Understanding the Yield Behaviour of Single Crystal Ni-based Superalloys
Nickel-based superalloys are the material of choice for turbine applications due to their superior high-temperature strength capabilities. This has been attributed to the γ' precipitates which have the L1₂-ordered crystal structure. Because of the ordered crystal structure, dislocations must travel in pairs, separated by an anti-phase boundary (APB), when moving through the precipitates. The dislocations are known to cross-slip from {111} glide planes onto {100} planes because the APB energy is lower on {100} planes. However, this theory remains unconfirmed. To better understand this unique property in superalloys, high resolution transmission electron microscopy (HRTEM) has been undertaken on tensile specimens of the single crystal Ni-based superalloys CMSX-4 under different deformation conditions. In a two-phase superalloy, during early stages of tensile deformation, dislocations nucleate in the γ matrix phase and glide in the narrow channels between precipitates. The yield point was found to correspond to the entry of dislocations into the γ' precipitates. Of the dislocations which enter into γ', only a small portion were observed to cross-slip. Due to the two-phase microstructure, dislocation loops are able to form which can shrink and annihilate. Atomic resolution of the dislocation pairs show the APB between the pair is on the {100} plane. On increasing the temperature, the dislocations in the γ channels are able to climb, and stacking fault shear of the precipitates becomes much more prevalent. Upon changing the strain rate of the tensile tests, deformation mechanisms akin to creep were observed which was associated with a drop in the stress. The stress drop is associated with the formation of stacking fault shear and was dependent on the composition of the alloy. The fault structure is the same as those observed in post-creep samples. Chromium and cobalt were also observed to segregate to the fault. The deformation structure of a fourth-generation superalloy, TMS-138A, was also analysed to understand the effects of additional rhenium and ruthenium. The yield point was lower and the dislocations are more rigidly confined to {111} slip planes in the γ channels. The observations and analyses included in this thesis improve the understanding of the tensile deformation process of two-phase, single crystal superalloys.
Tensile, single crystal, nickel superalloy, nickel-based, yield behaviour, dislocation
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