| Nickel-aluminide foams were synthesized from unalloyed nickel foams by using a two-step, high-activity pack-aluminizing process at 1273 and 1307 K. After processing, the nickel aluminide foams exhibited the same macro structure as the original nickel foams (open-cells with hollows struts). Single-phase NiAl foams, with average composition of 72 wt.% Ni (stoichiometry 68.5 wt.% Ni) and with 93–95% open porosity, were produced by first selecting the appropriate aluminizing time then annealing to homogenize the structure. Nickel wires and tubes were also aluminized at 1273 K and homogenized for various times to further investigate the aluminizing kinetics and the creation of Kirkendall pores.; NiAl foams with two different relative densities and cell size, 5.0 and 6.6% (20 and 30 pores per linear inch (ppi), respectively) were tested under compression creep conditions for temperatures ranging from 1073 to 1373K, and stresses ranging from 0.1 to 1.5 MPa. For stresses lower than 0.5 MPa the foams exhibit primary and secondary creep with power-law behavior, while at higher stresses power law breakdown was evident. In the former range, the creep exponent and the activation energy of the foams are 3.5 ± 0.33 and 200 ± 21 kJ/mole as calculated by linear regression of all data. These values are in good agreement with values reported for bulk nickel-rich NiAl.; A Finite Element model (FEM) of an idealized cell was implemented for two different geometric cell models with solid struts and 10.7, 5.5 or 5.0% relative density, and hollow struts with 5.5 or 5.0% relative density. The FEM results show reasonable agreement with the experimental data (20 ppi foams, 5.0–5.4% relative density), with a predicted minimum strain rate slower by a factor of 2.4. The Ashby Gibson analytical model predicts values of minimum strain rate up to 150 times faster than those of the experimental data. Based on the FEM results, a very simple analytical model is proposed, whereby struts in the vertical direction are in pure compression and the horizontal struts are only there to prevent buckling. This model produced results that fall close to the FEM predictions and could be used as an initial order-of-magnitude estimate for future tests in similar foams. |
