Nanocrystalline materials have been the subject of widespread research over the past couple of decades with significant advancement in their understanding especially in the last few years [3]. As the name suggests, they are single or multi-phase polycrystals with nano scale (1 · 10 9 –250 · 10 9 m) grain size. At the upper limit of this regime, the term ‘‘ultra- fine grain size’’ is often used (grain sizes of 250–1000 nm). Nanocrystalline materials are structurally characterized by a large volume fraction of grain boundaries, which may sig- nificantly alter their physical, mechanical, and chemical properties in comparison with conventional coarse-grained polycrystalline materials [4–6], which have grain sizes usually in the.
