Reaction mechanism of combustion synthesis of NiAl

Based on precise temperature measurements during combustion and microstructural analysis of quenched samples, the evolution of reaction of the NiAl combustion synthesis has been studied. The combustion reaction of a multilayer Ni/Al system takes place in a thermal explosion mode under near adiabatic conditions. The experimental results clearly show that the combustion reaction starts right after the melting of Al. From the start to completion, the reaction goes through three stages. In the first stage, the temperature rises from the melting point of aluminum to the decomposition temperature of the intermediate phase NiAl₃, i.e. 854°C. The reaction is the dissolution of nickel in liquid aluminum, with the formation of small amounts of intermediate phases NiAl₃ and Ni₂Al₃ at the solid-liquid interface. In the second stage, the temperature of the system increases from 854 to about 1300°C. The reaction is still the dissolution of nickel in liquid aluminum solution. However, due to supersaturation, solid NiAl precipitates out at about 1300°C, generates a great deal of heat and increases the temperature suddenly. The third stage starts at about 1300°C, and ends at the maximum reaction temperature. The reaction rate of this stage is much higher (two orders higher) than that of first and second stages. The final product, liquid NiAl, forms at this stage. © 2002 Elsevier Science B.V. All rights reserved.