Enhanced superplasticity and strength in modified Ti-6AI-4V alloys

Abstract

Although Ti-6A1-4V displays extensive superplasticity at 1200 K, lower superplastic forming temperatures are desirable. A study was conducted with the goal of modifying the composition of the Ti-6A1-4V alloy to lower the optimum superplastic forming temperature. Computer modeling results and previous experimental data suggested that additions to Ti-6A1-4V of beta-stabilizing elements which have high diffusivity in the beta-phase would permit lower superplastic forming temperatures. A series of modified alloys with 2 wt pct additions of Fe, Co, and Ni was prepared for experimental evaluation. The modified alloys achieved desirable microstructures for superplasticity at 1088 K,i.e., the grain size was approximately 5 µm and roughly equal volume fractions of the alpha- and beta-phases were present at the deformation temperature. The superplastic properties of the modified alloys were measured at 1088 K and 1144 K. The modified alloys produced values of flow stress, strain rate sensitivity, and total elongation at 1088 K approaching those of the base Ti-6A1-4V alloy at its standard superplastic forming temperature of 1200 K. In addition to lowering the superplastic forming temperature, the β-stabilizing additions also increased room temperature strength levels above those normally found for Ti-6A1-4V. Based on the room temperature and elevated temperature tensile properties, addition of selected beta-stabilizing elements to Ti-6A1-4V simultaneously raises resistance to deformation at room temperature and lowers resistance to deformation at elevated temperatures. This reversal in behavior is explained by considering the effect of beta-stabilizer additions on the deformation mechanisms at room temperature and at elevated temperatures.