Nanoindentation was used to probe the local slip resistance in CP-Ti deformed in compression to different extents. Changes in hardness in the deformed grains and twins were compared with the change in flow stress measured during deformation, with the aim to elucidate the relative contribution of slip and twinning to the work hardening of Ti alloys. The hardness values were calibrated with measurements on binary Ti-Al alloys. The hardness increased only slightly with deformation and cannot explain the observed work hardening. Although twinned regions were found to be harder than the parent grains, this increase was found to be small once the effect of crystal orientation was accounted for. The increase in hardness in the twins was slightly higher for compressive twins than for tensile twins. It is proposed that this modest hardness increase in the twins is more consistent with the presence of twinning stresses than with a change in the local flow stress caused by dislocation interactions. The implications of these findings to the work hardening of CP titanium are discussed.

SummaryBilateral midshaft femoral osteotomies were stabilized with a 3 mm fracture gap in 12 dogs using titanium bone plates and either smooth surfaced (SS) or porous surfaced (PS) titanium screws. Mechanical studies demonstrated that PS screws required a significantly greater torque to remove than the SS screws at both 8 and 16 weeks. At 16 weeks, screw removal torque increased by a factor of 1.3 over the insertion torque for PS screws and decreased by a factor of 2.9 for SS screws. Histologically, the amount of new bone formation and porosity, over the entire length of the femur, was significantly greater in the femora stabilized with PS screws; indicative of greater overall fixation rigidity. At both time periods, there was an increase in torsional stiffness in the osteotomies stabilized with SS screws. Radiographically, there was no difference in the osteotomy callus area for either screw type. From this study, there appears to be advantages to using PS bone screws instead of SS screws when subjected to dynamic loading such as gap fixation.