Crystal Viscoplasticity of a Ni-Base Superalloy in the Aged State

Abstract

Arising from long-term high temperature service, the microstructure of nickel-base (Ni-base) superalloy components undergoes thermally and deformation-induced aging characterized by isotropic coarsening and directional coarsening (rafting) of the c0 precipitates. The net result of the morphological evolutions of the c0 particles is a deviation of the mechanical behavior from that of the as-heat treated properties. To capture the influence of a rafted and isotropic aged microstructure states on the long-term constitutive behavior of a Ni-base superalloy undergoing thermomechanical fatigue (TMF), a temperature-dependent crystal viscoplasticity (CVP) constitutive model is extended to include the effects of aging. The influence of aging in the CVP framework is captured through the addition of internal state variables that measure the widening of the c channels and in-turn update the material parameters of the CVP model. Through the coupling with analytical derived kinetic equations to the CVP model, the enhanced CVP model is shown to be in good agreement when compared to experimental behavior in describing the long-term aging effects on the cyclic response of a directionally solidified (DS) Ni-base superalloy used in hot section components of industrial gas turbines.