Indentation Plastometry of Very Hard Metals

This investigation concerns the application of profilometry-based indentation plastometry (PIP) to metals with very high hardness, i.e., those with yield stresses of 1.5-3 GPa. The PIP procedure comprises (a) applying a force to an indenter ball, penetrating the sample to a preselected depth, (b) measuring the profile of the indent, and (c) iteratively running a finite element method (FEM) model to obtain the true stress-true strain curve giving optimal agreement between measured and modeled profiles. The procedure is no different when the sample is very hard, although the ball must remain elastic during the process. It is shown that this can be achieved using silicon nitride balls. These can fracture under some conditions, but it is shown that a "proof-testing" operation can be used to ensure that any particular ball will remain elastic under the complete range of service conditions. It is also shown, via systematic comparisons with the outcomes of uniaxial (tensile and compressive) tests, that reliable stress-strain curves can be obtained for very hard metals. Furthermore, PIP testing has advantages over uniaxial testing for obtaining information about their behavior at relatively high strains (approximate to 15%), as well as being much easier and simpler to implement.