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.