Dependence of microstructure, mechanical properties, and inter-granular corrosion behavior of Al-5.1Mg-3.0Zn-0.15Cu alloys with high temperature pre-treatment

The age-hardenable Al-5.1Mg-3.0Zn-0.15Cu (wt%) alloy in peak-aged condition has high strength but poor intergranular corrosion resistance. The effect of high temperature pre-treatment on mechanical properties and intergranular corrosion of the alloy has been investigated via hardness, tensile, inter-granular corrosion depth, and electrochemical testing. The morphology, distribution, and composition evolution of precipitates in the grain and along grain boundaries are characterized and analyzed using transmission electron microscopy. The relationships between mechanical properties, inter-granular corrosion resistance, and microstructure evolution during the high temperature pre-treatment and the subsequent aging process have been established. The intergranular corrosion resistance is enhanced with decreasing pre-treatment temperature. While the strength increases with increasing of pre-treatment temperature, and it maintains a similar value to that of the T6 temper, when the pre-treatment temperature reaches 410 degrees C. The 410 degrees C/1 h + T6 temper can improve the intergranular corrosion resistance approximately 55% and while simultaneously maintaining a similar strength to that of the T6 temper, which mainly results from preferentially segregated Mg and Zn atoms along grain boundaries and the preservation of the maintained solid solution state in grains after the 410 degrees C/1 h treatment, respectively. The discontinuity degree of grain boundary precipitates and the concentration difference of Cu element between grain boundary precipitates and adjacent zones have been found to be the key factors to improve the inter-granular corrosion resistance.