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 th e powder and the development of the microstructure with processing. In the finished powder the majority of the dispersoids present are eithe r alpha-alumina or aluminum nitride depending on the process condition s. The grain size in the as-milled powder is nanocrystalline. Two type s 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 obs erved to have occurred and the larger dispersoids appear to coarsen in a coupled manner with the grains via a dragging mechanism. Hot-presse d materials show much improved yield strength and ductility compared w ith the ingot-cast NiAl, indicating the effectiveness of mechanical al loying in improving the mechanical properties.