Porous and Foamed Amorphous Metals
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Amorphous Metals
Alan H. Brothers and David C. Dunand Abstract This article reviews the state of the art in the field of porous amorphous metals by describing current processing techniques, mechanical properties, and potential applications. In addition to the reduction in density, the main benefit of introducing porosity in amorphous metals is the improvement in compressive ductility and energy absorption. This ductilizing effect is explained by: (1) shear-band interruption by individual pores at low porosities and (2) stable plastic bending of thin struts at higher porosities, with cellular amorphous metals displaying compressive ductilities of up to 80%.
Introduction Of the many challenges inhibiting the use of amorphous metals in engineering applications, perhaps the most familiar and enduring is the problem of poor uniaxial ductility, which arises from the unstable propagation of shear bands.1 The approach commonly taken to mitigate this poor ductility is interruption of those shear bands by incorporation of second phases, either precipitated during solidification or introduced through composite processing techniques.2 Although these strategies lead to substantial improvements in compressive ductility, recent work has demonstrated that pores (which can be considered as a gaseous second phase) are equally effective in inhibiting catastrophic failures resulting from shearband localization. In highly porous amorphous metals, propagation of shear bands can even become stable, enabling macroscopic compressive strains of more than 80% without fracture.3 Here, we review the state of the art in the processing of porous and foamed amorphous metals as well as current understanding in the mechanical properties of these recently developed materials. Also, we evaluate these properties within the greater contexts of amorphous metals and porous crystalline metals and propose areas for future research and applications development.
Processing The first porous amorphous metal, made from Pd43Cu27Ni10P20, was described in 2003 by Schroers et al. and was produced
by expansion in the liquid alloy of water vapor bubbles generated from hydrated boron oxide flux powders, followed by quenching.4 This process, resulting in porosities of up to 85 vol% and pore sizes of ~200–1000 µm, was later modified to enable more stable bubble expansion in the low-temperature supercooled-liquid state and to create bubbles by mechanical air entrapment.5 Also in 2003, Wada and Inoue produced open-cell structures (with fully interconnected porosity, in contrast to the closed-cell structures of the bubble expansion method) with 65 vol% of 125–250 µm pores, by casting Pd42.5Cu30 Ni7.5P20 into beds of NaCl particles, quenching, and removing the salt by dissolution.6 Beginning in 2004, Wada and Inoue also foamed this alloy by the expansion of hydrogen bubbles precipitated from a supersaturated melt, yielding porosities of up to 71% with pore sizes of 80 µm and smaller.7–10 Inoue et al. later applied the method to a slightly different composition, Pd35Pt15
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