PHASE-TRANSFORMATION OF HEPTANE ADDED TI-AL MECHANICALLY ALLOYED POWDERS DURING HEATING

Phase transformation behaviors of Ti-Al mechanically alloyed (MA) powd ers during heating mere investigated with analytical TEM, DSC and XRD. Elemental Ti and Al powders with overall compositions of 33, 41 and 4 6 mol%Al were blended for mechanical alloying. N-heptane was used as a process control agent for the mechanical alloying. Irregular shaped a nd dislocation free Al supersaturated alpha-Ti nanocrystals mere forme d in the MA powders after 720 ks of milling. The mean grain size is ab out 25 nn. An hcp-->fcc-->hcp-->DO19 transformation in the Ti rich MA powders and an hcp-->fcc-->L1(0) transformation in the Al rich MA powd ers were observed during heating. The morphological features are as fo llows: (1) an fcc/hcp thin layered structure, (2) a composition invari ant, (3) a large number of defects, and (4) rapid grain growth. They s trongly suggest that the grains are formed by shear mechanism as well as short range diffusion, i.e., a massive transformation during heatin g. The fee phase formation can be explained by an hcp-->fcc structure change by the motion of a a/3[1 (1) over bar 001]type Shockley partial dislocation on every second plane of the basal plane of the hcp cryst al. Since an hcp structure is stable in Ti rich MA powders, the fee ph ase, which in turn transforms to the hcp phase by the motion of a a/6[ 11 (2) over bar] type partial dislocation. Finally, the DO19 (alpha(2) ) or L1(0) (gamma) phases form from the hcp or fee phases by the short range diffusion.