Influence of attrition milling on nano-grain boundaries

Abstract

Nanostructured materials have a relatively large proportion of their atoms associated with the grain boundary and the method used to develop the nano-grains has a strong influence on the resulting grain boundary structure. In this study; attrition milling iron powders and blends of iron powders produced micron-size particles composed of nano-size grains. Mechanical cold-working powder resulted in dislocation generation; multiplication; and congealing that produced grain refinement. As the grain size approached nano-dimensions; dislocations were no longer sustained within the grain and once generated rapidly diffuse to the grain boundary. Dislocations on the grain boundary strained the local lattice structure which; as the grain size decreased; became the entire grain. Mechanical alloying of substitutional aluminium atoms into iron powder resulted in the aluminium atoms substituting for iron atoms in the grain boundary cells and providing a grain boundary structure similar to that of the iron powder processed in argon. Attrition milling iron powder in nitrogen gas resulted in nitrogen atoms being adsorbed onto the particle surface. Continued mechanical milling infused the nitrogen atoms into interstitial lattice sites on the grain boundary which also contributed to expanding and straining the local lattice.