STRESS CORROSION CRACKING STUDIES AND ANALYTICAL ELECTRON MICROSCOPY OF ALUMINUM ALLOYS

This research program involved the stress corrosion study, scanning electron microscopy and transmission electron microscopy (TEM), X-Ray microanalysis (EDX) and Auger electron spectroscopy (AES) of ten 7475 aluminum alloys in two heat treated conditions. The alloys also had systematic variations in their Zr, Cr, Fe and Si content. Stress corrosion cracking (SCC) behavior was studied by obtaining V-K diagrams (plots of SCC velocity versus stress intensity) in various environments and temperatures. The peak-aged alloys (T651) showed an apparent activation energy for SCC of 23.5 KJ/mole in the high stress intensity Region II. For the thermomechanically processed (TMP) alloys, such an apparent activation energy was found to be 44 KJ/mole. Region II SCC velocity was typically 9.3 x 10('-8) m/s and 1.7 x 10('-8) m/s for the T651 and TMP alloys respectively at 60(DEGREES)C. This velocity decreased to below 10('-9) m/s at the SCC threshold K(,ISCC) after 2000 hours of exposure. Significant improvements in both K(,IC) and K(,ISCC) were found in the TMP alloys over the T651 alloys. The decrease in fracture resistance ranged from 10 percent in a TMP alloy to over 80 percent in a T651 alloy, depending on temperature. Analytical electron microscopy via TEM, EDX and AES allowed structure to property correlations. The TMP alloys had a more homogeneous and denser dislocaton morphology, larger grain boundary precipitates and interparticle spacing and smaller precipitate free zone widths than the T651 alloys. Grain boundary Zn and Mg concentrations decreased as Fe + Si content increased in the alloys; however, the Mg/Zn ratio decreased in the T651 alloys and increased in the TMP alloys with increasing Fe + Si. This resulted in a K(,ISCC) improvement with higher Fe + Si content in the T651 alloys but a decreased K(,ISCC) with increased Fe + Si content in the TMP alloys. Good correlation was obtained between the EDX and AES data, showing that judiciously applied AES can yield qualitative grain boundary chemistry information for alloys normally exhibiting ductile rupture. Data from these studies show strong evidences of hydrogen mechanisms in the SCC of 7475 aluminum alloys.