Critical issues related to instrumented indentation on non-uniform materials: Application to niobium subjected to high pressure torsion

Nanoindentation is a powerful tool for characterizing the mechanical properties of materials at small length scales. Since the loading rate can be accurately recorded and controlled during a nanoindentation test, the strain rate dependence of these properties can also be determined. However, there are still a few problems that need to be addressed when it is applied to some special materials. High pressure torsion (HPT) processed metals are examples of these materials with non-uniform microstructures and mechanical properties. In this work, commercially pure niobium disks with diameter of ~10.0 mm and thickness of ~1.0 mm were processed by HPT. Grain sizes from a few nanometers to a few micrometers were generated due to a strong radial strain gradient in the disk. Instrumented nanoindentation tests were conducted at different radial locations of the HPT niobium disk. During each test, the loading rate was controlled so that the indentation strain rate was kept constant. Some key issues associated with the nanoindentation experiment, such as contact stiffness, contact area and the effect of pile-up or sink-in were evaluated and discussed carefully. The work-based method was used in data processing and it was compared with other approaches. The effects of indentation location, i.e., grain size, as well as indentation strain rate were characterized.