Segregation-assisted climb of Frank partial dislocations: An alternative route to superintrinsic stacking faults in L12-hardened superalloys

In this work, we present the first observation and high-resolution analysis of Frank partial dislocations formed upon high temperature creep inside the L12 structure of a superalloy. The dislocations and associated planar faults are observed in a multinary Co-based superalloy after [001] tensile creep at 850 °C and 400 MPa up to a plastic strain of 4.6%. With their a/3〈111〉 Burgers vectors the Frank partial dislocations create superlattice intrinsic stacking faults (SISFs) by positive climb motion via vacancy condensation. High-resolution analysis of the opposite end of such an SISF revealed a terminating a/6〈112〉 Shockley partial dislocation, which is connected to an antiphase boundary (APB). This indicates that the whole defect configuration resulted from splitting of a Lomer-type a/2〈110〉 matrix dislocation which became incorporated into the γ′ phase. All observed SISFs exhibit solute segregation effects with enrichment of Co, Cr and W and depletion of Ni and Al. Leading Frank partial dislocations show pronounced Cottrell atmospheres of Co and Cr and asymmetric concentration profiles. The climb motion of the Frank partial is driven by an osmotic force induced by vacancy supersaturation and is assisted by solute segregation lowering the SISF energy. We discuss the forces acting on the aforementioned dislocations and their motion through the microstructure. Moreover, we propose a natural mechanism by which Lomer-type matrix dislocations can be generated enabling the formation and observed climb of Frank partial dislocations.