Modeling the Temperature-dependence of Tertiary Creep Damage of a Directionally Solidified Ni-base Superalloy

Directionally solidified (DS) Ni-base superalloys have become a commonly used material in gas turbine components. Controlled solidification during the material manufacturing process leads to a special alignment of the grain boundaries within the material. This alignment results in different material properties dependent on the orientation of the material. When used in gas turbine applications the direction of the first principle stress experienced by a component is aligned with the enhanced grain orientation leading to enhanced impact strength, high temperature creep and fatigue resistance, and improve corrosion resistance compared to off axis orientations. Of particular importance is the creep response of these DS materials. In the current study, the classical KachanovRabotnov model for tertiary creep damage is implemented in a general-purpose finite element analysis (FEA) software. Creep deformation and rupture experiments are conducted on samples from a representative DS Ni-base superalloys tested at temperatures between 649 and 982°C and two orientations (longitudinallyand transversely-oriented). The secondary creep constants are analytically determined from available experimental data in literature. The simulated annealing optimization routine is utilized to determine the tertiary creep constants. Using regression analysis the creep constants are characterized for temperature and stress-dependence. A rupture time estimation model derived from the Kachanov-Rabotnov model is then parametrically exercised and compared with available experimental data. INTRODUCTION Creep is defined as the time-dependent, inelastic deformation of a structural component at high temperature. Creep is temperature and stress dependent. Three distinct stages of creep are considered when examining creep strain of Ni-base superalloys. The first region, called primary creep, is due to strain-hardening where pre-existing dislocations encounter obstacles and becoming immobilized [1]. It initially occurs at a high rate but the eventual saturation of dislocation density inhibits further primary creep deformation. For Ni-base superalloys, primary creep is typically small. After this period, secondary creep is characterized by an almost constant strain rate due to a balance between strain-hardening and recovery mechanics. Increased mobility enhanced by thermal activity (temperature induced diffusion) can cause cross slip where dislocations can diffuse away from obstacles [2]. In this region, the nucleation of grain boundaries and grain boundary sliding occur. Finally, tertiary creep becomes dominant and is characterized by the rapid non-linear increase of strain rate until creep rupture. The coalescence of grain boundary voids induces microcracks. Unchecked growth of cracks coupled with a net area reduction leads to rupture. The advent of DS superalloys has lead to major advancements in the power generation industry where components experience high load and temperature environments [3]. Directional solidification involves the casting of a material so that grain boundaries are aligned at a desired orientation. The established manufacturing process is the Bridgeman vacuum casting process, where a directional heat flow is generated via remove of the shell mould from a hot zone to a cooling zone at some prescribed rate. The result of these casting processes is a component which exhibits enhanced strength, stiffness, and/or creep resistance in a set orientation. The creep deformation and rupture response of a material is highly dependent on the nature of the grain structure of the material. While grain boundaries parallel to the load direction impart enhanced material properties, perpendicularly oriented boundaries facilitate accelerated creep deformation and rupture. Proceedings of the ASME 2009 International Mechanical Engineering Congress & Exposition IMECE2009 November 13-19, Lake Buena Vista, Florida, USA