STUDY ON FORMATION OF LAMELLAR MICROSTRUCTURE OF Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y ALLOY BY HIGH TEMPERATURE LASER SCANNING CONFOCAL MICROSCOPE

In order to study the formation mechanism of fine fully lamellar microstructure of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy, the heat treatments of holding at 1450 degrees C for 35 s followed by cooling at the rates of 3-600 degrees C/min were conducted by a high temperature laser scanning confocal microscope. Evolution of lamellae were observed in situ, and the resultant microstructures were analyzed by scanning electron microscope and transmission electron microscope. The results show that the surface of the sample was contaminated by oxygen coming from the alumina crucible and the phenomena observed in situ are not as reliable as those occurred in the interior of the sample. The results observed in situ give some qualitative conclusions on alpha to lamellar colony transformation: gamma lamellae nucleate at alpha grain boundaries then propagate mainly from one end of grain to the other; at the same cooling rate, the start temperatures of the transformation are different for different alpha grains; with the increase of the cooling rate, the start temperature of the transformation decreases. The interior microstructures give some quantitative results: the alloy is in alpha+beta two phase region at 1450 degrees C; with the increase of cooling rate, the amount of B2 phase increases, the lamellar colony size and lamellar spacing decrease. When the cooling rate is higher than 30 degrees C/min, the nearly lamellar microstructure with volume fraction of B2 phase, lamellar colony size and lamellar spacing of similar to 2%, similar to 60 mu m, and about 100-500 nm, respectively, is obtained.