Creep deformation of nickel based superalloys

Since the mid 1940s, the need for more efficient and powerful jet engines and gas turbines have pushed the boundaries of high temperature materials to service conditions of up to 1100°C and mechanical loads in excess of 100MPa. The state of the art material in such conditions are nickel based superalloys, whose resistance to mechanical load at high temperatures stems from particles of a second phase (γ') embedded into the γ-matrix. The particles are usually not cut by single matrix dislocations as these would leave behind an Anti-Phase-Boundary with high energy. Dislocations pile up around the particles, leading to complex dislocation networks in the matrix which further strengthens the material.

Nonetheless, even superalloys slowly degrade and deform plastically in use. The complex deformation behaviour has been subject of research throughout the last 60 years, but a general, fully physics-based material model is still missing. It is well established that, climb and glide of dislocation is the main driver of macroscopic plastic deformation. Based on the Dyson flow-rule, a dislocation density based material model has been implemented in AceGEN, which allows for rapidly replacing and adjusting all constitutive equations. Physics-based models make use of constitutive equations which are derived from fundamental theoretical considerations. In regard of the complexity of dislocation interaction with the microstructure, such models also require a number of parameters whose, in contrast to phenomenological models, order of magnitude is known. The model takes into account the volumetric fraction of γ'-particles and temperature and is, therefore, ideally suited to describe the behaviour of polycrystals.

On the length-scale of the γ'-particles, we develop a fully implicit 3d eXtended Finite Element (XFEM) model. The γ'-particles alter shape under thermal and mechanical load, which is commonly denoted rafting. This will be captured by solving an additional equation for the level set propagation.