Shearing mechanisms of stacking fault and anti-phase-boundary forming dislocation pairs in the γ′ phase in Ni-based single crystal superalloy

Ni-based superalloys rely on high volume fractions of L12 ordered γ′ phase precipitates for strength against creep deformation at elevated temperatures. However, under certain conditions dislocations existing in the FCC γ matrix may enter the L12 γ′ phase in pairs. The shear motion of different combinations of dislocation pairs creates different planar defects in the γ′ phase, including anti-phase boundaries (APB) or stacking faults. The formation of an APB requires the shear distortion associated with an a/2〈110〉 dislocation and the formation of a stacking fault requires the shear distortion associated with a k〈112〉 dislocation. Given that the native dislocations in FCC structure are a/2〈110〉, the formation mechanism of k〈112〉 dislocations remains to be clarified. Different mechanisms have been suggested for the formation of stacking faults in the γ′ phase in the literature. In this study, the shearing motions of various partial dislocation pairs and the planar defects formed in the γ′ phase were investigated by means of transmission electron microscopy and the mechanisms were analyzed in terms of their crystallographic and energetic implications.