Power law steady state creep in α/β titanium alloys

Abstract

Secondary creep behaviour for two α/β titanium alloys, IMI318 and IMI685, is presented for temperatures in the range 670 to 825 K. The relationship between secondary creep rate, $\dot \in _S$, and stress, σ, is shown to take the form $\dot \in _S$=Aσ ⁿ exp (-Q _c/RT with n, the stress exponent, and Q _C, the activation energy for creep, having values that are much larger than can be explained by conventional recovery creep models. The anomalies, however, can be explained if $\dot \in _S$ is related to the effective stress (σ−σ ₀), with σ ₀ an experimentally determined friction stress. The exponent now has a value of about 3.5 and the activation energy for creep at constant (σ−σ ₀) is close to what is expected for diffusion in the matrix. It is suggested, therefore, that a recovery model is appropriate to describe the rate controlling process. Finally, it is shown that if the effective stress is normalized by the proof or yield stress of the alloy then the results for IMI318 and IMI685 fall on a universal line with data previously determined for nickel- and iron-based alloys.