High magnetic field induces preferential growth of α phases with specific orientation: Adjusting the alignment of α₂/γ lamellae for TiAl alloys

Based on α → α₂ + γ phase transition and crystallographic orientation relationship, as well as the theory of grain-preferred growth under high magnetic fields, this study proposes a novel method to adjust the alignment of α₂/γ lamellae for TiAl alloys through two-step treatment. The first step involved the pretreatment at 1350 °C (α single-phase region), followed by high magnetic field heat treatment at the same temperature. Results show that the fraction of α₂/γ lamellar orientation with an angle <30° respecting the magnetic field direction increases significantly to 72.74 % for pretreated Ti-42.91Al-0.69Fe alloy after holding for 30 min at 1350 °C with the 10 T magnetic field, compared with the reference sample. The preferred distribution of α₂/γ lamellae is attributed to the magnetic driving force generated at α grain boundaries, which predominantly dominates the preferential growth behavior of α grains by influencing the grain boundary migration during the short isothermal time under the 10 T magnetic field. After holding for 5 h at 1350 °C, the average grain size of Ti-42.91Al-0.69Fe alloy increases, and the grain boundaries become flatter. Consequently, the curvature driving force generated at the α grain boundaries is mostly lower than the magnetic driving force. The direction of grain boundary migration is thus primarily determined by the direction of magnetic driving forces during the holding time at 1350 °C under the 10 T magnetic field. Specifically, the grain boundaries of α grain move towards adjacent α grains and are swallowed up to grow as the 〈0001〉 orientation of α grains is normal to the magnetic field direction. Due to the unique habitus plane of γ and α phases, the α₂/γ lamellar orientation is approximately parallel to the magnetic field direction in the subsequent cooling progress. These results indicate that high magnetic field heat treatment at α single-phase region can adjust the alignment of α₂/γ lamellae.