Temperature dependence of strain rate sensitivity, indentation size effects and pile-up in polycrystalline tungsten from 25 to 950 °C

Abstract

Elevated temperature nanoindentation measurements were performed on polycrystalline tungsten to 950 °C under high vacuum conditions with very low thermal drift. The temperature dependence of the hardness, elastic modulus, strain rate sensitivity, activation volume and the indentation size effect in hardness were studied. More significant time-dependent deformation was observed from 850 °C. Strain rate sensitivity determined by analysis of indentation creep data increased with temperature. Activation volume reached a peak of ~50 b3 at 750–800 °C. Decreasing activation volume >800 °C was a consequence of the increased strain rate sensitivity. For a bcc metal lattice resistance depends on T/Tc (where Tc, the critical temperature, at which flow stress becomes insensitive to temperature, is 527 °C for W); size effects would be expected scale with this relative temperature. Stronger indentation size effects in hardness were found at elevated temperatures. The influence of the time-dependent deformation on the unloading data was accounted for by a viscoelastic compliance correction. After correction the elastic moduli were to within ~1% of literature values at 750–800 °C and to within 6% at 950 °C. These small remaining differences are consistent with AFM measurements which show that pile-up is significant in these high temperature indentations.