Title

Microstructural Stability in Grain Boundary Engineered Copper and Creep-Tested Inconel 617

Publication Date

8-2008

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Megan E. Frary

Abstract

The evolution of a material's microstructure can have dramatic effects on the mechanical and chemical properties exhibited throughout the lifetime of the material. In the present work, a study of microstructural stability in two face-centered cubic materials, commercially-pure copper and Inconel 617 (a nickel-based superalloy), is presented. One aspect of the research focuses on processing copper using a technique known as grain boundary engineering (GBE) and determining the subsequent stability of the microstructure at elevated temperatures. GBE is a thermomechanical process in which sequential straining and annealing cycles are used to increase the fraction of special grain boundaries (with Σ ≤ 29 according to the coincidence site lattice model). In addition to the GBE specimens, a conventionally-processed specimen is produced with a single strain step, equal to the total strain in the GBE specimen. The specimens are subjected to elevated temperatures for varying times in a controlled atmosphere. Microstructural characterization is performed using scanning electron microscopy (SEM) and concomitant analysis by electron backscatter diffraction (EBSD). An experimental method to analyze the microstructural changes induced by grain boundary engineering and isothermal aging is developed and used to characterize the evolution of the grain size, grain boundary character distribution, triple junction distribution, grain boundary network connectivity, and texture. The specimen that had been processed through four cycles of GBE exhibits the greatest microstructural stability, which is attributed to the specimen's high fraction of special boundaries and random texture. The other specimens experience abnormal grain growth. The insights gained from studying this straightforward system are projected to have implications for more complex alloys.

The other portion of the research assesses the role that grain boundary character plays in the evolution of a secondary-phase precipitate distribution in creep-tested IN617. In this investigation, samples of IN617 undergo creep testing at the Idaho National Laboratory. Again, microstructural characterization is performed using SEM and EBSD, with the added technique of energy dispersive spectroscopy (EDS). A novel method to characterize the distribution of secondary-phase precipitates is developed to analyze the microstructural changes induced by creep. Mo-rich precipitates are shown to be more prevalent at grain boundary triple junctions than Cr-rich precipitates; the size and diffusivity of the Mo atoms are believed to be the causes for the preferential precipitation. Among triple junction types, J2 triple junctions (i.e., those with two special boundaries) are most likely to be populated by precipitates (both Mo- and Cr-rich). A possible mechanism for the precipitation of the carbides at the J2 triple junctions is presented. The precipitation of the intergranular carbides appears to be suppressed on twin grain boundaries as compared to the general high-angle boundaries due to the grain boundary structures and relative energies. The analysis is inconclusive in determining whether or not precipitates redistribute to grain boundaries in tension versus those in compression as expected. The lack of conclusive results is attributed to the size of the areas analyzed; specifically, the areas chosen at random are too small to show the redistribution that is evident optically on a larger scale. Further analysis is needed.

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