Effect Of Zn On Mechanical Properties And Microstructure Of Al-Mg-Si Alloy

The Al-Mg-Si alloys are widely used in modern automotive industry as a result of their many advantageous properties obtained after heat treatment,such as high strength to weight ratio,excellent formability,high weldability and corrosion resistance.During proper heat treatment,a large number of nanometer-sized precipitates formed in the alloy effectively hinder the movement of dislocations,which leads to strengthen the materials.Therefore,understanding the evolution of precipitate microstructure in the heat treatment process is essential for improving the properties of the alloys.The Zn element,among those promising ones,has demonstrated positive alloying effects on automotive Al-Mg-Si alloys and therefore has received much attention in recent years.Although some studies reported that Zn addition to Al-Mg-Si alloys may cause the formation of extra precipitates,most of recent investigations have shown that Zn atoms only influence formation of the precipitates previously known in the Al-Mg-Si(-Cu)alloy system.The Zn addition leads to a finer and denser distribution of needle-shaped precipitates,resulting in improved peak hardness.In the present work,using microhardness test,tensile experiment,transmission electron microscopy and advanced high-angle annular dark field scanning transmission electron microscopy in association with first-principles calculations,based on alloy composition and ageing treatment,the thesis has studied the mechanical properties and precipitation behaviors of Al-Mg-Si-Zn alloy in detail.Zn contents have obvious effect on the age-hardening characteristics,strength and elongation of Al-Mg-Si-Zn alloy.Compared with the low Zn content(1.0 wt.%),the alloy with high Zn content(3.0 wt.%)not only has a more significant baking response and excellent age hardening effect,but also has good strength and plasticity.The microstructure of Al-Mg-Si-Zn alloy is affected by the Zn content,which brings apparent distinction in morphology,distribution and size of the precipitates.The alloy with high Zn content has smaller size and slower coarse rate of precipitates during ageing treatment process.In contrast,the precipitation strengthening effect of alloy with low Zn content is relatively weak.The impacts of Zn on the precipitates microstructure of Al-Mg-Si alloy are significant.In addition to the needle-like phases,a large number of lath phases were observed in the ageing process,and the twin Si phase was found in the slightly over-aged state.All the precipitates observed in the experiment,except for a small portion of?"-precipitates and Q'-precipitates,most of the precipitates in the alloy are composite precipitates containing several known structure units of Al-Mg-Si(-Cu)alloy system.The further observations clearly show that Zn atoms can enter all the hardening phases involved to occupy certain featured atomic sites of the original elements and modify their bulk structures,interfacial structures and morphologies in very characteristic manners to form Zn-containing?''_Zn,?'_Zn,U2_Znand Q'_Znphases.In addition,according to the atomic structure of some multiphase composite precipitates,the possible structural evolution of the?and Q series precipitates were discussed.