Research On The Microstructure, Mechanical Properties And Corrosion Resistance Of Mg-1.5Zn-xY-Zr Alloys

Magnesium alloys, as the lightest structural metallic materials, have greatpotential application in automotive, aerospace and electronic industries because oftheir low density, high specific strength, good damping capacity and easy recycling.However, the inherent properties of Mg alloys such as low strength, inferior ductility,poor thermal stability and corrosion resistance seriously hinder their application. Inrecent years, Mg-Zn-Y-Zr alloys have attracted significant interest because they havethe long period stacking ordered （LPSO） phases which have unique microstructure andextremely high strength. In this thesis, Mg-1.5Zn-xY-0.4Zr（x=3,4.5,6,7.5wt%）alloyswith LPSO phases were prepared by semi-continuous casting and hot extrusion. Theforming and strengthening mechanism of LPSO phases were investigated. Themicrostructure evolution and mechanical properties of Mg-1.5Zn-xY-Zr alloys indifferent ageing and annealing conditions were studied. The corrosion performances ofMg-1.5Zn-xY-Zr alloys were studied by immersion test and electrochemicalmeasurements in order to determine the corrosion rate and mechanism of the alloys.The results show that:The microstructure of Mg-1.5Zn-xY-Zr alloys hot extruded consistes of α-Mg andLPSO phases. The LPSO phases are mainly composed of18R-LPSO structure on thegrain boundary and some14H-LPSO structure in the Mg matrix. The volume fractionof LPSO phases are found to be greatly affected by the Y content: With the increase ofY content, the fraction of LPSO phase increases and the content of LPSO phases inMg_90.6Zn_1.5Y_7.5Zr_0.4alloy is about35.7%. LPSO phases are distributed along to theextrusion streamlined direction and kinks in18R-LPSO phase are found duringdeformation，which indicates that LPSO phases have good plasticity. In addition,dynamic recrystallization of α-Mg phase occured ant the degree of recrystallizationwas increased with the increase of Y content. During ageing and annealing heattreatment at300℃, the microstructures of Mg-1.5Zn-xY-Zr alloys were similar to thatof extruded alloys. However, after annealing at400℃,18R-LPSO phase is graduallydissolved and14H-LPSO began to precipitate within the Mg matrix. After annealing at500℃,18R-LPSO phase is completely dissolved into the matrix and a large number of14H-LPSO phase is formed throughout the grain as the main second phase inMg-Zn-Y-Zr alloys. At room temperature, Mg-1.5Zn-xY-Zr extruded alloys exhibit high strength andgood ductility, and the ultimate tensile strength increases with the increase of Ycontents, while the elongation increases first and then decreases. At elevatedtemperature （200℃）, the tensile strength decreases slightly, but the elongationincreases. Mg_90.6Zn_1.5Y_7.5Zr_0.4alloy has optimum mechanical properties: the yieldstrength, ultimate tensile strength and elongation were299MPa,360MPa and16.7%respectively at room temperature,282MPa,340MPa and23.5%respectively at200℃.Aging treatment leads to slight increase of the mechanical properties, while annealingtreatment at300℃does not. When annealed at400℃or higher temperatures,mechanical properties decline sharply. At ambient temperature, the fracture surface ofMg-1.5Zn-xY-Zr alloys both extruded and aged consists of tearing ridges, cleavageplanes and dimples, showing the characteristics of ductile fracture. While at elevatedtemperature （200℃）, Mg-1.5Zn-xY-Zr alloys as-extruded present ductile fracturefeatures with many dimples and a combination of high Strength and Toughness.The corrosion form is mainly localized corrosion which is observed in theinterface of LPSO phases and Mg matrix. The LPSO phases have a dual role incorrosion of alloys: It can not only promote micro-electric galvanic corrosion, but alsoimpede the corrosion of alloys because of the grid-like structure. Mg_90.6Zn_1.5Y_7.5Zr_0.4alloy shows the lowest corrosion rate among all the alloys tested due to the continuousdistribution of LPSO phases. The corrosion rate measured of Mg-1.5Zn-xY-Zr alloys is:Mg_90.6Zn_1.5Y_7.5Zr_0.4<Mg₍95.1Zn_1.5Y_3.0Zr_0.4<Mg₍92.1Zn_1.5Y_6.0Zr_0.4<Mg_93.6Zn_1.5Y_4.5Zr_0.4.Corrosion products of Mg-1.5Zn-xY-Zr alloys are mainly Mg （OH）₂, accompaniedwith a small quantity of LPSO phases uncorroded.