ORIENTATION DEPENDENCE OF CYCLIC DEFORMATION IN HIGH-PURITY TITANIUM SINGLE-CRYSTALS

Cyclic deformation was studied in a series of high purity titanium single crystals with different orientations. The multiple step test method was employed in low cycle fatigue with a closed-loop electro-hydraulic servo-controlled Instron machine and cyclic stress-strain curves were obtained. The surface morphology of the fatigued specimens was observed with scanning electron microscopy, and dislocation structure was analysed with transmission electron microscopy. The results show that the cyclic responses and microstructures of titanium single crystals are sensitive to orientation. In the crystals oriented near the center of the (0001) stereographic projection, single slip and/or cross slip occur, planar dipole arrays and dislocation loops exist, and the cyclic strain hardening rate is low; whereas for crystals, in which orientations are favorable for double and multiple slip, cell structure is found and the cyclic strain hardening rate is high. Cyclic twins with different morphologies and fine structures are observed in almost all specimens, and the higher the content of twins, the higher is the strain hardening rate. Stacking faults are found in fatigued titanium especially in the region of twins in spite of the high stacking fault energy.