Stress corrosion cracking (SCC) behaviors of extruded 7050 series aluminum alloys in T6 and RRA (Retrogression and re-aging) tempers were studied as compared with that of the conventional 7075 aluminum alloy. 7050 and 7055 aluminum alloys in T6 showed a superior balance among strength, fracture toughness and SCC resistance above 7075–T6. The threshold stress intensity
KISCC for SCC initiation can be ranked in the order of 7075 < 7050 < 7055, together with the SCC growth rate d
a/d
t in the order of 7050 < 7055 < 7075. The alloys in RRA1 having a higher tensile strength than each of them in T6 exhibited an improved d
a/d
t of 1/30∼1/50 below that of T6, and 7050–RRA2 showed a more reduced level below 1/100. Based on the test results and metallographic observation,
KISCC of the alloys in T6 can be derived to agree with the
KI condition when the crack tip plastic zone expands over the diameter in the
R (radial) direction of fiber formed unrecrystallized grains, so that it is supported the SCC process in T6 materials is controlled by hydrogen embrittlement due to the dislocation transport of hydrogen and accumulation at grain boundaries. The contribution of anodic dissolution, on the other hand, is evident in RRA materials.
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