Effects of segregation in nickel-base superalloys: Dendritic stresses

Abstract

Residual stresses in the dendritic structure of single-crystal nickel-base superalloys were investigated. Dilatometric measurements showed that the thermal contraction of the dendrite arms (DAs) is larger than that of the interdendritic regions (IRs). The reason is, that the γ' phase, precipitating during cooling, has a smaller lattice parameter than the γ-matrix and that this γ/γ' misfit is higher in the DA than in the IR. In the "DA-IR-compound" the different thermal contractions cause residual stresses. The stress distribution within the dendritic cell was analyzed theoretically by finite element (FE) modeling. The stresses were proved directly by X-ray diffraction (XRD) and indirectly by the transformation of the γ/γ' morphology observed after load free annealing. It was found that dendritic stresses influence the structural and mechanical behavior of superalloys during high temperature creep: rafting and plastic deformation start in the primary dendrite arms (PDA).