Preparation, Structure And Properties Of Ni-Cr-Fe Superalloy Foams

As a kind of new multifunctional materials, porous foam metals exhibit many excellent properties, such as lightweight, heat insulation, shock resistance, impact resistance and electromagnetic shielding, and have a widely apllied prospect in the areas of aviation and aerospace, energy and so on. With the high density or being lack of oxidation-resistance problems, the traditional superalloy materials haven't met the urgent needs of aviation or aerospace areas where desire lightweight superalloy materials.In this paper, reticulated lightweight Ni-Cr-Fe superalloy foams were synthesized from unalloyed nickel foams by using a two-step, high-activity pack-ferrochromizing process at 1050℃and homogenization at 1200℃. The morphology and microstructure of Ni-Cr-Fe superalloy foams were observed and analyzed by optical microscope and scanning electron microscope (SEM). Mechanical properties of Ni-Cr-Fe superalloy foams were tested in room and high temperature, to study the influence of composition on mechanical properties of Ni-Cr-Fe superalloy foams. Finally, the oxidation-resistance properties of Ni-Cr-Fe superalloy foams were tested at 800℃, 900℃and 1000℃separately.At 1050℃, the Cr and Fe atoms in the solid-powder evenly deposited onto the surface of pure Ni foam, formed an alloy layer which would get more thicker with the extension of time. The homogenization at 1200℃/48h could remove concentration gradients and to solutionize the alloyed struts, formed Ni-Cr-Fe superalloy foams which well kept the morphology of three-dimensional reticulated structure of the original pure Ni foam, the struts became thicker and the pores get smaller.Compared to the pure Ni foam, the Ni-Cr-Fe superalloy foams show better properties, the compressive strength and energy absorption capacity under normal and high temperature have been greatly improved. With the increase of Cr and Fe content, the relative density, microhardness, compressive strength and energy absorption capacity Ni-Cr-Fe superalloy foams increased. The compressive strength and energy absorption capacity dropped dramatically as the temperature increased. At 800℃, the Ni-Cr-Fe superalloy foam is relatively stable, after 50h oxidation, the weight gain(Δω) was 0.009⁰.012g,Δω/m0 was 0.52⁰.76% and shown excellent oxidation-resistance. At 900℃, there had been a protective layer- Cr2O3 forming at the surface of Ni-Cr-Fe superalloy foam, it also shown excellent oxidation- resistance, but at 1000℃, the Cr₂O₃ layer wrinkled and ruptured, resulting in the oxidation-resistance Ni-Cr-Fe superalloy foams weakened.The oxidation kinetics of Ni-Cr-Fe superalloy foams followed the parabolic law, (Δω)²=kpt, as the temperature increased, the oxidation rate of Ni-Cr-Fe superalloy foams speed up, and the oxidation-resistance declined; at the same temperature, the pre-oxidation rate was 5³0 times higher than the middle and late stage.