In-situ observation of deformation micromechanisms in a rafted γ/γ′ superalloy at 850 °C

In-situ straining experiments were performed in a transmission electron microscope at temperatures ranging from 800 to 850 °C on a MC2 γ/γ′ nickel-based superalloy single crystal with a rafted microstructure. Perfect and partial dislocations were observed in motion. It is shown that deformation starts in the γ phase, which is the weaker phase at this temperature, although interaction of dislocations with point defects is still effective. The γ/γ′ interfaces play a different role depending on the propagation direction of the dislocations. From γ to γ′, interface crossing often acts as an additional obstacle that can be overcome by pile-ups of dislocations. From γ′ to γ, an increase of the dislocation speed is almost always observed, with no noticeable waiting time at the interface. Some dislocation multiplication processes are observed when interfaces are crossed from γ′ to γ. Unusual dissociation of superdislocations involving super stacking faults is observed in γ′, along with microtwinning. This may be taken as an indication of the low stacking fault energy of the MC2 alloy.