Microstructure And Mechanical Properties Of Mg-Sn Alloys

The microstructure and phase constituent of Mg-Sn-Zn-Ca and Mg-Sn-Al-Zn-Ca alloys were examined by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The mechanical properties of the alloys were tested by universal electronic material testing machine. The fracture surfaces were analyzed by using SEM. The important results are summarized as follows:The phases of as-cast Mg-5Sn-5Zn-xCa(x=0, 0.5, 1, 2)alloys are composed ofα-Mg, Mg2Sn andα-Mg+MgZn phases. While another CaMgSn phase is formed after Ca addition, and ternary eutectic phasesα-Mg+MgZn+Mg2Ca are precipitated when Ca content is about 2wt%. The dendrite microstructures with refined arm spacing due to Ca addition were observed for the four alloys.α-Mg+MgZn phase precipitates in plate or particle form; and Mg2Sn and CaMgSn phase have no obvious changes after solution and aging treatment. The Brinell hardness of four alloys increases firstly and then decreases with the aging time. Addition of Ca to the Mg-5Sn-5Zn alloy can obviously improve mechanical properties. The tensile strength of the Ca-containing alloys decrease with Ca increasing, and the coarse CaMgSn phases leads to an adverse effect on the tensile strength. The tensile strength and elongation of as-cast Mg-5Sn-5Zn-0.5Ca alloy reach to 184.72MPa, 3.29% respectively. After solid solution of 400℃, the tensile strengths and elongations of 6h, 12h, 24h and 48h aging time were 196MPa, 225MPa, 255MPa, 212MPa and 5.59%, 6.07%, 2.23%, 3.98%, respectively.The phases of as-cast Mg-5Sn-xAl-1Zn-yCa (x=3, 6 and y=1, 2) alloys except Mg-5Sn-3Al-1Zn-2Ca are composed ofα-Mg, CaMgSn and Mg32(Al, Zn)49. The Mg2Sn is formed in the two alloys with Ca content is 1wt%. Mg32(Al, Zn)49 phase rather than Mg32(Al, Zn)49 phase formed in Mg-5Sn-3Al-1Zn-2Ca alloy. The tensile strength of alloys with 1wt%Ca is better than that with 2wt%Ca alloy, and the ultimate tensile strength of Mg-5Sn-6Al-1Zn-Ca alloy is 149.35MPa. The Mg32(Al, Zn)49 phase is precipitated as the second phase for three alloys after solution and aging treatment, and the Brinell hardness of four alloys increases firstly and then decreases with the aging time. With the aging time increasing, the second phases precipitated in phase boundary are increased for the Mg-5Sn-6Al-1Zn-1Ca alloy, and the second phases coarsened with 48 hour aging. After solid solution of 400℃, the tensile strengths and elongations of 6h, 12h, 24h and 48h aging time were 169MPa, 189MPa, 231MPa, 191MPa and 1.54%, 1.06%, 2.54%, 2.72% ,respectively.