Single Stage Ageing Property and Precipitation Variation of a High-Zinc Al–Zn–Mg–Cu Alloy

Abstract

In order to analyze single stage ageing behavior of a high-zinc Al–Zn–Mg–Cu alloy, the microstructure and property variations of the alloy treated by various temperatures were explored by hardness, conductivity, tensile properties, transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM) techniques. The results indicated that a long time was needed to reach a peak hardness value for the alloy treated at 100 and 120 °C and the peak hardness value could sustain a relatively long time interval. The alloy treated at 120 °C adopted a shorter time to reach peak hardness values and its peak hardness value was larger compared with that at 120 °C. The hardness treated at 140 °C reached the peak rapidly and then diminished continuously. With ageing time prolonging, the conductivities of the alloy treated at various temperatures increase persistently. After a peak ageing regime of 120 °C/24 h, the alloy possessed fine and dispersedly distributed matrix precipitates (MPs) and the ultimate tensile strength (UTS), yield strength (YS), elongation and electrical conductivity were 697 MPa, 655 MPa, 12.9% and 17.3 MS m−1, respectively. With the alloy temper varying from under-ageing to peak-ageing to over-ageing, the variety of MPs changed from GPI zone, GPII zone and η′ phase to GPII zone and η′ phase to η′ phase. During this process, the size distribution of MPs became broader and average precipitate size became larger.