THE EFFECT OF RRA ON THE MICROSTRUCTURE AND PROPERTIES OF A NOVEL Al - Zn - Mg - Cu - Zr ALLOY

A novel Al -7.5 Zn -1.6 Mg -1.4 Cu -0.12 Zr alloy was subjected to the retrogression and re-ageing (RRA) treatments at a lower range of retrogression temperatures from 170 to 200°C. The effect of RRA on the mechanical properties, electrical conductivity and the microstructure of the alloy has been investigated. The results indicate that an increase in the yield strength of the alloy can be achieved after RRA treatment. With increase of the retrogression temperature, the retrogression time for keeping the strength levels similar to T6 temper decreases rapidly. When submitted to RRA at 170°C for 90-120 min, the alloy can obtain a good performance on both SCC and mechanical strength; the electrical conductivity was above 40%IACS, reaching values typical for T76 condition and the tensile yield strength values were 552-570 MPa which is higher than that of the T6 temper. The microstructure is a very fine distribution of GP zones and η′ precipitates inside grains, similar to T6 condition and η precipitates on grain boundaries distributed similarly to T7 temper.