Strain Effect on the γ′ Dissolution at High Temperatures of a Nickel-Based Single Crystal Superalloy

The kinetics of the γ′ phase dissolution have been studied at 1473 K and 1523 K (1200 °C and 1250 °C) for CMSX-4® alloy. Interrupted creep tests at 1323 K (1050 °C) and pure thermal aging at 1373 K (1100 °C) have been used to vary the initial γ′ morphology. Subsequent dissolution studies were conducted with or without an applied load. Differences in γ′ dissolution kinetics were observed between the dendrite cores and the interdendritic regions, resulting from the chemical microsegregations remaining after the standard heat treatments. It is shown that the initial γ′ morphology, the relaxation of the coherency stresses and the accumulated plastic strain are critical parameters controlling the dissolution kinetics. An increase of the accumulated plastic strain leads to an increase of the dissolution kinetics, whatever the location in the dendritic structure. In addition, once a given accumulated creep strain is reached prior to the dissolution tests (i.e., a given dislocations density), the dissolution kinetics are the same at 1473 K and 1523 K (1200 °C and 1250 °C). A modified equation of the recently developed Polystar model is proposed to incorporate the role of the accumulated plastic strain on the γ′ dissolution kinetics and to achieve a better predictability of the creep deformation under non-isothermal loading paths.