The stress corrosion cracking (SCC) of underaged, peakaged and overaged Al 7010 in either water vapour saturated air (WVSA) or in salt water has been investigated using transmission electron microscopy (TEM). The crack tip and near crack tip regions formed in Double Cantilever Beam (DCB) samples are described. A range of behaviours will be outlined in relation to the observed grain boundary microstructures and it will be shown that fracture is caused by either pure mechanical rupture, pure corrosion or SCC.
The high-strength 7XXX series aluminum alloys can be susceptible to SCC in some tempers. SCC is far more severe in peak aged alloys, for example, than in either their under- or overaged counterparts but there are relatively little direct evidence about the microstructural development of SCC. Historically, however, there has always been considerable interest in SCC since Dixl proposed an anodic dissolution model more than fifty years ago. Since then many studies have centered on the macroscopic characterization of SCC2 but it is only more recently that TEM has been used successfully to examine the phenomenon3*4. Here we describe the SCC microstructures formed in 7010 Al as a function of alloy temper and SCC environment in an attempt to unravel the complex relationship between the way in which cracks propagate and the microstructure of the alloy.
EXPERIMENTAL PROCEDURE
An industrially processed 70 mm thick plate of Al 7010 (Al-6.15 Zn-2.34 Mg-1.59 Cu (wt%) and within the specifications of the Aluminum Association for Fe, Si and Zr) received in the T35 1 condition (stretched and naturally aged) was used to prepare DCB specimens according to Standard IS0 66 at 114 thickness and cut so that the crack propagates in the L-LT plane, which is most susceptible to SCC5> and in the L direction, Some specimens were aged in the laboratory to the T65 1 (peakaged) and T745 1 (overaged) tempers. An underaged specimen was also prepared. The DCB specimens were initially mechanically pre-cracked then exposed to a corrosive environment, either WVSA at 50°C or salt water (57 g/l NaCl). The specimens were removed as soon as SCC had propagated significantly, typically after a few hours, so that the stress intensity factor was still in the plateau region of the da/dt - K curve6 (a = crack length). Figure 1 illustrates how the TEM foils were prepared parallel to the L-ST and LT-ST planes using dimpling and ion milling techniques. The samples were then examined using a Philips CM30 TEM operated at 200 kV.
