Experimental study on pure titanium subjected to different combined tension and torsion deformation processes

The results of the mechanical properties, microstructure and fracture analysis of the pure titanium deformed by combined tension and torsion simultaneously (TT1), combined torsion-tension successively (TT2) and combined tension-torsion successively (TT3) are investigated by micro-indentation (MI), optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). The MI test indicates the improvement of micro-indentation hardness (H) for TT1 is the most obvious of all studied processes and the increment of H depends on the increase of tensile stress. But the H shows a different variation trend with different proportions of tensile stress during TT2 and TT3. OM observation shows the grain elongation degree on longitude section can be weakened with the increase of tensile stress during TT1. However, the grain morphology on longitude section is mainly controlled by the torsion deformation during TT2 and TT3. The results of SEM indicate tensile stress plays a different role in the process of fracture during TT1, TT2 and TT3. Different types of dimples with different sizes and depths are observed in TT1, TT2 and TT3. TEM observation shows that different dislocation structures gather in the surrounding of the grain-boundary trijunctions in TT1, TT2 and TT3. Finally, the effects of different dislocation movements caused by TT1, TT2 and TT3 on microstructure evolution are discussed.