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 investig ated using analytical transmission electron microscopy (TEM/EDS). A ne w 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 con sisted 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 th e MA powder was heated up to the consolidation temperature, 1173 K, a massive transformation of the or phase to a non-equilibrium gamma(gamm a(M):L1(0)) phase took place, followed by decomposition of the gamma(M ) to equilibrium gamma grains, alpha(2) and Ti2AlC particles. The orie ntation 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 fo rmation of the (alpha(2)+gamma) microduplex structure in titanium alum inide compacts made by mechanical alloying.