Effects of 1 at.% Ta substitution for 1 at.% W on the coarsening kinetics and deformation behavior of γʹ-strengthened CoNi-base superalloys

Abstract

The coarsening behavior of the γʹ precipitates and compression deformation mechanism of the γʹ-strengthened CoNi-base superalloys are investigated. By substituting 1 at.% Ta for 1 at.% W, the 4W2Ta alloy exhibits a higher γʹ solvus temperature of 1218 °C and a lower mass density of 8.93 g/cm3. During the aging process from 900 to 1000 °C, both of the 5W1Ta and 4W2Ta alloys show stable γ/γʹ two-phase microstructure. The coarsening behavior of the γʹ precipitates in the two alloys follows the classical Lifshitz-Slyozov-Wagner (LSW) model. The coarsening rate constant of the 4W2Ta alloy is approximately two times lower than that of the 5W1Ta alloy, which is attributed to its larger lattice mismatch due to the addition of Ta. Besides, compared with the 5W1Ta alloy, the 4W2Ta alloy has higher high-temperature strength and shows different deformation behavior. When the deformation temperature is lower than 800 °C, pairs of dislocations shear the γʹ precipitates and γ matrix. At 800 °C, stacking faults (SFs) appear in the γʹ precipitates and increase with the increasing temperature, whereas most of dislocations bypass the γʹ precipitates at 1000 °C.