Bovard, Francine Susan
(2006)
Environmentally Induced Cracking of an Al-Zn-Mg-Cu Alloy.
Master's Thesis, University of Pittsburgh.
(Unpublished)
Abstract
Although numerous mechanistic studies have been conducted over the past three decades that have correlated stress corrosion cracking (SCC) susceptibility with various parameters, a comprehensive mechanistic understanding of the SCC susceptibility of aluminum-magnesium-zinc-copper alloys is still lacking. Most mechanistic theories support either an anodic dissolution type of mechanism or a hydrogen embrittlement type of mechanism but within each of these two types of environmentally induced cracking (EIC) mechanisms there is little consensus on the mechanistic details and it seems likely that both anodic dissolution and hydrogen embrittlement phenomena play critical roles in the EIC of 7xxx aluminum alloys. From 1996 to 2000, a multidisciplinary research program was carried out by researchers from University of Virginia, Lehigh University, and Alcoa Technical Center to study the EIC mechanisms in 7xxx series aluminum alloys. The results of this program provided new evidence to support critical roles of both hydrogen embrittlement and anodic dissolution. The role of hydrogen was supported by establishing a correlation between hydrogen uptake and Stage II crack growth rates (CGR), demonstrating Arrhenius dependence of crack growth in humid air, and establishing a correlation between changes in the activation energy for bulk or volume diffusion of hydrogen with temper and corresponding changes in activation energy for crack growth. The primary evidence for an anodic dissolution contribution to the SCC process is the observation that development of an aggressive localized environment at the crack tip is a pre-requisite for high crack growth rates. In this study, crack growth rates in humid air (90%RH) were measured for AA7050 isothermally aged at 325¢ªF for various times. Ln-ln plots of CGR as a function of aging time show linear relationships. However, a distinct change in slope after ~10 hours of aging indicates a change in the rate controlling process for environmentally induced cracking (EIC). TEM analyses of AA7050 under the same aging conditions and mechanically strained 2% indicate that the change in the slope of the CGR behavior coincides with the transition in the dislocation glide behavior from predominantly particle shearing and planar slip to homogeneous, wavy slip. The correlation between a change in the rate controlling process for EIC and the change in the slip character suggests that crack velocity may be controlled by the rate of dislocation transport of hydrogen. Quantitative evaluation of the results shows the dependence of CGR on the precipitation and growth processes via the Avrami equation. A comprehensive mechanism for EIC of Al-Mg-Zn-Cu alloys that is consistent with the experimental data in this study and in the literature is proposed.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
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| Date: |
31 January 2006 |
| Date Type: |
Completion |
| Defense Date: |
7 December 2005 |
| Approval Date: |
31 January 2006 |
| Submission Date: |
6 December 2005 |
| Access Restriction: |
No restriction; Release the ETD for access worldwide immediately. |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Swanson School of Engineering > Materials Science and Engineering |
| Degree: |
MSMSE - Master of Science in Materials Science and Engineering |
| Thesis Type: |
Master's Thesis |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
chloride; glide bands; humidity; hydrogen embrittlement; isothermal aging; slip bands; slip charachter; slip planarity; stress dorrosion cracking; temper |
| Other ID: |
http://etd.library.pitt.edu/ETD/available/etd-12062005-090939/, etd-12062005-090939 |
| Date Deposited: |
10 Nov 2011 20:08 |
| Last Modified: |
15 Nov 2016 13:53 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/10090 |
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