CREEP DEFORMATION OF SOLUTION STRENGTHENED NICKEL-BASED SUPERALLOYS

Author

Daniel Lee Spader, University of Rhode IslandFollow

Date of Award

2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering and Applied Mechanics

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Hamouda Ghonem

Abstract

Materials selected for high temperature applications, as seen in aerospace and energy generation, must possess exceptional strength, toughness, and microstructural stability. Achieving these properties at elevated temperatures requires the use of increasingly complex alloys to cope with the extreme environment. One of the most prevalent families of materials used in these environments are nickel-based superalloys. With the increase in time, temperature, and microstructural complexity, creep deformation and its associated diffusional processes make prediction of long-term deformation increasingly challenging. As the operational life of some of these materials approach 60 years, one-to-one testing cannot be conducted in laboratory environments. As the mechanical behavior is dependent on the microstructural state, long-term predications of in-service deformation requires a fundamental understanding of how temperature and stress will affect the microstructural stability of these materials, the rate at which these changes will occur, and how these changes will impact the deformation characteristics during operation.

The γ’-precipitate strengthened nickel-based alloys have proven beneficial in many industrial applications of superalloys through the resulting increases in yield strength seen in low to moderate temperature environments. With increasing service temperatures, the γ’ phase also increases creep resistance by acting as a barrier to dislocation motion, forcing dislocations to loop, shear, and/or climb, thereby increasing the high temperature strength and prolonging the life of the material. However, at temperatures exceeding ~760°C, the benefits of the phase are lost as the γ’ precipitate phase dissolves back into the matrix. In applications at or above the γ’ solvus, the most common precipitates which form within the matrix and along the grain boundaries in nickel-based superalloys, such as Inconel 617, are the meta-stable MC and M₂₃C₆ carbide formations.

Inconel 617, a candidate material for high temperature applications exceeding 900°C, is a solid solution strengthened nickel-based super alloy owing its high temperature strength to misfit strains induced through the addition of elements such as Cr, Mn, Fe, and Co to the Ni, FCC, matrix. At these elevated temperatures, the solubility of these elements is exceeded within the solid solution resulting in segregation, precipitation, and coarsening of secondary phases in both intragranular and intergranular regions. The elements Cr and C most abundantly favor the precipitation of carbides of the M₂₃C₆ type, which precipitate in a dispersed, discrete fashion in the early stages of creep (early on in the service life exposure). These carbides have been shown to dissolve and re-precipitate within the grain boundary as service time increases. In the grain boundary, M₂₃C₆ carbides can form discrete networks with discernable spacing or continuous slabs which appear as “sheets” of carbides between grains.

The presence of discrete carbides along grain boundaries and in the matrix has shown to provide high temperature strength as they act as obstacles for dislocation motion; suppressing grain boundary sliding and associated microstructural changes. While carbides remain in a stable state, long-term predictions of creep deformation can be extrapolated from initial microstructural conditions. When carbides become unstable, prediction of time-dependent creep deformation becomes a difficult task.

Mechanisms of creep for crystalline materials are generally described in terms of dislocation motion and/or the diffusion of vacancies through the matrix and along the grain boundary. At lower temperatures and higher stress, creep deformation is governed by the glide of dislocations along slip planes in what is known as dislocation creep. With an increase in temperature, aided by diffusion, dislocation climb can contribute to dislocation mobility. At high temperatures and low stress, creep deformation is facilitated by the diffusion of vacancies through the matrix (Nabarro-Herring) and along the grain boundary (Coble). At lower temperatures, the increased free energy associated with the addition of an interface (i.e. grain boundary) reduces the activation energy required for diffusion compared to that of the bulk. As such, diffusion along grain boundaries occurs at lower temperatures and dominates the creep response. As temperatures exceed 0.6-0.7 T_m, the equilibrium concentration of vacancies, as well as their mobility, within the matrix increases due to the increase in thermal energy.

The following work investigates the role of carbides on the long-term creep response of the solid solution strengthened nickel-based superalloy, Inconel 617, for use in the next generation of nuclear reactors which are expected to operate at temperatures exceeding 900°C. The viability of this material in these applications requires: 1) the understanding of the dominant deformation mechanisms which will be acting during operation, 2) the influence of microstructure on deformation, and 3) the effects of the loading environment on microstructural stability. The first two chapters in this dissertation describe and simulate the role of carbides on the creep rate of Inconel 617 in terms of grain boundary sliding. The third chapter identifies the significance of grain boundary carbides as they pertain to creep deformation, as well as, how exposure to high temperatures and stress influence the morphology and locality of grain boundary carbides. The fourth chapter of this work examines how stress influences the precipitation of grain boundary carbides and provides a mechanism to explain the precipitation behavior and model for predicting carbide stability.

Recommended Citation

Spader, Daniel Lee, "CREEP DEFORMATION OF SOLUTION STRENGTHENED NICKEL-BASED SUPERALLOYS" (2024). Open Access Dissertations. Paper 1723.
https://digitalcommons.uri.edu/oa_diss/1723