Effects of chromium addition on aging phenomena in Al-Zn-Mg alloy systems

Abstract

Aging phenomena of Al-Zn-Mg-(Cu) alloys with a small addition of Cr were investigated by means of hardness and electrical resistivity measurements, EPMA analyses and transmission electron microscopy. Various specimens based on the (α + T) type Al-Zn-Mg-(Cu) alloys were solution-treated at 470-580°C for 4hrs, quenched into iced water and aged at various aging temperatures, TA. The results obtained are as follows:(1) In the Al-Zn-Mg alloy, the initial aging rate decreased when TA was below about 125°C, as Cr contents increased and solution-temperatures were lowered. Increases of Cr contents and high temperature aging above 150°C suppressed the age-hardenability remarkably. EPMA analyses showed that the solute contents of Zn and Mg in the E phase were much higher than those in the matrix. Thus, the Zn and Mg contents in the matrix inevitably decreased as the amount of Cr addition increased and solution-temperatures were lowered. This fact was considered to explain the aging suppression at low TA in the Cr containing alloys. Further, an electron microscopic examination revealed that the preferential precipitation of the T phase took place on the E phse at high TA. This suggests that the solute contents in the matrix were further decreased by the T phase precipitation. It was considered that the extremely low age hardenability of the Cr bearing alloys at high TA was caused by this fact together with the simple decreasing effect of the solute content by Cr additions.(2) In the Al-Zn-Mg-Cu alloy, the suppression of the initial aging rate at low TA and the low age hardenability at high TA were also introduced by a small addition of Cr. These were considered to be due to the same effect of Cr as in the ternary alloy.