Mechanomaking of nanostructured hypereutectic white irons: Powder fabrication

Abstract

White irons containing 5·7 wt-%C were produced by room temperature, high energy milling from iron and carbon elemental powders. Both iron and cementite phases have crystal sizes of less than 10 nm in the powders which have fully dense particles of 50 μm mean size. The gas content is 0·5 wt-% and the powder is stable up to 550°C. Above 550°C decomposition of cementite occurs, involving a decrease in combined carbon content from 5·7 to 3·5 wt-% (annealing at 1050°C). The crystal growth kinetics has two temperature regions with lower activation energies in the high temperature (above 800°C) region. Crystal sizes still below 100 nm were observed for annealing up to 800°C. The size distribution in mechanosynthesised powders was modified by tumbling (dry) and attritor (wet) milling down to mean sizes of 4 μm and 1·4 μm respectively. Whereas tumbling milling does not alter the properties of MS powders, wet attritor milling produced higher gas content (7 wt-%) and much decreased thermal stability. Mechanosynthesised and tumbling milled powders can be degassed prior to consolidation, while wet attritor milled ones cannot.