Defect Healing Mechanisms during Sinter/HIP of Polyphase Materials

Abstract

The Sinter/HIP — Process is a technology for sintering and post — densification in one cycle with low pressures. It is a beneficial and economical method for the production of dense and homogeneous materials. Sinter/HIP may be a favored technology for polyphase materials, such as cemented carbides, heavy metal alloys or ceramics, which consist at the sintering temperature of hard grains and a highly mobile or viscous phase. For industrial applications, however, it is necessary to investigate the densification behavior and the defect healing mechanisms to establish the conditions under which a dense and homogeneous microstructure is obtained. In tungsten with nickel and in alumina with anorthite-glass the densification behavior and the elimination of large defects were investigated during Sinter/HIP. The defects were similar to macropores which may result from differential sintering of agglomerated powders or after debinding of injection molded greens. To verify the densification behavior, creep parameters and the viscosity of the systems were measured in a load dilatometer by single sphere deformation. The predictions made from the deformation experiments are in good agreement with the measured densities of HIPed samples. Macropore elimination by a simultaneous flow of grains and the highly mobile intergranular phase is obtained by changes of the amount of intergranular phase and the application of different Sinter/HIP — conditions. The conditions necessary for a mixed flow during postdensification with low pressures by HIP are quantified and discussed as a function of temperature, time, pressure and initial gram size.