STRUCTURE EVOLUTION OF DIRECTIONALLY SOLIDIFIED Ti--50Al ALLOY AND LAMELLAR ORIENTATION CONTROL

Ti–Al alloys as the high temperature structural material with the most prospective development are widely used in aerospace. Further study should been conducted on their formation of fully lamellar structure in directional solidification and lamellar orientation control for a good balance of mechanical properties. Directional solidification experiments were conducted for Ti–50Al (atomic fraction, %) alloy in a relatively wide range of growth rates. The effects of growth rate on interfacial morphology, microstructure evolution and formation of lamellar structure were investigated. A single–phase growth of cellular α was observed in a growth rate range of 1—5 μm/s, and finally a fully lamellar structure was formed. When the growth rate reached 10 μm/s, a single–phase growth of cellular α was also observed during a relatively long distance after initial solidification, but as solidification proceeded, intercellular solute enrichment became so severe that γ phase precipitated from liquid appeared between α cells, and finally a full lamella can not be formed. When the growth rate was higher than 15 μm/s, a dendritic growth of α phase and γ phase between α dendrites were observed. * ) 50771041 50801019, * 20080430909, LBH–Z08127 ! HITQNJS.2008.018 +, # : 2009–04–27, #$# : 2009–09–18 -AE%! : $*, #, 1979 , $ , 11 %%. : & & Ti–50Al '&/0'()AE! &'1 1337! The analysis on the final lamellar orientations at different growth rates showed that the lamellar orientation is history–dependant on the orientation of as–cast grain at the started interface of direcitonal solidification. Based on the above rules, Ti–50Al alloy, also as seed, was solidified under controlling the lamellar orientation, and a relatively low growth rate of 8 μm/s was chosen to ensure a single–phase growth of α. Finally, a fully lamellar structure with an orientation parallel to the growth direction was obtained.