Processing and microstructural evolution of powder metallurgy Zn-22 Pct Al eutectoid alloy containing nanoscale dispersi
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INTRODUCTION
DISPERSIONS of fine particles in metal matrices have been shown to enhance the strength and heat resistance of materials. The concept originated in a discovery made by Irmann,[1] who reported extraordinary high-temperature properties in hot-pressed aluminum compared to cast aluminum in 1949. With the advent of new processing techniques in the 1970s including ball milling,[2] it has become possible to achieve a much finer dispersion distribution, which has led to a better balance between strength and ductility. Ball milling involves repeated welding, fracturing, and rewelding of powder particles, in contact with a highly energetic ball charge.[3] More recently, this process has attracted considerable interest, primarily because of its potential to generate nanocrystalline and other nonequilibrium structures in large quantities.[4,5,6] In addition to structural refinement, another inherent characteristic associated with milling is the formation of nanoscale dispersions by the promotion of in-situ reactions between the powder matrices and environmental gases such as oxygen, or milling media such as nitrogen. For example, Luton et al.[7] demonstrated that a dispersion of nanoscale aluminum oxy-nitride particles formed in Al powders during ball milling in liquid nitrogen Y. XUN, Graduate Research Assistant, and R. RODRIGUEZ, Research Associate, are with the Department of Chemical Engineering and Materials Science, University of California, Irving, CA 92697-2575. E.J. LAVERNIA, Professor, is with the Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. F.A. MOHAMED, Professor, is with the Department of Chemical Engineering and Materials Science, University of California, Irving, CA 92697-2575. Contact e-mail: [email protected] Manuscript submitted January 20, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
(cryomilling). Following milling, milled powders need to be consolidated to reduce porosity and produce bulk materials. The present article reports and discusses the results of a detailed investigation on the processing and microstructural evolution of powder metallurgy (PM) Zn-22 pct Al. In the investigation, the powder material was produced through spray atomization,[8,9] and then cryomilled in liquid nitrogen.[7] Finally, the milled powder particles were consolidated by hot isostatic pressing (“hipping”) followed by thermomechanical treatment, resulting in a two-phase microstructure. The objective of the investigation is twofold: (1) to monitor the microstructural evolution of PM Zn-22 pct Al that accompanies the preparing of the alloy using the above processing route; and (2) to investigate the effects of features such as dispersion particles and porosity, if present in the microstructure as a result of processing, on deformation under steady-state creep conditions. II. EXPERIMENTAL PROCEDURES A. Material The target material, Zn-22 pct Al, was selected for three main reasons. First, the material is a eutectoid alloy that consists of equal volume fraction
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