Publication Date

7-10-2013

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Megan Frary

Advisor

Michael Hurley

Advisor

Clyde J. Northrup

Abstract

Inconel 617 is a candidate material for use in the intermediate heat exchanger of the Next Generation Nuclear Plant. Because of the high temperatures and the fluctuations in stress and temperature, the fatigue behavior of the material is important to understand. The goal of this study was to determine the influences of the microstructure during fatigue crack propagation. For this investigation, Inconel 617 compact tension samples, fatigue tested by Julian Benz at the Idaho National Laboratory, were obtained. The testing conditions included two environments at 650 °C (lab air and impure-He) and varied testing parameters including: loading waveform (triangular, trapezoidal), loading frequency (0.01, 0.05 Hz), and maximum stress intensity factor, Kmax (20, 25, 30 MPa√m). The value of Kmax had the greatest influence in the crack growth rate followed by the testing environment. In this study, electron backscatter diffraction was used in order to relate the crack path to the microstructure on the scale of microns. Using this information the crack was found to crack in the {001} family of planes and to the family of directions with the least propensity to propagate in the family of directions. This supports that crack growth in Inconel 617 at 650 °C propagates with a ‘quasi-cleavage’ mechanism. Also in this study it was found that the character of fatigue crack deflections within a grain versus that at a grain boundary differs with statistical significance. Within a grain, the deflection angle had a unimodal distribution with a mean of 16° ±15°. The low angle of deflection suggests that the plane of highest stress is the highest influencing factor. Also, the deflections at the grain boundary were found to have a different distribution as it was multimodal, which suggests multiple mechanisms for fatigue crack deflection. Though, the fatigue crack growth rate was found to be highly influenced by the testing atmosphere and loading parameters, they were found to have no statistical significance on the fatigue crack path on the microstructural level. At these testing conditions, the crack grew at a rate slow enough for the microstructure to have influence on its path, though it was still constrained to remain close to the plane of highest stress. Developing a deeper understanding of the influence of microstructure on fatigue crack propagation will support selection of materials and design of the intermediate heat exchanger for the next generation nuclear plant.

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