On the massive phase transformation regime in TiAl alloys: The alloying effect on massive/lamellar competition

The influence of alloy composition on the solid-state transformations which occur during continuous cooling of TiAl-based alloys has been studied with the aim of defining the alloy compositions which most strongly influence the massive transformation. It has been found that heavy elements such as Nb and Ta decrease the cooling rates required to form massive gamma by suppressing the formation of feathery and lamellar structures to even lower cooling rates. It is proposed that the low diffusivity of such heavy elements retards these diffusion-controlled phase transformations (feathery and lamellar) so that the massive transformation, which does not require diffusion, but which occurs at a lower temperature can take place. The significance of the selection of alloying elements, which allow the formation of massive gamma, is briefly discussed in terms of the refinement of the microstructures of castings by heat treating massively transformed samples.