Hardness, strength, ductility and toughness of nanocrystalline materials

Considerable effort has been devoted over the last decade to preparing nanocrystalline powder and to consolidating this to bulk form while maintaining a nano-scaled microstructure. It has been clearly shown that very high room temperature hardness or compression strength can be obtained in powders which can be retained to a large extent after careful consolidation. Very little attention has been devoted to the examination of other important mechanical properties, such as tensile strength, ductility or toughness. This is particularly disappointing since a stated general objective of many research programmes is to ''improve toughness'' or to ''ductilise'' rather brittle materials. The present overview examines the information published to-date on the strength and hardness and on the ductility and toughness of materials in nanocrystalline form, concentrating especially on metallic materials and their properties at room temperature. In particular the question of whether the results obtained are: processing-limited, that is determined to a large extent by structures and defects that depend on the conditions of preparation, or are intrinsic properties of these materials in their nano-structured forms, will be examined. Since the properties of the final materials are so dependent on the conditions of processing, in a first stage the methods used for processing of such materials are reviewed, including a summary of the characteristics associated with each method of production. In a second stage, what may be called ''fracture-independent'' properties (hardness, compression strength) will be examined. Finally, those properties where fracture or ductility may play an important role, such as tensile flow stress, maximum strength or ductility, or bend strength or ductility, will be examined.