Microstructure And Superplastic Behavior Of CoCrFeNiMn High-entropy Alloy Prepared By Friction Stir Processing

The eutectic high-entropy alloy is considered as a material with wide application potential because of it has good strength and plasticity,excellent wear resistance and thermal stability.As a typical metal with face-centered cubic structure（FCC）,CoCrFeNiMn eutectic high entropy alloy exhibits good plastic deformability at room temperature,which provides a great possibility for its application in aerospace field.However,the coarse grains in as-cast CoCrFeNiMn alloy limit its machinability and industrial application.Friction stir processing（FSP）technology is a new type of severe plastic deformation technology,which has the characteristics of environmental protection and simple process.However,the friction heat generated during FSP inevitably leads to grain coarsening in the processing zone.In order to further refine the grains and strengthen the strength of the alloy,researchers put forward the technique of forced cooling friction stir processing（SFSP）for rapid and effective reduction of the friction heat during processing,so as to prepare fine-grained or even ultrafine-grained materials.Therefore,this paper intends to modify the microstructure of as-cast CoCrFeNiMn high-entropy alloy using once/twice SFSP technology by taking this kind of alloy as the research object,with a view to exploring its microstructure and mechanical properties under different processing technologies,as well as the high-temperature thermal stability and superplastic deformation behavior of twice SFSP samples.The main conclusions are as follows.Fine-grained CoCrFeNiMn high entropy alloy with excellent mechanical properties is successfully prepared by SFSP technology in this paper.The results manifest that SFSP memorably refines the grain size of the alloy,with the average size of the primary and secondary SFSP samples refined from 110μm of the base metal to2.6μm and 1.7μm respectively.Also,a small amount of new phase with body-centered cubic（BCC）structure is precipitated at the grain boundary.In addition,the tensile strength of CoCrFeNiMn high-entropy alloy is significantly enhanced after SFSP processing,with 606 MPa and 938 MPa obtained for primary and secondary SFSP samples,respectively.No significant reduction in the elongation of the two samples is observed,which is owing to the further refined grains and improved comprehensive mechanical properties caused by the increase of processing passes.As good thermal stability of the alloy is a prerequisite for superplastic deformation,the microstructure stability of SFSP samples at different temperatures is investigated.It is demonstrated that after holding at 650℃,700℃and 750℃for 5h,10h and 15h respectively,the grain size of the alloy is slightly coarsened,and the Cr-rich precipitated phase with BCC structure precipitates along the grain boundaries,pinning the grain boundaries and preventing the abnormal grain growth.The occurrence of superplastic deformation of fine-grained CoCrFeNiMn high entropy alloy is found at all test temperatures due to its uniform and fine structure and good thermal stability.Especially,at the deformation temperature of 650℃（?0.5T_m,T_m represents the melting point of CoCrFeNiMn）and the strain rate of 5×10^-4 s^-1,the fine-grained CoCrFeNiMn high-entropy alloy obtains the optimal elongation（328（4））,showing good low-temperature superplasticity.During superplastic deformation,dynamic recrystallization is induced by thermo-mechanical coupling,resulting in grain refinement.Meanwhile,the generation of BCC phase and twins inhibits grain growth and promotes superplastic elongation.The strain rate sensitive value m of superplastic deformation of fine-grained CoCrFeNiMn high-entropy alloy is calculated as 0.31under the optimal superplastic deformation conditions,which indicates that the superplastic deformation mechanism is mainly grain boundary slip.