Path dependence and strength anisotropy of mechanical behavior in cold-compacted powders

Abstract

The problem of compaction of powders at low homologous temperatures has been studied over the last twenty years in many fields including powder metallurgy, ceramics, pharmaceutical, agricultural, and mining. Recent emphasis of research efforts has been on the use of phenomenological models that are capable of predicting compaction loads and density distributions in the final product. However, the mechanical properties of the compact cannot be predicted from current models since they consider strength as a function of density alone. A number of studies have shown that strength is dependent on other variables besides density, including the stress path used for consolidation. In prior work, path dependence in ductile powders has been shown experimentally. In this thesis, a ceramic, dibasic calcium phosphate, was consolidated using a variety of stress paths, ranging from nearly isostatic to nearly closed-die. Yield loci were shown to be dependent on stress path as well as compact density. Strength anisotropy in ductile and brittle powders was shown to exist after closeddie compaction and is dependent on compact density. Ductile powders become increasingly anisotropic with density. Brittle powders exhibit anisotropy during the early stages of compaction, but this diminishes as densification continues. Separate mechanisms to explain these behaviors are proposed and supported with experimental data from tensile strength testing, SEM fracture surface analysis and surface area testing. Finally, path dependence and strength anisotropy are shown to have a common origin, namely, directionality of microstructure resulting from initial particle morphology and particle deformation during compaction.%%%%Ph.D., Materials Science and Engineering – Drexel University, 2005