Creep and superplasticity in nanocrystalline materials: current understanding and future prospects

Nanocrystalline materials (nc-materials), which are characterized by a grain size in the range 1–100 nm, have emerged as a new class of materials with unusual structures. In recent years, creep and superplasticity in nc-materials have been the subject of a number of studies. In particular, experimental studies have focused on (a) whether grain boundary diffusion creep, which is expected to be dominant in coarse-grained materials at moderate temperature (0.4–0.6 Tm) and low stresses, becomes significant at low temperatures (for example, room temperature), and (b) whether micrograin superplasticity, which has been observed at high temperatures (T>0.5 Tm) and moderate strain rates (10−5–10−2 s−1), can be observed at low temperatures and high strain rates. A review of these studies shows that data obtained by different investigators are either not consistent in trend or too limited in scope. Various aspects characterizing these consistencies and limitations are identified. It is suggested that a number of issues need to be fully addressed in order to provide a better understanding of the deformation processes which control the behavior of nc-materials.