Grain size effect on mechanical behavior of thin pure titanium foils at elevated temperatures

Abstract

To contribute to process design of microforming at elevated temperatures, the effect of grain size on mechanical behavior of pure titanium foils was studied. Uniaxial tensile tests and microbending tests were performed at different temperatures of 298, 433, 573, and 723 K and at different strain rates of 10−3, 10−2, and 10−1 s−1. Pure titanium (Ti) foils with various grain sizes of 2.7–42.4 µm were used. As results, flow stress of Ti foils with a larger grain size showed stronger dependency on the temperature and strain rate conditions. In particular, both the reduction rate of the yield stress and that of the ultimate tensile strength relative to that at 298 K increased with increasing grain size at higher strain rates of 10−2 and 10−1 s−1. However, only at a higher temperature and at a lower strain rate of 10−3 s−1, the flow stress increased for the foils with larger grain sizes. This is attributed to the dynamic strain aging, which is known as hardening behavior due to the diffusion of additional inclusion under higher temperatures. Furthermore, in microbending tests, the reduction rate of springback angle by increasing temperature also increased with increasing grain size. This is attributed to the increased reduction of strain gradient of the foils with a larger grain size. To clarify the mechanism of grain size effect on the mechanical behavior of pure Ti foils deformed at high strain rates, a composite constitutive model involving statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) was built with the capability of being employed at elevated temperatures. The effect of strain gradient on the material behavior was discussed in terms of its relationship with the density of GNDs.