Abstract
The effect of the Zn/Mg ratio on microstructures, mechanical properties and corrosion performances of Al-Zn-Mg alloys was studied. Microstructures were characterized using the optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Tensile tests, intergranular corrosion (IGC) and stress corrosion cracking (SCC) tests were conducted to study the properties. Microstructures results indicated that with the decrease of the Zn/Mg ratio, the recrystallization proportion and the fraction of second phase decreased, while the size of η’ (MgZn2) phases in grain interior also significantly decreased. The number density of η’ phases in grain interior increased and grain boundary precipitates developed discontinuous distribution with the decrease of the Zn/Mg ratio. These microstructures contributed to the significant improvement of the strength and corrosion resistance. The tensile strength and yield strength increased by 34.1% and 47.4%, respectively, with the Zn/Mg ratio decreased from 11.4 to 6.1. Calculating results indicated that the enhancement of strength mainly contributed from the solid-solution strengthening, grain-boundary strengthening and precipitation strengthening. The intergranular corrosion degree was greatly relieved and the stress corrosion sensitivity index decreased from 0.031 to 0.007 with the Zn/Mg ratio decreased from 11.4 to 6.1.
Highlights
Al-Zn-Mg alloys have been widely used in vehicles, buildings, bridges and other fields as light material because of their lower density, higher strength, better weldability and formability [1–3].It is well known [4] that this alloy is an age-hardening alloy and its precipitation sequence is: the supersaturated solid solution (SSS) → the segregation zone (GP zone) → transition phases η’(metastable phases MgZn2 ) → equilibrium phases η (MgZn2 )
The proportion of grain boundary with large angle gradually decreased with the decrease of the Zn/Mg ratio
The fraction of the coarse second phaseswith significantly decreased with theTherefore, decrease the fraction of the coarse second phases significantly decreased with the decrease of the Zn/Mg ratio
Summary
Al-Zn-Mg alloys have been widely used in vehicles, buildings, bridges and other fields as light material because of their lower density, higher strength, better weldability and formability [1–3]. It is well known [4] that this alloy is an age-hardening alloy and its precipitation sequence is: the supersaturated solid solution (SSS) → the segregation zone (GP zone) → transition phases η’. In order to improve the corrosion resistance of 7xxx series aluminum alloys, many researches have been carried out by regulating heat treatment processes and optimizing the composition of alloys. The RRA (retrogression and re-aging) process can regulate the grain boundary precipitates to discontinuous distribution, improve the corrosion resistance [6,7]. The relationship of microstructures and properties is built to provide a reference for the composition optimization and performance control of the Al-Zn-Mg extrusion profiles
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
