The effects of test temperature, temper, and alloyed copper on the hydrogen-controlled crack growth rate of an Al-Zn-Mg-(Cu) alloy

The hydrogen-environment embrittlement (HEE)-controlled stage II crack growth rate of AA 7050 (6.09 wt pct Zn, 2.14 wt pct Mg, and 2.19 wt pct Cu) was investigated as a function of temper and alloyed copper level in a humid air environment at various temperatures. Three tempers representing the underaged (UA), peak-aged (PA), and overaged (OA) conditions were tested in 90 pct relative humidity (RH) air at temperatures between 25 °C and 90 °C. At all test temperatures, an increased degree of aging (from UA to OA) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed an Arrhenius-type temperature dependence, with activation energies between 58 and 99 kJ/mol. For both the normal-copper and low-copper alloys, the fracture path was predominately intergranular at all test temperatures (25 °C to 90 °C) in each temper investigated.Comparison of the stage II HEE crack growth rates for normal- (2.19 wt pct) and low- (0.06 wt pct) copper alloys in the peak PA aged and OA tempers showed a beneficial effect of copper additions on the stage II crack growth rate in humid air. In the 2.19 wt pct copper alloy, the significant decrease (∼10 times at 25 °C) in the stage II crack growth rate upon overaging is attributed to an increase in the apparent activation energy for crack growth. In the 0.06 wt pct copper alloy, overaging did not increase the activation energy for crack growth but did lower the pre-exponential factor (v0), resulting in a modest (∼2.5 times at 25 °C) decrease in the crack growth rate. These results indicate that alloyed copper and thermal aging affect the kinetic factors that govern stage II HEE crack growth rates. The OA, copper-bearing alloys are not intrinsically immune to hydrogen-environment-assisted cracking, but are more resistant due to an increased apparent activation energy for stage II crack growth.