DEFECT HEALING DURING SINTER/HIP

The Sinter/HIP process combines sintering of a green body and postdensification by HIP in one cycle. During Sinter/HIP of polyphase materials which contain a viscous or a liquid phase for an enhanced densification, such as in cemented carbides or structural ceramics, during HIPing the separation of grains and viscous phase can occur in residual macropores which may result from inadequate powder processing of differential sintering of agglomerated powders. This effect was studied by investigating the elimination of artificial spherical macropores in W-Ni and Al2O3-anorthite during Final Stage HIP. To characterize the densification behavior, creep parameters and viscosities were measured in a load dilatometer. The healing mechanisms were investigated and compared to viscoelastic models. Microstructural observations showed that phase separation will mainly occur in materials with large grains or extended grain growth during HIPing. Pore healing can be described by defect healing diagrams which indicate the dominating healing mechanism for different pore sizes at special HIP conditions. The diagrams are based on a model which relates the diffusional creep of the solid phase to the flow of the viscous phase through channels present at triple junctions. The model for the prediction of the volume content of the viscous phase in previous macropore spaces as functions of materials and HIP parameters are in good agreement with the experimental results.