The Effect of the Retrogression and Reaging Treatments on the Mechanical Properties and Susceptibility to Stress Cracking of AA 7039 Al Alloy

RRA 처리가 AA 7039 Al 합금의 기계적 성질과 내응력 부식성에 미치는 영향

To investigate the effect of the Retrogression and Reaging (RRA) treatment on the strength and the stress corrosion cracking in AA 7039 the measurement of hardness, tensile properties and the time to failure in a constant rate method were measured. Transmission electron microscope was used to examine the microstructural changes within graln and grain boundary. The results show that the RRA treatment of the T6 condition results in a significant increase in the time to failure without sacrifying the original T6 strength. It is believed that high density and even distribution of precipitates in RRA condition were observed within grain so that the RRA condition could have similar strength to the T6 condition. The presence of fine dispersion of semicohernt ${\eta}^{\prime}$ transition phase is also believed to contribute to that effect. Examination of the grain boundary microstructure shows that the RRA treatments increases significantly the average size of the grain boundary preciptates. It is suggested that the benificial effect of the RRA treatment on the susceptibility to SCC be due to the increase in the size of grain boundary precipitates obtained during the retrogression treatment.