Enhanced solid solution effects on the strength of nanocrystalline alloys

Solid solution strengthening in nanocrystalline alloys is studied using sputtered Ni-W as a model system. In the composition range of 0-20 at.% W, the sputtered alloys have a nanocrystalline structure with a grain size that is independent of composition. Nanoindentation of these alloys shows that solute addition increases strength to very high levels, almost in proportion to the solute content. This behavior is not expected based on traditional solid solution strengthening mechanistic models of local dislocation pinning at solute atoms, but can be explained by further considering a global effect of solute on the average properties of the Ni lattice. The new strengthening term arises by considering grain boundaries as pinning points for dislocation motion in nanocrystalline materials and incorporating the effect of solutes on such a mechanism. Our discussion surrounding Ni-W also provides insights into other solid solution nanocrystalline systems, a variety of which we show can be accurately described using the same concept. These developments also explain the origin of solid solution softening in some nanocrystalline alloys.