This book concentrates on both understanding and development of nanocrystalline materials. The original relation that connects grain size and strength, known as the Hall-Petch relation, is studied in the nanometer grain size region. The breakdown of such a relation is a challenge. Why and how to overcome it? Is the dislocation mechanism still operating when the grain size is very small, approaching the amorphous limit? How do we go from the microstructure information to the continuum description of the mechanical properties?

The influence of grain size or cell size on the mechanical behavior of metals is well known, although debate continues about details of defect control mechanisms. In the past, difficulty in producing bulk samples with grain sizes smaller than about 1 [mu]m limited mechanical behavior studies in finer-grained materials. Expressions relating strength to grain size, determined from studies of coarse-grained materials, suggest that reducing grain size into the sub-micrometer range results in increased mechanical strength at low homologous temperatures. At high temperatures, diffusional creep effects may lead to increased ductility. This paper reports the major results to date of an ongoing study of the mechanical behavior of nanocrystalline metals produced by the inert-gas condensation method; some results have been reported elsewhere. Studies of the tensile strength, low-temperature creep and Vickers microhardness of Cu, Pd and Ag reported here are complemented in this broader study by processing studies, x-ray grain-size and strain analyses, and high resolution microscopy studies of nanostructure and microstructure. The work provides a basis for predicting low-temperature mechanical behavior of ultrafine-grained metals, subject to some significant constraints imposed by the processing conditions. 16 refs., 2 figs., 3 tabs.

Ultrafine grained and nanocrystalline metals have attracted increasing interest, both scientific and commercial, in recent years because of their potentially superior mechanical properties. Their properties, such as very high strength, primarily arise from the change in the underlying deformation mechanisms. Experimental and simulation studies have shown that because of the extremely small grain size conventional dislocation plasticity is curtailed in these materials and grain boundary mediated mechanisms become more important. Although the deformation behavior and the underlying mechanisms in these materials have been investigated in depth, relatively little attention has been focused on the inhomogeneous nature of their microstructure and its influence on their macroscopic response.

There has been a growing research interest in understanding the mechanical behaviors and the deformation mechanisms of nanocrystalline metals and alloys in the past a few decades, due to their extraordinary mechanical prosperities, such as high strength, hardness, and wear resistance, which have great potentials in engineering applications. As grain sizes in crystalline metals and alloys transit down to the lower end of the nanometer range, the plastic deformations are no longer dominated by the intragrain dislocation activities. Instead deformations assisted by grain boundary start to play a more important role in deciding the mechanical response of the bulk materials, as the interfacial volume fraction increases with the reduction of grain sizes. A polycrystalline constitutive theory is developed in the form of the extend aggregate Taylor model of Asaro and Needleman for the nanocrystalline metals. The plastic deformation description is based on the Asaro, Krysl and Kad (AKK) model, which considers deformation mechanisms such as the emission of perfect, partial dislocations and deformation twins from grain boundary and grain boundary sliding when the grain size is sufficiently small in the nanometer regime (less than 100nm), and their transitions are governed by the factors such as grain size, stacking fault energy, temperature, and strain rate, etc. Therefore the effect of grain size distributions in addition to the mean grain size is considered important on the mechanical response in this constitutive theory. The grain size distributions can be simulated with the experimentally determined lognormal distributions for the electro-deposited nanocrystalline metals for example. Numerical simulations are carried out for nanocrystalline Ni, Cu, Al and Pd, and the simulated phenomena include the mechanical response of these materials when subjected to uniaxial tension and compression under different deformation rates, texture development under high pressure torsion (HPT), and the grain growth effect during nanoindentation, etc, where the contribution of each deformation mechanism is carefully studied. The obtained numerical results are in reasonably good agreement with the experiments. Due to the fact that the deformation mechanisms in nanostructured materials are not yet fully understood, this constitutive theory will need to be further improved with the future findings of deformation mechanisms, which this theory has the flexibility to easily incorporate.

This book concentrates on both understanding and development of nanocrystalline materials. The original relation that connects grain size and strength, known as the Hall-Petch relation, is studied in the nanometer grain size region. The breakdown of such a relation is a challenge. Why and how to overcome it? Is the dislocation mechanism still opera

In an attempt to meet the demand for new ultra-high strength materials, the processing of novel material configurations with unique microstructure is being explored in systems which are further and further from equilibrium. One such class of emerging materials is the so-called nanophased or nanostructured materials. This class of materials includes metals and alloys, ceramics, and polymers characterized by controlled ultra-fine microstructural features in the form oflayered, fibrous, or phase and grain distribution. While it is clear that these materials are in an early stage of development, there is now a sufficient body of literature to fuel discussion of how the mechanical properties and deformation behavior can be controlled through control of the microstructure. This NATO-Advanced Study Institute was convened in order to assess our current state of knowledge in the field of mechanical properties and deformation behavior in materials with ultra fine microstructure, to identify opportunities and needs for further research, and to identify the potential for technological applications. The Institute was the first international scientific meeting devoted to a discussion on the mechanical properties and deformation behavior of materials having grain sizes down to a few nanometers. Included in these discussions were the topics of superplasticity, tribology, and the supermodulus effect. Lectures were also presented which covered a variety of other themes including synthesis, characterization, thermodynamic stability, and general physical properties.

The mechanical behavior of nanocrystalline metallic, intermetallic, and multiphase materials was investigated using tension, compression, and disk-bend techniques. Nanocrystalline NiAl, Al-Al3Zr, and Cu were synthesized by gas condensation and either resistive or electron beam heating followed by high temperature vacuum compaction. Disk- bend tests of nanocrystalline NiAl show evidence of improved ductility at room temperature in this normally extremely brittle material. In contrast, tension tests of multiphase nanocrystalline Al- Al3Zr samples show significant increases in strength by substantial reductions in ductility with decreasing grain size. Compression tests of nanocrystalline copper result in substantially higher yield stress and total elongation values than those measured in tensile tests. Implications for operative deformation mechanisms in these materials are discussed.