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Cold Swaging and Recrystallization Annealing of Ti-Nb-Ta-Zr-O Alloy - Microstructure, Texture and Microhardness Evolution

Metastable β-Ti alloy Ti-35.3Nb-7.3Zr-5.7Ta-0.7O (wt. %) shows properties desirable for use as an implant material. However, very large grains (with the size of 0.5 – 3 mm) negatively affect the strength and fatigue resistance. Combination of cold-swaging and recrystallization annealing is used to refine the microstructure. Microstructure after cold swaging and after annealing is studied using scanning electron microscope by electron back-scatter diffraction measurements. Grain size and texture is determined and homogeneity of deformation inside the grains is discussed. Microhardness maps are measured in the cold-swaged samples. It is found that microhardness and thus also the deformation is higher in the center of each rod however the grains retain their original size. Annealing leads to recrystallization that is complete or nearly complete at the temperatures of 1000°C and more. Annealing at 1000°C for 15 min yields grain size of around 100 μm. This grain size is believed to increase the fatigue performance to satisfactory values.