Mechanical Behavior of Ultrafine-Grained Ti-6Al-4V Alloy Produced by Severe Warm Rolling: The Influence of Starting Microstructure and Reduction Ratio

To provide insight into the mechanical behavior and microstructural evolution of bulk ultrafine-grained (UFG) Ti-6Al-4V alloys, we produced Ti-6Al-4V alloy sheets with grain size smaller than 300 nm through severe warm rolling of three different starting microstructures (i.e., lamellar, equiaxed, and hybrid, that is half equiaxed plus half lamellar microstructures) with various reduction ratios (i.e., 60, 70, 80, and 90 pct) at 873 K (600 °C). Accordingly, the tensile behavior, microhardness, grain size, and dislocation density of the UFG Ti-6Al-4V alloys with different starting microstructures and reduction ratios were comparatively analyzed. Our results show that, following the continuous enhancement of tensile strength and hardness as the rolling reduction ratio increased from 0 to 70 pct, there was a saturation state in which the values of strength and hardness remained constant as the reduction ratio further increased from 70 to 90 pct for all the alloy samples with different starting microstructures. In terms of microstructural evolution, although grain size decreased and dislocation density increased continuously as the rolling reduction ratio increased from 0 to 70 pct, grain size and dislocation density did not change significantly when the reduction ratio further increased from 70 to 90 pct. Our results suggest that, whereas the starting microstructure influences the early stages of grain refinement and mechanical performance, this influence diminishes as the rolling reduction ratio is increased beyond a critical value. This behavior was rationalized on the basis of the limits of grain boundary and dislocation strengthening during severe warm rolling.