Exploiting interactions between structure size and indentation size effects to determine the characteristic dimension of nano-structured materials by indentation

It was shown that yield (or flow) stress is determined by a critical dimension and the reciprocal sum of component critical dimensions (such as indentation size, structure size and dislocation density) combine into a single critical dimension as predicted by slip distance theory (Hou et al 2012 Acta. Mater. 60 4128). This suggests that ?length determines strength? and all lengths contribute at all times to the critical value. We have already shown that Cu hardness increases when grain size falls below six times the indentation contact radius (Hou et al 2008 J. Phys. D: Appl. Phys. 41 074006). In this paper, we test the inverse case (indent size greater than grain size), by indenting two different metallic glasses (NiAl and ZrTiAlCuBe). We show that the indentation size effect (ISE) does indeed become observable even when the indent size is larger than the grain size by up to an order of magnitude. The indentation depth (size) at onset of the ISE is proportional to the characteristic structure size of these nano-structured materials and suggests a novel use of ISE as a determinant of structure size. These findings have implications for the design of hardness reference blocks and the use of hardness mapping to determine materials property variations.