Strain rate and temperature effects on the selection of primary and secondary slip and twinning systems in HCP Zr

We investigate the temperature and rate dependence of slip, twinning, and secondary twinning in high-purity hexagonal close packed α-Zr over a wide range of temperatures and strain rates (from 76 K to 673 K and 0.001 s−1 to 4500 s−1). To reliably identify the dominant deformation mechanisms for each condition, we employ electron-backscattered diffraction (EBSD), dislocation theory, multi-scale polycrystal constitutive modeling, and a thermally activated dislocation density evolution based hardening law. We demonstrate with direct comparison with measurement that the constitutive model, with a single set of intrinsic material parameters, can predict the underlying texture evolution, primary and secondary slip and twin activity, and twin volume fraction associated with the different loading orientations and applied temperatures and strain rates. We find that the {101‾2}〈101‾1‾〉 twin is the preferred tension twin, either as a primary or secondary twin depending on the sample orientation, over the broad temperature and strain rate range tested. In contrast, we show that the preferred contraction twin, whether {112‾2‾}〈112‾3‾〉 or {101‾1}〈101‾2‾〉, is sensitive to temperature but insensitive to strain rate and whether it is a primary or secondary twin. Based on the concomitant changes in the dominant slip mode predicted by the model and revealed by the texture development, we rationalize that the temperature-induced transition in contraction twinning is due to the increased predominance of basal 〈a〉 slip at high temperatures (>673 K). Last, our analysis implies that all twin modes studied are rate insensitive and so the strong influence of strain rate and temperature on twinning is due to the rate-sensitivity of slip.