High temperature creep in Ni-based superalloys

The high creep resistance of Ni-based superalloys at elevated temperatures distinguishes them from other materials and enables them to be utilized in a wide range of high temperature applications. Their exceptional performance can be attributed to their unique microstructure, which consists of ordered cuboidal precipitates of γ′ phase that are aligned along elastically soft <100> directions in a disordered FCC γ matrix. However, if the working conditions are harsh, such as a complicated mixture of high temperatures and stresses over an extended period of time, it can result in microstructural evolution with the combination of diffusion of alloying elements and creep deformation. To model the complex behavior of superalloys at service conditions, we provide a systematic and in-depth analysis of creep behavior in superalloys by incorporating thermodynamics, material diffusion, and elasto-plasticity into the multi-phase-field framework. The creep deformation of superalloys is performed using the strain gradient dislocation density based crystal plasticity model. The model is validated against experiments at high temperature, low stress creep up to 1% creep strain. N- and P-type rafts are produced by applying tension and compression, respectively. The model is able to reproduce the right kinetics of microstructure evolution with up to 1% creep strain, which is in agreement with experimental data. In order to capture the kinetics of the microstructure evolution as a diffusion-controlled process, we used an effective diffusivity taken from the slowest diffusing element, Rhenium. Furthermore, creep simulations of Ni-based superalloys for various γ′ particle sizes are performed to determine an optimum γ′ particles size which can yield better creep properties. Furthermore, it is demonstrated that loss of coherency between the γ matrix and γ′ precipitates is linked to the minimum creep rate and allows the precipitates to coalesce and initiate the rafting process. Coherency between the ordered γ′ precipitates and disordered γ channels precludes the emergence of geometrically necessary dislocations. Loss of coherency enables the generation of the GNDs and creates the non-recoverable crystal lattice rotation of γ matrix known as 'Schmid rotation'. Creep simulations of superalloys at low stress and high temperature regimes and measuring creep deformation and local rotations can further support this argument. Additionally, it has been observed that the rafting direction depends upon the state of the stress in the material. Normal tensile stress produces N-type rafts, whereas, mix mode or higher local stresses divert the direction of the rafting. The stresses close to the rupture zone of the tensile sample are extremely high, which causes the rafting to form in a zigzag pattern. Simulating creep process at higher stresses can create conditions comparable to those found near the rupture zone in experimental samples and could alter the rafting direction. Moreover, the crystallographically informed crystal plasticity model assists in determining the minimum creep deformation required to influence the rafting morphology.