Measurement and modeling of creep in open-cell NiAl foams
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11/9/03
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Measurement and Modeling of Creep in Open-Cell NiAl Foams ANDREA M. HODGE and DAVID C. DUNAND Nickel-rich NiAl foams, consisting of open cells with hollow struts and exhibiting two relative densities (5.0 and 6.6 pct) and average cell sizes (1.27 and 0.85 mm), were creep tested between 800 °C and 1100 °C under compressive stresses between 0.10 and 1.50 MPa. For stresses lower than 0.50 MPa, the foams exhibit secondary creep with power-law behavior characterized by creep exponents and activation energies close to those of bulk, nickel-rich NiAl. A three-dimensional (3-D) finite-element model (FEM) was implemented for a cell consisting of hollow struts on a cubic lattice, which predicts creep rates in reasonable agreement with the experimental data. Based on these numerical results, a simplified analytical model is proposed, whereby struts parallel to the applied stress deform by creep in a purely compressive mode, while perpendicular struts prevent buckling but provide no direct loadbearing capacity. This simple model produces results that are very close to the predictions of the complex numerical model and in good agreement with the experimental data. By contrast, an existing model considering creep bending of struts within the foam predicts strain rates that are too high by approximately two orders of magnitude.
I. INTRODUCTION
TO date, research on the mechanical behavior of cellular metals has mainly been focused on the room-temperature behavior of aluminum-based open- and closed-cell foams, as reviewed in References 1 through 4. Only a few reports exist on the high-temperature mechanical behavior of metal foams, which examine exclusively aluminum foams.[5,6,7] A simple analytical model based on creep bending of struts within the foam was developed by Gibson and Ashby[1] and was found to be in general agreement with creep data for open-cell alumina foams (with a 10 to 21 pct relative density, between 1300 °C and 1500 °C[8]), for aluminum foams with open cells (relative density of 6 to 14 pct, tested between 275 °C and 350 °C [5] ), and with closed cells (relative density of 8.7 pct, tested between 260 °C and 350 °C[6]). These studies show that, at low stresses, metallic foams exhibit the same creep exponent and activation energy as the bulk metal, as predicted theoretically by the strut creep-bending model of Gibson and Ashby.[1] At high stresses, anomalously high stress exponents have been measured in closed-cell aluminum foams crept at 300 °C[6,7] and are linked to localized high-stress regions (crush bands) where the material exhibits powerlaw breakdown.[7] Recently, we have developed a new process for the synthesis of nickel aluminide foams, based on pack aluminization of pure nickel foams.[9,10] Homogenization at elevated temperatures resulted in foams with a single, homogenous NiAl phase, which retained the open-cell architecture and hollow struts of ANDREA M. HODGE, formerly Graduate Student, Department of Materials Science and Engineering, Northwestern University, is Postdoc
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