The role of self-fluidization in combustion synthesis of porous and granular Ni-Al intermetallics

Abstract

A wide range of capillary processes participates in the structural transformation of the reacting metal powders during the combustion synthesis of intermetallics. Using additives that decompose in the combustion wave allows controlling the formation and interaction of melts that open new opportunities for direct manufacturing of materials with desired shape and structure, which are ready for applications without additional processing steps. This paper studies the combustion mechanism of Ni+Al powder mixtures (average size of particles 10 µm) doped with CaCO3. The methods of high-speed imaging, pyrometric temperature measurements, and reaction quenching were used. It was found that CaCO3 decomposes in the preheat zone of the combustion wave resulting in the fluidization of the powder mixture. In the reaction zone of the combustion wave, an ensemble of moving microdroplets with a diameter of 0.1–0.2 mm forms. Some microdroplets drift in a preheat zone of the combustion wave, which stimulates the fluidization process. The microdroplets ultimately form larger droplets with a diameter of up to 2 mm that crystallize and form spherical struts of the gas-permeable scaffold of the combustion products. Synthesis of intermetallic compounds couples with the formation of calcium aluminates, which affects the processes of reaction coalescence and the structure of combustion products. For the CaCO3 concentration below 5–8%, the combustion products represent strong porous scaffolds consisting of tightly fused spherical-like intermetallic struts forming a net of mm-sized interconnected pore channels. Brittle scaffolds comprised of detached intermetallic granules with a diameter of 25–200 µm separated by oxide layers form if CaCO3 concentration exceeds 14%. The washing-out of the oxide phases with 5% hydrochloric acid allows for obtaining the powder of the remaining granules. The effect of self-fluidization on the structure of the synthesized alloys has been discussed in detail.