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

Nanocrystalline materials (nc-materials), which are characterized by a grai n 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-mater ials have been the subject of a number of studies. In particular, experimen tal studies have focused on (a) whether grain boundary diffusion creep, whi ch is expected to be dominant in coarse-grained materials at moderate tempe rature (0.4-0.6 T-m) and low stresses, becomes significant at low temperatu res (for example, room temperature), and (b) whether micrograin superplasti city, which has been observed at high temperatures (T > 0.5 T-m) and modera te 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 limite d in scope. Various aspects characterizing these consistencies and limitati ons are identified. It is suggested that a number of issues need to be full y addressed in order to provide a better understanding of the deformation p rocesses which control the behavior of nc-materials. (C) 2001 Elsevier Scie nce B.V. All rights reserved.