Effects of anisotropic pore structure and fiber texture on fatigue properties of lotus-type porous magnesium
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We studied the effects of anisotropic pores and fiber texture on the fatigue strength and fracture surface of lotus-type porous magnesium fabricated through unidirectional solidification in pressurized hydrogen and argon atmospheres. The fatigue strength in the direction parallel to the longitudinal axis of pores is higher than that in the perpendicular direction. Not only anisotropic pores but also fiber texture grown along the pore direction contributes to the anisotropy in the fatigue strength. The fatigue strength at finite life of lotus magnesium is closely related to the ultimate tensile strength; the fatigue strength is proportional to the ultimate tensile strength for both loadings parallel and perpendicular to the pore direction. The fracture surface of lotus magnesium is not flat, which originates from porous structure. For parallel loading, fiber texture in lotus magnesium also contributes to the irregular surface, i.e., a combination of texture and pore structure affects fracture surfaces.
I. INTRODUCTION
A porous magnesium can be fabricated by a thixomolding casting method, casting method using a polymer or wax precursor, and accumulative roll-bonding method, etc.1–3 Magnesium itself is lightweight and shows high damping capacity and high specific strength.4 Thus, porous magnesium whose weight is reduced by pores is ultralight weight and shows various functions such as permeability of fluid, sound absorption, etc. originating from porous structure.5,6 However, the mechanical strength of conventional porous magnesium is low because the pore shape is irregular and the distribution of pores is random.7,8 Recently, lotus-type porous magnesium (lotus magnesium) possessing cylindrical pores elongated in one direction have been fabricated by a unidirectional solidification method in pressurized hydrogen atmospheres.9 Because of the elongated pores, stress barely concentrates around pores for loading in the longitudinal direction of the pores. 10–14 Thus, the Young’s modulus in the direction decreases linearly with increasing porosity.15 Thus, lotus magnesium is expected to be used as structural materials with various functions originating from porous structure. For lotus magnesium to be used as structural materials, it is important to clarify its fatigue properties. Lotus magnesium is fabricated by the unidirectional a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0385 3120
J. Mater. Res., Vol. 22, No. 11, Nov 2007
solidification method. Therefore, fiber texture exists in the matrix.9 Furthermore, the mechanical properties of a magnesium crystal show strong anisotropy originating from the hexagonal close-packed (hcp) structure.16–19 Thus, the fatigue strength of lotus magnesium probably depends not only anisotropic pore structure but also fiber texture (crystalline orientation). In a previous paper, the fatigue properties of lotus copper with fiber texture have been studied.20 The texture does not affect the fatigue strength of lotus copper, while t
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