Study On Surface Alloying And Surface Strengthening Mechanism Of Titanium/nickel Based Materials

Surface alloying of titanium alloy and pure nickel was studied by high current pulsed beam irradiation(HCPEB).The surface alloying was characterized by X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and three-dimensional laser scanning microscopy(3D LSM).The microstructures of the surface layer after irradiation alloying were characterized.The microhardness,friction and wear properties and electrochemical corrosion properties of the alloyed samples were tested.By establishing the internal relationship between the microstructures and properties,the surface alloying mechanism and modification mechanism were analyzed and summarized.In order to improve the surface hardness,friction,wear and corrosion resistance of TC4 titanium alloy,the surface of TC4 titanium alloy was Cr alloyed by HCPEB technology.The surface of the sample irradiated by HCPEB melted and formed a large number of eruptive pits.Its density and size decreased with the increase of alloying times.A large number of alloying elements melt into the matrix to form alloying layer.The thickness of alloying layer increases with the increase of alloying times.There are beta-Ti equiaxed grains and a'-Ti martensite in the alloy layer of Cr-TC4 with the size of 2-5?m.There are also a large number of Cr₂Ti nanoparticles in the alloy layer.The results show that the surface hardness of the irradiated samples is significantly enhanced,which is due to the formation of nanocrystalline Cr₂Ti(Laves)phase and nanocrystalline structure induced by the surface layer of the irradiated samples.The surface friction and wear properties of the irradiated samples are significantly improved,which is related to the surface roughness of the samples.The decrease of the surface hardness and the increase of the thickness of the alloy layer are related to the increase of the surface hardness.At the same time,the corrosion resistance of the alloy layer is also improved obviously,which is due to the formation of continuous and dense Cr₂O₃ oxide film on the surface of the Cr-TC4 alloyed sample.In order to study the alloying of some insoluble metals as beta phase stabilizers,Mo and Nb were selected as alloying elements.After irradiation with HCPEB,the surface grains of Mo-TC4 alloying system were refined obviously,and a large number of nanocrystalline structures were formed in some areas.Lamellar martensite was also found,twins and high-density dislocations were found in martensite.The thickness of Mo alloy layer formed after irradiation is about 3.5?m.A large number of dispersed Al₅Mo nanoparticles are formed in the alloy layer,which plays a role of dispersion strengthening for the improvement of surface hardness.In order to increase the content of alloying elements in the alloy layer,a metallurgical Nb-rich alloy layer was formed on the surface of TC4 alloy by mechanical alloying(MA)combined with HCPEB irradiation technology.The alloy layer was continuous and compact,and the surface properties were improved.Surface alloying of pure nickel as matrix and Cr,W and Sn as alloying elements was studied by HCPEB technology.Among them,after HCPEB alloying the samples of Cr/W-Ni system,a large number of eruptive craters formed on the surface of the samples,and the number of craters decreased with the increase of irradiation times.During HCPEB irradiation,a large number of alloying atoms melt into the matrix to form alloy layer.In addition,a variety of crystal defect structures,such as grain boundaries,dislocations,dislocation cells/walls,twins and so on,are induced in the surface layer,which provide rapid diffusion channels for alloying elements.The formation of supersaturated solid solution after HCPEB irradiation results in the precipitation of fine particles,and the second phase particles tend to distribute near grain boundaries and crystal defects.The phase structure of the alloy layer in the Cr-Ni system is composed of Ni(Cr)solid solution and intermetallic compound Cr₃Ni₂,while the alloy layer in the W-Ni system samples forms Ni(W)solid solution,GP region,Moire fringe particles and Ni W phase.The results of performance test after HCPEB alloying show that the surface hardness of the alloyed samples has been significantly improved,which is mainly attributed to the combined effect of stress strengthening,sub-crystal strengthening,dispersion strengthening and solution strengthening of the rich micro-configuration in the alloy layer.Proper alloying process can significantly improve the surface strength and corrosion resistance of the samples.In addition,Sn alloying was applied to the surface of Ni matrix,and only the phase structure,hardness and surface morphology were characterized and measured.The results show that HCPEB irradiation is also suitable for surface alloying modification of low melting point elements,and the alloy layer can be significantly strengthened.The solid solution strengthening of three alloying systems of Cr,W and Sn on Ni surface was simulated by first-principles calculation.The results show that the atomic radius of alloying elements will affect the lattice constant of Ni-based solid solution in the alloy layer,and then lead to the solid solution strengthening of the matrix,which is characterized by the increase of hardness.The simulation results show that the larger the atomic radius of alloying elements is,the more significant the strengthening effect is,which is consistent with the experimental data.