The structure and mechanical properties of metallic nanocrystals

Metallic nanocrystals are ultrafine-grained polycrystalline solids with grain sizes in the range of 1 to 10 nm in at least one dimension. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located at the grain boundaries, and thus, they possess novel, and often improved, properties over those of conventional polycrystalline or glassy materials. In comparison to more conventional materials, nanocrystalline materials show a reduced density; increased thermal expansion, specific heat, and strength; a supermodulus effect; and extremely high diffusion rates. Traditionally brittle materials can be made ductile by nano-structure processing. At present, there is considerabe confusion on the nature of the micro-structure and mechanical properties of the nanocrystalline materials, especially of the equiaxed (three-dimensional, 3-D) type. The present article reviews the current understanding of nanocrystals and evaluates the data available on structure and mechanical properties of nanocrystalline metals. This invited overview is based on a presentation made in the symposium “Structure and Properties of Fine and Ultrafine Particles, Surfaces and Interfaces” presesnted as part of the 1989 Fall Meeting of TMS, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Structures Committee of ASM/MSD.