Journal of Alloys and Compounds 434–435 (2007) 92–96 Thermo-plastic expansion of amorphous metallic foam Marios D. Demetriou a,∗ , Chris Veazey a , Jan Schroers a,b , Jay C. Hanan c , William L. Johnson a a Keck Laboratory, California Institute of Technology, Pasadena, CA 91125, United States b Liquidmetal Technologies, Lake Forest, CA 92630, United States c Mechanical and Aerospace Engineering, Oklahoma State University, Tulsa, OK 74106, United States Available online 19 October 2006 Abstract Amorphous Pd 43 Ni 10 Cu 27 P 20 foam is produced at 38%, 49%, and 70% porosity by isothermally expanding a 25%-porosity amorphous precursor in the supercooled liquid state for varying durations. The foam morphologies exhibit good spatial homogeneity as well as good size uniformity of bubbles, which is a consequence of the high viscosity of the supercooled liquid state which inhibits floatation and dampens the growth kinetics. The expansion capability of amorphous metals into high-porosity foam demonstrated in this study is attributed to the plastic deformability of the supercooled liquid state, which enables large plastic membrane elongations during foaming. © 2006 Published by Elsevier B.V. Keywords: Amorphous material; Metals; Liquid quenching; X-ray diffraction; Strain 1. Introduction Families of metallic glass-forming systems exhibiting a remarkable kinetic stability against crystallization have been developed over the last decade [1,2]. Such systems are charac- terized by supercooled liquid states (metastable states to which glass relaxes before undergoing crystalline transition) that main- tain stability for periods of up to 10 3 s at temperatures extending more than 100 ◦ above the glass transition [3]. The viscosity characterizing these states varies smoothly and predictably from viscous fluid to solid, taking values that typically range between 10 5 and 10 12 Pa s [4]. The kinetic stability characterizing these systems combined with their excellent rheological properties opens the possibility for isothermal forming processes similar to those employed in the processing of plastics or conventional glasses [5]. This “thermo-plastic” forming capability of amorphous metals constitutes a key quality that can be exploited to considerably advance their plastic processing [6]. Thermo- plastic forming performed on vitrified feedstock effectively decouples the forming stage from the vitrification stage, which requires rapid quenching of the product. Such decoupling ∗ Corresponding author. E-mail address: marios@caltech.edu (M.D. Demetriou). renders the requirement for product rapid quenching redundant, and hence the limitations on size imposed by the rate of heat removal (referred to as critical casting thickness) are relaxed. Expansion of closed-cell amorphous metal foam appears to be an attractive application of thermo-plastic forming [7]. The challenge in producing high quality metal foam is accomplishing a uniform and homogeneous foam morphology that would give rise to enhanced structural and functional characteristics [8]. Since crystalline metals cannot undercool substantially, foaming can be performed in either the equilibrium liquid or the crys- talline solid state. Foam processing in the equilibrium liquid state can result in highly porous foams [9], whose morpholo- gies however would evolve essentially “uncontrolled”, owing to very low viscosities promoting under-damped growth and bubble sedimentation. By foaming in the crystalline solid state, substantial control over the evolution of morphology is intro- duced, however, the limited superplastic deformability of crys- talline metals imposes constraints on the attainable porosity [10]. Conversely, the highly viscous supercooled liquid states ren- der glass-forming metallic systems much more attractive for metallic foam processing [11]. The high viscosity characteriz- ing supercooled liquids can inhibit sedimentation and dampen growth giving rise to “controlled” expansion. Moreover, the outstanding “plastic” deformability of the supercooled liquid 0925-8388/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.jallcom.2006.08.323