Segregation-assisted creep in nickel-based superalloys: experiments, theory and modelling

Mid-temperature creep deformation in the range of 600 to 850°C is assuming greater importance in Ni-based superalloys. This is because the design operating temperature of the combustion cycle is increasing as the new generations turbofan engines become more efficient. The temperature at the rim of turbine disks and the root of turbine blades can be in this critical range of temperatures for significant portions of the mission cycle, leading to a complex time-dependent mode of plasticity called "microtwinning"'. Unfortunately this kind of coupled displacive-diffusive deformation mechanism is not yet well understood, and even the range of temperatures and stresses where microtwinning occurs is not clearly defined. This work explores the fundamentals of this phenomenon, from the kinetics of microtwinning to its influence on the mechanical behaviour of the material. To achieve this objective, coupled computational-experimental studies have been carried out. First, the contribution of microtwinning mechanism to the creep deformation of a single crystal superalloy is studied. The accumulated creep strain computed from quantitative stereology of the tested samples supports the role of this mechanism in conferring plastic deformation. Second, the chemical composition of the microtwins is analysed by means of atomic-resolution characterisation techniques (APT and TEM). Segregation of γ'-stabilisers to the growing faults is found to be crucial for the understanding of the creep mechanisms in this range of temperatures. Third, a model for diffusion-controlled growth of microtwins is proposed and used to recover the experimental creep strain rates. This then provides the basis for a thermodynamically consistent constitutive model developed on the basis of crystal plasticity theory. The constitutive model is subsequently implemented into a finite element code to study the activation of the different plastic mechanisms within single crystal and polycrystalline aggregates depending on the crystal orientation. With the support of this model, a relation between the rotations of the crystal and the creep life of the different crystal orientations is established. The numerical and experimental results ultimately reveal the critical role of the microtwinning on the asymmetric behaviour of the alloy and thus, its influence on the mechanical performance.