EFFECTS OF ALUMINUM CONCENTRATION AND COMPACT THICKNESS ON REACTION SYNTHESIS OF NI3AL-NIAL ALLOYS

The effects of aluminum concentration and green compact thickness on the reaction synthesis of powder compacts containing 25 to 40 at. %Al were studied under a uniaxial compressive stress of 10 MPa. The results indicate that both reaction product and material density are sensitive to these two parameters. For all powder compacts with a thickness of 5 mm, no temperature increase was detected during reaction synthesis, indicating no formation of transient liquid phases and thus no self-sustaining combustion reaction. For 6- and 7-mm-thick compacts containing 40% Al, a full synthesis reaction took place, resulting in production of the NiAl alloy with a high material density (>98%) and a fine grain size (14 mum). Other compacts containing 25 and 30% Al showed a coexistence of partially and fully reacted zones, with the reaction zone size strongly dependent on aluminum concentration and compact thickness. All the results are discussed in terms of local heat accumulation and heat transfer as well as thermodynamic analyses of exothermic reactions during each reaction step.