Date of Award

12-2003

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Dr. Peter K. Liaw

Committee Members

Dr. Raymond A. Buchanan, Dr. John D. Landes, Dr. Louis K. Mansur

Abstract

As the candidate target container material of the new Spallation Neutron Source (SNS) being designed and constructed at the Oak Ridge National Laboratory (ORNL), 316 low-carbon nitrogen-added (LN) stainless steel (SS) will operate in an aggressive environment, subjected to intense fluxes of high-energy protons and neutrons while exposed to liquid mercury. The SNS is an accelerator-based neutron source that provides pulsed beams of spallation neutrons by bombarding a mercury target with 1 GeV protons. The function of the SNS is to convert a short pulse (< 1 ms, 60 Hz, 17 kJ/pulse), high-average-power (1 MW), 1-GeV proton beam into 18 lower-energy (< 1eV), short-pulsed (~ tens of ms) neutron beams optimized for use by neutron-scattering instruments.

The current project is oriented toward materials studies regarding the effects of test environment and frequency on the fatigue behavior of 316 LN SS. Class 316 LN SS is a low-carbon, nitrogen-added austenitic stainless steel, which possesses excellent resistance to both wear and corrosion, and is widely used in the nuclear industry. However, this material hasn’t been systematically investigated for its feasibility in the Spallation Neutron Source with a mercury target. In order to study the structural applications of this material and improve the fundamental understanding of the fatigue damage mechanisms, fatigue tests were performed in air and mercury environments at various frequencies and R ratios (R = smin/smax, smin and smax are the applied minimum and maximum stresses, respectively).

Fatigue data were developed for the structural design and engineering applications of this material. Specifically, high-cycle fatigue tests, fatigue crack-propagation tests, and ultrahigh cycle fatigue tests up to 109 cycles were conducted in air and mercury with test frequencies from 10 Hz to 700 Hz. Microstructure characterizations were performed by optical microscopy (OM), scanning-electron microscopy (SEM), and transmission-electron microscopy (TEM). Fractographic studies characterized the crack-initiation and propagation behavior of the alloy. It was found that mercury doesn’t seem to have a large impact on the crack-initiation behavior of 316 LN SS. However, the crack-propagation mechanisms in air and mercury are different in some test conditions. Transgranular cracks seem to be the main mechanism in air, and intergranular in mercury.

A detailed study on the dislocation structure of 316 LN SS after fatigue was performed, parallel to a collaborative work on the residual-stress evolution of the material using the neutron-scattering technique with researchers ORNL. The study showed that most dislocations in 316 LN SS after high-cycle fatigue are of an edge type, which corresponds to the result of the theoretical calculation by researchers at ORNL.

A significant specimen self-heating effect was found during high-cycle faituge. Theoretical calculation was performed to predict temperature responses of the material subjected to cyclic deformation. The predicted cyclic temperature evolution seems to be in good agreement with the experimental results.

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