This paper presents a study of the microstructures of A1-5Mg alloys with additions of the alloying elements So, Zr, and Ag and the effects of these elements on the polarization and stress corrosion cracking behavior of this material in a 3.5%NaCI solution (pH=8) at 50°C. The results show that Sc+Zr additions have a stronger effect on inhibiting recrystallization than does Zr alone, and that in a sample aged for 24 hours the Zr+Sc alloy had less precipitation of AI3Mg2 along the grain boundaries relative to the alloy that contained only Zr additions. In the alloy that contained Ag and Zr additions, there was significantly more precipitation of A13Mg2 along the grain boundaries than in the aUoy that contained only Zr. Additions of Sc and Ag improve the protective nature of the passive film in A1-5Mg-0.15Zr alloys in 3.5%NaCI solution. Addition of Sc to an AI-5Mg-0.15Zr alloy improves stress corrosion cracking resistance, while addition of Ag causes the alloy to become more susceptible to stress corrosion cracking in the quenched (510°C) and aged (175°C/24h) conditions.
AI-5Mg alloys are of interest in the automotive industry due to their light weight and high strength. However, these alloys are susceptible to stress corrosion cracking (SCC) if the Mg content is higher than 3.5% (wt) t~,2]. During the aging of these alloys at temperatures ranging from 100 to 200°C, AI3Mg2 (IB-phase) will precipitate if the Mg content is greater than 3.5% (wt) [31. It is generally accepted that AIaMg2 precipitates that have formed along grain boundaries provide an active path for SCC 0-41, although other mechanisms for failure have been proposed, such as hydrogen induced cracking [5] and degradation of the passive film at the crack tip trl. Doig and Edington t71 indicated that the corrosion potential of AI3Mg2 is several hundred millivolts more anodic than that of the matrix; consequently there will be a localized corrosion reaction at the precipitate/matrix interface. Cui and Wu tsl found that the SCC resistance is improved in AI-Mg alloys when the 13-phase along the grain boundaries changes from a continuous film of precipitates to discrete precipitates.
Various studies have shown that the addition of alloying elements such as Zr, Sc and Ag can change the microstructure and the mechanical properties of AI-Mg alloys [9-121. The addition of small amounts of Sc causes the formation of fine, coherent, spherical precipitates of AI3Sc which can pin grain boundaries and inhibit recrystallizationt91; this can result in a great strengthening effect 001. Zr plays a similar role, but it is not as effective in inhibiting recrystallization [131. The addition of Ag promotes age- hardening in the AI-Mg systems 01, ~21 However, less is known about the effects of these alloying additions in small amounts on the polarization and the stress corrosion cracking response of AI-5Mg alloys. In this paper we will present a study of the electrochemical polarization and stress corrosion cracking of these alloys in 3.5% NaCI solution (pH=8) at 50°C and also describe their microstructures after specific heat treatments.
EXPERIMENTAL PROCEDURE
The materials we used in this study were AI-5Mg-0.15Zr-0.05Sc, A1-SMg-0.15Zr, A1-5Mg-0.2Ag and AI-5Mg-0.15Zr-0.2Ag alloys. Table 1 shows their chemical compositions. The ingots were extruded at 450°C to bars with rectangular cross-sections (65 mm x 9 mm). The heat treatment procedure involved solutionizing at 510°C for 30 minutes, followed by water quenching. The samples were then aged at 1750C for 24 hours. Microstructures were examined in detail before and atter the heat treatment by optical microscopy (OM) and transmission electron microscopy (TEM). Samples f
