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Coherency loss marking the onset of degradation in high temperature creep of superalloys: phase-field simulation coupled to strain gradient crystal plasticity
A dislocation density based crystal plasticity — phase-field model is applied to investigate directional coarsening during creep in CMSX-4 Ni-based superalloys in the high temperature and low stress regime. Coherency between the ordered γ′ precipitates and the disordered γ channels prevents the generation of geometrically necessary dislocations, since the precipitate can be considered undeformable in the low stress regime. After coherency loss between the γ matrix phase and the γ′ precipitates the constraint against generation of geometrically necessary dislocations is relaxed, causing rotation of the crystal lattice under uniaxial load, known as “Schmid rotation”. As a consequence, the creep rate in the matrix increases, whereby degradation can be measured by the number density of geometrically necessary dislocations. The state of coherency loss is associated with the minimum creep rate in a creep experiment under constant load. The presented simulations start from a coherent γ′ precipitates distribution with random size and position generated during a precipitation heat treatment process. Simulations of N-type and P-type rafting under tensile and compressive load respectively are presented. The effect of coherency loss, coalescence of precipitates and lattice rotation due to generation of geometrically necessary dislocations is discussed in correlation with experimental findings.