Quench sensitivity of Al-Zn-Mg-Cu alloys

Abstract

The quench sensitivity of an Al-7.5Zn-1.7Mg-1.4Cu-0.12Zr alloy was determined by temperature-time-property (TTP) curve by an interrupted quench method with the measurement of the hardness as aged and electrical conductivity as quenched compared with traditional 7B04 alloy and 7150 alloy. The results indicate that the nose temperature of TTP curve and the corresponding incubation period of the novel alloy, 7150 alloy and 7B04 alloy are about (290°C, 4.5 s), (320°C, 2.6 s) and (335°C, 0.1 s), respectively, the nose temperature of the alloy is the lowest among three alloys and the critical time at the nose temperature is the longest for the alloy, which is obvious that the supersaturated solid solution of the alloy is the most stable, exhibiting the alloy has the lowest quench sensitivity. Further, TEM analysis results show that, with the prolongation of keeping time at the nose temperature, the quench-induced precipitation phenomenon becomes obvious. The quench-induced η precipitates nucleate and precipitate at grain and sub-grain boundaries by preference, and the quench-induced precipitates appear to nucleate mostly on the pre-existing Al3Zr dispersoids. After aging treatment, these η precipitates were surrounded by a precipitate-free zone. The composition and microstructure morphology of alloy play an important role in the quench sensitivity of the alloy quenched. For the novel alloy, appropriately decreasing the cooling rate may be helpful to relieve the residual stresses.