Study On Microstructure Regulation And Strengthening Mechanisms Of CrMnFeCoNi High Entropy Alloy

CrMnFeCoNi high entropy alloy（HEA）has more slip systems and higher ductility due to its inherent face-centered cubic（FCC）structure,but its yield strength is low at room temperature,about 200 MPa.Therefore,how to improve its strength without large loss of plasticity is a major research problem.The mechanical properties of materials are determined by their microstructure,in this paper,CrMnFeCoNi HEA was selected as the research object,and its microstructure was regulated by cold rolling and post deformation annealing（CR-PDA）,ultrasonic nanocrystal surface modification（UNSM）and Si element alloying to achieve a significant increase in its property.The main research contents include:（1）The CrMnFeCoNi HEA was homogenized at 1473 K-12 h to obtain the base sample.Rolling with a total rolling amount of 50%was carried out on the base sample at room temperature,followed by annealing at 1073 K-0.5 h to obtain the CR-PDA sample.The grain size of the alloy decreases from 209.5 to 4.6μm after CR-PDA treatment.The dislocation density increases from 0.72×10¹⁵ to 5.49×10¹⁵ m^-2.The microhardness increases from 164 to 210 HV_0.1,the yield strength increases from 209to 352 MPa,but the total elongation decreases from 59.3 to 43.8%.The self-corrosion potential increases from-0.309 to-0.283 V,and the self-corrosion current density decreases from 0.208 to 0.148μA cm^-2.Smaller grain size and larger residual compressive stress value improve the corrosion resistance of the alloy.（2）UNSM treatment was carried out on the CR-PDA sample.The UNSM sample with gradient structure layer was obtained.The grain size of the surface decreases from 4.6 to 2.7μm and the depth of severe plastic deformation layer is about160μm after UNSM treatment.The surface microhardness of the alloy increases from210 to 300 HV_0.1,and the depth of the hardened layer is about 400μm.The yield strength of the alloy increases from 352 to 447 MPa,and the total elongation increases from 43.8 to 46.4%.The synchronous increase in strength and plasticity is mainly attributed to the coordinated deformation of fine and coarse grains in gradient nanostructures.The self-corrosion potential of the alloy increases from-0.283 to-0.276V,and the self-corrosion current density decreases from 0.148 to 0.070μA cm^-2.The improved corrosion resistance of the alloy is attributed to the smaller grain size,surface roughness and larger residual compressive stress value.（3）On the basis of CrMnFeCoNi HEA,two kinds of HEA samples with different Si element contents（0.3 and 5.7 at.%）were designed,named as CrMnFeCoNi Si_0.3 and CrMnFeCoNi Si_5.7 sample.With the addition of Si element,the second phase particles with different sizes are precipitated on the surface of CrMnFeCoNi Si_0.3 alloy,and the microstructure of CrMnFeCoNi Si_5.7 alloy is dendritic.The microhardness of the alloy increases from 164 to 414 and 222 HV_0.1,respectively.The yield strength of the alloy increases from 209 to 324 and 240 MPa,respectively.But the total elongation decreases from 59.3 to 5.0 and 6.6%,respectively.The self-corrosion potential decreases from-0.309 to-0.328 and-0.385 V,and the self-corrosion current density increases from0.208 to 0.547 and 0.684μA cm^-2,respectively.The second phase with poor corrosion resistance decreases the corrosion resistance of the alloy.The CrMnFeCoNi Si_0.3alloy has good oxidation resistance at low temperature,but as oxidation time and temperature increase,the oxidation resistance of the alloy decreases.The results show that CR-PDA treatment improves strength while retaining good plasticity of CrMnFeCoNi HEA and the main strengthening mechanisms are fine grain strengthening and dislocation density strengthening.UNSM treatment simultaneously improves the strength and plasticity.The strengthening mechanisms are mainly the fine grain strengthening,dislocation density strengthening and twinning strengthening.Si element observably improves the microhardness and strength of the alloy and the strengthening mechanisms are mainly solid solution strengthening and second phase strengthening,but the plasticity and the corrosion resistance of the alloy are reduced.