Study On The Microstructure And Mechanical Properties Of High Strength Mg-Y-Bd-Zn-Zr Cast Alloys

Abstract:Mg-Nd alloys are very attractive for aerospace, racing automotive and electronics industries because of their excellent mechanical properties, thermal stability and casting performance. Y element has significant strengthening effect on magnesium alloys than Nd. Zn is one of the cheap alloy elements, and the magnesium alloys containing trace amount of it possess excellent solid solution strengthening and age hardening effect. At present, the effects of adding Y and Zn element in Mg-Nd alloys on microstructure and mechanical properties have not been systematic researched. Therefore, in this article, based on the Mg-Nd alloys, the new type of Mg-Y-Nd-Zn-Zr alloys are designed by adding Y(0-6.0wt.%) and Zn(0.2-1.5wt.%) element content, the microstructure?mechanical properties and strengthening mechanisms of Mg-Y-Nd-Zn-Zr alloys were investigated deeply and systematically. The main conclusions can be summarized as follows:As cast Mg-2.4Nd-0.2Zn-0.5Zr alloy are composed mainly of a-Mg, Mg12Nd phases. Mg24Y5phase appear in Mg-2.5Y-2.4Nd-0.2Zn-0.5Zr alloys with increasing Y content. Mg-4.2Y-2.4Nd-0.5Zr-(0.5-1.5)Zn alloys contains Mg12YZn phase besides that in Mg-4.2Y-2.4Nd-0.2Zn-0.5Zr alloy. After solution treatment, Mgi2Nd and Mg24Y5phases in Mg-Y-Nd-Zn-Zr alloys almost disappear.In225?peak-aged condition, the main precipitate phases of Mg-Y-Nd-Zn-Zr alloys are ?' phase. With increasing Y content, the volume fraction of ?' phase increase; with increasing Zn content, the volume fraction of ?' phase first increase then decrease. The precipitation sequence of Mg-6.0Y-2.4Nd-0.2Zn-0.5Zr alloy is:a-Mg (S.S.S.S)??"??'??1?? during ageing at250?.At room temperature, the yield strength of the peak-aged Mg-Y-Nd-Zn-Zr alloys increase with increasing Y content; the yield strength of alloys first increase then decrease with increasing Zn content. Mg-4.2Y-2.4Nd-0.2Zn-0.5Zr alloy exhibits the optimal mechanical strength, the value of ultimate tensile strength (UTS), yield strength (YS) and elongation (?) are330MPa,265MPa and5.8%, respectively. The Mg-6.0Y-2.4Nd-0.2Zn-0.5Zr alloy exhibits the optimal mechanical strength, with UTS of280MPa, YS of235MPa and s of11.0%at250?tensile temperature. The high mechanical properties of Mg-Y-Nd-Zn-Zr alloys are mainly attributed to the high volume fraction, the prismatic ?' precipitate of big aspect ratio.The secondary phase strengthening dominates the strengthening of as cast Mg-Y-Nd-Zn-Zr alloy in yield strength. The solid solution strengthening and precipitation strengthening both make the important contribution in strengthening the alloy in as solution treated and aged condition.