Phase transformation and deformation behavior of a TiAl-Nb composite under quasi-static and dynamic loadings

To improve the high-temperature deformability of TiAl intermetallics, a TiAl-Nb intermetallic matrix composite was prepared by powder metallurgy to try to introduce the deformable beta phase. The microstructure and deformation behavior of the TiAl-Nb intermetallic matrix composite under quasi-static and dynamic conditions were systematically investigated. It is found that the microstructure of the composite is composed of a near gamma TiAl matrix and Nb-rich regions. In the Nb-rich regions a coexistence of beta(o), omega(o) and gamma phases was identified where omega(o) and gamma phases were supposed to directly precipitate in the beta(o) matrix. The ultimate tensile strength of the composite exhibits different temperature-dependent behaviors under quasi-static and dynamic loading conditions. It increases with temperatures up to 650 degrees C and then declines with temperature under quasi-static loading, while it does not decrease from 650 to 850 degrees C under the dynamic loading. In the deformed microstructures, the Nb-rich regions at all conditions are still composed of beta(o), omega(o) and gamma phases, indicating that the solvus temperature of the omega(o) phase could be above 850 degrees C. Additionally, the TiAl matrix appears to not significantly contribute to the plastic deformation of the composite, where the propensity of dislocations and mechanical twins do not significantly change when varying loading conditions. However, the gamma particles formed in Nb-rich regions contain substantial mechanical twins and dislocations under both quasi-static and dynamic loading conditions. Thus, the gamma particles directly transformed from the beta(o) phase exhibit better deformability than the gamma phase in the matrix. This could be probably explained by a decreased stacking fault energy for the newly formed gamma phase due to a decrease in Al fraction and a significant increase in Nb fraction. Meanwhile, twin intersections occurred in the gamma particles, and the induced stress concentration in the intersection regions was relieved by lattice distortion and detwinning processes. This twin intersection behavior could be beneficial for improving the composite strength without sacrificing much ductility.

Automated summary

By introducing Nb into TiAl intermetallics, a TiAl-Nb intermetallic matrix composite was successfully prepared with improved high-temperature deformability. The composite exhibited different temperature-dependent behaviors in ultimate tensile strength, with Nb-rich regions showing better deformability.