Experimental and theoretical evidence of displacive martensite in an intermetallic Mo-containing γ-TiAl based alloy

In this study the martensitic transformation behavior of a Mo-bearing γ-TiAl based alloy was investigated. Therefore, a homogenization treatment within the single β-phase field region followed by water quenching has been carried out, whereby the majority of the disordered β-phase transforms into hexagonal α ₂′-martensite during cooling. Since the β to α transformation in the intermetallic β-solidifying Ti-44Al-3Mo-0.1B alloy (at%) occurs at very high temperatures causing enhanced diffusional processes, a very locally diffusion-controlled transformation together with a displacive, hence purely martensitic transformation take place. This work investigates the displacive martensite formation in the high-temperature regime using state-of-the-art experimental methods as well as modelling concepts from the phenomenological theory of martensite crystallography. The high temperature at which the transformation takes place suggests the preference of plastic slip over twinning. This fact has also been verified by transmission electron microscopy, as no twinning has been observed after generation of single martensite variants forming an invariant interface plane with the initial β-lattice. Such invariant interfaces are formally possible according to the phenomenological theory of martensite crystallography, if the Bain strains of the martensite variants are superimposed by additional simple shear.