Microstructural evolution of Ti–47Al–2Cr–2Nb–0.8B alloy prepared by semi-solid process

The refinement of large boride ribbons in the as-cast TiAl alloy is the essential issue for aerospace industry application, which is difficult to avoid by classical casting techniques. The present paper seeks to explore the possibility of the semi-solid process in manufacturing Ti–47Al–2Cr–2Nb–0.8B (at%) alloy. An important result is that, except forming a nondendritic globular structure, the semi-solid process also plays a crucial role in refining large borides for the TiAl alloys with boron. In the current alloy, the long ribbon borides can be successfully converted to fine, equiaxed particles with an average size of 2–5 μm. Furthermore, subsequent proper heat treatment is necessary to control and achieve a fine, nearly fully lamellar microstructure. The key microstructure-refinement mechanism is due to the fine borides obtained during semi-solid process, which act as the effective nucleation centers for the formation of interdendritic fine γ grains, providing the pinning effect on prior α-grain boundaries. These results prove that semi-solid process can be an ideal candidate for the microstructure refinement in TiAl alloys.