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 ?' strengthening phase and the corresponding high tem-perature 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 ?' 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) charac-terization and the thermodynamic calculation, respectively. The dislocations shearing into the ?' precipitates is the primary deformation mechanism in both alloys when deformed at 600 degrees C. However, when increasing the deformation temperature to 800 degrees 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 degrees 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.