Compared with other kinds of metallic materials for biomedical applications, Ti-24Nb-4Zr-8Sn (abbreviated as Ti2448) alloy possesses much low elastic modulus, high strength, excellent corrosion resistance and biocompatibility. The mechanical properties of Ti2448 alloy are sensitive to both Sn and Zr contents originated by their significant suppression contribution on martensitic transformation. This thesis focuses on investigating the effect of martensitic transformation on microstructual evolution and mechanical properties of the Ti2448 with low Sn content, in hope of achieving the balance of low Young’s modulus and high strength by heat treatment.
Microstrructre analysis by TEM showed that the cold-rolling results in forming nanocrystallines with increase of deformation ratio. Tensile test at room temperature revealed much weak strengthening during cold-rolling as compared with conventional metallic materials. Both tensile and dynamical Young’s moduli decrease with the increase of cold-rolling. Texture analysis showed that the cold-rolling favors both {112}<110> and {001}<110> texture of the parent phase with body-centered cubic crystal structure as well as both {100}<001> and {110}<001> texture of the ″ martensite with orthorhombic crystal structure along the rolling direction. Since Young’s modulus along <110> direction is much lower than that along <111> direction of the BCC crystal, the cold-rolled sheets have lower Young’s modulus along the rolling direction.
After annealing treatment in single β phase field, the texture of the phase changes to {112}<110>, {011}<112> and {001}<110>. Due to recrystallization of nanocrystallines, the Young’s modulus is higher than that of cold-rolled alloy. Due to the favour contribution of grain growth on matensitic transformation, the stress- induced phase transformation during tensile test damages seriously the yield strength. After annealing at temperatures ranged from 600 to 650°C, Young’s modulus is higher than that of cold-rolled alloy due to the precipitation of phase. Since the softening effect of grain growth prevails over the strengthening effect of precipitation, the yield strength is still lower than that of cold-rolled alloys but the ductility increases significantly. Tensile test also showed that, with the increase of cooling rate after high temperature annealing, the alloy has better ductility and lower strength and Young’s modulus.
Ageing at a temperature below 500°C after high temperature annealing leads to increase both strength and Young’s modulus but decrease the ductility due to the precipitation of phase during ageing treatment. Ageing at 450°C for 2-4 hours is an optimized treatment to reach good balance of low Young’s modulus and high strength. Under identical heat treatments, the radial direction of cold-rolled sheets has higher Young’s modulus than that of rolling direction. After recrystallization annealing plus aging, the smaller deformation ratio leads to higher strength but higher Young’s modulus and lower elongation.
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