STUDIES ON THE ALPHA-]GAMMA PHASE-TRANSFORMATION MECHANISM IN TI-48 AT-PERCENT AL-ALLOY BY HIGH-RESOLUTION ELECTRON-MICROSCOPY

The alpha --> gamma phase transformation, which occurs during quenching and produces extremely fine alpha(2)-gamma lamellae, is discussed in relation to the analysis of the alpha(2)-gamma interface structure. In a high-resolution image obtained from the growth edge of a gamma plate in the alpha(2) matrix, steps that were always found to be (0002)(alpha 2) or (111)(gamma) planes high were observed at the alpha(2)-gamma interface. This indicates that the alpha --> gamma phase transformation is performed by the migration of (a/3) [1 (1) over bar 00](hcp) Shockley partial dislocations on alternate (0001)(hcp) planes to change the stacking sequence AB... to ABC.... Chemical analysis by analytical transmission electron microscopy with energy-dispersive X-rays showed that the alpha(2) and gamma plates have the same composition, so that the alpha --> gamma phase transformation which occurred during quenching can be interpreted as being composition invariant when these results are taken into consideration, it is suggested that the transformation is martensitic.