The Influence of Oxygen and Nitrogen Contamination on the Densification Behavior of Cryomilled Copper Powders during Spark Plasma Sintering

It has been found difficult to fully densify some mechanically milled pure metal powders by spark plasma sintering (SPS). In this study, the densification behavior of cryomilled, nanostructured (NS) Cu powders during SPS was related to changes to the chemistry of the powders. The results showed that the presence of very small amounts of O and N in the powders, which were introduced during cryomilling and handling, significantly influenced the densification response. Moreover, reduction/removal of O/N via thermal annealing of the powders before SPS led to complete densification of the powders during subsequent SPS. The mechanisms responsible for this behavior were ascertained: O and N existed in the cryomilled powders in the form of thermally unstable compounds, and the subsequent thermal decomposition of these compounds during SPS generated the gaseous species, leading to porosity formation and incomplete densification; annealing of the powders before SPS removed the gases which resulted from thermal decomposition, thereby facilitating complete consolidation during subsequent SPS

Microwave assisted processing of ceramics

Microwave heating can provide cost and time effective processing routes for ceramic systems. Microwave heating assisted drying of green compacts, low temperature sintering of certain oxides, and partial or complete nitridization, carburization or reduction of hemi-oxides have been attempted recently. Using a multi-mode microwave cavity, we were able to reduce MeOx (where Me = Cu and Ag) to their sub-oxides or base metal at close to room temperature within minutes. The product sub-oxides or metals were identified and characterized for their crystallinity and phase purity. The results suggest that the microwave heating can provide an alternative, cost effective route for alloying for certain metals. The criteria for effectiveness of microwave heating and the coupling between the materials and the electromagnetic wave are discussed

High-Pressure Torsion-Induced Grain Growth and Detwinning in Cryomilled Cu Powders

Two mechanisms for deformation-induced grain growth in nanostructured metals have been proposed, including grain rotation-induced grain coalescence and stress-coupled grain boundary (GB) migration. A study is reported in which significant grain growth occurred from an average grain size of 46 nm to 90 nm during high pressure torsion (HPT) of cryomilled nanocrystalline Cu powders. Careful microstructural examination ascertained that grain rotation-induced grain coalescence is mainly responsible for the grain growth during HPT. Furthermore, a grain size dependence of the grain growth mechanisms was uncovered: grain rotation and grain coalescence dominate at nanocrystalline grain sizes, whereas stress-coupled GB migration prevails at ultrafine grain sizes. In addition, detwinning of the preexisting deformation twins was observed during HPT of the cryomilled Cu powders. The mechanism of detwinning for deformation twins was proposed to be similar to that for growth twins