In-situ processing of NiAl–alumina composites by thermite reaction

NiAl–Alumina composites have been fabricated by reactive sintering of ball-milled and compacted powder mixtures containing NiO and Al. Dense NiAl–Al2O3 composite with an interpenetrating Al2O3 network was fabricated after 2 h of vacuum hot pressing at 1200°C and 50 MPa. The thermite reaction mechanism, as well as phase and microstructural development of the composites, were investigated by quenching the compacts from different temperatures during vacuum hot press. It was found that different intermediate phases are formed depending on the temperatures; Ni formed first at around 600°C, followed by NiAl3, Ni2Al3 and NiAl as temperature increased. The formation of Al2O3 phases during reactive hot press is believed to be a three-stage process. Initially, a small amount of Al2O3 was produced by the reduction of NiO with solid Al. As temperature increases, Al2O3 particles forms by the reduction of NiO with liquid Al. Finally, a NiO/Ni/Al2O3/NimAln layer structure forms via solid state displacement reaction between NiO and nickel-aluminides (NiAl3, Ni2Al3, NiAl) leading to the formation of interpenetrating Al2O3 network in the final product.