MICROSTRUCTURE OF A TI-45 MOL-PERCENT-AL MECHANICAL ALLOYED POWDER AND ITS ALPHA-]GAMMA MASSIVE TRANSFORMATION DURING CONSOLIDATION

The microstructure of a mechanically alloyed (MA) Ti-45 mol% Al powder, in the as-milled state as well as after heat treatment, was investigated using analytical transmission electron microscopy (TEM/EDS). A new method for preparing the TEM samples was employed which resolved the microstructure of the MA powders, whose particle sizes were more than 10 mu m in diameter. The MA powder milled for 720 ks was composed of nanocrystals, of which the grain size was approximately 20 nm, and consisted of an Al supersaturated alpha-Ti(HCP) phase and a small amount of alpha(2)(DO19) phase. No evidence was observed for the existence of other phases, such as carbides, oxides or an amorphous phase. When the MA powder was heated up to the consolidation temperature, 1173 K, a massive transformation of the or phase to a non-equilibrium gamma(gamma(M):L1(0)) phase took place, followed by decomposition of the gamma(M) to equilibrium gamma grains, alpha(2) and Ti2AlC particles. The orientation relationship between gamma(M) and alpha(2) strongly suggested that the gamma(M) nucleated at the alpha/alpha(2) interface boundary. The massive transformation, therefore, has an important role in the formation of the (alpha(2)+gamma) microduplex structure in titanium aluminide compacts made by mechanical alloying.