Improvement of Stress Corrosion Cracking (SCC) Resistance of a 7150 Al-Zn-Mg-Cu Alloy by Retrogression and Reageing (RRA) Treatment

A 7150 Al-Zn-Mg-Cu alloy is artificially aged at 120 oC for varying time. The peak hardness (T6 temper) is obtained at about 24 h at that temperature. Further, the T6 temper is subjected to retrogression and reageing (RRA) treatment. Slow strain rate test (SSRT) has been carried out on the T6 and RRA tempers. SSRT results indicated that the RRA temper have higher resistance to SCC compared to that of T6 temper. SCC behaviour of the alloy tempers have been explained with the help of microstructural features studied by transmission electron microscope (TEM). The large, discrete and discontinuous grain boundary precipitates observed in the microstructure of the RRA temper are believed to be the responsible factor for achieving higher SCC resistance. Further, SEM fractographs and crack morphology have also been analyzed to evaluate the SCC behaviour of the alloy tempers.