Microtomography of Amorphous Metal during Thermo-plastic Foaming

Low density metallic foams of consistent controlled morphology offer improved unique material and mechanical properties, yet they have eluded traditional metallurgical engineering methods. A new method to produce metallic foam through a thermo-plastic expansion process has been discovered and characterized with microtomography. The new foaming process enables porosities unachievable through traditional metallurgical processes. The metal foams exhibit excellent morphological characteristics as evidenced by the microtomography analysis. The high porosities, exceeding 70%, contribute to enhanced plastic yielding. The expansion of supersaturated gas bubbles in an amorphous precursor by lowering ambient pressure was observed as a function of expansion time with X-ray microtomography at 2-BM of the Advanced Photon Source. The porosity distribution was observed and used to confirm a “balloon-growth” model governing bubble evolution. Two-dimensional and three-dimensional methods of visualizing the foam structure are presented and compared along with measurements enabled by microtomography. INTRODUCTION Foam has long been a subject of micro-tomography (μ-CT) study [1]. These foams, or cellular structures, are composed of solid and gaseous phases with an open or closed form to the cells. Polymer foams are common, but metal and ceramic foams may also be produced. Closed-cell metal foams attract particular interest for their suitability in structural and impact applications as they are characterized by high specific strength, high stiffness, and high-energy absorption [2]. More importantly, metal foams are suitable for loading under conditions and temperatures at which conventional polymer foams fail. The challenge in producing high quality metal foam is accomplishing a uniform and homogeneous foam morphology that would result in enhanced structural and functional characteristics. Along with the interesting mechanical characteristics and three-dimensional volume structure, advantages to the non-destructive micro-tomography * Corresponding author, e-mail: [email protected] 85 Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002