Processing - structure - property relationships in nanocrystalline and microcrystalline niobium carbide based cermets.

摘要

In recent years, there has been a great deal of interest in nanocrystalline materials. Indications are that these materials often possess some extraordinary properties relative to their microcrystalline counterparts. Some researchers have found greatly improved strengths and fracture toughnesses. A common method for improving the toughness of brittle microcrystalline materials has been to toughen the material with a ductile phase. One such class of materials is cermets, or brittle ceramics toughened by ductile metals. In this work, we explore the combined effects of ductile metal toughening of brittle ceramics and the microstructural scale on the mechanical properties of a variety of cermets.; Specifically, the processing, structure, and mechanical properties of NbC based cermets has been studied. Room temperature mechanical alloying of elemental powders was used to produce nanocrystalline cermet powders where the brittle phase was NbC, and the ductile phase was either Fe, Cu, or a combination of these elements. The high melting temperature NbC was produced from its elemental constituents at room temperature. These cermet powders were consolidated by hot-isostatic-pressing at low temperatures and moderate pressures in evacuated steel cans. Analysis of the consolidated materials revealed a fully dense bulk material that retained a structure on the nano-scale.; These bulk nanomaterials were studied by electron microscopy and x-ray techniques. Mechanical properties were approximated by microindentation and notched bend bar tests. Post consolidation heat treatment of the nanomaterials was carried out to coarsen the fine structures into the more conventional micro-regime. Electron microscopy, x-ray diffraction, and mechanical property tests were performed on the materials at various stages of microstructural development to create a continuous picture of the development of the materials from the nano-scale to the micro-scale. Detailed characterization of the microstructures through this microstructural evolution was performed.; The mechanical properties (namely fracture toughness) were compared with existing models for conventional micro-scale materials. These models consider the effects of the ductile phase and its apparent size on toughening of the brittle ceramic matrix. Correlation with these models and the experimental fracture toughness values were reasonable. This comparison has allowed these models, developed for conventional microcrystalline materials, to be effectively utilized in modeling materials of a microstructural scale an order of magnitude or more smaller.