IN-SITU HIGH-RESOLUTION ELECTRON-MICROSCOPE OBSERVATION OF OMEGA-PHASE PRECIPITATION IN BETA-TYPE TITANIUM-ALLOYS

In order to investigate dynamic events concerning atomic behavior of p hase transformation and precipitation, in-situ high resolution electro n microscope (HREM) observation methods are useful. In;situ HREM obser vation and observation by dark-field imaging have been carried our on the formation and growth of aged omega-phase in a Ti15Mo5Zr alloy sing le crystal. The effect of stresses on the formation of the omega-phase has also been investigated by HREM observations. The main results obt ained are as follows; (1) omega-phase crystals in thin foil specimens, which were aged for 100 ks at 623 K, did not appear. However, omega-p hase crystals appeared in a specimen, which was aged in the bulk state , thinned and then aged. Also, when a specimen was deformed by rolling and then aged for 3.6 ks, omega-phase crystals appeared in the whole of a deformation band. omega-phase appeared in a specimen deformed by tension. These results suggest that some kinds of stresses are necessa ry to form the aged omega-phase. (2) The results of in-situ HREM obser vation on the formation of aged omega-phase crystals suggest that the omega-phase grows by the formation of atom-pairs with 0.28 nm distance , which are formed by movement of two neighboring atom lines in opposi te [111] directions as required by the model.