Design of near α-Ti alloys using δ-parameter

In this study, to obtain a high solid-solution strengthening effect, new Ti alloys with bimodal structures were designed using the δ-parameter, which represents the average atomic radius misfit of the constituent elements used in high-entropy alloys. The δ-parameter calculated by the alloy composition was used to estimate the average solid-solution strengthening of the alloys. In particular, to clarify the effect of Zr and Sn in Ti alloys, Ti alloys were designed with different amounts of Zr and/or Sn using the δ-parameter. The compressive strength and creep behavior of the designed alloys were investigated to determine whether the δ-parameter is useful for designing strengthened alloys. The compressive strength of the designed alloys was comparable to or higher than that of the commercial TIMETAL834 alloy up to 650 °C. This indicates that the δ-parameter is a useful parameter for estimating the average solid-solution strengthening of the alloys. The compressive strengths of the designed alloys were approximately proportional to the 4/3 power of the δ−parameter. An increase in the strength by precipitation strengthening by α₂-Ti₃Al was also observed in the Sn-added alloys, whereas Ti₃Al was not formed in the Sn-free alloys. Microstructural factors, such as the volume fraction of the equiaxed α phase and lamellar spacing in the bimodal microstructure, significantly contributed to the minimum creep strain rate, in addition to solid-solution and precipitation strengthening.