Past approaches to modelling the creep behaviour of engineering alloys have been either totally empirical or, while having functional forms consistent with current understanding of deformation and fracture mechanisms, have been calibrated by comparison with an available creep database. They have not specifically included quantitative measures of the microstructural features that are thought to impart creep resistance to the alloys. The present paper reviews and extends a microstructure-specific model of creep in particle-strengthened alloys in which the model parameters are directly related to measurable characteristics of the microstructure of nickel-base superalloys. The model accounts for three previously identified changes in microstructure that occur during the creep of superalloys: (i) coarsening of the precipitate; (ii) the progressive increase in mobile dislocation density with accumulated creep strain; and (iii) the development of grain boundary cavities. The relative dominance of each will be demonstrated using a set of commercial alloys with specific alloy microstructures. It is emphasised that the model-generated curves are true predictions using input microstructural characteristics and are not empirically fitted to the creep data.

The (111) intrinsic stacking fault energy γISF in Ni and Ni-Co alloy was calculated and compared using two different ab initio methods, viz., the supercell approach and the axial interaction model (AIM), based on density functional theory. The supercell approach uses energies of crystal structure in slab geometry with and without the stacking fault. In the AIM approach, the problem is mapped to a 1D spin-model and the interaction parameters are obtained using energies for ordered structures, thus obviating the need to handle faulted structure. For elemental Ni, the calculated values of γISF from AIM and supercell approaches differ by not more than by 2%, and compares well with experimental value. For Ni-Co alloy, AIM predicts a slightly faster decrease in γISF with increasing Co concentration compared to supercell approach and experimental data. Overall, there is good agreement between the two approaches.

An ab initio method based on density functional theory has been employed to compute the zero-temperature anti-phase boundary (APB) energies for Ni3Al1−xRx (R = Nb, Ta, Ti) system over a range of compositions. The computation is limited to the APB on the (1 1 1) plane for L12 crystal structure, allowing only the volume relaxation, appropriate for the γ′ precipitate in Ni-based superalloy. For the limiting case of the binary system Ni3Al, the APB energy has also been calculated for the (1 0 0) plane. We find that the APB energy for the (1 1 1) plane in Ni3Al is 181 mJ m−2, and substitution of Nb, Ta or Ti at the Al site increases the APB energy to over 600 mJ m−2, leading to higher strengths. While the peak APB energy values for all the ternary systems are quite similar, they are achieved over very different compositional ranges. Nb and Ta are found to have almost identical strengthening effect on Ni3Al. The selected compositional space is of direct relevance to the commercially important family of Ni-based superalloys, and our results provide important guidelines for alloy design strategies.

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