Tuning Mechanical Properties Of CoCrNi-Based Medium-Entropy Alloys And Their Strengthening Mechanisms

The CoCrNi alloy with superior corrosion resistance,irradiation resistance and toughness at room and cryogenic temperatures.However,the as-cast CoCrNi alloy with low strength is hard to be used in the fields of aerospace and nuclear industry.Currently,architecting multi-scale structure via tuning composition and fabrication method is effective to increase high strength.Unrevealing the evolution patterns of crystal defects under different conditions,such as grain boundaries,dislocations,SFs and twins is important for the design and fabrication of high-strength CoCrNi alloys.Based on the regulation of alloy composition,this paper aims to improve the comprehensive mechanical properties of CoCrNi-based alloy（room and high temperatures）by optimizing the preparation methods and cold/hot machining processes.The correlation laws between the hierarchical microstructure,including therefinedheterogeneousgrainstructure,homogeneous/non-homogeneous nano-scale precipitates,and multi-scale latttice defects,and the mechanical properties of CoCrNi-based alloy were systematically investigated.Correspondingly,the strengthening-plasticization mechanisms of CoCrNi-based alloy were explored in depth.The results are as follows:（1）The connection between microstructure/mechanical properties and fabrication methods and subsequent plastic deformation and heat treatment were investigated.The results indicated that compared with the lower yield strength of the as-SPSed CoCrNi alloy（352.1 MPa）,the yield strength of as-SLMed CoCrNi alloy reached up to 690.3 MPa,which was attributed to the formation of dislocation-formed cells.Additionally,the high-density lattice defects,such as dislocations,SFs and nano-scale deformation/annealing twins,could be introduced into the refined heterogeneous grains of as-SPSed/SLMed CoCrNi alloys via tuning cold-rolling and annealing processes,which could improve the comprehensive mechanical properties of the alloys,the yield strength and fractured stain reached up to 1160.9 MPa and 31.5%.Correspondingly,the strength of the CoCrNi alloy could be improved to some extent by obtaining grain refinement strengthening and dislocation strengthening.（2）It is hard to improve the mechanical properties of CoCrNi alloy to a large extent via a single regulation of alloy composition.The formation of semi-coherentμprecipitates and grain refinement effect were introduced into CoCrNi alloys successfully via doping Mo element.The yield strength of as-SPSed and as-SLMed CoCrNi alloys reached up to 1020.2 MPa and 1342.5 MPa,respectively.The refined heterogeneous grain structure that contains nano-scale heterogeneousμprecipitates in the alloy delivered effective grain refinement and precipitation strengthening effects,which enhanced the strength and work-hardening ability.Although the formation of SFs,nano-scale deformation twins and annealing twins was able to improve the ability of accommodating dislocations in the alloy,the formation of hard-and-brittleμprecipitates led to the poor plasticity,corresponding fractured strains were 14.3%and12.2%,respectively.（3）Based on the above results,the formation of coherent γ’precipitates（2～5 nm）and stable dislocation-formed cells in the as-SLMed CoCrNi alloys via doping Ti and Al elements.The yield strength and fractured strain of as-aged alloy were 1305.6 MPa and 19.9%at room temperature,718.6 MPa and 39.4%at 700℃,respectively.The CoCrNi alloy exhibited superior comprehensive mechanical properties at room and high temperatures without subsequent plastic deformation.Compared with CoCrNi alloy,Ti element segregated at cell boundaries of the dislocation-formed cells in the（CoCrNi）₉₄Ti₃Al₃ alloy decreases its reduction rate;Superior precipitation strengthening and dislocation strengthening were the main reasons for the enhanced strength at room and high temperatures,on the basis of remained outstanding plasticity.Meanwhile,the formation of multi-scale high-density lattice defects（including SFs,LCs,deformation/annealing twins and 9R structure）could also improve the co-deformation capabilities of the alloy further.Besides,the process that contains cold-rolling and annealing could decrease the grain size and increase dislocation densities effectively,which deliver superior comprehensive mechanical properties（YS:1661.3 MPa;FS:10.7%）.Therefore,this thesis mainly focus on the challenges of low strength at room and high temperatures of CoCrNi-based alloy with superior plasticity and ductility,architecturing a typical hierarchical microstructure that contains heterogeneous grains-nanoscale precipitates-multiscale lattice defects and acquiring superior comprehensive mechanical properties.It is a guideline for the design and preparation of medium/high entropy alloys with high strength and plasticity at room and high temperature.