Profilometry-based Indentation Plastometry Testing of Tungsten at High Temperature

This work concerns application of PIP (Profilometry-based Indentation Plastometry) to pure tungsten, at temperatures up to 800˚C. PIP involves extraction of stress-strain curves from measured indent profiles, via (automated) inverse Finite Element Method (FEM) modelling. This metal is brittle at room temperature, but plastic deformation (with little work hardening) becomes prevalent above about 200˚C. PIP-derived curves are consistent with tensile results in this regime. For lower temperatures, PIP allows extraction of yield stress values and work hardening characteristics, but brittle fracture precludes this for tensile testing. Attention is paid to effects induced by exposure to air at high temperature, with both oxide layer formation and the absorption of oxygen being investigated. At 800˚C, a highly porous oxide rapidly forms, with approximately linear growth kinetics. However, this has little effect on measured indent profiles (and hence on inferred stress-strain curves), both because time at high temperature can be kept short during PIP testing and due to the weak and porous nature of the oxide. A check on the role of creep revealed that it was significant at 800˚C (for typical quasi-static strain rates), but had only a relatively minor effect on the PIP-inferred stress-strain curve.