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

Elevated temperature nanoindentation measurements were performed on polycrystalline tungsten to 950 degrees 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 degrees C. Strain rate sensitivity determined by analysis of indentation creep data increased with temperature. Activation volume reached a peak of similar to 50 b(3) at 750-800 degrees C. Decreasing activation volume >800 degrees C was a consequence of the increased strain rate sensitivity. For a bcc metal lattice resistance depends on T/T-c (where T-c, the critical temperature, at which flow stress becomes insensitive to temperature, is 527 degrees 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 similar to 1% of literature values at 750-800 degrees C and to within 6% at 950 degrees C. These small remaining differences are consistent with AFM measurements which show that pile-up is significant in these high temperature indentations. (C) 2018 Elsevier Ltd. All rights reserved.