Metal-gas eutectic growth during unidirectional solidification
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RODUCTION
A new method was developed by Shapovalov[1] for producing a type of porous metals whose long cylindrical pores are ordered. This method was called the ‘‘Gasar’’ process in which an invariant reaction, the so-called ‘‘metal-gas eutectic reaction,’’ was used to fabricate a regular porous structure. Gasar metals with different porosity and structure may be used for filters, metal-matrix composites, bearings, brakes, and damping elements.[2] Compared with the traditional fabrication methods, this process allows an effective control of porosity, pore size, and orientation, so it is thought to be a revolutionary method. Since its discovery, this new method has been extensively studied.[3–11] Although Gasars have been extensively studied in the last decade in view of their fabrication, the unusual properties and potential applications, comprehensive mathematical model, and a fundamental understanding of their production process are still limited. Shapovalov[12,13] gives simple mathematical descriptions of some particular details of the entire process. In addition, Drenchev[14] presents an analytical/numerical analysis of Gasar growth. An approach for evaluation of Gasar porosity is given in References 15 and 16. Although metal(solid)-gas eutectic growth resembles the traditional solid(metal)-solid(metal or nonmetal) eutectic growth or solid(metal)-liquid(metal) monotectic growth, many differences exist because one of the two phases in coupled growth is a gas phase instead of the solid or liquid phase. So, the metal-gas eutectic growth rule is an interesting fundamental problem. In our early studies,[17,18,19] the bubble nucleation in the melt, the pore spacing, and a new model predicting the porosity were discussed. Based on these previous studies, by applying a solution procedure similar to that in the classical Jackson– Hunt eutectic growth model,[21] a new model was developed in this article to predict or explain the influence of processing parameters such as gas pressure and solidification rate on the pore diameter, interpore spacing, and porosity. These theoretical analyses are helpful for understanding the metal-gas eutectic growth. YUAN LIU, Doctor, YANXIANG LI, Professor, and JIANG WAN and HUAWEI ZHANG, Doctoral Students, are with the Department of Mechanical Engineering, Tsinghua University, Beijing, 100084, People’s Republic of China. Contact e-mail: [email protected] Manuscript submitted February 18, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS A
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EXPERIMENTAL PROCEDURE AND RESULTS
A schematic of the fabrication apparatus for lotus-type porous metals is presented in Reference 19. It consists of a graphite crucible (100-mm inside diameter and 200-mm high) surrounded by a heating coil and a ceramic mold with a water-cooled copper bottom and a side-heating coil. These are installed in a high-pressure chamber. High-purity 99.99 pct magnesium was melted in the crucible in vacuum, and then high-pressure hydrogen or gas mixture of hydrogen and argon was introduced into the chamber. The purity of th
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