Microstructure formation process and grain refinement of heptane added Ti-Al mechanically alloyed powders

Microstructure formation process during heating of n-heptane added mechanically alloyed (MA) Ti-Al compacts, as well as the effects of aluminum content and heating rate on the grain refinement of the MA compacts whose compositions were Ti-33-53 mol%Al were investigated. Aluminum supersaturated α nanocrystals were formed in Ti-rich Ti-Al powders, while the mixture of an amorphous phase and aluminum supersaturated α nanocrystals was formed in the MA powders with the aluminum contents more than 49 mol%. TEM/EDS examination of Ti-41, 46 and 51 mol%Al MA powders revealed that the α phase in the MA powders underwent a massive transformation to a non-equilibrium γ phase at the heating between 873 K and 973 K. In the case of Ti-51 mol%Al MA powder, crystallization of the amorphous phase to an α phase took place prior to the massive transformation. In the case of Ti-46 mol%Al MA powder, higher heating rate yielded finer grain size due to the finer dispersion of Ti2AlC particles. Increase in aluminum content resulted in the finer grain microstructure because of the increase in Ti2AlC particles in the vacuum hot pressed compacts sintered at 1173 K for 3.6 ks. Thus, grain refinement in the Ti-Al MA compacts can be accomplished by controlling the heating rate and the aluminum content. (author)