Effect of Solidification Condition and Alloy Composition on Formation and Shape of Pores in Directionally Solidified Ni-
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POROUS metals exhibit various unique features such as low density, fluid permeability, and energy and sound absorption.[1] Therefore, porous metals are expected to be applied in structural and functional materials such as filters, heat sinks, and impact energy-absorbing materials. Thus, extensive studies have been performed to improve their properties and develop new fabrication techniques.[2–8] Recently, porous metals with cylindrical pores oriented along a single direction, which are referred to as lotus-type porous metals (lotus metals)[3] or GASAR metals,[4] have attracted much attention because of the various unique features originating from their directional pores.[9–13] For example, the mechanical properties along the orientation direction of the cylindrical pores are much superior to those of metallic foams with spherical or isotropic pores[9–11] because stress rarely concentrates when loaded along the orientation direction. Therefore, porous metals with oriented cylindrical pores are expected to be useful as lightweight functional and structural materials in some engineering fields. TAKUYA IDE, Member of the Board of Directors Chief Technology Officer, is with Lotus Alloy Co., Ltd., Osaka, Japan, also with The Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Osaka, 567-0047, Japan. MASAKAZU TANE, Associate Professor, is with The Institute of Scientific and Industrial Research, Osaka University. Contact e-mail: [email protected] HIDEO NAKAJIMA, Director, is with the Wakasa Wan Energy Research Center, Tsuruga, Japan, and also with The Institute of Scientific and Industrial Research, Osaka University. Manuscript submitted November 30, 2012. Article published online May 23, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
In general, porous metals with cylindrical pores are fabricated by unidirectional solidification of melt containing dissolved hydrogen, which exploits the different solubility of hydrogen in solid and liquid metal. In the pressurized gas method,[3] a metal or alloy is melted in hydrogen atmosphere so that hydrogen is dissolved in the melt. Then, the melt containing hydrogen is solidified unidirectionally. During unidirectional solidification, hydrogen, which is insoluble in solid metal, is rejected by the solid/liquid interface. The rejected hydrogen then forms pores, which grow along the direction of solidification. Using this fabrication process, various kinds of porous metals with cylindrical pores, including porous TiAl and NiAl intermetallic compounds,[14–16] have been fabricated. Previous research efforts on pure metals[3] revealed that uniform cylindrical pores along the solidification direction form during solidification. On the other hand, for binary or ternary alloys,[17–19] irregular pores form because primary crystals with dendritic structures form at the solidification front and prevent the growth of cylindrical pores. However, for TiAl and NiAl intermetallic compounds, relatively uniform cylindrical pores form, similar to those in pure metals, although the ori
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