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

Cyclic deformation was studied in a series of high purity titanium sin gle crystals with different orientations. The multiple step test metho d was employed in low cycle fatigue with a closed-loop electro-hydraul ic servo-controlled Instron machine and cyclic stress-strain curves we re obtained. The surface morphology of the fatigued specimens was obse rved 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 a re 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 cycli c strain hardening rate is low; whereas for crystals, in which orienta tions are favorable for double and multiple slip, cell structure is fo und and the cyclic strain hardening rate is high. Cyclic twins with di fferent morphologies and fine structures are observed in almost all sp ecimens, and the higher the content of twins, the higher is the strain hardening rate. Stacking faults are found in fatigued titanium especi ally in the region of twins in spite of the high stacking fault energy .