Detailed assessment of indentation size-effect in recrystallized and highly deformed niobium

The nanoindentation hardness H of polycrystalline niobium in the recrystallized condition and after up to eight room temperature 90° equal-channel angular pressing passes (equivalent deformation ∼9) has been studied in detail. An important indentation size effect (ISE) that progressively disappears as the indentation depth h decreases has been observed at all pre-deformation levels. Qu et al. [J Mater Res 2004;19:3423] have explained such behaviour as a deviation from the ISE expected for conical or pyramidal indenters (a H2 − 1/h proportionality) because of the rounded shape of real tips. The Nix and Gao ISE model [J Mech Phys Solids 1998;46:411], further refined by taking into account the tip roundness and other unnoticed contributions to ISE, has been compared with the niobium results. Quantitative agreement between observed and predicted ISE requires the true dislocation pattern under the indentation being much denser than the very idealized geometrically necessary dislocations pattern assumed in the Nix and Gao model.