Dislocation motion during high-temperature low-stress creep in Ru-free and Ru-containing single-crystal superalloys

Creep deformation of the two experimental single crystal superalloys at high-temperature low-stress (1140 °C/137 MPa) has been analyzed through transmission electron microscopy. Emphasis is placed on elucidating the dependence of dislocation motion on microstructural evolution. The detailed analysis demonstrated that the stacking fault energy of the γ matrix significantly decreased with the addition of Ruthenium (Ru). The stacking faults presenting in the γ matrix after heat treatment has been rarely reported previously. During the primary creep stage, the dislocations can easily cross-slip on the different {1 1 1} planes in the horizontal matrix and leave 60° dislocation loops on the (0 0 1) γ/γ′ interfacial plane. Furthermore, calculations demonstrated that it is difficult for the slipping dislocations to bow into the vertical γ matrix channel. In the early stages of steady state creep, the interfacial dislocations reoriented slowly from the 〈1 1 0〉 slipping direction to the 〈1 0 0〉 well misfit stress relief direction. On the other hand, few dislocations shearing into the rafted γ′ phase have been observed. In fact, during the middle stages of the steady state creep, although perfect dislocation networks have formed, some dislocations shearing into the γ′ phase have also been observed. In addition, the a 〈0 1 0〉 type superdislocations (some with non-compact core) have also been observed in the two experimental alloys. At last, the Ru-containing alloy possesses more negative lattice misfit, denser γ/γ′ interfacial dislocation networks and higher microstructural stability, thus can maintain a minimum creep rate in the steady state stage and have a longer creep life.