Stress corrosion evaluation of powder metallurgy aluminum alloy 7091 with the breaking load test method

摘要

The stress corrosion behavior of powder metallurgy (PM) aluminum alloy 7091 was evaluated using a technique called the breaking load test method. Direct tension specimens machined from extruded material in the T7E69 and T7E70 conditions were tested in both the longitudinal and transverse orientations. Specimens were exposed to a 3.5 percent NaCl solution in alternate immersion for up to 9 days at stress levels as high as 90 percent of the material yield strength. Optical and scanning electron microscopy were used to evaluate specimen fracture surfaces to determine the extent of stress corrosion and to identify attack by other mechanisms. Breaking stress data were analyzed with extreme value statistics to determine threshold stress levels for stress corrosion cracking, probability of survival at specific stress levels, and 99 percent survival stresses. The results are in agreement with reported data, and indicate that PM 7091 aluminum is highly resistant to stress corrosion cracking for the orientations tested. Preliminary data analysis indicates that the effects of test variables such as heat treatment, specimen orientation, and exposure can be better discriminated by the breaking load test method.