Stress corrosion cracking of Al-Zn-Mg-Cu alloys: effects of heat-treatment, environment,and alloy composition

Abstract

Stress corrosion cracking (SCC) is a common damage mechanism, which usually involves slow crack growth in materials that are exposed to specific environments at stresses that are much lower than those required to produce fast fracture in inert environments. SCC of high-strength precipitation-hardened aluminium alloys occurs in aqueous and moist air environments, and continues to be a problem, particularly in ageing aircraft. Thus, SCC can compromise structural integrity, and increase maintenance costs and aircraft ‘downtime’. In newer aircraft, aluminium alloy compositions and heat-treatments with a high resistance to SCC are now used, and these heat-treatments, such as retrogression and re-ageing (RRA), have been used retrospectively for some aluminium alloy components in ageing aircraft, as described in the introduction in Chapter I. The microstructures of precipitation-hardened Al-Zn-Mg-Cu alloys are complex, exhibiting nano-scale strengthening intragranular precipitates, somewhat coarser scale manganese-, chromium-, zirconium-rich dispersoid particles, coarse iron- and siliconcontaining constituent particles, grain-boundary precipitates, precipitate free zones adjacent to grain-boundaries, and segregated alloying elements at grain-boundaries. The grain-boundary characteristics are particularly important, because SCC is predominantly intergranular, especially in rolled plates stressed normal to the short-transverse crack-plane orientation, where the grain-boundaries are mostly normal to the applied stress. Details of the development of typical microstructures are summarised in Chapter I, section 2. SCC is also a very complex phenomenon, and Chapter I, section 3 includes a description of the fundamentals and the effects of variables such as the stress intensity factor (the mechanical driving force), environment (pH, halide-ion concentration, etc.), electrochemical potential, temperature, and alloy composition/ageing condition. Controversies regarding the mechanisms and rate-controlling steps for SCC are also outlined. For Al-Zn-Mg-Cu alloys, the survey of the literature indicated that there is no consensus regarding the mechanism of SCC, except that ‘hydrogen embrittlement’ is probably involved. There is also no general agreement regarding the relative importance of various microstructural features on rates of SCC. Understanding SCC requires some knowledge of electrochemistry and corrosion in the absence of stress, and some of these aspects are outlined in Chapter I, section 4. An understanding of fast fracture in inert environments is also required, and this topic is reviewed in Chapter I, section 5. The aims of the present work (Chapter II) were to obtain a better understanding of the effects of microstructure and alloy composition on SCC, and to determine whether remedial heat-treatments could be applied to a wider range of alloys used in ageing aircraft than was previously thought possible. There is a view in the literature that only alloys with greater than 1 wt.% copper can be given overaging or RRA-treatments to improve SCC resistance, but few studies appear to have been carried out to substantiate this claim. In the present work, the effects of quenching rates (from solution treatment temperatures), overaging, and RRA have been studied for two low-copper (0.6-0.9 wt.%) Al-Zn-Mg-Cu alloys (7079 and 7022) and a relatively high-copper (1.4 wt.%) alloy (7075). Another aim of the work was to obtain a better understanding of the mechanisms of SCC in aluminium alloys. To achieve the above aims, a wide range of experimental techniques, described in Chapter III, has been used, e.g. optical microscopy, scanning electron microscopy and transmission electron microscopy (including micro-chemical analysis), atomic force microscopy, Auger electron microscopy, electrochemical tests, and hardness and conductivity measurements. SCC velocities were determined using a standard method, viz. optical measurements of crack length versus time on the side surfaces of bolt-loaded double-cantilever beam specimens immersed in saturated NaCl solution or exposed to moist air (43-75% relative humidity). Specimens were cut from 76 mm thick plate at various positions through the thickness. The aims of the work were largely achieved, as described in the results section (Chapter IV) and the discussion (Chapter V). For example, the results suggested that SCC resistance of Al-Zn-Mg-Cu alloys in aqueous environments at high stress intensity factors is largely determined by the composition of grain-boundary precipitates (especially copper content) – not by grain-boundary precipitate size or spacing, matrix precipitate size and spacing/slip-mode, precipitate free zone width, or grain-boundary segregation as has been variously proposed previously. For the low-copper 7079 and 7022 alloys, it appeared that the copper content of grain-boundary precipitates depended on the quench-rate from solution-treatment temperatures – slower quench-rates giving higher copper contents and higher resistance to SCC. For the high-copper 7075 alloy, quench-rate appeared to be less important. The results also indicated that the rate-controlling process for SCC at high stress intensity factors in aqueous environments was the rate of crack-tip electrochemical reactions – specifically the rate of dissolution of grain-boundary precipitates, which is strongly affected by their copper content. The rate of this anodic reaction controlled the cathodic reaction, viz. hydrogen generation, and the amount of hydrogen available for embrittlement. The rate-controlling process for SCC in moist air, where electrochemical reactions cannot occur, was probably associated with film rupture and re-passivation at crack-tips, which controlled the amount of hydrogen, generated by chemical dissociation of water, available. The difference in rate-controlling processes in aqueous and moist air environments resulted in 10-100 times faster rates of aqueous SCC for the rapidly quenched peak-aged 7079 and 7022 alloys than for the peak-aged 7075 alloy, but 10-100 times slower rates of SCC for 7079 and 7022 alloys than for the 7075 alloy in moist air environments (43% relative humidity). The present results and those in the literature also suggested that the mechanism of SCC in both aqueous and moist air environments involved generation of hydrogen at crack-tips, diffusion of hydrogen ahead of cracks, and weakening of inter-atomic bonds across grainboundaries. Such a process accounts for the observations of crack arrest markings on fracture surfaces, which suggest that SCC is a discontinuous process. Based on the present work, remedial heat-treatments could be applied not only to the high copper 7075 alloy, but also to the low-copper 7079 alloy components in ageing aircraft, and the practicalities are discussed in section 6 of the discussion. The results also showed that fracture toughness was increased by remedial heat-treatments, and possible reasons for this improvement are discussed. Finally, suggestions for future work are outlined in Chapter VII.