Characterization of microstructural changes in an Al-6.8 wt.% Mg alloy by electrical resistivity measurements
Introduction
Wrought Al-Mg alloys have been considered for use in a wide variety of automotive and marine applications due to their excellent properties, such as high strength to weight ratio, good formability, corrosion resistance and weldability. It is well known [1], [2], [3], [4], [5], [6] that the strength level of Al-Mg alloys increases with increasing Mg content due to the solid solution hardening effect. On the other hand, the benefit of increasing the strength can be accompanied by decreasing corrosion resistance of high Mg-containing Al-Mg alloys. The solid solution in Al-Mg alloys with ≥3 wt.% Mg is supersaturated at room temperature and the excess Mg solute atoms tend to precipitate out as highly anodic β-phase (Mg5Al8) particles, preferably distributed along the grain boundaries [1], [2], [6]. Since the corrosion potential of the β-phase (−1.24 V) is more negative than that of the Al-matrix (−0.87 V), dissolution of the anodic β-phase particles can occur in an appropriate corrosive solution. Precipitation sequences during decomposition of supersaturated solid solution have been reported previously [7], [8] as follows: α-Al matrix → GP zones → β′-phase → β-phase (Mg5Al8).
This process occurs slowly during a very long time at room temperature, or in a short exposure period at slightly elevated temperatures (60–180 °C). Al-Mg alloys containing ≥3 wt.% Mg become unstable with time and susceptible either to intergranular corrosion (IGC) or stress corrosion cracking (SCC) [2], [3], [6], [9]. These phenomena are usually referred to as “sensitization”. The rate of sensitization increases with increasing Mg content and with prior plastic deformation [9]. However, susceptibility of the Al-Mg alloys to different forms of corrosion depends not only on the presence of β-phase particles, but especially on their form and distribution throughout the structure. Microstructures with continuous layers of β-phase precipitates along the grain boundaries are highly susceptible to corrosion, while randomly distributed β-phase particles provide high corrosion resistance [9], [10], [11]. In order to provide good mechanical properties and high corrosion resistance, optimization of the chemical composition and microstructure development during thermo mechanical treatment (TMT) are required.
Understanding of the precipitation behavior in high Mg-containing Al-Mg alloys appears to be very important because of their wide range of application. The aim of this study was to follow the β-phase precipitation and variation of the concentration of Mg solute atoms in the matrix of a highly alloyed Al-6.8 wt.% Mg sheet, which occur under different TMTs and during sensitization treatment. The precipitation or ageing processes were investigated by electrical resistivity measurement which was found to be reliable technique for microstructural characterization of the Al alloys because of its simple procedure and high sensitivity to detect different microstructural features, including dissolution/precipitation processes [1], [7], [12], [13], [14], [15], [16], [17].
Section snippets
Material
The Al-Mg alloy used in this study was supplied in a fully annealed condition (O temper, YS = 161 MPa, UTS = 343 MPa, El = 26%), with the chemical composition given in Table 1.
The material was fabricated by casting and a homogenization treatment at 440 °C/10 h and 490 °C/12 h, with furnace cooling (FC) to room temperature. It was preheated at 460 °C/9 h and 480 °C/2 h prior hot rolling, and subsequently hot rolled down to 7 mm. The hot rolled sheets were cold rolled to 3 mm and finally annealed at 320 °C for 3 h.
Tensile properties
Variations of the yield strength (YS) of the tested AlMg6.8 alloy after different TMTs are shown in Fig. 1 (closed symbols). It can be seen that thermal exposure of the cold rolled samples in the lower temperature range from, 225 °C to 265 °C, was followed by a decrease of the yield strength from YS = 334 MPa, 382 MPa and 426 MPa in the cold rolled condition after 30%, 50% and 70% deformation, respectively, to values in the range 230–260 MPa, depending on the annealing temperature and deformation.
Discussion
During annealing of highly alloyed Al-Mg alloys, concurrently with recovery and recrystallization processes, a change in the concentration of Mg solute in the solid solution occurs. The variation of the amount of Mg solute in the Al-matrix of the tested AlMg6.8 alloy was followed by electrical resistivity measurement, as a very sensitive tool to detect the amount of Mg solute in/out of the solid solution [1], [7], [12], [13], [14], [15], [17]. However, the fact that the electrical resistivity
Conclusions
Microstructure development under different TMTs and after sensitization treatment of AlMg6.8 alloy was followed through mechanical properties, optical metallography and electrical resistivity measurements.
It was observed that electrical resistivity can be employed as indirect evidence of the concentration of Mg solute in the solid solution and of an increased dislocation density in the structure. The effect of the dislocation density on the electrical resistivity was less pronounced compared to
Acknowledgements
The authors are grateful to the Ministry of Science of the Republic of Serbia and IMPOL-SEVAL Aluminum Rolling Mill, Sevojno for the financial support and supplying of the material used in this investigation, respectively.
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