Enhancing stress corrosion cracking resistance in Al–Zn–Mg–Cu–Zr alloy through inhibiting recrystallization

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Abstract

The aim of the work was to inhibit recrystallization in Al–Zn–Mg–Cu–Zr alloy and to evaluate the stress corrosion cracking (SCC) behavior of the alloy in the peak aged condition. For this purpose, scandium addition was made to an Al–Zn–Mg–Cu–Zr alloy as the former inhibits recrystallization. The scandium-containing alloy was heat-treated to peak aged condition and compared with the base peak aged alloy which contained recrystallized grains. The SCC susceptibilities of the alloys were evaluated using slow strain rate testing (SSRT) and U-bend techniques. While, the base alloy having recrystallized grains showed drastic loss in ductility in the corrosive environment (3.5 wt.% NaCl solution), the scandium-containing alloy having un-recrystallized and fine grains showed no significant loss in ductility in the similar environment. The fracture surface analysis revealed typical intergranular cracking of recrystallized grains in the base alloy, whereas in the scandium-containing alloy predominant ductile failure was observed. Thus, the study clearly indicated that inhibiting recrystallization in AlZnMgCuZr alloy through scandium addition, the SCC resistance of the alloy can be substantially improved even in the peak aged condition.

Introduction

There is a high demand for aluminium alloys possessing high strength to weight ratio for applications in the aircraft and aerospace industries, and hence research is constantly directed towards imparting higher strength to aluminium alloys. However, it is noted that high strength aluminium 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 Al–Zn–Mg–Cu–Zr (7 0 1 0) alloy subjected to multi-step ageing treatments i.e. two-step peak aged and three-step over aged treatments, in air and 3.5 wt.% NaCl solution using slow strain rate testing (SSRT) method [5], [6]. 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 wt.% NaCl) and elongation dropped from 10% (in air) to 3% (in 3.5 wt.% NaCl). However, the 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 wt.% NaCl) and there was no drop in the elongation (∼10%) when tested at a strain rate of 10−6 s−1. It should be noted that this improvement in the SCC resistance was made possible at an expense of 10–13% loss in UTS due to over ageing. Thus, the alloy showed a UTS of 490 MPa in over aged condition when subjected to 10−6 s−1 strain rate in 3.5% NaCl solution. The high SCC resistance of over aged alloy was attributed to the coarsening of the anodic grain boundary precipitates (GBPs) in the over aged alloy (which inhibits easy crack propagation) as opposed to fine and continuous distribution of GBPs in the peak aged alloy and the higher amount of copper in the GBPs of over aged alloy (which ennobles the GPBs) as compared to that of the peak aged alloy [5], [6]. Interestingly, the study also showed that the recrystallized grains are more prone to SCC [6]. Hence, the aim of the work was to inhibit the recrystallization in Al–Zn–Mg–Cu alloy and evaluate the SCC behavior in the peak aged condition.

Literature survey shows that scandium additions to aluminium alloys not only inhibits the recrystallization but also enhances the strength level through grain refinement [7], [8], [9], [10], [11]. Combined additions of scandium and zirconium to aluminium alloys have been shown to have a high number density of homogeneously distributed Al3ScxZ1−x – dispersoids, coherent with aluminum-matrix, and are thermally stable i.e. the dispersoids coarsen slowly and thus enabling these alloys to maintain strength at high temperatures [8], [9], [10]. Recent studies have shown that 0.25 wt.% scandium addition to an Al–Zn–Mg–Cu–Zr (7 0 1 0) alloy improves the peak aged strength properties without any compromise in the ductility of the alloy [11]. The tensile properties of the base alloy (Al–Zn–Mg–Cu–Zr) and 0.25 wt.% scandium-containing alloy are listed in Table 1.

Only a limited study has been made on the effect of scandium on the SCC behavior of high strength aluminium alloys. Wu et al. [12] and Elagin et al. [13] have stated that the scandium improves the SCC resistance of high strength aluminium alloys. However, the SCC mechanism related to grain-recrystallization was not discussed. Hence, in order to understand whether inhibition of recrystallization can improve the SCC resistance, a comparative study on the alloy containing un-recrystallized grains (i.e. scandium-containing Al–Zn–Mg–Cu–Zr alloy) and the alloy containing recrystallized grains (i.e. base alloy), both in peak aged condition, were studies in air and in corrosive environment (3.5 wt.% NaCl solution) using slow strain rate testing (SSRT) and U-bend techniques.

Section snippets

Experimental procedure

The chemical composition (in wt.%) of the alloy (Al–Zn–Mg–Cu–Zr) examined in the present work is Zn(6.30)–Mg(2.30)–Cu(1.55)–Zr(0.14)–Fe(0.9)–Si(0.6)–Al(bal). The material in the form of 5 mm thick sheet was produced from ingots of Al–Zn–Mg–Cu–Zr alloy by Defence Metallurgical Research Laboratory, Hyderabad, India. The material was then heat-treated to two-step peak ageing (solution treated at 465 °C, followed by water quenching at room temperature and aged at 100 °C/8 h and 120 °C/8 h) condition.

Results

Fig. 3a–d represent the optical micrographs revealing grain structure of the base and the scandium-containing alloys obtained in both longitudinal and long-transverse directions. The longitudinal view of the base alloy shows network of small equiaxed recrystallized grains as well as the original large grains (Fig. 3a). At similar magnification, the scandium-containing alloy does not reveal the features very clearly; though on a closer look reveals fine grains. In long-transverse direction (at

Discussion

In order to quantify the SCC susceptibility of the alloys, the SCC susceptibility indices (ISCC) were calculated based on the ratio of a particular mechanical property value (such as elongation) measured in a SSRT test in corrosive environment (i.e. in 3.5 wt.% NaCl) to its corresponding value in an inert environment (i.e. in air). A low ISCC suggest high SCC susceptibility, and when the ISCC approaches unity this means that there is no effect due to the test environment or in other words the

Conclusions

The work suggests that the alloy (Al–Zn–Mg–Cu–Zr) in the peak aged condition is prone to SCC. The cracking in this alloy is found to be preferentially occur along the grain boundaries of the recrystallized grains. The anodic nature of the precipitates makes the recrystallized grain boundaries preferential site for the crack growth. Inhibiting recrystallization in the Al–Zn–Mg–Cu–Zr alloy through scandium addition enhances the SCC resistance of the alloy significantly. The improvement in the SCC

Acknowledgment

The authors would like to thank Dr. A.K. Mukhopadhyay, Defence Metallurgical Research Laboratory, Hyderabad, India for providing the materials.

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