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 alpha grains and transformed beta areas can be regarded as a kind of "dual-phase" structure. In this study, the volume fraction of primary a grains was systematically modified by changing the intercritical annealing temperature in the alpha + beta 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 alpha 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 (similar to 12%) was obtained at an intermediate annealing temperature (910 degrees C) where the volume fractions of two components were nearly equal and thereby the length density of the interface between them reached a maximum.