Study On Columnar To Equiaxed Crystal Transformation And Properties Of Cocrfeni System High Entropy Alloy Prepared By Laser Deposition

Compared with traditional alloys,the multicomponent characteristics of high entropy alloys have brought new ideas for component design.The resulting high entropy effect,lattice distortion,slow diffusion,and "cocktail" effect also make high entropy alloys exhibit excellent properties such as high strength,toughness,high wear resistance,and high corrosion resistance,and have excellent application prospects in various extreme service environments.Compared to other preparation methods,laser deposition technology can effectively reduce the component segregation phenomenon during the solidification process of high entropy alloys.However,its extremely high temperature gradient and cooling rate also lead to the formation of coarse columnar crystals in the solidification structure of laser deposited high entropy alloys,limiting the application of laser deposited high entropy alloys.In this paper,CoCrFeNi based high entropy alloys were studied.CoCrFeNi based high entropy alloys were prepared by laser deposition process.The effects of alloying elements such as Al and Ti on the solidification behavior of laser deposited CoCrFeNi based high entropy alloys were studied.TiC was used as heterogeneous nucleation particles to explore the columnar equiaxed crystal transition behavior of laser deposited CoCrFeNi based high entropy alloys under the effect of component supercooling induced heterogeneous nucleation,The effects of grain morphology on tensile,friction,wear,and corrosion resistance of laser deposited high entropy alloys were analyzed,and the following main conclusions were obtained:(1)The laser deposited CoCrFeNi high entropy alloy has a single FCC solid solution phase structure,and its solidification structure is a coarse columnar crystal perpendicular to the outer edge of the substrate;When more than 10 at%Al or Ti is added,the phase of laser deposited CoCrFeNi-(Al,Ti)high entropy alloy changes from a single FCC to a FCC+BCC dual phase.With the increase of Al/Ti content,the average grain size of the solidified structure of laser deposited high entropy alloy decreases,and the proportion of equiaxed grains in the microstructure increases significantly;When the addition amount is the same,Ti can make laser deposition of CoCrFeNi based high entropy alloys produce a more significant reduction in nucleation undercooling and an increase in growth limiting factors,indicating that adding Ti alloy elements can produce a more significant component undercooling effect.(2)The average grain size of laser deposited CoCrFeNi is 234 μm.The columnar crystal ratio is 71.7%,and the average grain size of(CoCrFeNi)0.92Ti0.08 is 91 μm.The proportion of columnar crystals is 40.4%;The average grain size of laser deposited CoCrFeNi high entropy alloy with 5 wt%TiC added is 39 μm.The proportion of columnar crystals is 37.95%;For laser deposited(CoCrFeNi)0.92Ti0.08 high entropy alloys with Ti addition and 5 wt%TiC heteronuclear particles,the average grain size is the smallest,only 28 μm.The proportion of columnar crystals is the lowest(12.56%),and the microstructure is basically composed of equiaxed crystals;A good coupling effect can be generated between component undercooling elements and heterogeneous nucleation particles,thereby realizing the columnar to equiaxed crystal transition of laser deposited CoCrFeNi based high entropy alloys.(3)The effect of solidification structure morphology on the properties of laser deposited CoCrFeNi based high entropy alloys shows that with the refinement of the grains in the solidification structure,laser deposited high entropy alloys can achieve a better strength plasticity match.The tensile strength of laser deposited(CoCrFeNi)0.92Ti0.08 high entropy alloys with 5 wt%TiC added can reach 921 MPa,which is 110.8%higher than that of laser deposited CoCrFeNi based high entropy alloys,and the elongation(21.6%)of the former is only 32.7%lower than that of the latter;As the grain size of the solidified structure decreases,the friction and wear properties of laser deposited CoCrFeNi based high entropy alloys increase,and the wear mechanism changes from abrasive wear(laser deposited CoCrFeNi high entropy alloys)to fatigue wear(laser deposited(CoCrFeNi)0.92Ti0.08 high entropy alloys with 5 wt%TiC added);The electrochemical corrosion test results show that the addition of Ti and the refinement of grain size can effectively improve the corrosion resistance of laser deposited high entropy alloys.