Effect of ageing treatment on microstructure and stress corrosion cracking of an Al-4.2Zn-1.6Mg alloy

The effect of T4, T6, T77 and T73 ageing treatments on microstructure and stress corrosion cracking of an Al-4.2Zn-1.6Mg alloy was investigated. TEM study revealed that there is no eta' phase on the matrix or grain boundary in the T4 sample, and the precipitates on the grain boundaries of the T6 sample were small and continuously distributed, while the precipitates of the T77 and T73 samples discontinuously distributed on the grain boundary. Stress corrosion cracking resistance test in in the 50 degrees C 3.5% NaCl solution and fractured surface observation revealed that the T4 sample have higher tendency of stress corrosion cracking compared with that of the T6 sample, while the T77 and T73 samples show good resistance of stress corrosion cracking. It is worth to note that the strength of the T77 sample is very close to that of the T6 sample.