Hydrogen Assisted Fatigue Cracking of High Strength Aluminum Alloys.
RENSSELAER POLYTECHNIC INST TROY N Y DEPT OF MATERIALS ENGINEERING
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High cycle tension-tension fatigue tests have been performed on a 7075-T6 alloy in air, in 0.5N NaCl, pre-corroded in 0.5N NaCl, re-heat-treated and tested in air. Additionally fatigue tests were performed on a high purity analogue alloy, Al-5.5Zn 2.5Mg-1.5Cu in air, in 0.5N NaCl and in NaCl with applied cathodic currents hydrogen charged. Corrosion fatigue of the high-purity alloy resulted in intergranular crack initiation with a shift to transgranular cracking as the crack propagated. High charging currents and high cyclic stresses tended to reduce the relative amount of intergranular cracking. Transgranular fatigue fracture surfaces differing from those observed in air tests in that they were highly faceted crystallographic and exhibited cleavage-type markings. These results indicate that corrosion fatigue of high-strength aluminum alloys is a hydrogen embrittlement process with hydrogen being produced by corrosion of the alloy. Transgranular cracking occurs when high cyclic stresses induce mobile dislocations which cause hydrogen migration into the grains in fatigue-generated slip bands.
- Properties of Metals and Alloys