Combustion synthesis and near-net shape forming of nickel aluminides

In this dissertation, the combustion synthesis of nickel aluminides was investigated with a focus on reaction thermodynamics, reaction mechanism, and product net shape forming method. First, the final state of the combustion synthesis of Ni-Al system was predicted by thermodynamic adiabatic temperature calculations, and was verified by experiments. The results indicated that in most conditions the adiabatic reaction temperature was equal to or higher than the melting temperature of the reaction products. Therefore, the products were in a semi-solid or liquid state after the reaction.; Second, the reaction mechanism of (3Ni +Al) and (Ni +Al) systems was studied by using laminated reactants and in a near adiabatic condition. The evolution of the reaction and the events which occurred during the reaction were successfully revealed in the temperature (T)-time (t) and temperature change rate (dT/dt)-T plots. The intermediate reaction products were identified by quenching techniques. Furthermore the intermediate reaction states and reaction path were confirmed by thermodynamic calculations. The results showed that the reaction of both (3Ni+Al) and (Ni +Al) started at the melting temperature of Al, then proceeded to 854°C with the dissolution of Ni in liquid Al and the formation of intermediate solid compounds, Al₃Ni and Al ₃Ni₂. Afterwards the reaction in (3Ni+Al) is dominated by solid-state reactions, the sequential formation of solid interfacial phases, Ni₂Al₃, NiAl and Ni₃Al. On the other hand, the (Ni +Al) system proceeded with the continuous dissolution of Ni and precipitation of NiAl.; Third, a novel net shape forming method of intermetallics based on the combination of combustion synthesis and semi-solid forming techniques was proposed and explored. The key of the method is to obtain semi-solid intermetallics by combustion synthesis. Various techniques controlling the liquid fraction in combustion synthesis products were presented. As an example, the liquid fraction as a function of the initial reaction temperature and the amount of dilutes has been calculated for NiAl and Ni₃Al. Experimentally, semi-solid Ni₃Al has been produced by the combustion synthesis of Ni and Al powders. Semi-solid Al₂O_3p/Ni₃Al composites with uniform reinforcement distribution have been obtained too.