Preparation of CoAl intermetallic compound by combustion synthesis in self-propagating mode

The production of cobalt aluminide (CoAl) from elemental powder compacts was conducted by self-propagating high-temperature synthesis (SHS) in this study. Effects of the initial sample density, particle size of the reactants, and initial sample temperature on the combustion characteristics, as well as on the composition and morphology of final products were studied. Unlike most of the intermetallic reactions, the combustion on the CoAl powder compact is self-sustained upon initiation even without any preheating prior to ignition, implying that the reactions are highly exothermic. The combustion process was preceded by the fast propagation of the flame-front, followed by vigorous bulk combustion. The flame-front propagation velocity and combustion temperature were found to increase with increasing sample green density. The use of fine particles of Al (10 μm) also increased the flame-front velocity. In addition, the combustion products synthesized from the samples using fine Al particles were slightly shrunk or retained the original shape. However, when the coarse particles of Al (350 mesh) were used the sample exhibited a volume expansion after combustion, resulting in an increase in the sample porosity. According to the XRD analysis, combustion products obtained in this study were all made up of a single-phase composition of CoAl without the presence of any secondary phases or unreacted constituent elements. Based upon the measured data of this study, the activation energy with a value of 121.7 kJ/mol was deduced for the synthesis of CoAl by SHS.