Effect Of Heat Treatment On Microstructure And Properties Of Medical Ti-35Nb-7Zr-10Cu Alloy

The Ti-35Nb-7Zr-10Cu alloy has achieved functionalization of antibacterial properties and long-term durability,and has obtained good mechanical properties.However,the elastic modulus value of the alloy is still higher than that of human bones（0-30 GPa）,which needs to be addressed through appropriate heat treatment.In this study,the Ti-Nb-Zr-based alloy was selected as the research object to investigate the effects of heat treatment on the microstructure,mechanical properties,corrosion resistance,friction and wear properties,and biocompatibility of medical Ti-35Nb-7Zr-10Cu alloy.The study also attempted to obtain the precipitation mechanism of the copper-containing phase under different heat treatment processes and establish the correlation between the Cu-containing phase and its properties.The research results were presented in accordance with academic paper standards.Theβtransformation temperature（T_β）of the as-cast Ti-35Nb-7Zr-10Cu alloy prepared by vacuum were melting furnace was determined to be approximately 790℃using d-electron theory formula and DSC.Combined with Ti-Cu and Cu-Zr phase diagrams,the annealing temperature was controlled above T_βwithin the range of 800-1000℃to control the precipitation structure in the alloy.Different alloy structures were obtained by cooling the annealed samples using water cooling（WC）,air cooling（AC）,and furnace cooling（FC）.The research results were presented in accordance with academic paper standards.The phase analysis and morphology observation of Ti-35Nb-7Zr-10Cu alloy after heat treatment showed that the alloy was mainly composed ofβ-Ti because of the high content ofβ-stable elements and the temperature of heat treatment was kept above T_β.At 800℃to 900℃,the Cu-containing phase was mainly Ti_xCu_y phase andα"martensite phase appears.In addition to Ti_xCu_y phase,Cu_xZr_y phase appears at 950℃and 1000℃.With the increase of heat treatment temperature,the main copper containing phase Ti_xCu_y decreased,while Cu_xZr_yincreased.At 950℃and 1000℃,the copper containing phase at the grain boundary ofβ-Ti phase produced heterogeneous layered structure（heterogeneous laminated structure,HLS）.With the decrease of cooling rate,the Ti_xCu_y phase increaseed gradually.The copper-bearing phase was mainly distributed in the grain boundary ofβ-Ti phase,and with the increase of heat treatment temperature,the copper-bearing phase at the grain boundary ofβ-Ti phase converges and finally forms spherical copper-bearing phase.With the increase of heat treatment temperature,the form of copper-bearing phase will gradually change from lamellar to spherical.With the decrease of cooling rate,the eutectoid reaction has becomed more full,a large number of eutectoid structures appear in the alloy,and the copper phase at the grain boundary of theβ-Ti phase decreases,and some copper phase precipitates in theβgrain of the matrix.The mechanical properties of Ti-35Nb-7Zr-10Cu alloy after heat treatment show that the alloy exhibits superelasticity after solution treatment at 800℃,850℃and 900℃.At 950℃,the yield platform decreaseed and the yield strength increaseed to 1319 MPa.The HLS structure of the alloy at 1000℃was the reason for the excellent compressive deformation rate of the alloy.With the increase of heat treatment temperature,the elastic modulus of the alloy decreaseed from69 GPa to 47 GPa and the hardness increaseed from 352 to 503 HV.The yield strength of the alloy increased from 916 MPa to 1232 MPa with the cooling rate decreasing.The elastic modulus increased from 47 GPa to 73 GPa and the hardness decreased from 503 HV to 294 HV.The alloy has higher yield strength and hardness and lower elastic modulus under water cooling at 1000℃.Experimental results of electrochemical corrosion of Ti-35Nb-7Zr-10Cu alloy after heat treatment show that:With the increased of heat treatment temperature,the self-corrosion current density of the alloy decreased gradually,from 1.214×10^-7 A/cm² to 1.080×10^-7A/cm²,and the alloy at 1000℃has the lowest value(1.214×10^-7 A/cm²).At this time,the impedance value of the alloy is 91.24×104Ω/cm²,and the corrosion resistance of the alloy increases with the increased of heat treatment temperature.With the decreased of the cooling rate,the self-corrosion current density of the alloy decreases from 1.214×10^-7 A/cm² to 6,5791×10^-7 A/cm²,and the passivated film impedance decreases from 91.24×104Ω/cm² to 5.02×104Ω/cm².The alloy has the best corrosion resistance under WC conditions.The alloy had good corrosion resistance under water cooling at 1000℃.Experimental results of friction and wear of Ti-35Nb-7Zr-10Cu alloy after heat treatment show that:At 800-950℃,the friction coefficient of the alloy didn’t show a great change,the value of COF is about 0.54,but when the temperature rises to 1000℃,the friction coefficient of the alloy decreases to 0.4466.With the increased of heat treatment temperature,the wear width and depth of the alloy gradually decrease,and the wear resistance gradually increases.The alloy at 1000℃had the lowest wear rate（0.82‰）.With the decreased of the cooling rate,the friction coefficient of the alloy increases from 0.4466 to 0.5620,the wear width and wear depth of the alloy gradually increase,and the wear resistance decreases.Under WC condition,the alloy had the lowest wear rate（0.82‰）.The main wear mechanism of the alloy is adhesive wear,fatigue wear and abrasive wear.The alloy had good wear resistance at 1000℃and WC.The antibacterial activity of Ti-35Nb-7Zr-10Cu alloy after heat treatment showed that the antibacterial rate of Ti-35Nb-7Zr-10Cu alloy against Escherichia coli decreased from 70.6%to56.9%and from 48.5%to 41.4%with the increase of heat treatment temperature.The antibacterial rate of the alloy against Escherichia coli increased from 56.9%to 91.7%,and that of Staphylococcus aureus increased from 48.5%to 59.6%with decreasing cooling rate.The main mechanism of antibacterial activity is the change of microcrystalline morphology and eutectoid products.Ti_xCu_y phase content was correlated.With the increase of heat treatment temperature and cooling rate,the content of copper phase of the alloy decreases,showed the antibacterial property decreases.In summary,Ti-35Nb-7Zr-10Cu alloy,solid solution treatment at 1000℃and heat treatment at water cooling condition can make the alloy have higher yield strength and hardness,lower elastic modulus,better corrosion resistance and wear resistance,but it has certain effect on the inhibition of Escherichia coli and Staphylococcus aureus.This study lied a theoretical foundation for the regulation of Cu-containing phase in Ti-Nb-Zr-Cu alloys.