Research On Strengthening And Toughening Mechanism Of CoCrNi Medium Entropy Alloy By Laser Additive Manufacturin

Twinning Induced Plasticity/Transformation Induced Plasticity（TWIP/TRIP）medium entropy alloy is a new kind of alloy with excellent strength and plasticity matching proposed in recent years,which has great potential applications in the fields of aerospace,national defense science and technology,rail transit and engineering materials.If the additive manufacturing and damage remanufacturing of mechanical parts are realized,this machine will have important significance and value.However,the internal structure of the mechanical parts prepared by laser hardening is usually non-equilibrium solidification structure,the microstructure uniformity is poor,the microstructure is prone to segregation,and the defects such as pores and cracks are easy to occur in the alloy.therefore,it is difficult to maintain high strength and good plasticity at the same time.Aiming at the difficulty of controlling the properties of TWIP/TRIP medium entropy alloy,five kinds of TWIP/TRIP medium entropy alloy powders were designed in this paper,that are CoCrNi Mo_x（0.8at%,1.6at%）and CoCrNi（TiAl）_x（2.6at%,4at%,6at%）.The microstructure and properties of laser deposited state were studied,the effects of laser additive and doping a small amount of chemical elements in CoCrNi medium entropy alloy were discussed,finally,the effects of hot forging,heat treatment and surface sandblasting on the microstructure and properties of laser additive cocrni medium entropy alloy were systematically discussed.The specific research work is as follows:The medium entropy alloy powder of CoCrNi system for laser additive was designed.On the premise of fully considering the strengthening and toughening mechanism of medium entropy alloy in CoCrNi system and the defect formation mechanism of laser additive parts,Alloy powder for laser additive based on solution strengthening and precipitation strengthening was carefully designed and optimized,a total of 5 kinds of cocrni medium entropy alloy powders for laser additive.The medium entropy alloy in defect-free laser additive CoCrNi system was prepared,and the microstructure and mechanical properties of medium entropy alloy in laser deposition state of CoCrNi system were characterized and analyzed.Through the reasonable control of laser parameters,there is no serious segregation in the microstructure of the two kinds of as-deposited alloys,and there are no defects such as pores and cracks in the alloy.For the solid solution strengthened alloy doped with Mo in CoCrNi medium entropy alloy,the laser deposition state structure is typical equiaxed crystal structure,the overall element distribution is uniform,and there is no obvious macrosegregation;the ultimate tensile strength,yield strength and elongation of CoCrNi Mo_1.6laser deposition state alloy respectively are 849±15 MPa,510±20 MPa and 47±4%,and the CoCrNi Mo_1.6medium entropy alloy has good work-hardening ability in the whole plastic deformation stage.For the precipitated strengthened alloy doped with TiAl in CoCrNi medium entropy alloy,the laser deposition state microstructure with low doping content is typical cellular structure and has the characteristics of directional solidification,and the laser deposition state microstructure with high doping content is obvious precipitated structure of L1₂coherent particles.The ultimate tensile strength,yield strength and elongation of CoCrNi（TiAl）₆laser deposition state alloy respectively are 1100±18 MPa,750±15 MPa and 28±2%.The fracture is typical of ductile fracture and has high work-hardening ability.The influence mechanism of typical alloy elements on the microstructure and properties of CoCrNi medium entropy alloy by laser direct deposition was discussed.The doping of Mo element can achieve solid solution strengthening and effectively improve the mechanical properties of the alloy;the appropriate increase of the amount of Mo element doping can more fully strengthen the solid solution effect,increase the lattice distortion and the dislocation density,and significantly improve the strength and plasticity of the alloy,compared with CoCrNi Mo_0.8deposited alloy,the ultimate tensile strength,yield strength and elongation of CoCrNi Mo_1.6deposited alloy increased by88.67%,132.88%and 20.51%,respectively.TiAl element doping can achieve precipitation strengthening.L1₂coherent phase can be precipitated in FCC matrix phase of CoCrNi medium entropy alloy.L1₂coherent precipitation can reduce the stress concentration at the interface between precipitated phase and matrix phase,increase the critical stress of crack initiation and propagation,and the pinning effect of L1₂particle phase can effectively hinder the slip of dislocations and grain boundaries,thus effectively improve the strength and plasticity of the alloy.As a result,the strengthening and toughening of CoCrNi medium entropy alloy is realized.It is worth noting that with the increase of TiAl content,the strength of the alloy increases,but the plasticity decreases,the ultimate tensile strength and yield strength of high TiAl doping CoCrNi（TiAl）₆alloy increased by 40.66%and 48.22%respectively compared with low TiAl doping CoCrNi（TiAl）_2.6alloy,but the plasticity decreased by 28.20%.The effects rules of hot forging,aging and surface sandblasting post-treatment on the microstructure and properties of medium entropy alloy of CoCrNi system in laser direct deposition were revealed.The typical conclusions are as follows:the hot forging post-treatment process can effectively refine the grain size of the alloy,which effectively increases the strengthening effect of the grain boundary,makes the dislocation slip difficult,the strength increases effectively,and the plasticity decreases greatly in theory,but benefit from the performance of the low stacking fault energy of the CoCrNi medium entropy alloy,there are a large number of annealing twins in the alloy after hot forging,which provides a new channel for dislocation slip.Therefore,the alloy has high strength and good plasticity at the same time.After hot forging,the yield strength of CoCrNi Mo_1.6medium entropy alloy is increased by 132.88%with only 17.94%loss of plasticity.The aging post-treatment process is mainly aimed at the CoCrNi-TiAl medium entropy alloy with precipitation characteristics,L1₂particle phase coherent with the matrix FCC phase is precipitated in the aging post-treatment process.When the alloy is deformed,the dislocations are not easy to gather around the L1₂precipitates,which can restrain the local stress concentration and effectively improve and stabilize the work-hardening ability of the alloy.With the increase of aging temperature,the precipitated particles at the grain boundary become larger,the strength increases,but the plasticity decreases.In addition,with the increase of the amount of TiAl doping,the strength after aging is increased but the plasticity decreases.The ultimate tensile strength of the CoCrNi（TiAl）₆medium entropy alloy after aging treatment reaches the order of 1.2GPa,and the plasticity is lost by 14.29%,but still has an elongation of 24±2%.The surface sandblasting post-treatment process can form a nano-gradient hardened layer of several hundred microns on the alloy surface,and effectively improve the uneven stress concentration on the alloy surface,delay the initiation and propagation of surface cracks,and maintain a high work-hardening ability,and effectively improve the strength of the alloy without affecting the plastic properties.Co MPared with the samples without surface sandblasting post-treatment of CoCrNi Mo_1.6medium entropy alloy.the yield strength of the surface sandblasting process samples increased by 37.45%,and the elongation increased by 9.4%.