The rate-dependent deformation and localization of fully dense and porous Ti-6Al-4V

The rate-dependent deformations of the titanium alloy Ti-6A1-4V are investigated using a combination of standard testing machines and compression and torsional Kolsky bars. Both homogeneous and localized deformations of both non-porous (‘fully dense’) and porous (7.6% porosity) versions of the alloy are studied. The microstructures of the two materials were made similar through heat-treatment, with the primary difference being the presence of the porosity. The fully dense Ti-6Al-4V (for this Widmänstatten microstructure) is found to show negligible strain hardening in compression (either quasistatic or dynamic), while the porous Ti-6Al-4V shows significant strain hardening in compression (both quasistatic and dynamic). The difference is believed to be because of the progressive compaction of the pores with increasing compressive strain. Both the fully dense and porous Ti-6Al-4V are observed to be rate-sensitive, with an increase in flow stress of ~ 30% over a six decade increase in strain rate. The strength-reduction due to porosity is ~ 20% for both quasistatic and dynamic strain rates. The constitutive response of the fully dense Ti-6Al-4V alloy in torsion is consistent with data in the literature, and again shows negligible strain hardening. The porous Ti-6Al-4V also shows negligible strain hardening in torsion, in contrast with the strong hardening evident in the compression data. This influence of the stress state on the behavior of the porous metal is due to the sensitivity of the pores to the mean (hydrostatic) stress. Specimens of the fully dense Ti-6Al-4V develop a deformation instability (during compression) in the form of a localized shearing failure in a plane at about 45° to the compression axis. The localized shearing failure in the fully dense metal consists of ‘deformed’ shear bands 3–10 μm in width which develop after a critical strain of 8–10%. In contrast, the porous Ti-6Al-4V specimens never fail in compression, showing homogeneous deformation even up to 30% strain. The relative stability of the porous metal is believed to be a result of the hardening mechanism afforded by the progressive compaction of the pores with increasing compressive strain. Shear bands are developed in both the fully dense and porous Ti-6Al-4V alloys after dynamic torsion. The critical shear strains to failure are ~ 14% for the fully dense metal and ~ 7% for the porous metal. Thus, the presence of the porosity destabilizes the deformation in shearing, but appears to stabilize the deformation in compression.