Alan’s research centers on the processing and properties of a brand new class of materials called amorphous metal foams. These are foam materials (i.e., materials with large fractions of empty space inside them, like sponge, bone, or cork) made from a special class of metal known as a bulk metallic glass (BMG). Bulk metallic glasses are sophisticated metal alloys that, when cooled very quickly from the liquid state, retain an amorphous atomic structure. They are characterized by a number of unique properties, including exceptional strength and elasticity, high hardness/wear resistance, and good corrosion resistance. Members of the successful Zr-based BMG family, which are the focus of my research, typically have strengths around 2 GPa in compression, and stiffness ca. 80-100 GPa.
In bulk form, however, BMG alloys are also brittle, making them dangerous in structural applications where their high strength could be useful. Normally, the approach taken to improve the toughness of BMG is incorporation of second phases (i.e., composites processing), either deliberately added or precipitated out of the alloy. Recently, however, many researchers have noted the high inherent ductility of thin BMG pieces (like wires or foils) during bending, a fact which suggests that low-density BMG foams (whose struts are naturally thin and prone to bending deformation) could be ductile despite the brittle nature of the alloys in bulk form. In addition to allowing for high ductility in the amorphous metal, foaming techniques will also impart the natural advantages of foam architectures, such as improved density-compensated mechanical properties, acoustic damping, energy absorption at constant stress, and many others. The goal of our research is to create and study these foams to see how well these predictions stand up.
A number of methods have been investigated for foaming of BMG, but both successful methods are based on melt infiltration. The generic process is shown schematically in Figure 1. We made the first Zr-based BMG foam using an infiltration pattern made from hollow carbon microspheres. An example of “syntactic” foam made using these spheres is shown in Figure 2.
FIGURE 1: Schematic method for producing foam from the BMG alloy ‘Vit106.’ The method is based on melt infiltration of a porous pattern material (e.g., a bed of hollow carbon microspheres), followed by immersion in a bath of quenchant. |
FIGURE 2: Images showing the polished surface of an amorphous foam made from the alloy Vit106 using the method shown in Figure 1. Panel (a) shows a low-magnification view of the whole surface, showing that the structure of the material is uniform. Panel (b) shows a closer view of the same surface, where individual hollow spheres are visible within the amorphous matrix. The arrow indicates a fragment of a broken sphere wall. |
The second type of foam has an open-celled, rather than closed-celled, structure. The infiltration pattern in this method is a sintered refractory salt. Unlike the hollow carbon spheres, this pattern has to be removed after infiltration, and an example of the foam after removal of the salt in acid is shown in Figure 3.
FIGURE 3: A Vit106 foam made by infiltration of a soluble refractory salt, as shown in Figure 1, followed by removal of the salt in acid. The pores in this foam are on the order of 212-250 microns in size, and are fully interconnected.
These two methods are (to the author’s knowledge) the first and only ones ever developed for making Zr-based amorphous metal foams, and much still needs to be done to investigate the properties of these new materials. Results are coming in all the time, though, so check back again! In the meantime, please see the articles below for more details. For pictures of other foams I have made in the past, see “Melt Infiltration Processing of Foams Using Glass-Forming Alloys” (MRS Proceedings).
Related Publications
- C. San Marchi, A.H. Brothers, D.C. Dunand. “Melt Infiltration Processing of Foams Using Glass-Forming Alloys.” Materials Research Society Symposium Proceedings 754 CC1.8 2003.
PDF - A.H. Brothers, D.C. Dunand. "Syntactic Bulk Metallic Glass Foam", Applied Physics Letters, 84 (7) 1108-1110 (2004).
PDF - A.H. Brothers, D.C. Dunand. "Ductile Bulk Metallic Glass Foams." Advanced Materials 17 (4) 484-486 (2005).
PDF - A.H. Brothers, R. Scheunemann, J.D. DeFouw, D.C. Dunand. "Processing and Structure of Open-Celled Amorphous Metal Foams," Scripta Materialia, 52, 335-339 (2005).
PDF - A.H. Brothers and D.C. Dunand. "Plasticity and Damage in Cellular Amorphous Metals." Acta Materialia. 53 (16) 4427-4440 (2005).
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Financial Support
This research is funded by DARPA.
