Change in Morphology of Mechanically Alloyed Nb/Al and Ti/Al Powders and Their Material Properties.

Abstract

Milling processes of powder mixtures of elemental Nb and Al and those of Ti and Al which resulted in the formation of amorphous and crystalline intermetallic compounds were investigated from the viewpoint of change in morphology and microstructure of milled powder particles during milling. The morphological and microstructural change was observed by optical and scanning electron microscopy. The formation of alloy phase from the elements was examined by microhardness measurements made on the milled powder particles. The amorphization behavior was monitored by X-ray diffraction and thermal analyses. As a result, the milling processes of both Nb/Al and Ti/Al were found to go through the follow steps; in the initial stage of milling flaky and plate-like particles with a characteristic structure consisting of an Al-matrix with small Nb or Ti fragments embedded in it were formed; subsequently, lamellar structure developed in these particles due to their plastic deformation; large globular particles were formed by coalescence of the lamellar particles; refinement of the lamellar structure and the eventual formation of an amorphous or crystalline alloy phase occurred in surface regions of the globular particles; in the final stage fine amorphous or crystalline powder broke off from the surfaces of the globular particles. A glass forming range of Nb/Al determined by X-ray diffraction analysis in the present experiment was 60-90wt.%Nb. The amorphization of Ti/Al powder was found to be strongly dependent on the milling ball size. TEM study on the amorphous Ti/Al powder particles revealed that the amorphous particles includes crystallites of nanometer size in an amorphous matrix.