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 γ′ precipitates. The net result of the morphological evolutions of the γ′ 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 γ 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.

Employing sequential experiments to gain insights on the interaction between creep and fatigue damage in Ni‐base superalloys

“…[3][4][5][6][7] For common blade alloys, the volume fraction of the γ 0 precipitates is quite high, ranging from 0.6 to 0.75. Since the γ/γ 0 structure readily evolves at higher temperatures with the evolution dependent on the stress level coupled with a limited amount of inelastic deformation, [8][9][10][11][12][13][14][15][16][17] the creep behavior of single-crystal superalloys can be put in one of three categories tied to the morphology and state of the γ/γ 0 structure which can be generally grouped by stress level and temperature, as illustrated in Figure 1. Microstructure evolution leading to rafting (directional coarsening) occurs at high temperatures and low stress influenced by misfit stresses at the γ/γ 0 interphase.…”

A viscoelastic – viscoplastic material model for superalloy applications

“…The work of Zhao and Tong [13] is similar to that of Cornet et al, however damage terms are not considered and the main focus of the work is the application of a power law viscoplastic model to study local crack tip behaviour. Viscoplasticity has also been applied in finite strain formulations, as evidenced in the recent work by Kirka and Neu [14]. Here, crystal plasticity models were developed for the γ and γ phases that often form in Nickel superalloys.…”