Pack-Aluminization Methods
FIGURE 1: 20 ppi Ni-9.1Al (wt.%) foam (ρ* = 2.6%): a) prior to deformation, b) deformed at 0.23 MPa at 725 °C, showing a macroscopically homogeneous deformation.
Dr. Heeman Choe
Reticulated ceramic foams have found applications as metal casting filters due to their high permeability and melting point, and as high-temperature insulation in oxidative environment. However, ceramic foams are difficult to use for structural, load-bearing applications at elevated temperature, due to their low ductility, toughness and thermal shock resistance. A ductile metallic foam, consisting of an oxidation- and creep-resistant alloy, could find numerous load-bearing applications, e.g., as the core of sandwich structures in engines and furnaces, or as high-temperature catalyst substrate, filter, or heat exchanger subjected to mechanical loads.
Nickel-base superalloys are the preferred alloy system for high-temperature structural applications in air up to ca. 1050 °C. The two most important alloying elements in superalloys are Cr and Al, with Cr providing solid-solution strengthening of the γ-Ni, FCC matrix, and Al (sometimes partially replaced with Ti) providing precipitation strengthening by forming coherent, γ’ Ni3Al precipitates with the L12 structure. Also, both Cr and Al provide oxidation resistance by creating a native, adherent oxide film. Many other transition elements can be further alloyed, mostly to provide solid-solution strengthening of both γ and γ’ phases. Thus, a nickel-base superalloy with good creep and oxidation resistance could be the basis for metallic foams with structural applications at temperatures as high as 1050 °C.
We have demonstrated that the pack-aluminization process developed for intermetallic NiAl foams can be adapted to the production of Ni-8~9 wt.% Al foams, which, after heat-treatment, exhibit the γ / γ’ structure typical of superalloys. We have also shown that Cr can be further alloyed to these Ni-Al foams by a similar pack–chromizing technique, resulting in a foam with Ni-14~17 wt.% Cr-5~9 wt.% Al composition and γ / γ’ structure. The mechanical properties of these Ni-Al and Ni-Cr-Al foams have been identified at both ambient and elevated temperatures, and we have compared these observations to predictions of analytical models assuming strut deformation by creep bending or creep compression.
Related Publications
- A.M. Hodge, D.C. Dunand, “Synthesis of Nickel Aluminide Foams by Pack Aluminization of Nickel Foams” Intermetallics, 9, 7, 581-589 (2001).
PDF - D.C. Dunand, A.M. Hodge, C. Schuh, “Pack Aluminisation Kinetics of Nickel Rods and Foams” Materials Science and Technology, 18, 326-332 (2002).
- A.M. Hodge, D. C. Dunand “Measurement and Modeling of Creep in Open-Cell NiAl Foams” Metallurgical and Materials Transactions A ,34, 10, 2353-2363 (2003).
PDF - H. Choe, D.C. Dunand. “Synthesis, Structure and Mechanical Properties of Ni-Al and Ni-Cr-Al Superalloy Foams”, Acta Materialia, 52, 5, 1283-1295 (2004).
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H. Choe, D.C. Dunand.“Mechanical Properties of Oxidation-Resistant Ni-Cr Foams”, Materials Science and Engineering, 384, 184-193 (2004) .
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Financial Support
This research is funded by NASA.
Ni-base Foams for Solid-Oxide Fuel Cell Interconnects
Dr. John DeFouw, Dr. Yuttanant Boonyongmaneerat [Vee]
Dr. John DeFouw has demonstated the synthesis of Ni-based superalloy foams through casting. The commercial superalloy IN792 can be cast into a sintered pattern of SrF2 (Tm = 1473 °C) particles. The SrF2 is then removed with an HCl solution that does not harm the alloy. This alloy is heat treated to form the desired γ/γ’ microstructure and is currently being tested to assess its ambient and high temperature (creep) mechanical properties. This process has been extended by Yuttanant Boonyongmaneerat [Vee] to producing foams using the J5 Ni-based alloy with a high-melting ceramic as the removable pattern material for the application of interconnects of solid oxide fuel cells.
Financial Support
This research is funded by GE Global Research and NASA.
Magnetic Shape-Memory Ni-Mn-Ga Foams
Dr. Yuttanant Boonyongmaneerat [Vee]
Vee has used the casting replication methods to produce Ni-Mn-Ga foams. Our collaborators at Boise State University (Prof. Peter Mullner and his student Markus Chmielus) measured a magnetic shape-memory strain in the polycrystalline Ni-Mn-Ga foam (0.12 %) which is increased sixty-fold as compared to bulk, non-foamed polycrystalline Ni-Mn-Ga (with a very small strain of 0.002%). This strain is developed as a magnetic field moves twin boundaries in a fully reversible manner and is repeatable over millions of magnetic cycles. The shape-memory strain in the foam is comparable, in terms of magnitude and response time, to the elastic strain exhibited by the best commercial magnetostrictive material, Terfenol D, when exposed to a magnetic field. However, Ni-Mn-Ga foams have lower density and contain less expensive metals than Terfenol D. Their open porosity may lead, beyond light-weight actuators, to micro-pumps without moving parts or magnetic refrigeration near room temperature.
The strain improvement in the foams is due to grains spanning whole struts in the foam in a so-called bamboo structure, thus reducing the internal constraints present between adjacent grains in non-foamed bulk polycrystalline Ni-Mn-Ga. Thus, each foam strut is similar to a single crystals, but is constrained at its nodes, so it exhibits a lower strains than a free single crystal with can achieve up to 10%. Further improvements in foams architecture will probably lead to higher strains. Foamed polycrystalline Ni-Mn-Ga can be easily cast by conventional methods, unlike Ni-Mn-Ga single crystals.
A paper based on this research was accepted for publication in Physical Review Letters in October 2007: "Increasing magnetoplasticity in polycrystalline Ni-Mn-Ga by reducing internal constraints through porosity" (by Yuttanant Boonyongmaneerat, Markus Chmielus, David C. Dunand and Peter Mullner)
