Stress-Corrosion Cracking of Aluminum Alloys

Abstract

The stress corrosion behaviour of precipitation hardened Al-9Mg, Al-22Zn and Al-3Mg-6Zn alloys has been studied in aqueous environments and ethanol. The stress corrosion susceptibility defined as the reciprocal of failure time has been investigated as a function of alloy-environment system, isothermal aging treatment, microstructure, applied tensile stress, and temperature using smooth and notched specimens. Constant load tests, load-relaxation tests and tensile tests in different environments have been used to evaluate the stress corrosion characteristics of aluminum alloys. A limited study of Mg-9A1 has also been carried out in aqueous environments. The process of stress corrosion generally consisted of three parts: 1) A slow initiation stage 2) a rapid propagation stage 3) mechanical fracture due to tensile overload. With a few exceptions, the initiation time was greater than the propagation time. The crack initiation and propagation rates were stress and thermally activated and could be expressed by a general equation of the form Rate = A q a 11. exp (^|) where a is the applied tensile stress, Q is the apparent activation energy of the rate controlling process and A q and n are constants for a given alloy-environment system. The apparent activation energy of the rate controlling process was different in the two environments. It also changed between initiation and propagation stages. Page 4.4. Variation in Stress Corrosion Susceptibility with the Alloy System 4.5.1. Initiation and Propagation of Stress Corrosion Cracks in NaCl/K^CrO^. Solution 123 4.5.2. The Relative Lengths of Initiation and Propagation Times in NaCl/K^CrO^ Solution 129 4.6. The Effect of Different Environments on the Stress Corrosion Behaviour of Aluminum Alloys 130 4.6.1. Stress Corrosion in Alcohols 136 4.6.2. Effect of Environment on Mechanical Properties of Al-2.6Mg-6.3Zn 137 4.6.3. Effect of Environment and Strain Rate on Mechanical Properties of ; Al-21.5Zn 4.6.4. Effect of Strain Rate on Serrated Yielding 4.7. Temperature Dependence of the Stress Corrosion Process 4.8. The Hydrogen Mechanism 4.9. The Requirements of a Satisfactory Model... 4.9.1. Models Involving Either Dissolution or Deformation and Their Drawbacks 4.10. The Proposed Model 158-ix-Page 4.10.1. The Observations Explained by the Proposed Model 4.11. Conclusions 167 4.12. Suggestion for Further Work 168 Appendix I 1^9 Appendix II 177 Bibliography 180-X-LIST OF FIGURES Figure Page 1. Types of tensile and stress corrosion specimens used in the present work 2. Stress corrosion cells used in the the present work 3. Modified creep stand for constant load tests 4. Precipitation hardening of Al-9Mg, aged at 200°C 5. Precipitation hardening of Al-3Mg-6Zn, aged at 200°C. 6. Precipitation hardening of Mg-8.6A1, aged at 200°C... 7. Failure time vs aging time for Al-9Mg aged at 200°C. 8. Applied stress vs failure time for Al-9Mg aged for 7 hours at 200°C. 33 9. Effect of aging time at 160°C on the failure time of Al-3Mg-6Zn in NaCl/K 2 Cr0 4 10. Applied stress vs failure time for th...