Microstructural Evolution and Bio-Corrosion Behavior of a CP-Ti Processed by Multi-Pass of Severe Plastic Deformation

The microstructure evaluation and phase transformation of multi-pass friction stir-processed (FSP) commercially pure titanium were investigated using optical microscopy, scanning electron microscopy equipped with a backscatter detector, and electron backscatter diffractometer. The microstructure characterization shows that the grain refinement mechanism changed with the increasing number of passes. The equiaxed alpha grains in the untreated sample changed to lath-shaped grains surrounded by serrated grain boundaries, and produced Widmanstatten morphology during the cooling cycle of FSP. The higher micro-hardness values and bio-corrosion resistances of the FSP-treated samples are due to their lower grain size compared to the untreated samples. Furthermore, the presence of lath-shaped Widmanstatten morphology shows a higher micro-hardness value and lower bio-corrosion resistance.