Microstructure evolution and tensile deformation behavior of Ti-45Al-2.5Nb-1.5Cr-0.2 C alloy processed by dynamic zone annealing heat treatment

In the present study, in order to overcome the deterioration of microstructure and mechanical properties induced by the interface reaction between the alloy melt and the ceramic mold shell when preparing columnar grain TiAl alloys using the directional solidification technology, it was proposed to control the directional migration of grain boundaries through dynamic zone annealing heat treatment (also known as directional heat treatment, DHT) in the α single-phase region. thereby transforming the equiaxed grains of the as-cast TiAl alloy into columnar grains along the direction of the heat zone in the solid state. After subjecting to DHT, the Ti-45Al-2.5Nb-1.5Cr-0.2 C alloy was composed of aligned columnar grains, and the original nearly full lamellar microstructure of transformed into dual microstructure, accompanying network carbide distributed along the colony boundary. Compared to the as-cast TiAl alloy, the tensile ultimate strength and yield strength of the DHT-TiAl sample at room temperature were increased by 20 % and 19 %, respectively. The propagation path of the main crack changed from trans-lamellae in the as-cast TiAl alloy specimen to trans-lamellae plus inter-lamellae in the DHT-TiAl alloy specimen. Stacking faults were generated at the lamellar interface due to the dissociation of interfacial misfit dislocations induced by high cooling rate inherent to DHT. With the assistance of stacking faults acting as heterogeneous nucleation sites, the energy barrier for twinning deformation during room-temperature tensile testing was effectively lowered. Both dislocation-controlled and twinning-controlled plastic deformation mechanisms were activated in the DHT-TiAl sample. The interaction between twins and dislocation, as well as the intersection of primary twins and secondary twins contributed to an excellent combination of tensile strength and elongation of the DHT-TiAl sample.