Abstract The indentation size effect observed during indentation testing is studied in pure Ni, Fe and two different Ni–Fe alloys as well as in a Ni–Fe diffusion couple. The hardness as a function of depth in these solid solutions is modeled in terms of geometrically necessary dislocations (GND). It is found that, for indentation depths varying from 50 to 5000 nm, the depth dependence of hardness could be modeled with a parameter f . Parameter f allows scaling of the storage volume of GND with indentation depth, thus yielding the effective contribution of GND to the hardness in a specific alloy. For the alloy studied, parameter f decreases with solid solution strengthening. Frictional stresses or changes in stacking fault energy might reduce the storage volume for GND. Using these observations, it is shown that diffusion couples and nanoindentation allow macroscopic solid solution hardening to be determined in a combinatorial manner.