Mechanical properties of an ultrafine-grained Al-7.5 Pct Mg alloy
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IN the present article, results on the mechanical properties of a bulk cryomilled ultrafine-grained (UFG) Al-7.5 pct Mg alloy, which was tested both in tension and compression, are reported and analyzed. This investigation was motivated by several considerations. First, the field of nanotechnology has engendered interest in phenomena that take place at nanometer (10⫺9 m) length scales.[1] In the area of structural materials, this has led to the development of synthesis techniques for producing materials with grain sizes of 10 to 500 nm. In general, for grain sizes that reach the 300 nm to 1 m range, the materials are described as ultrafine-grained materials (UFMs), while for grain sizes less than 200 nm, the materials are referred to as bulk nanostructured materials (BNMs). According to this general classification, UFMs exhibit a range of grain sizes that represents a transition domain between micrograined materials and nanograined materials. Careful investigation of the mechanical behavior of UFMs, such as that of Al-7.5 pct Mg used in the present study, can provide information that may be applied to shed light on the underlying phenomena occurring in BNMs. Second, in order to establish a clearer picture of the nature and origin of deformation processes in BNMs and UFMs, large bulk specimens free from porosity and other flaws need to be tested. Inspection of the scientific literature shows that there are many techniques that can be used to produce nanostructured materials, including inert-gas condensation
B.Q. HAN, Senior Research Associate, and F.A. MOHAMED, Professors, Department of Engineering and Materials Science, University of California, Irvine, CA 92697-2575. Contact e-mail: [email protected] Z. LEE, Graduate Research Assistant, and S.R. NUTT, Professor, Department of Materials Science and Engineering, are with the University of Southern California, Los Angeles, CA 90089. E.J. LAVERNIA, Professor, Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616-5294. Manuscript submitted April 11, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
or chemical-vapor condensation,[1,2,3] pulse electron deposition,[4] plasma synthesis,[5] crystallization of amorphous solids,[6] severe plastic deformation,[7] and mechanical alloying or cryomilling.[8] However, only a few of these techniques, such as equal-channel angular pressing (ECAP) (grain sizes of 200 to 1000 nm),[7] electrodeposition,[4] and cryomilling (grain sizes of 30 to 500 nm),[8] generate nanostructures with sufficient thermal stability to permit the fabrication of bulk materials. The Al-Mg alloy tested in the present investigation was produced in the bulk form using cryomilling. Accordingly, a study of the mechanical behavior of the alloy would provide data that can be compared with conventional coarsegrained materials. Third, the few reports of the room-temperature mechanical properties of BNMs reveal some conflicting results.[9–13] The first trend is related to strain-hardening. On the one hand, strain-
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