Morphological analysis of pores in directionally freeze-cast titanium foams
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Synchrotron x-ray tomography was performed on titanium foams with aligned, elongated pores, initially created by sintering directionally freeze-cast preforms using two different powder sizes. Three-dimensional reconstructions of the pore structures were analyzed morphologically using interface shape and interface normal distributions. A smaller powder size leads to more completely sintered titanium walls separating the dendritic pores, which in turn created a more compact distribution of pore shapes as well as stronger pore directionality parallel to the ice growth direction. The distribution of pore shapes is comparable to trabecular bone reported in the literature, indicating the foam’s potential as a bone replacement material. I. INTRODUCTION
Metallic foams have an interesting combination of properties, such as high specific strength and stiffness when incorporated into sandwiches and high gas permeability with high thermal conductivity. This makes them useful in low-weight structural applications, filtration, battery electrodes, and acoustic damping.1 Titanium-based foams, in particular, also combine the advantages of outstanding mechanical strength with low density, high corrosion resistance, and surface oxide biocompatibility, which make them especially promising for use in medical implants as a bone replacement material.2–5 Thus, it is of great interest to create titanium foams that exhibit the same aligned, elongated pore architecture as bone, which gives it both its structural and mechanical anisotropy. Directional freeze-casting is a method that relies on directional solidification to create aligned pores in ceramics6–10 and, recently, in titanium.11 First, a liquid, usually water, is mixed with solid powders to make slurry. The slurry is then subjected to directional solidification so that the porous structure is determined by the growth of the solid ice crystals, rejecting the powders to the interdendritic space. After solidification, the structure is freezedried, sublimating the ice out of the sample, leaving large aligned, elongated pores separated by walls of lightly bound solid powders, corresponding respectively to the dendritic and interdendritic regions. These powders are then sintered to create dense walls separating the pores. X-ray tomography has been used previously to visualize ice crystals in the fields of food science and glaciology.12–16 For instance, directional solidification of a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0023 J. Mater. Res., Vol. 24, No. 1, Jan 2009
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ice into meat was studied using tomography, but the information was not used to create three-dimensional (3D) reconstructions.16 A more comprehensive study analyzed the microstructural evolution of snow in three dimensions, over time, while holding temperature constant,15 calculating porosity, specific surface area, anisotropy, and curvature distribution of the snow crystals. Optical and x-ray tomography have a
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