The hardness and modulus of polycrystalline beryllium from nano-indentation

Abstract Nanoindentation was used to compare properties of four industrial beryllium grades with different purity. An extremely high variation of hardness was observed in all samples which obscured differences between samples. Analysis of the nanoindentation data in combination with SEM/EBSD measurements demonstrated that the crystallographic orientation of the indented grain was the major source of the wide variation in hardness, which was 2.5 times higher when the indentation direction was close to the [0001] c-axis of beryllium compared to indentation along the [ 11 2 ¯ 0 ] or [ 1 1 ¯ 00 ] directions. The most noticeable difference between tested grades were observed for the “soft” orientations: hardness of less pure structural grades was 15–30% higher compared to the pure nuclear grade. Crystal plasticity finite-element (CPFEM) simulations indicated how the hardness anisotropy of beryllium arises from the anisotropy in the plastic deformation. Experiments and simulations also demonstrated that localised plastic deformation of the surface around the indent (pile-up or sink-in) was highly crystallographically dependent: during indentation into “soft” orientations, pile-up dominated increasing the contact area; while sink-in behaviour was dominant during indentation into “hard” orientation reducing the contact area. This implies that the hardness values calculated from indenter displacement and indenter profile using the standard Oliver-Pharr approach, without considering pile-up/sink-in effects, will be incorrect. Several contact area correction methods were applied and compared. In contrast the indentation modulus was similar for all investigated grades and was not found to have any strong crystallographic dependence. Crystal plasticity finite element analysis indicates that this is due to the complex 3-dimensional nature of the elastic interaction between the indenter and the sample, and also since, for the chosen indentation depth, the elastic interaction volume is much larger than the materials’ grain size.