The effects of Ti and Ta on the microstructure and high temperature deformation mechanisms of the CoNi-based superalloys.

The roles of the alloying elements, Ti and Ta, on the γ' strengthening phase and the corresponding high temperature deformation responses of the CoNi-based superalloys have been systematically investigated. The results indicate that addition of either 4 at.% Ti (4Ti) or 2 at.% Ta (2Ta) can significantly increase the volume fraction of the γ' phase. Detailed analysis of the deformation mechanisms and Gibbs free energies of each phase in the two alloys at elevated temperatures were carried out by using the transmission electron microscopy (TEM) characterization and the thermodynamic calculation, respectively. The dislocations shearing into the γ' precipitates is the primary deformation mechanism in both alloys when deformed at 600 °C. However, when increasing the deformation temperature to 800 °C, the dislocation cross-slipping and by-passing mechanism dominants the 4Ti alloy's deformation, while the additional stacking faults (SFs) were introduced to the 2Ta alloy due to the lower barrier energy of the SFs formation. Continuing increasing the deformation temperature to 1000 °C will promote the dislocation-based mechanisms by lower the dislocation propagation resistance in both alloys. The process will also induce the mechanical twins in the 2Ta alloy which can further strengthening the alloy. The alloying induced deformation mechanism transition and the related high-temperature mechanical performance of the CoNi-based superalloys have been further discussed. • Adding Ti or Ta to the CoNi-based alloy will improve its γ' volume fraction to 60% • Ti and Ta will improve the flow strength at medium and high temperature, respectively • Ta can reduce the energy barrier for stacking fault formation at high temperature • Deformation mechanisms at elevated temperatures can be altered by the alloy elements [ABSTRACT FROM AUTHOR]