Peritectic Solidification Kinetics and Mechanical Property Enhancement in a Rapidly Solidified Ti-48 at% Al-8 at% Nb Alloy via Hierarchical Twin Microstructure

The strength and ductility trade-off dilemma has limited the wide application of TiAl-based alloys. Here, a new insight into the potential for increasing the strength and ductility of a hypoperitectic Ti-48 at% Al-8 at% Nb alloy is accomplished by the electromagnetic levitation (EML) technique. Moreover, a systematic analysis of the primary and subsequent peritectic solidification kinetics is conducted in the undercooling range of 308 K. Assisted by a high-speed camera, in situ observation of the liquid-solid (primary beta-Ti phase and peritectic alpha-Ti phase) interface migration is accomplished. When the alloy melt is undercooled to 240 K, high-ordered nanotwins are observed in the Ti-48 at% Al-8 at% Nb alloy, which form a unique hierarchical microstructure. Upon further increasing the undercooling, the density of these nanotwins is significantly enhanced. The room-temperature compression results reveal that the strength and ductility increase up to 140% and 150%, respectively. This is mainly ascribed to the remarkable grain refinement, formation of nanotwins with various orientations, accumulation of dislocations and stacking faults, and retention of the metastable gamma-phase. The superior combination of strength and ductility indicates the possibility to fabricate high-ordered nanotwins via rapid solidification, thus improving the performance of gamma-TiAl-based alloys.