Improvement of mechanical properties in Ti alloys using heterogeneous elemental distribution

Most of practical titanium alloys contain some alloying elements such as Nb and Mo, which are prone to segregation. In this study, we have tried to change the distribution of these alloying elements in two types of titanium alloys through thermomechanical treatment. Ti-Al-Nb base inter-metallic alloy, Ti-25Al-14Nb-2Mo-1Fe (mol %), was hot-forged and bar-rolled at 1273K in the (B2+α₂) two phase region. After the rolling, Nb and/or Mo-rich region forms swirly band structure. Due to the segregation of these β stabilizing elements, we can obtain heterogeneous distribution of α₂ phase particles after the heat treatment at 1273K, Room temperature tensile elongation value was improved from nearly 0 to 6 %, which can be attributed to this heterogeneous distribution of α₂ phase particles. We also applied similar thermomechanical treatment on corrosion resistant β-type Ti-12Mo (mass %) alloys, and obtained Mo-rich swirly band-like structures. This led to heterogeneous precipitation of ω phase after aging at temperatures lower than 723K due to the local variation in β phase stability. Room temperature tensile elongation value after aging at 523K for 3. 6ks was improved from 4% to 19% with tensile strength of around 1000MPa.