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00368 The Effects of Temper, Test Temperature, and Alloyed Copper on the Hydrogen-Controlled Crack Growth Rate of an AI-Zn-Mg-(Cu) Alloy

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Product Number: 51300-00368-SG
ISBN: 00368 2000 CP
Author: John R. Scully and George A. Young Jr.
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The hydrogen embrittlement controlled stage II crack growth rate of AA 7050 (6.1% wt. Zn, 2.1% wt. Mg, 2.2% wt. 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, peak aged, and overaged conditions were tested in 90% relative humidity (RH) air at temperatures between 25 and 90 °C. At all test temperatures, an increased degree of aging (from underaged to overaged) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed Arrhenius-type temperature dependence with activation energies between 53 and 97 kJ/mol. For both the normal copper and low copper alloys, the fracture path was predominately intergranular at all test temperatures (25-90 °C) in each temper investigated. Comparison of the stage II crack growth rates for normal and low copper alloys in the peak aged and overaged tempers showed the beneficial effect of copper additions on stage II crack growth rate in humid air. In the 2.2 wt.% copper alloy, the significant decrease (-10 times at 25 °C) in 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.% copper alloy, overaging did not increase the activation energy for crack growth but did lower the pre-exponential factor, vo, resulting in a modest (-2 times at 25 °C) decrease in crack growth rate. These results indicate that alloyed copper and thermal aging affect the kinetic factors which govern stage II crack growth rate. Overaged, copper bearing alloys are not immune to hydrogen environment assisted cracking but are intrinsically more resistant due to an increased apparent activation energy for stage II crack growth.
The hydrogen embrittlement controlled stage II crack growth rate of AA 7050 (6.1% wt. Zn, 2.1% wt. Mg, 2.2% wt. 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, peak aged, and overaged conditions were tested in 90% relative humidity (RH) air at temperatures between 25 and 90 °C. At all test temperatures, an increased degree of aging (from underaged to overaged) produced slower stage II crack growth rates. The stage II crack growth rate of each alloy and temper displayed Arrhenius-type temperature dependence with activation energies between 53 and 97 kJ/mol. For both the normal copper and low copper alloys, the fracture path was predominately intergranular at all test temperatures (25-90 °C) in each temper investigated. Comparison of the stage II crack growth rates for normal and low copper alloys in the peak aged and overaged tempers showed the beneficial effect of copper additions on stage II crack growth rate in humid air. In the 2.2 wt.% copper alloy, the significant decrease (-10 times at 25 °C) in 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.% copper alloy, overaging did not increase the activation energy for crack growth but did lower the pre-exponential factor, vo, resulting in a modest (-2 times at 25 °C) decrease in crack growth rate. These results indicate that alloyed copper and thermal aging affect the kinetic factors which govern stage II crack growth rate. Overaged, copper bearing alloys are not immune to hydrogen environment assisted cracking but are intrinsically more resistant due to an increased apparent activation energy for stage II crack growth.
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