Effect Of Micro-alloying Elements On The Microstructure And Mechanical Properties Of Mg-RE-Zn Alloy

The trends in the development of modern aeronautics and astronautics devices, weapons and precision instrument is to be more lightening, high speed and high power. However, the strength and toughness of existing magnesium alloys can not fully meet the requirement of the main load-carrying structure in these products. Therefore, to carry out the studies in the high strength magnesium alloys become more urgent. Due to the compound strengthening of LPSO phase and precipitation hardening phase, Mg-RE-Zn alloys exhibit excellent mechanical properties. Nevertheless, the application and development of these alloys is still simplified and mainly concentration on the controlling of the ratio of element Zn and RE reasonably to obtain different strengthening effects. Micro-alloying has very important significance in the process of ferrous and nonferrous materials. However, studies on the impact of Micro-alloying to Mg-RE-Zn alloys are rarely reported. In accordance with this present situation, this article illustrates the controlling on the precipitation behavior of the main strengthening phases in Mg-RE-Zn alloys by Micro-alloying to obtain better mechanical properties.First, the impact of grain refiners such as Mn, Zr on the microstructure and mechanical properties of Mg-9Gd-4Y-1Zn alloy has been studied. Traces of Mn, Zr can effectively refine as-cast structure without changing the the phase composition of the alloy. Mn can effectively promote the dynamic recrystallization and large amounts of Zr-rich ring which caused the fracture of the alloys can be found in Zr-containing alloys. The ultimate tensile strength and elongation of as-aged bar are 475 MPa, 6%.Second, the controlling of Sn, Al on the the morphology of LPSO phase was investigated. Sn can promote the precipitation of rod, lamellar LPSO phase. Yet, Al will inhibits the formation of rod LPSO phase and slightly promotes the the precipitation of lamellar LPSO phase. The strength of the as-extruded alloy has been enhanced by the emerging of the kinking grains. After extrusion, the Mg-9Y-5Zn-1Sn alloy obtain UTS of 420 MPa and elongation of 10%.Third, the influence of Sn content on the phase composition and the mechanical properties of Mg-9Gd-4Y-1Zn-0.8Mn-xSn(x=0~2, wt.%) alloy series was investigated. Results show that trace Sn has no effect on the phase category of the alloy and the Sn4(Gd,Y)5 phase appeared as the Sn content reaches 1 wt%. After annealing, the lamellar LPSO phase which lead to the distorted grains during extrusion precipitated in the matrix of the Sn-doped alloys. With increasing the content of Sn addition, the volume fraction of distorted crystal grains increased, but decreased as the Sn content reaches 2 wt%. UTS and EL were increased considerably with addition of 0.5 wt% Sn then decreased with increasing the content of Sn addition which attributed to the presence of the coarse Sn4(Gd,Y)5 phase. After extrusion, the 0.5 wt% Sn-doped alloy shows an UTS of 412 MPa with an elongation-to-failure over 15.6%. The UTS of the alloy reaches 502 MPa with an elongation-to-failure over 6% after aging treatmentBesides, the influence of Ag, Nd on the precipitation behavior of Mg-9Gd-4Y alloy was investigated. Results show that Ag, Nd addition was favored to the mechanical properties improvement. The peak hardness was increased from 122 HV to 127 HV while the strength of as-aged alloy increased 23 MPa after adding 1 wt% Nd. The peak hardness was increased from 122 HV to 134 HV while the strength of as-aged alloy increased 50 MPa after adding 1 wt% Ag.At last, the influence of Ag content on the phase composition and the mechanical properties of Mg-9Gd-4Y-1Zn-0.8Mn-xAg(x=0~1.5, wt.%) alloy series was investigated. Results show that trace Ag could effectively promote the precipitation of the lamellar LPSO phase. With increasing the content of Ag addition, the white stripe LPSO phase which contains Ag elements was obtained. The aging at 200? for 50 h after extrusion could make the Mg-9Gd-4Y-1Zn-0.8Mn-1.5Ag(wt.%) alloy obtain UTS of 540 MPa, elongation of 9%.