Hall–Petch relationship in nanocrystalline Ni and Be–B alloys

An overview of the plastic deformation of crystalline solids with different grain sizes are presented. Special emphases are on materials with a grain size less than ∼20 nm. This is the region where the classical Hall–Petch (H–P) relationship breakdowns (or the inverse H–P relationship) are often reported. In the present paper, two alloy systems, pure nickel and Be–B binary alloys, are studied and the results discussed. The nanocrystalline nickel and Be–B alloys were produced by electrodeposition and sputter deposition, respectively. In the case of n-Ni, we used both nanohardness and nanoscratch experiments to demonstrate successfully a H–P breakdown at a grain size of about 14 nm. In the case of Be–B alloys, we illustrated that an apparent H–P breakdown is, in fact, an artifact. The apparent inverse HP relation was actually caused by the presence of relatively soft amorphous Be–B phases when the grain size of Be was significantly refined by B alloying.