Hot Deformation Behaviors And Heat-treatment Technology Of Biomedical Ti-6al-7nb Alloy

Ti-6Al-7Nb alloys have been widely used in surgical prostheses due to their elastic modulus similar to human bone, excellent biocompatibility and synthetical mechanical properties,. Surgical implants must be undergone hot-forming and heat treatment before finally application, and the processing parameters have great effects on the ability of plastic deformation and microstructure of the materials. The present paper aims to optimize hot work technology and heat treatment technology in order to make the alloy have better capability of hot forming and enhance its mechanical properties.Hot compression tests were carried out in the strain rate range 0.001 s-1～10s-1 and the temperature range 750℃～900℃. The deformed microstructures and heat-treated microstructures were observed by OM and TEM. By calculatation of apparent deformation activation energy the deformation mechanisms in hot compression process were analyzed. The results show that the true stress—true strain curves behave the flow softening after undergoing primary work-hardening stage. Softening mechanism is attributed to the dynamic recrystallizations of a phase at low strain rate 0.001-0.1s-1 and adiabatical effect at high strain rate 0.001-0.1s-1. The apparent activation energy at 750℃～850℃calculated based on Arrhenius Equation is about 200kJ/mol which is close to self diffusion activation energy ofα-Ti and suggests that the dynamic recrystallizations ofαphase by Ti self diffusion is the rate-controlling step. The apparent activation energy at 900℃is 130 kJ/mol, less than self diffusion activation energy inβ-Ti. This indicates that the deformation is controlled by dynamic recovery inβphase. Superplastic deformation occurs in the condition of temperature 750℃and the strain rate slower than 0.001s-1. Considering workability and microstructural control, the optimum hot deformation conditions are determined in the temperature range 750℃-900℃and strain rate range 0.01 s-1-0.1s-1.The wear-resistance property of Ti-6Al-7Nb alloys are relatively insufficient as surgical high-stress implants, the present paper is to find appropriate method of increasing hardness by different heat treatment including conventional annealing, dual annealing and solution-aging. Dual annealing can effectively increase the hardness of Ti-6Al-7Nb alloy. Considering cost and mechanical properties, a dual annealing processing is 700℃/lh/AC+500℃/4h. The hardness of Ti-6Al-7Nb alloy increases with increasing temperature of solution and aging. The highest hardness reaches 330HV at the condition of 950℃solution+600℃aging/6h.