REACTIVE SINTERING OF NI3AL UNDER COMPRESSION

Reactive sintering of Ni3Al was performed with uniaxial compressive stresses ranging from 0 to 120 MPa, using elemental powders with the stoichiometric composition preheated to 620-degrees-C in a vacuum of 7 x 10(-3) Pa, It was shown that both compressive stress and heat flow strongly affected the reaction process and, hence, the structure and density of reaction-sintered products. Without compression, reaction-sintered products had a relative density up to 98% and were mainly composed of Ni3Al with uniformly distributed fine pores and large shrinkage cavities located in the center. Green density has little effect on densification. There was no effect from a compressive stress when it was applied to specimens after the onset of the self-sustaining reaction. When a green compact self-ignited under a pre-loaded compressive stress (50 MPa), a highly densified product (relative density as much as 99.3%) was obtained. In addition, the product, which was composed of Ni3Al, NiAl and Ni, did not contain large shrinkage cavities. A detailed description of the entire reaction process is given on the basis of electron probe microanalyses and thermodynamic considerations.