Using four kinds of α+β titanium alloys (Ti-Al-V-Sn-Zr-Mo-Cr-Fe system), the effects of test temperature and volume fraction of α phase on superplastic properties were investigated.
(1) Total elongation of each alloy was more than 400% in the temperature range between 1023 K and 1123 K at the strain rate of 6.7×10-4 s-1. Maximum flow stress increased with decreasing temperature rapidly in the temperature range lower than 1073 K. From these results, the temperature range between 1073 K and 1123 K was considered to be suitable for superplastic forming of these alloys at the strain rate of 6.7×10-4 s-1.
(2) The effects of test temperature and volume fraction of α phase on the strain rate sensitivity, m, were examined by multiple regression analysis. Following regression equation was obtained.
m=-17.128(Θ-1.095)2-0.514(Vα-0.426)2+0.422
Θ : Test temperature K×10-3, Vα : Volume fraction of α phase
From this equation, titanium alloys whose volume fraction of α phase is 0.426 at the temperature of 1095 K are considered to give the maximum m value and the best superplastic properties at a strain rate of 6.7×10-4 s-1.
(3) α phase was formed at the surface of specimen due to the reaction with oxygen during high temperature tensile test in an argon atmosphere. Such α phase was expected to be detrimental for superplastic forming and to reduce ductility of the material.
