Microstructure and Mechanical Properties of As-Cast -TiAl Alloys with Different Cooling Rates

-TiAl alloys composed of Ti-45Al-5Nb-1W-1B (at.%) solidified at three different cooling rates by vacuum electromagnetic induction melting under a controlled degree of vacuum of under 2Pa. The influence of the cooling rate on the microstructure and mechanical properties of the -TiAl alloy samples was investigated. A relatively large temperature gradient at the beginning of solidification led to a thin fine-grained layer around the graphite mold, and undercooling had two main effects: (a) promoting nucleation in the grain-refining surface layer and (b) promoting the growth of columnar crystals beneath the surface. Moreover, all prepared ingots of this alloy could be clearly divided into Regions S, M and C from the surface to the core. The colony size of Region M was larger than those of the other two regions due to composition segregation and the growth of columnar grains. The existence of tungsten led to micro-inhomogeneity, and the mechanical properties of the alloy changed according to concentration fluctuations. The degrees of homogeneity were expressed as H-0.1 and H-0.2. The alloy with the highest values, which possesses the best mechanical properties, solidified at a relatively low cooling rate. The deformation mechanism is also discussed.