Preparation and mechanical properties of highly densified nanocrystalline Al
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I. INTRODUCTION
NANOCRYSTALLINE (NC) materials are a very attractive research topic in the materials science at present. In the last few years, over 100 articles examining NC materials have been published each year. However, a relatively small amount of work has involved the study of the macroscopic mechanical properties of NC materials, especially their tensile properties. For example, 570 articles were published in the Fourth International Conference on Nanostructured Materials, i.e., NANO’98, while the only seven articles involved studies of their mechanical properties.[1] The reason for the lack of articles on the mechanical properties of NC materials is mainly due to the difficulty in producing fully dense bulk samples large enough to conduct a mechanicalproperty test. Inert gas condensation (IGC), with subsequent in situ consolidation under conditions of high vacuum, is one of the earliest methods used to produce bulk NC materials.[2–6] In the early works performed by this method, the prepared NC samples contain many residual defects and the relative densities usually do not exceed 97 pct.[7,8] Recently, with improvements of this preparation technique, the relative densities of prepared samples have exceed 98 pct.[9,10] However, since the productivity of NC materials in this method is relatively low, it is still difficult to produce larger bulk samples suitable for mechanical-property tests. Although there exist still some other techniques to produce bulk NC materials, such as ball milling followed by hot pressing or hot forging[11–15] and severe plastic deformation (SPD),[16–21] they have certain limitations. Owing to the difficulty of sample preparation, many scientists have been forced to conduct experiments such as hardness and compression tests,[11,13–15,22,23] miniaturized disk bend tests,[15,24]
and three-point bend tests,[22] using smaller specimens, in order to study the mechanical properties of NC materials. The strength values of some NC materials are to be calculated from the microhardness data of small compacts with nearly full density, according to the evident relation between the hardness and strength.[11,23] The results of these experiments and calculations could not fully reflect the mechanical properties of NC materials, especially their tensile properties. Recently, the tensile properties of NC Cu, Pd, Au, Ag, Al, and an Al alloy have been studied,[10,25–28] in which the results given by Sanders et al.[10] and McFadden et al.,[28] up to now, could be the best experimental results reported in literature. However, it would seem that all these results still could not completely reflect the intrinsic tensile properties of the NC materials, because their samples are too small or contain many processing defects. It can be seen from the aforementioned that today, knowledge of the mechanical properties of NC materials is limited. Many problems are not yet fully clear: for example, whether the intrinsic mechanical properties of NC materials could be extremely enhanced, as expected by the Hall–Petc
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