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.
The short-term age softening characteristics of isothermal annealed and post weld heat treated 2-1/4Cr-1Mo steel have been investigated over the temperature range 454 to 727°C (850 to 1340°F). Significant degradation in flow strength is observed for times and temperatures within the current service regime of the commercial alloy. Age softening is atypical: flow strength degradation is most rapid at some intermediate temperature. The reduction in flow strength is greatest, and occurs most rapidly, at 575°C (1067°F). The strength degradation is attributed to time-dependent redistribution of molybdenum within the proeutectoid ferrite. It is predicted that thermal aging in the temperature range 427 to 593°C (800 to 1000°F) will result in similar strength decrements for annealed 2-1/4Cr-1Mo steel regardless of prior thermal history above 704°C (1300°F).
Observations concerning the statistical evaluation of creep data are presented. Methods currently employed in the determination of stress rupture regression lines can result in conflicting and necessarily invalid results. Anomalous behavior is principally associated with the selection of the dependent variable. However, it is the least squares method of curve fitting which introduces regression bias. Methods to improve the validity of least squares regressions are suggested.
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