness of the Al_xCr_2-xCo Fe Ni(x=0.25?0.75)alloys will significantly increased.Prolonging the annealing time will decrease the Al content in the FCC or BCC phases and leads to decreaseing of the alloy hardness.Compared with the thermodymically calculated results based on the Pan HEA high-entropy alloy database,it was found that the phase constituent of the as-cast Al-Co-Cr-Fe-Ni alloys can be predicted according to the vertical cross-section solidus.The influence tendency of the Al content on the decomposition temperature of the?phase can also be predicted,but the real decomposition temperature of the?phase cannot be predicted well,which is about 114?lower than the experimental value.In addition,thermodynamic predictions overestimated the phase stability of FCC in Al_xCr_2-xCo Fe Ni(x=1.25?1.5)alloys.Therefore,it is necessary to thermodynamically optimize the?phase in the Al-Co-Cr-Fe-Ni system based on more experimental data to obtain more accurate thermodynamic model parameters to guide the composition and processing design of this type alloys.Moreover,in the Al/CoCrFeNi and Al/AlCoCrFeNi diffusion couples,the diffusion rate of the elements towards the liquid Al side is Cr>Fe>Co>Ni.Two kinds of layers,Al-Ni-riched B2 layer and Fe-Cr-riched?layer,formed in the 1000?Al/CoCrFeNi diffusion couples which can decrease the diffusion rate of these elements.