Determination of superlattice stacking fault energies in multi-component superalloys

Superalloy single crystals, used in the manufacture of gas turbine blades, can accumulate a substantial amount of plastic strain in a relatively short time when subjected to conditions that favour primary creep. This presents a challenge when aero engines are operated at full power during take-off, climb to cruising altitude and thrust reversal whereby these materials are subjected to comparatively high stresses. These stresses are not sufficiently high to allow the cutting of the L12 ordered intermetallic phase precipitates by paired a/2<110> dislocations bounding antiphase boundaries, as is observed during macroscopic yield. Instead, the precipitates are sheared by widely extended a<112> dislocations that form low-energy superlattice stacking faults (SSFs). The susceptibility of superalloys to primary creep is strongly dependent on their composition. Understanding of the compositional effects on the SSF energies is therefore of great importance to the design of future alloys. Ab initio calculations can provide limited insight into these effects, but are computationally expensive. In this work we employ Transmission Electron Microscopy (TEM) in conjunction with the Phase Field Model of Dislocations (PFMD) [1] to investigate the formation of SSF nodes [2] on superdislocation networks in Ni- and Co-Al-W-based superalloys. We use PFMD to evaluate the effect of stacking fault energies on the geometry of the SSF nodes and apply this insight to experimental evaluation of SSF energies from TEM imaging of the nodes in order to investigate the compositional dependencies and influence on primary creep behaviour.