Creep-induced planar defects in L12-containing Co- and CoNi-base single-crystal superalloys

Precipitate shearing mechanisms during single-crystal tensile creep of L12-containing Co- and CoNi-base alloys have been investigated. Interrupted creep tests were conducted at 900 °C under vacuum at constant load, and stresses varied between 275 and 345 MPa. Transmission electron microscopy diffraction contrast analysis revealed superlattice intrinsic and extrinsic stacking faults in the γ′ precipitates in the Co-base alloys. However, in alloys containing significant additions of nickel (CoNi-base alloys), antiphase boundaries formed by shearing of a/2〈1 1 0〉 dislocations on octahedral planes. The shearing mechanisms in the Co- and CoNi-base γ′ phase are compared to commercial Ni-base alloys, and the relative fault energies inferred from these observations are discussed.