Measurement and Modeling of Creep in Open-Cell NiAl Foams

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.