On the Season Cracking of High Tensile Al-Alloys and its Prevention

Environment-Induced Cracking of High-Strength Al-Zn-Mg-Cu Aluminum Alloys: Past, Present, and Future

“…[14] in 1919 and Rosenhain et al [15] and Rosenhain [16] in 1921 reported mechanical property degradation for several high-zinc containing age-hardened aluminum alloys, including Al-18Zn-2.5Cu-0.35Mg-0.35Mn alloy (UTS 534 MPa, Yield Stress 473 MPa), held under tensile stress while exposed to sea-water or dilute saline solutions, such a tap water. The resultant cracking phenomenon called 'Intercrystalline Fracture under prolonged application of Stress' [14] was likened to the 'Season Cracking' of brass [8,13]. Then in 1939 Grogan and Pleasance [49] applied constant-load tests to similar alloys,( including Al-Zn binary alloys in laboratory air and 5% NaCl, and found reduced susceptibility with lower quench rates after solution-heat-treatment (SHT) and increased susceptibility with exposure to more aggressive test environments, Figure 3.…”

“…Al-Zn-Mg-Cu alloy development in Japan, starting in 1935, resulted in patent applications for highstrength aluminum alloys containing up to 14% Zn, 2-5% Mg, and 1% Cu that had maximum strengths of up to 560 MPa with a 10% elongation [53]. Igarashi and Kitahara [8] based on studies at Sumitomo Light Metals restricted the claimed chemical compositional range to 8-10% Zn, 1.5% Mg, 2.5% Cu, 0.25% Cr, and 0.5-1.5% Mn, where suitable heat-treatment could provide enhance resistance to 'Season Cracking', along with excellent forgeability and maximum tensile strengths up to 598 MPa with around 14% elongation. Alloy compositions designated as Extra Special Duralumin (ESD) were patented [9] and during 1938 introduced into the wings of the Japanese 'Zero Fighter' plane [10,54].…”

“…Igarashi and Kitahara [8] based on studies at Sumitomo Light Metals restricted the claimed chemical compositional range to 8-10% Zn, 1.5% Mg, 2.5% Cu, 0.25% Cr, and 0.5-1.5% Mn, where suitable heat-treatment could provide enhance resistance to 'Season Cracking', along with excellent forgeability and maximum tensile strengths up to 598 MPa with around 14% elongation. Alloy compositions designated as Extra Special Duralumin (ESD) were patented [9] and during 1938 introduced into the wings of the Japanese 'Zero Fighter' plane [10,54]. 'Season Cracking' was used in Japan to describe EIC until at least the mid 1940's [10,11].…”

“…The lack of EIC references from aluminum alloy usage pre-1940 results from the use of alternate descriptors, including: 'Spontaneous disintegration' [4,5], 'Brittleness' [6,7], 'Season Cracking of Aluminium' [8][9][10][11][12] (due to the apparent similarities to season cracking of brass [13]), 'Intergranular fracture under prolonged application of stress' [14][15][16], 'Stress cracking' [17,18], and 'Acceleration of corrosion under stress' [18].…”

“…Webber's [17] depiction of a 'peculiar' cracking mode promoting structural failure in high zinc containing Al-Zn-Mg-Cu alloys exposed to corrosive conditions while under high stress, as 'stress cracking' in 1933 was further complicated in 1941 when Nock [18] noted that these alloys were also potentially susceptibility to another cracking phenomenon he called 'Accelerated Corrosion under Stress'. Meanwhile, in Japan during the late 1930's, Igarashi and Kithara having observed EIC in high-zinc content (8-10 wt.%) Al-Zn-Mg-Cu alloys also used the term 'Season Cracking' adding that the cracking could occur in water vapour [8]. In 1940 Dix [27] advocated use of the terms 'stress corrosion' or 'stress corrosion cracking (SCC)' to describe the spontaneous failure of metals under the combined action of high stress and corrosion, and during the 1944 ASTM/AIME Symposium on the Stress Corrosion of Metals held in Philadelphia proposed these terms should supersede 'season cracking' [28].Wassermann [22] also confirmed the early Japanese findings that water vapor alone promoted rapid crack growth in Al-Zn-Mg-Cu alloys.…”