Fabrication of porous magnesium with directional pores through use of hydrogen thermally decomposed from MgH 2 powders d
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Porous magnesium with directional cylindrical pores (or “lotus-type” porous magnesium) was fabricated through the use of hydrogen decomposed from MgH2 powders during unidirectional solidification. Liquid magnesium was cast into a mold in which MgH2 powders were placed and was unidirectionally solidified, which achieved growth of pores elongated along the direction of solidification. The effect of the amount of the MgH2 powders on the pore structure (porosity, diameter, and number density of pores) and the change in the pore structure along the pore growth direction were clarified. The porosity and number density of pores increase with increasing amount of MgH2 powder, and the average diameter of pores decreases with increasing amount of MgH2 powder. The pore structure changes with the growth of pores along the solidification direction.
I. INTRODUCTION
Porous metals receive much attention because of their unique features, such as light weight, permeability of fluid, and energy absorption.1,2 Because the unique features of porous metals depend on their pore structure, control of the pore structure is important for improvement of various properties of porous metals. Porous metals with cylindrical pores oriented in one direction, so-called “lotus-type” porous metals (or “gasar” foams),3,4 gather much interest because of their unique anisotropic pore structure. Because stress concentration hardly occurs for loadings along the longitudinal direction of the pores, the mechanical strength in that direction is superior to that of conventional porous metals with isotropic pores.5,6 Furthermore, the unique structure provides various functions, such as energy absorption and sound absorption.3 Therefore, lotus metals are expected to be used as light-weight structural materials with various functions. A lotus metal can be fabricated through unidirectional solidification of a metal in pressurized hydrogen atmosphere, when the hydrogen solubility in the liquid of the metal is much larger than that in the solid, i.e., hydrogen solubility gap between the liquid and solid exists in equilibrium state.3,7,8 By melting a metal in pressurized hydrogen atmosphere, the hydrogen is dissolved in liquid a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0105 J. Mater. Res., Vol. 23, No. 3, Mar 2008
metal up to the equilibrium solubility. When the liquid metal is solidified unidirectionally, a part of the hydrogen dissolved in the liquid is rejected at the solid–liquid interface because of the hydrogen solubility gap. The rejected hydrogen then forms pores, and the pores grow unidirectionally along the solidification direction. In this method, pressurized hydrogen gas is used, to be dissolved in liquid metal. Recently, Nakajima et al. developed a method for fabricating lotus metals without using pressurized hydrogen gas.9 The fabrication method, so-called “thermal decomposition method,” uses hydrogen thermally decomposed from gas-forming compounds in place of pressurized hydrogen gas for
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