Decomposition of Massively Transformed Ti-Al MA Powders

Phase transformation during heating of mechanically alloyed (MA) Ti-Al powders whose compositions were Ti-40, -45 and -50 at.% Al were investigated using analytical transmission electron microscopy (TEM/EDS). An Al supersaturated alpha -Ti nanocrystals were formed in Ti-rich Ti-Al powders, while an amorphous phase was formed in the Ti-50 at.% Al powder after 720 ks MA process. TEM /EDS examination revealed that when the powders were heat treated the alpha phase in these MA powders underwent a massive transformation to a nonequilibrium gamma phase at the temperature range between 873-973K. In addition, in the Ti-50 at.% Al MA powder, crystallization of the amorphous phase to an alpha phase took place prior to the massive transformation. A large number of defects such as stacking faults, micro-twins and local strain contrasts were observed in the massively transformed gamma ( gamma sub M ) grains. The gamma sub M phase decomposed to equilibrium gamma , alpha sub 2 and Ti sub 2 AlC phases in the Ti-40 and 45 at.% Al MA powders, and to equilibrium gamma and Ti sub 2 AlC phases in the Ti-50 at.% Al MA powder during heating > 973K. The decomposition behavior of the gamma sub M phase and microstructure of the consolidates were strongly influence by the heating rate.