Microstructures,properties And Preparation Of CoCrFeNiTiNb_x-based Medium-/high-entropy Alloy Coatings On Pure Titanium Substrate By Laser Cladding

CoCrFeNiTiNb_x-based medium-/high-entropy alloy coatings with good metallurgical bonding were successfully prepared on surfaces of pure titanium sheets by pulsed laser cladding.Phase constitutions and microstructural characteristics of the coatings were characterized and analyzed by combined use of XRD,EBSD,ECC,EDS and second electron image techniques.In addition,the hardness and wear resistance of the coatings were studied,and the reasons for the changes in surface properties were analyzed in depth based on the microstructure characteristics.The main conclusions are as follows:(1)Affected by the ultrafast cooling rate during the pulsed laser cladding process,three zones(cladding zone,bonding zone and heat affected zone)with distinct microstructures are formed on the surface of the Co Ni Ti coating during the non-equilibrium solidification process.The cladding zone is comprised of dendritic(body centered cubic solid-solution phase,BCC phase)-interdendritic(Ti₂Ni type phase)structures with many nanoparticles(Ti₂Co type phase)dispersed inside the interdendritic structures.The bonding zone consists of fine acicular grains resulted from martensitic transformation while heat affected zone beneath the bonding zone is composed of irregular-shaped bulk grains.While fine cellular grains(average grain size of 0.91±0.67?m)are formed in the Cr Ni Ti coating which correspond to body centered cubic solid-solution phase.Inside most of the cellular grains,irregular-shaped Cr₂Ti Laves phase with C14-type appears while intergranular structures are determined to be Ni Ti intermetallics.Hardnesses tests reveal that the Co Ni Ti and Cr Ni Ti coatings have hardness of 571±46HV and 940±35 HV,which is?5 and?8 times that of the substrate(114±4 HV),respectively.Wear tests show that the specific wear rates of the Co Ni Ti and Cr Ni Ti coatings are calculated to be 1.7×10^-5 mm³·N^-1·m^-1 and 7.4×10^-5 mm³·N^-1·m^-1,far lower than that of the substrate(6.3×10^-4 mm³·N^-1·m^-1).Besides,the wear mechanism of Co Ni Ti coating is adhesive wear,accompanied by oxidative wear.The main wear mechanism of Cr Ni Ti coating is oxidative wear,accompanied by abrasive wear.Comprehensive analyses show that such remarkable surface performance of the coatings can be attributed to combined contributions from the solid-solution hardening and grain refinement hardening of the body centered cubic phase,as well as second phase hardening produced by Ti₂Ni and Ti₂Co type intermetallics,Cr₂Ti Laves phase and Ni Ti intermetallics.(2)Under the effect of high-entropy effect,both Co Cr Ni Ti and Cr Fe Ni Ti coatings consist of BCC phase.In addition,due to the rapid cooling rate,grains in the molten pool can not grow up in time,which refines the grains to a certain extent.Also,elements do not have enough time to diffuse,resulting in a certain degree of element segregation and the formation of second phase.Therefore,Cr₂Ti Laves phase(C14 type)is mainly formed in the intergranular structure.The hardness of Co Cr Ni Ti coating is estimated to be 762±32 HV,the wear rate is1.7×10^-5 mm³·N^-1·m^-1,while the hardness of Cr Fe Ni Ti coating is about 820±34 HV,the wear rate is 2.8×10^-5 mm³·N^-1·m^-1.The main wear mechanism of both the coatings is adhesive wear,accompanied by oxidative wear.Comprehensive analysis shows that the improvement of surface performance can be attributed to the joint effect of the solid solution strengthen,grain refinement strengthens of the body centered cubic phase,and the second phase strengthen produced by intermetallic compound Cr₂Ti Laves phase.(3)CoCrFeNiTi coating is consisted of BCC phase with equiaxed bulk grain morphology and Cr₂Ti Laves phase(C14-type hexagonal structure)with fine interdendritic lamellar morphology.After adding 1 at.%Nb,in addition to the body centered cubic solid-solution phase and the Cr₂Ti Laves phase,Cr₂Nb Laves phase with C15-type cubic structure also appears in the interdendritic region.The CoCrFeNiTi and CoCrFeNiTiNb₁ coatings have hardness values of?790±52 HV and?1008±56 HV,respectively,significantly higher than that of the pure titanium substrate.Besides,the specific wear rates of two coatings are only 1.0×10^-5 mm³·N^-1·m^-1 and 0.6×10^-5 mm³·N^-1·m^-1,respectively,which is much lower than that of the substrate.The main wear mechanism of CoCrFeNiTi and CoCrFeNiTiNb₁ coatings is oxidative wear.(4)The final results show that the six coatings are dominated by BCC phase,and various intermetallic compounds are formed.Besides,Ti element contributes to the formation of BCC phase,and Nb element contributes to the formation of fine lamellar structures.The CoCrFeNiTiNb₁ coating has the highest hardness,exceeding 1000 HV,and its wear rate is also the lowest,nearly two orders of magnitude lower than that of the pure Ti substrate.It can be seen that by selecting appropriate elements and supplementing with reasonable processing parameters,high-performance laser cladding high-entropy coating has been successfully prepared on the pure Ti surface.