STP821

    Utilizing Various Test Methods to Study the Stress Corrosion Behavior of Al-Li-Cu Alloys

    Published: Jan 1984


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    Abstract

    The stress corrosion (SC) behavior of two powder metallurgy (P/M) processed (Al-Li-Cu) alloys, with and without Mg addition, in aqueous 3.5% NaCl solution has been investigated. Three test techniques were employed in the investigation: (1) the alternate-immersion testing of tuning fork specimens, (2) slow crack growth tests using fracture-mechanics specimens, and (3) the slow-strain-rate testing of straining electrode specimens. The corrosion conditions investigated include cathodic and anodic potentiostatic control and free-corrosion under constant-immersion and alternate-immersion conditions. The heat treatment and the orientation of the alloys were varied to establish the more susceptible material conditions. Scanning electron microscopy and optical metallography were used to demonstrate the character of the interaction between the Al-Li-Cu alloys and the selected environment.

    Both Al-Li-Cu alloys are susceptible to SC in an aqueous 3.5% NaCl solution under certain electrochemical and microstructural conditions. Each test method was found to yield important information on the character of the SC behavior. Under all conditions investigated, second-phase particle stringers along the extrusion direction in the alloys were rapidly attacked, and they played a principal role in the SC process. With time, larger pits developed from these rows of smaller pits, and under certain electrochemical conditions surface cracks initiated from the larger pits and contributed directly to the fracture process. Evidence to support slow crack growth was observed in both the slow-strain-rate tests and the sustained-immersion tests of precracked fracture-mechanics specimens. The possible role of H2 in the stress-corrosion cracking process is suggested.

    Keywords:

    aluminum-lithium-copper, stress corrosion, aqueous sodium chloride solution, alternate immersion, threshold stress, threshold stress intensity factor, potentiostatic polarization, slow-strain-rate test, slow crack growth


    Author Information:

    Pizzo, PP
    Associate Professor, San Jose State University, San Jose, Calif.

    Galvin, RP
    Foothill/De Anza Community College Work Engagement Program, and Chief, Materials Test Engineering Branch, NASA-Ames Research Center, Moffett Field, Calif.

    Nelson, HG
    Foothill/De Anza Community College Work Engagement Program, and Chief, Materials Test Engineering Branch, NASA-Ames Research Center, Moffett Field, Calif.


    Paper ID: STP34433S

    Committee/Subcommittee: G01.11

    DOI: 10.1520/STP34433S


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