Date of Award

2019

Degree Type

Thesis

Degree Name

Master of Science in Mechanical Engineering and Applied Mechanics

Department

Mechanical, Industrial and Systems Engineering

First Advisor

Hamouda Ghonem

Abstract

Creep deformation of nickel-based superalloys at elevated temperatures is inherently dependent on the microstructural state of the material. Carbides have been observed to suppress intragranular and intergranular deformation rates at elevated temperatures by impeding dislocation motion within the grain and along the grain boundary. First, the rate controlling effects of intra- and intergranular carbides as it relates to the grain boundary sliding are examined. A microstructurally sensitive, viscous description and model of grain boundary sliding is then presented. This work provides the concepts and mathematical formulation to model the rate controlling processes governing the grain boundary sliding associated with creep of carbide-strengthened superalloy Inconel 617 (IN617). The framework of the model considers both the microstructural state (size, volume fraction, and spacing of carbides) in the matrix, as well as along the grain boundary when determining the overall sliding rate. The model accounts for the role of carbides as they pertain to dislocation arrival/absorption into the grain boundary and the rate at which they glide and climb along the grain boundary plane, resulting in grain boundary sliding. It is considered that grain boundary sliding necessitates the supply of extrinsic dislocations from the matrix to facilitate sliding and, as such, the rate at which dislocations arrive to and are absorbed into the grain boundary dictates the overall sliding rate. Carbides along the grain boundary are then modeled as accumulation points for backstress which suppresses grain boundary sliding as a function of their size and spacing. At elevated temperatures, carbides within the matrix and along the grain boundary are subjected to diffusional processes resulting in time-dependent microstructure and mechanical response, requiring a detailed understanding of the rate controlling properties of both intra- and intergranular carbides as they pertain to grain boundary sliding and creep deformation.

Following this, a method of using stress relaxation tests carried out at 780 °C on IN617 specimens of various aging exposure times to examine the effect that the matrix microstructure exerts over the material’s deformation at elevated temperatures is explored. Included in this experimental work are quantitative microstructural assessments of IN617 specimens of various exposure times through the use of scanning electron microscopy (SEM). Once the microstructure of the material has been evaluated, the elevated temperature stress relaxation tests are utilized as a means of producing accelerated creep behavior via interconversion of stress relaxation data to creep strain data.

At 780°C, above the solvus temperature of gamma prime (γ’), but below that of the chromium-rich M23C6 carbides, the microstructure could be regarded as constant. In doing this, the time dependent nature of the M23C6 carbide evolution was able to be eliminated, thereby allowing a “snapshot in time” with fixed carbide radius, volume fraction, and spacing values which could be quantified via SEM. As the inelastic sliding of the viscous grain boundary is asserted to provide the means of stress relaxation, holding constant the grain and grain boundary microstructure allowed for determination of the number of grain boundary dislocations (ngb) required to produce the corresponding amount of grain boundary displacement. This is achieved by analyzing the matrix and grain boundary dislocation behavior within the framework of a physics-based deformation model which couples the matrix dislocation release (nm) to the grain boundary dislocation population which – through prevailing glide and climb processes internal to the boundary that occur at the experimental stress and temperature – facilitate grain boundary sliding. A unification of the influence of the matrix and grain boundary microstructure on the creep behavior of IN617 is then provided, proffering a comprehensive and efficient tool for consideration in the design and analysis of carbide precipitate strengthened nickel-based superalloys for high temperature applications.

Available for download on Thursday, October 31, 2019

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