Can local hot spots induce α2/γ lamellae during incomplete massive transformation of γ-TiAl alloys?

Oil quenching is applied to a Ti-45Al-7.5b alloy (in at%) in order to investigate the occurrence of local “hot spots” during rapid cooling from the single α-phase region. After quenching the microstructure consists of large α2-Ti3Al grains and a small volume fraction of massively transformed γm-TiAl particles. The majority of the γm-particles show a featureless α2/γm interface, whereas in the immediate vicinity of some γm-particles lamellar α2/γ-structures exist, exhibiting a Blackburn orientation relationship. The extension into the α2-parent grain is in the range of 1–10 μm. In this study it is investigated, if the diffusion-controlled formation of the γ-laths is triggered by the release of latent heat during the α → γm transformation, which leads to a local increase in temperature by a hot spot effect. A theoretical model is presented, which describes the temperature conditions around the hot spot. The model predicts that the life time of the hot spot is much too short to generate γ-laths observed with a length of 1–10 μm.