Microstructure evolution and strengthening mechanisms in commercial-purity titanium subjected to equal-channel angular pressing

High-resolution electron backscatter diffraction (EBSD) was applied to examine grain refinement in commercial-purity titanium Grade 4 subjected to equal-channel angular pressing (ECAP) via the Conform technique. This approach enables the production of long-length billets and thus has the potential for commercial application. Microstructure evolution was found to be a relatively-complex process which included several stages. At relatively-low accumulated strains, microstructure changes were markedly influenced by mechanical twinning. However, the concomitant grain refinement suppressed this mechanism, and subsequent microstructure development was dictated by the evolution of deformation-induced boundaries which developed preferentially near the original grain boundaries. The final material produced after an effective strain of ~ 8.4 was characterized by a mean grain size of 0.3µm, high-angle boundary fraction of 55 pct., a texture of moderate strength, and a yield strength of ~ 1050MPa. Based on the detailed microstructural analysis, the contributions of various strengthening mechanisms were quantified. The rapid material strengthening during the early stages of ECAP was explained in the terms of a major increase in dislocation density and the extensive formation of the deformation-induced boundaries. With further increments in accumulated strain, however, the dislocation as well as grain-boundary density reached a saturation, thus reducing the hardening efficiency of ECAP at high strains.