STRAIN HARDENING BEHAVIOUR OF AN INITIALLY TEXTURED TI6AL4V TITANIUM ALLOY AS A FUNCTION OF STRAIN RATE AND COMPRESSION DIRECTION

This work shows some first results of a study aiming to determine the deformation mechanisms of a commercially available titanium alloy Ti6Al4V as a function of initial texture and strain rate. In particular this work will deal with the strain hardening behaviour of the material. Cylindrical specimens were fabricated from a textured extruded bar in mill annealed condition. One set of cylindrical samples ( LD) had its axis aligned to the axis of the rod, a second set ( TD) had its axis perpendicular to the rod axis and parallel to the majority of the c-axes of the alpha-phase. Both sets were tested under uni-axial compression loading for a wide range of strain rates ( 0.001/s up to 1000/s) using a servo-hydraulical testing machine ( lower strain rates) and a SHPB-setup ( higher strain rates). The LD-samples showed lower flow stresses than the TD-samples for the same amount of strain, as could be expected from the lower CRSS available slip systems. The LD-samples also supported much more strain before final fracture. The strain hardening of the LD-samples showed a clear tendency to increase significantly during deformation, this tendency being less pronounced for the TD-samples. This was clearly illustrated by the concave-up true stress - true strain curves. Friction effects could be ruled out as being the source of this behaviour and after the origin of this hardening behaviour could be attributed to extensive twinning as illustrated by optical microscopy, but to be confirmed by TEM analysis. Texture measurements ( OIM and XRD) before and after testing showed for both sets rotating of the c-axes of the alpha-phase towards the compression axis.