Strength and torsion fracture mechanism of commercially pure titanium with ultrafine-grained structure

The mechanical behavior and the mechanism of torsional fracture of medical grade titanium Grade4 with an ultrafine-grained (UFG) structure in comparison with a coarse-grained (CG) structure were investigated. CG titanium (d(av) = 25 mu m) was investigated in the hot-rolled state. To obtain the UFG structure, two processing regimes were used. In the first regime, homogenization annealing was carried out at a temperature of 680 degrees C for 1 hour, then equal-channel angular pressing (6 passes) according to the "Conform" scheme (ECAP-C) at a workpiece and tool temperature of 250 degrees C (route Bc, phi=120 degrees). The average grain size is equal to 0.4 mu m. In the second regime, after homogenization annealing and six ECAP-C passes, the workpieces were drawn at a temperature of 200 degrees C (ECAP-C+D). The average grain size is equal to 0.2 mu m. The torsion test of samples with a diameter of 3 mm (close to the diameter of screws for fixing plates in traumatology) was carried out on a KTS 403-20-0.5 installation in accordance with GOST 3565-80. The torque, number of revolutions and angle of twisting of the samples made from CG and UFG titanium, as well as the mechanical properties of titanium during torsion were evaluated. Torsion tests have shown that the torsional strength and the torsional yield strength of UFG titanium increase, and the relative shear decreases in comparison with similar properties of CG titanium. In addition, the mode of nanostructuring (ECAP-C+D) provides higher values of the torsional strength properties of titanium compared to ECAP-C, which is favorable from the standpoint of resistance to fracture of titanium screws by torsion. On the surface of the fractures, there are three regions differing in microrelief: the central part of the fracture, the middle (transitional) and the peripheral parts. In the central part of the fractures, the microrelief consists of equiaxed dimples and structureless areas. In the middle part, shear dimples dominate, and in the peripheral part of fractures, areas of shear dimples alternate with areas of the rubbed surface.