Investigation On Microstructure And Corrosion Resistance Of The Co-Fe-Mn-Ni And Co_xCrFeMnNi_2-x Alloys

CoCrFeMnNi alloys are the most typical FCC-type high-entropy alloys and have been widely studied.However,the influence of element contents and annealing treatment on the microstructure and corrosion resistance of these alloys is not perfect.In the present work,three series of alloys,Co_xCr Fe Mn Ni_2-x,Co_xFe Mn Ni_3-x,Co_yFe_3-yMn Ni and Co Fe_zMn-Ni_3-z,with varying only two element contents,were prepared by arc melting and then vacuum annealed at 600～1000°C.The microstructure and corrosion resistance of them were studied by means of XRD,OM,SEM-EDS,EBSD and EIS.Experimental results indicate that,except alloy Co_1.75Cr Fe Mn Ni_0.25,which is composed of the FCC and HCP phases,all the other as-cast Co-Fe-Mn-Ni and Co_xCr Fe Mn Ni_2-xhigh-entropy alloys are composed of single FCC phase.And they have woven network dendrite microstructure.With the increase of Co content,the dendrites of the as-cast alloy become much coarser.Electrochemical test suggests that,as for Co_xCr Fe Mn Ni_2-xalloys（with 20 at.%Cr）,the corrosion resistance of the as-cast alloys is greatly affected by Cr and Ni content.Due to the high Cr content,the corrosion resistance of the alloy is significantly improved and becomes better than that of 304L stainless steel.As for Co-Fe-Mn-Ni high-entropy alloys without Cr,the corrosion resistance of as-cast alloys is mainly affected by Ni content.With the decrease of Ni content,the corrosion resistance of the alloys significantly decreases.After vacuum annealing at 600～1000°C,the changes of the microstructure of the Co_xCr Fe Mn Ni_2-xand Co-Fe-Mn-Ni alloys are similar.The corrosion resistance of these alloys decreased after 600°C annealing and then increased after 800°C or 1000°C annealing.After annealing at 600°C,obvious dendrite structure still maintains,but the dendrite trunks have been truncated,which makes the dendrite begin to fade and refine.These unstable grain boundaries reduce the corrosion resistance of the alloys,which becomes poorer than that of the as-cast alloy.After being annealed at 800°C for 120 h,the dendrites in the Co_xCr Fe Mn Ni_2-xalloys disappeared and transformed into coarse equiaxed crystals containing a large number of sub-grains.Although the grain boundary is coarse and tortuous,the corrosion resistance of the Co_xCr Fe Mn Ni_2-xalloys is still greatly improved.As for the Co-Fe-Mn-Ni alloys without Cr element,the dendrite structure still exists after increasing the Co content or decreasing the Ni content,which indicates that the atomic diffusion is relatively slow and higher annealing temperature is required to make the dendrite disappear.After annealing at1000°C,coarse equiaxed grains are formed in the Co_xCr Fe Mn Ni_2-xalloys,the grain boundaries become smooth and fine,and the corrosion resistance is significantly improved.In the Co-Fe-Mn-Ni based high-entropy alloys,the grain size of the 1000°C annealed alloys is larger than250μm.Only a few large grains can be observed in one view field.However,due to the absence of Cr element,the corrosion resistance of the annealed Co-Fe-Mn-Ni alloys is still lower than that of 304L stainless steel.In addition,prolonging the annealing time can significantly improve the corrosion resistance of the Co_xCr Fe Mn Ni_2-xannlys,but will reduce the corrosion resistance of the Co-Fe-Mn-Ni alloys without Cr element.These results will provide a basis for designing and microstructure controlling of the FCC-type high-entropy alloys.