COMBUSTION SYNTHESIS OF NI3AL AND NI3AL-MATRIX COMPOSITES

The self-propagating mode of combustion synthesis (SHS) of Ni3Al starting from compacts of stoichiometrically mixed Ni and Al powders readily forms fully reacted structures with about 3 to 5 pct porosity, if green density of the compacts is greater than about 75 pct of theoretical. SHS-produced Ni3Al matrix composites with up to 2 wt pct Al2O3 whiskers also have relatively low porosity levels. Porosity increases rapidly with lower green densities, higher Al2O3, or SiC whisker contents, and the degree of reaction completeness diminishes. The SiC whiskers undergo reaction with the matrix, while Al2O3 whiskers are nonreactive. All of these observations correlate well with temperature measurements made during the course of the reaction. The SHS mode can be achieved with agglomerated particle size ratio D(Al)/D(Ni) greater-than-or-similar-to 1, larger than the limit established from studies of the thermal explosion mode of combustion synthesis, D(Al)/D(Ni) congruent-to 0.3.