Exothermic effects during sintering of a mixture of nickel and aluminum powders. II

When a mixture of nickel and aluminum powders is sintered there is an exothermic effect which involves a rise of several hundred degrees in the temperature of the compact in a few seconds. The magnitude and nature of the exothermic effect depend on the aluminum content of the mixture, the degree of dispersion of the powders, and the initial porosity of the specimens. The temperature at which the exothermic effect begins moves towards lower temperatures with increase in the degree of dispersion, with increase in the concentration of aluminum in the mixture, and with decrease in the porosity. The magnitude of the heat effect increases with increase in the aluminum content.It is shown that the exothermic effect in the sintering process arises mainly from decomposition of the liquid phase and crystallization of chemical compounds with large heats of formation. A definite role is played by the metallothermic reduction of nickel oxide by aluminum present in the liquid phase and by the formation of intermetallic compounds at the boundaries of dissimilar particles of powder during sintering in the solid phase.