A study was made of the formation of deformation twins during the high-temperature creep of a high volume fraction monocrystalline superalloy. A correlation with the nature of the superlattice stacking faults was also observed. In general, it was found that the formation of high-temperature twinning could always be associated with the loading orientation in which SESF formation was expected from a determination of the sign of the shear stress. The reason for the twinning being associated with extrinsic stacking faults could be rationalized by considering the critical radius of stacking-fault loop nucleation. In particular, calculations performed at 1223K demonstrated that a minimum
radius was associated with 4 overlapping faulted planes for the extrinsic case. Although the process by which twin formation occurred had not been observed, it was shown that twin formation could take place via the passage of extrinsic stacking faults within the precipitate, which operated on every alternate plane of the structure. If formation was considered to occur via a pole mechanism, similar to that in a face-centered cubic structure, a mechanism for this could be proposed due to the fact that climb of the helix structure as it rotated around a pair of suitable matrix dislocations would amount to twice that which occurred in the single-pole case.

Superlattice Stacking Fault Formation and Twinning during Creep in γ/γ' Single Crystal Superalloy CMSX-4. D.M.Knowles, Q.Z.Chen: Materials Science and Engineering A, 2003, 340[1-2], 88-102