17 th European Conference on Fracture 2-5 September , 2008 , Brno , Czech Republic Enhancing Stress Corrosion Cracking Resistance in Al-Zn-Mg-Cu Alloy through Inhibiting Recrystallization

In this work, the effect of recrystallization inhibition in Al-Zn-Mg-Cu (7010) alloy towards the stress corrosion cracking (SCC) behavior was studied. For this purpose, Sc addition (0.25 wt. %) was made to Al-Zn-Mg-Cu alloy and the SCC behavior of the base and Sc-bearing alloys in peak-aged condition was examined using slow strain rate testing (SSRT). The base 7010 Al alloy showed 10 % elongation, 9.9 % reduction in area and 561 MPa ultimate tensile strength (UTS), when tested in air. The ductility of the base alloy dropped to 3 % and 3.3 % in terms of elongation and reduction in area, respectively, when tested in 3.5 % NaCl solution, showing its high susceptibility to SCC. On the other hand, the 0.25 wt. % Sc containing alloy showed a significant improvement in ductility not only in air but also in 3.5 % NaCl solution, without any loss in the UTS. Thus, the 0.25 wt. % Sc containing alloy exhibited 13.4 % elongation, 15.8 % reduction in area and 560 MPa UTS in air and 12.5 % elongation, 16.4 % reduction in area and 560 MPa UTS in 3.5 % NaCl solution. The study shows that inhibiting recrystallization in 7010 alloy through Sc addition, improves the SCC resistance substantially even in the peak aged condition. Introduction There is a high demand for Al alloys possessing high strength to weight ratio for applications in aircraft and aerospace industries, and hence research is constantly directed towards imparting higher strength to Al alloys. However, it is noted that high strength Al alloys are prone to stress corrosion cracking (SCC), particularly at near peak strength condition [1, 2]. Their resistance to SCC can be increased by over ageing, but with a loss of strength [3, 4]. Recently, we studied the SCC behavior of 7010 Al alloy subjected to multi-step aging i.e. two-step peak aged and three-step over aged [5]. The peak aged alloy showed high susceptibility to SCC, that is the reduction in area dropped from 9.9 % (in air) to 3.3 % (in 3.5 % NaCl) and elongation dropped from 10 % (in air) to 3 % (in 3.5 % NaCl). However, over aged alloy showed a significant improvement in the SCC resistance. For example, the reduction in area dropped from 28.1 % (in air) to just 24.4 % (in 3.5 % NaCl) and there was no drop in the elongation (~10 %) when tested at a strain rate of 10/s. It should be noted that this reduction in SCC susceptibility was made possible at the expense of 10-13 % loss in UTS due to over ageing. Thus, it showed a UTS of 490 MPa in over aged condition when subjected to 10/s strain rate in 3.5 % NaCl solution. The study showed the coarsening of the grain boundary precipitates (GBPs) in the over aged alloy as opposed to fine and continuous distribution of GBPs in the peak aged alloy has improved the SCC resistance in the former. Further, the study showed that recrystallized grains are more prone to SCC [6]. Literature shows that Sc additions improve the strength of various Al alloys. In 1971, Willey [6] patented that small addition of Sc results in enhancing specific strengthening of Al