Synthesis of nickel–aluminide foams by pack-aluminization of nickel foams

Nickel–aluminide foams were synthesized from unalloyed nickel foams by using a two-step, high-activity pack-aluminizing process at 1273 K. After processing, the nickel–aluminide foams exhibited the original structure of the original nickel foams (open-cells with hollows struts and low density). Single-phase NiAl foams, with average composition within 1 wt.% of stoichiometry and with 92% open porosity, were produced by first selecting the appropriate aluminizing time, and then annealing to homogenize the structure. Foams of average Ni3Al composition were produced by the same method, but multiple intermetallic phases remained due to large variations in strut thickness and thus local composition. Nickel wires and tubes were also aluminized at 1273 K and homogenized for different times to further investigate the aluminizing kinetics and the creation of Kirkendall pores. For aluminization depths up to about 100 μm, Kirkendall pores can be avoided, leading to pore-free struts in the foam.