FORMATION AND CHARACTERISTICS OF NANO-CRYSTALLINE TIAL

We have attempted to set up a crystallization kinetics for the formation of nano-crystalline TiAl and a HIP map for the full densification of amorphous alloy powders and also to characterize the fully densified nano-intermetallics. A nano-crystalline Ti49Al51 exhibiting a single exothermic DSC peak is characterized by a two-phase structure in a metastable equilibrium which consists of gamma(TiAl) with alpha(2)(Ti3Al) and an amorphous phase. With decreasing temperature to 795 K, the average gamma grain size (d) decreases to 10.4 nm and at the same time the amorphous volume increases to 32.4%. The Johnson-Mehl-Avrami analysis permits us to derive the Avrami exponent of 3 for the middle stage of nano-crystallization of Ti49Al51, indicating a cellular growth mechanism for the saturated nucleation sites. The HIP mechanism diagram which is constructed using the viscous flow parameters for amorphous Ti49Al51, and is subjected to full structural relaxation predicts the full densification prior to the onset of crystallization. For the fully densified nano-crystalline Ti49Al51, the Vickers hardness (H-V) follows a Hall-Fetch relation of proportional to d-(0.5) above a critical grain size (d(c)) of approldmately 15 nm. Below d(c), a relation of H-V proportional to d(0.5) is formulated based on the mixture rule.