The identification of hydrogen trapping states in an Al-Li-Cu-Zr alloy using thermal desorption spectroscopy
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I. INTRODUCTION
Al-Li-Cu-Zr alloys have been shown to be susceptible to environmentally assisted cracking (EAC) when exposed to aqueous chloride-containing environments.[1–4] However, there has been little research to distinguish dissolution-based processes from hydrogen-embrittlement processes for this system. For instance, the presence of T1 in Al-Li-Cu-Zr alloys has been argued to result in both aqueous dissolution and hydrogen embrittlement under similar conditions.[5] However, the role of T1 in each of these processes has not been isolated. For Al-Li-Cu-X alloys, intergranular, intersubgranular, and transgranular EAC paths in aqueous solutions have been attributed to boundary-T1 dissolution,[6] T1 dissolution with hydrogen uptake,[7] AlLiH4 formation and cracking,[8,9] dissolution of copper-depleted zones,[10] and intrinsic hydrogen embrittlement involving mobile and or trapped hydrogen.[11] Which of these or other metallurgical factors control hydrogen-assisted cracking has not been clearly established. Consequently, knowledge of important factors controlling EAC and governing mechanisms are not available. To gain insight on the interaction of absorbed hydrogen with Al-Li-Cu-Zr alloys, electrochemically precharged specimens were examined by an ultra-high-vacuum gas extraction technique, thermal desorption spectroscopy (TDS). Thermal desorption spectroscopy has been used to measure the binding energies of surface-adsorbed species since the mid-1950s.[12,13,14] This technique was not applied to the analysis of species that were absorbed in the bulk of a STEPHEN W. SMITH, formerly Graduate Research Assistant, Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903, is Senior Engineer, Lockheed Martin Engineering and Sciences, Hampton, VA 23681-2199. JOHN R. SCULLY, Associate Professor, is with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. Manuscript submitted March 9, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
material and trapped at microstructural sites of varying binding energy until the early 1980s.[15,16] When this technique is used to measure bulk-absorbed species, detrapping and lattice diffusion must precede surface desorption. As the temperature of a sample is increased, the rate of gas evolution will increase, resulting in an instantaneous rise in gas density within a fixed volume. If a mass spectrometer is used to measure the rise of pressure within the chamber, the flux of any desorbed species can be determined. It is this ability to dynamically measure evolved gases and to discriminate the evolved species that makes this tool more powerful than traditional fusion methods used to determine the total concentration of absorbed species.[17] To fully understand the process of hydrogen embrittlement, one must establish a fundamental understanding of the ingress, transport, and trapping of hydrogen in a material system.[18,19] This article serves as a first step to developing such an understanding.
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