TRANSMISSION ELECTRON-MICROSCOPY CHARACTERIZATION OF MECHANICALLY ALLOYED NIAL POWDER AND HOT-PRESSED PRODUCT

NiAl intermetallic powder has been synthesized by mechanical alloying of elemental powders in an attritor mill using controlled atmospheres. The powders contain a bimodal distribution of dispersoids with sizes ranging from 5 to 200 nm. Analytical transmission electron microscopy has been used to establish the types of dispersoid phase present in the powder and the development of the microstructure with processing. In the finished powder the majority of the dispersoids present are either alpha-alumina or aluminum nitride depending on the process conditions. The grain size in the as-milled powder is nanocrystalline. Two types of hot pressing have been used for consolidation: a simple pressing using an argon cover gas and a vacuum hot pressing. After hot pressing to greater than 97% density, both grain and dispersoid growth are observed to have occurred and the larger dispersoids appear to coarsen in a coupled manner with the grains via a dragging mechanism. Hot-pressed materials show much improved yield strength and ductility compared with the ingot-cast NiAl, indicating the effectiveness of mechanical alloying in improving the mechanical properties.