The effect of two-step aging on the quench sensitivity of an Al-5 Pct Zn-2 Pct Mg alloy with and without 0.1 Pct Cr

Abstract

The effect of two-step aging on the quench sensitivity of an Al-5 pct Zn-2 pct Mg alloy with and without 0.1 pct Cr has been studied. Results show that the quench sensitivity effect can be eliminated in thin samples of these alloys by two-step aging if the slow cooling during quenching does not allow the precipitation process to proceed too far. Lack of achievement of full strength in the aged condition due to a slow quench rate can be attributed to 1) loss of vacancies during quenching and 2) formation of incoherent-type precipitates during quenching. The trend to lower strength due to the loss of vacancies can be reversed by two-step aging; however, if incoherent-type precipitates form, some strength potential of the alloy is permanently lost. A 0.1 pct Cr addition increases the quench sensitivity effect by accelerating incoherent-type precipitation during quenching. These incoherent precipitates, which appear in the form of bands within the grains and in the grain boundaries, lead to an increase in ductility.