Factors determining room temperature mechanical properties of bimodal microstructures in Ti-6Al-4V alloy

The bimodal microstructure in Ti-6Al-4V alloy comprised of equiaxed primary α grains and transformed β areas can be regarded as a kind of “dual-phase” structure. In this study, the volume fraction of primary α grains was systematically modified by changing the intercritical annealing temperature in the α + β two phase region and the tensile properties were evaluated at room temperature. It was found for the first time that the relative nano-hardness of the two components changed depending on their volume fractions which were determined by the intercritical annealing temperature. The change in the relative hardness was explained from a viewpoint of solid solution strengthening of α phase by aluminum. Moreover, the change in the relative nano-hardness was found to have a significant influence on mechanical properties of the bimodal microstructures through affecting the strain distribution behavior. A peak uniform elongation (~ 12%) was obtained at an intermediate annealing temperature (910 °C) where the volume fractions of two components were nearly equal and thereby the length density of the interface between them reached a maximum.