Study On Structure Regulation And Strength-ductility Mechanism Of CoCrFeMnNi High Entropy Alloys

At present,the widely studied CoCrFeMnNi high entropy alloys（HEAs）are face-centered cubic structure HEAs,which usually have good ductility and excellent low temperature fracture properties.However,it usually have low room temperature strength,which greatly limits their application and development as structural materials.For this reason,two kinds of CoCrFeMnNi HEAs with equiatomic and non-equiatomic were studied in this paper.The methods of nitrogen alloying,friction stir processing（FSP）,cold rolling and annealing were used to solid solution strengthening and refine grain strengthening,the microstructural evolution and the mechanism of strength-ductility of HEAs under different processing conditions were compared and studied.In addition,the technical ways to improve the strength-ductility of such HEAs were explored.The microstructural evolution and mechanical properties of non-equiatomic CoCrFeMnNi HEAs during FSP were studied.The results showed that the grain size were significantly refined with an average grain size of 2.94μm in the processing zone.Discontinuous dynamic recrystallization was the main mechanism of grain refinement during FSP.Since recrystallization nucleates near shear deformed grains,the recrystallized microstructure in the processing zone still retained deformation shear texture G（{001}<110>）.FSP can significantly improve the microhardness and tensile properties of non-equiatomic CoCrFeMnNi HEAs,which the microhardness was 201 HV,the yield strength was 336 MPa,the ultimate tensile strength was 615 MPa and the uniform elongation was 21.89%.The microstructure and mechanical properties of nitrogen alloying as-cast and homogenized equiatomic CoCrFeMnNi HEAs were researched.It was found that there existed obvious dendritic segregation in both nitrogen alloying and nitrogen free as-cast HEAs,which Fe,Co and Cr were enriched in the dendritic regions,whereas Mn and Ni were enriched in the interdendritic regions.In addition,it was found that nitrogen atoms were enriched in the interdendritic regions of nitrogen alloying as-cast HEAs.After homogenization treatment,all elements tended to be uniformly distributed.Due to the solid solution strengthening effect of nitrogen atoms,nitrogen alloying significantly improves the mechanical properties of as-cast and homogenized CoCrFeMnNi HEAs.The microstructures and mechanical properties of nitrogen alloying equiatomic CoCrFeMnNi HEAs treated by cold rolling and subsequent annealing were investigated.It was indicated that the microstructure characteristics of nitrogen free HEAs during cold rolling process were mainly deformation twins and shear bands,while the nitrogen alloying HEAs showed uniform distribution microbands.Moreover,the main texture components of nitrogen free cold-rolled HEAs were G and B textures,while the main texture components of nitrogen alloying cold rolled HEAs were G/B textures.This may due to nitrogen alloying can increase the stacking faults energy of CoCrFeMnNi HEAs,and thus promoted the slip of dislocations during cold rolling.During annealing at773-873 K,the deformation twins of the nitrogen free HEAs were partially annihilated,while the microstructure of nitrogen alloying HEAs showed high stability.With the annealing temperature increasing,both nitrogen free and nitrogen alloying HEAs were recrystallized.The drag effects of nitrogen atoms promoted the grain refinement of nitrogen alloying HEAs during annealing.At the same time,a large number of Cr₂N precipitates were formed in the recrystallization region.Compared with the cold rolled state,the microhardness and tensile strength of the annealed nitrogen alloying HEAs increased abnormally at 773-873 K.This was due to the dragging effect of segregation nitrogen atoms,which in the regions with high density dislocations.After annealed at 973K,the nitrogen alloying HEAs shows the best combination of tensile strength and ductility with the yield strength,ultimate tensile strength and uniform elongation reached 872 MPa,1025 MPa and 9.57%,respectively.The multi-mechanism in terms of nitrogen solid-solution strengthening,precipitation strengthening and refine grain strengthening resulted from the nitrogen alloying can account for it.The microstructure and mechanical properties of nitrogen alloying equiatomic CoCrFeMnNi HEAs during FSP were studied.Due to nitrogen alloying can improve the dynamic recrystallization nucleation rate during FSP and thus delays grain growth,the grain size in the processing zone was significantly refinement than that in the nitrogen free HEAs.A significantly increase in the microhardness and tensile strength of nitrogen alloying HEAs after FSP,in which the microhardness,yield strength,tensile strength and uniform elongation were increased by 42%,19%,20%and 15%,respectively.These improvements could be attributed to solid solution strengthening through nitrogen alloying and grain boundary strengthening by FSP.