Optimizating Grain Boundary Character And Properties Of Biomedical Co-Cr Alloy Fabricated By Selective Laser Melting

Co-Cr alloys have been widely used in biomedical implants and prostheses such as hip implants,oral prostheses,and heart stents,due to its high corrosion resistance,biocompatibility,and mechanical properties.Selective laser melting(SLM)technology has the characteristics of individualization and precision,and has gradually replaced the preparation methods of casting and computer numerical control machining(CNC).SLM has become the main processing method of biomedical Co-Cr alloy implants and restorations.However,due to the large temperature gradient and fast solidification rate during the SLM shaping process,the As-SLM Co-Cr alloy has prominent residual strain and microstructure anisotropy,which damages the dimensional accuracy and service performance.In addition,the biomedical Co-Cr alloy is affected by the corrosive environment and complex forces when useed in the human body,which may cause corrosion or even fracture,and finally resulting in treatment failure.Therefore,in order to ensure the stability and safety of SLM fabricated Co-Cr alloy at long-term service,it must have excellent tensile properties,fatigue properties,corrosion resistance,and stress corrosion cracking resistance.Generally,the Co-Cr alloy fabricated by SLM is a polycrystalline material,and its properties are closely related to the grain boundary character distribution.Grain boundary engineering(GBE)use theromechical process to increase the fraction of special boundaries(Σ3~n,n=1,2,3)in the material and optimize its distribution characteristics.The GBE treatment can improve the mechanical and chemical properties of the material,such as stress corrosion cracking,creep,electrical conductivity,fatigue performance et.al.By optimizing the distribution of grain boundary characteristics of the material,its physical and chemical properties can be greatly improved without changing its chemical composition.In this work,we used heat treatment to optimize the grain boundary characteristics of As-SLM Co-Cr alloy by utilizing the academic concept and research results of GBE,in order to improve the physical and chemical properties of SLM fabricated Co-Cr alloy.Firstly,we study the microstructure and forming mechanism of As-SLM Co-Cr alloy.Then,we optimize the grain boundary characte distribution of As-SLM Co-Cr alloy by heat treatment to obtain the heat-treated SLM Co-Cr alloy with grain boundary engineered structures.In this work,the microstructure and grain boundary character evolution of As-SLM Co-Cr alloy during heat treatment are investigated in detail,which provides a theoretical guide line for the further application of AM-compatible GBE strategies and fabrication of high-performance of biomedical Co-Cr alloy.Finally,we investigated the tensile properties,fatigue properties,corrosion resistance and stress corrosion cracking behavior of GBE-60 Co-Cr alloy,in order to reveal the influence of microstructure and grain boundary character on physicochemical properties.To further reveal the influence of microstructure on physicochemical properties and verify the superiority of SLM shaping process,we investigate the mechanical and chemical properties of Cast Co-Cr alloy under the same experimental conditions.The main research results of this paper are as follows:1.Due to the high temperature gradient and fast solidification rate in the SLM shaping process,the As-SLM Co-Cr alloy exhibits a cellular grain growth mode,and has prominent residual strain.The As-SLM Co-Cr alloy contains a large number of stacking faults and subgrains(size range of 300-450 nm)with element of W,Mo,and Si segregation.The subgrain boundaries hinder the slip of dislocations,which improve the high yield strength(800 MPa)of As-SLM Co-Cr alloy.In addition,owing to the point-by-point,line-by-line,and layer-by-layer shaping methods for the SLM technology,the As-SLM Co-Cr alloys have microstructure anisotropy.2.In this paper,based on As-SLM Co-Cr alloy,by introducing the phase transformation process during heat treatment to make it undergo complete recrystallization,we obtained the grain boundary engineered Co-Cr alloy(GBE-60 Co-Cr alloy)with high fraction of special boundaries and optimized grain boundary character distributions.The GBE-60 Co-Cr alloy have the fraction of 85.78%special boundaries(among the fraction of twin boundary(Σ3)is 79.5%),which successfully interrupts the connectivity of random boundaries,and form large grain clusters.The high density of stacking faults in As-SLM Co-Cr alloy generated the complex-shaped twin boundaries in GBE-60 Co-Cr alloy by growing,sliding and extending the stacking faults during recrystallization.3.After optimizing the grain boundary character distributions and introducing a large number of twin boundaries,the obtained GBE-60 Co-Cr alloy has high static tensile properties and dynamic fatigue properties.The tensile and fatigue properties of GBE-60 Co-Cr alloy are superior to Cast Co-Cr alloy with the same chemical composition.The large fraction of twin boundaries(up to 79.5%)in GBE-60 Co-Cr alloy can simultaneously improve the tensile and fatigue properties by grain refinement and crack deflection.In contrast,the large sized second phase in Cast Co-Cr alloy strongly hinder the sliping of dislocations and causes stress concentration,reducing the tensile and fatigue properties.4.Because the second-phase particles with small size and uniform distribution in GBE-60 Co-Cr alloy cannot make up effective corrosion galvanic cell with the metal matrix,its corrosion resistance is better than that of Cast Co-Cr alloy.On the contary,the coarse island-like second phases in Cast Co-Cr alloy casue severe composition segregation.Meanwhile,the large sized precipitates and metal matrix can make up an effective corrosion galvanic cell,resulting in pitting corrosion and decreasing the corrosion resistance.The high frequency of special boundaries and stress-induced martensitic transformation during deformation resulted in high stress corrosion cracking resistance of GBE-60 Co-Cr alloy in saline solution.The high fraction of twin boundaries in GBE-60 Co-Cr alloy resulted in crack deflection and crack bridge,which shield the stress in the crack tips and reduce the driving force of crack propagation.At the same time,the stress-induced martensitic transformation will constrain the surrounding unfractured materials and cracks,further improving the resistance for local necking and crack propagation.