The structure and mechanical properties of metallic nanocrystals

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I.

INTRODUCTION

N A N O C R Y S T A L L I N E materials (also referred to as nanostructures, nanophase materials, or nanometer-sized crystalline solids) are single-phase or multi-phase polycrystals, the crystal size of which is of the order of a few (typically 1 to 10) nanometers (1 nm = 1 0 - 9 meter) in at least one dimension, t~-TjThus, they can be basically equiaxed in nature [1,5,7~ and will be termed nanostructure crystallites [three-dimensional (3-D) nanostructures], or they can consist of a lamellar structure, fS~ and will be termed a layered nanostructure [one-dimensional (l-D) nanostructure], or they can be filamentary in nature r9~[twodimensional (2-D) nanostructure]. Table I shows this classification and Figure 1 illustrates the three types of nanostructures schematically. The magnitudes of length and width are much greater than thickness in the layered nanocrystals, and length is substantially larger than width in filamentary nanocrystals. The nanocrystalline materials may contain crystalline, quasicrystalline, or amorphous phases and can be metals, ceramics, or composites. It has been shown that these materials have properties often superior to those of the conventional coarse-grained polycrystalline materials, tl,5,Tl A great deal of effort has gone into the synthesis and characterization of these novel materials, with emphasis on the nanostructure crystallites and, to a lesser extent, on the layered structures. Relatively few investigations have been reported on the filamentary nanocrystals. Recently, greater attention is being paid to the development of nanocrystalline ceramics, since it has been demonstrated that they can be sintered at relatively low tem-

C. S U R Y A N A R A Y A N A and F.H. FROES, Professor and Director, respectively, are with the Institute for Materials and Advanced Processes, University of Idaho, Moscow, ID 83843-4195. This invited overview is based on a presentation made in the s y m p o s i u m "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 A S M / M S D . METALLURGICAL TRANSACTIONS A

peratures t~~ and can also exhibit improved ductility, tJll and even potential superplasticity, ~ in the nanocrystalline state. A number of research groups are currently devoting their efforts to understanding of the nature and properties of the nanocrystalline materials and exploiting them for industrial applications, t;-7] Although useful properties (e.g., lowered sintering temperature and improved fabricability) have been reported for ceramic materials with nanometer-sized grains, the present review will restrict itself to a discussion of the nanocrystalline metals. More specifically, structure and mechanical properties will he considered in detail, with emphasis on rationalizing conflicting reports in this area.

II.

CHARACTERISTICS

A schematic representation of a hard-sphere model of an equiaxed nanocrystal

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