Mitigating Intergranular Stress Corrosion Cracking in Age-Hardenable Al-Zn-Mg-Cu Alloys
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INTRODUCTION
COPPER-CONTAINING age-hardenable aluminum Al-Zn-Mg-Cu (7xxx series) alloys are widely used in the aerospace sector due to their high specific strength. However, these alloys undergo environmentally assisted cracking (EAC), leading to premature failures, especially in the peak-aged conditions.[1–5] Therefore, the alloys of 7xxx series are commonly treated to overaged (OA) condition in order to resist premature EAC failures.[6] The mechanical behavior of these alloys depends on the interaction of dislocations with the material microstructure (solute atoms, matrix precipitates, grain boundary precipitates (GBP), and grain boundary microchemistry) obtained through various thermal aging cycles. Thermal aging cycles produce changes in the precipitate-free zone width, size, volume fraction of precipitates in the matrix, microchemistry of grain boundary precipitates, and changes in the slip mode.[7] The grain boundary microchemistry and its subsequent electrochemistry play a vital role in the stress corrosion cracking (SCC) resistance, as the failures reported under monotonic loading conditions are intergranular in nature.[8] That is, in peak-aged alloys, where
M. AJAY KRISHNAN, V.S. RAJA, and SHWETA SHUKLA are with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Mumbai 400076, India. Contact e-mail: [email protected] S.M. VAIDYA is with Godrej Aerospace, Vikhroli West, Mumbai, Maharashtra 400076, India. Manuscript submitted November 21, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
SCC is predominant, the anodic phases on the grain boundaries make the alloys highly susceptible to SCC.[9] It is widely accepted that the microstructures produced through overaging have enriched Cu content on the grain boundaries, providing significant SCC resistance in relation to peak-aged temper.[8,10–15] Overaged Al-Zn-Mg-Cu alloys, albeit having higher SCC resistance, lose 10 to 15 pct of their strength levels (yield strength and ultimate tensile strength (UTS)) under service conditions. To overcome this problem, complex aging cycles[16,17] were developed, including retrogressed and reaging (RRA) tempers. Although, RRA tempers can provide strength levels equaling peak-aged condition and also offer good SCC resistance, they are practically difficult to implement.[18–23] RRA temper has a higher reversion temperature, thereby reducing its applicability to thicker section blocks and lesser quench-sensitive alloys. For several decades, the thermal aging technique has been popular among researchers for developing SCC-resistant microstructures, and vast developments in this area are a testimonial for the same.[7,16] However, development of a successful thermal aging cycle that not only provides superior SCC performance but also provides higher strength with feasible applicability remains a challenge. This article reports a novel heat treatment cycle developed for aluminum 7xxx series alloys that not only provides strength levels in relevance to peak-aged temper but also exh
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