A review of high-strength nanolaminates and evaluation of their properties

Nanolaminates are composed of nanoscale-thick alternating layers of different materials and their properties are dependent on the individual layers, the layer thickness and the interfaces between the layers. Nanolaminates composed of cubic crystal structured metals are usually ductile compared to nanolaminates containing hexagonal crystal structured metals. Mechanical properties such as strength and hardness of nanolaminates increase with a decrease in individual layer thickness down to a few nanometers and they become independent when the thickness of individual layers is less than a couple of nanometers. This review provides a detailed analysis of the effects of individual layer thickness and the interface structures on the strength and the strengthening mechanisms of nanolaminates, their ductility and fracture behavior in terms of structural variations including grain morphologies, nanotwins, amorphous phases and crystal structures of the layers. The principles for designing nanolaminates with exceptionally high mechanical and physical properties and their fabrication are also highlighted. Some contradictory issues such as strengthening mechanisms, elastic modulus dependency on individual layer thickness and the effect of a thin amorphous layer on the strength are discussed. This review also provides future research directions in designing the high-strength nanolaminates that will facilitate practical engineering applications through analyzing up-to-date research efforts.