Investigation of the influence of the sign of the g/g'-lattice misfit on the creep behaviour of single-crystal superalloys with g/g'-microstructure

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The g/g'-lattice misfit is an important structural parameter of Ni- and Co-base superalloys which determines the efficiency of precipitate strengthening and the thermal stability of the g/g'-microstructure. In commercial Ni-base superalloys the misfit at service temperatures is negative, but in new Co-base superalloys it is positive. This reopens the fundamental debate about the question which sign of the misfit is beneficial for the mechanical properties of superalloys with g/g'-microstructure. The experimental and theoretical investigation of this question is the topic of the present proposal. It starts with investigating the influence of the misfit sign on the creep behaviour of single-crystal superalloys with g/g'-microstructure. Here Ni-base superalloys will be used, first, because the g/g'-partitioning behaviour of the alloying elements in Ni-base superalloys allows to design alloys with negative and positive misfit as well and second, because a huge database is available for Ni-base superalloys, allowing to model misfit and creep. Three comparative nickel-base superalloys with negative, zero and positive misfit will be designed using our model, which predicts the misfit in dependence of alloy composition and temperature. The target misfit values correspond to the service temperature of a turbine blade, in our case 1000°C. The misfit will be controlled by substitution of Cr by Ni and vice versa, i.e. the atomic concentrations of other elements are kept constant. This compositional variation changes the misfit significantly but has only a small influence on the g'-volume fraction and the creep resistance of g- and g'-phase, as shown in our preliminary work and follows from literature. Such a principle of alloy design is necessary in order to separate the effect of composition on the misfit from its effects on other properties, such as solid solution strengthening. The sign of the misfit can influence the creep in two ways: first, via the inversion of the sign of the misfit stresses at all temperatures and second, via the change of the rafting direction at high temperatures. In order to separate both effects, [001] oriented single-crystals of the designed alloys will be tested under creep conditions at a lower temperature of 850°C, where the microstructure remains cuboidal, as well as at a higher temperature of 1000°C, where rafting occurs. The experiments will be accompanied by modelling of creep rate for microstructures with different given g'-morphologies and misfit signs. The planned investigations will clarify which sign of misfit is beneficial for the creep behaviour of superalloys. Depending on the results, it could be a first step to the targeted development of superalloys with positive misfit.