Study About Influence Of Nd And Sm On Microstructure And Mechanical Properties Of Mg-Zn-Y Alloys

Mg-Zn-Y as-cast alloy is an important research field of magnesium alloy. This dissertation studied about the microstructure, mechanical properties and the most suitable heat treatment system of Mg-Zn-Y-RE as-cast alloys after added rare earth metal Nd and Sm in Mg-6Zn-1.5Y-0.8Zr alloy.The phase structure, micromorphology, phase transition temperature, element distribution and space lattice structure were studied by experimental methods as X-ray diffraction (XRD), optical microscope, differential scanning calorimeter (DSC), transition electron microscopy (TEM), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), the mechanical property of different alloys were measured by material testing machine. The results indicate:When the content of Nd and Sm in Mg-6Zn-1.5Y-0.8Zr alloy separately is 3% and 2%, the crystalline grains of corresponding alloys named MN03 and MS02 are well refined and mechanical properties of the two alloys reach highest point. Compare to the alloy without Nd and Sm, the ultra tension strength (UTS) of MN03 alloy have increased by 25% to 207.5Mpa and the elongation have increased by 86% to 6.5%, while the UTS of MS02 alloy have increased by 28% to 212Mpa and the EL have increased by 90% to 6.75%.The Nd and Sm element added into the alloy have replaced some of Y in the inhere W-phase(Mg3Y2Zn3), Z-phase(Mg3YZn6) and X-phase(Mg12YZn) of the alloy and form W'-phase(Mg3(RE)2Zn3), Z'-phase(Mg3REZn6) and little X'-phase(Mg12REZn) without any change in space lattice structure. The excessive content of Nd and Sm may cause second phase enriched in grain boundary and the boundary coarsen, which will reduce the mechanical properties of the alloy.The study about heat treatment system of MN03 and MS02 alloys indicate that the best solution temperature of two alloys are both 530℃. The best solution time of MN03 alloy is 10 hours and its suitable heat treatment system is solution treatment for 530℃×10h with aging treatment for 250℃×24h. The UTS of MN03 alloy increased by 14.7% to 238Mpa and EL have increased by 15.4% to 7.5%. The best solution time of MS02 alloy is 8 hours and its suitable heat treatment system is solution treatment for 530℃×8h with aging treatment for 250℃×24h. The UTS of MS02 alloy increased by 14.2% to 242Mpa and EL have increased by 18.5% to 8%.During heat treatment process, W'-phase decompose and convert to Z'-phase and little X'-phase. Z'-phase is icosahedral quasicrystal phase with good pinning effect, which can strengthen the alloy. Long time solution treatment will cause the grain boundary dissolve and grains merger and coarsen, which has diverse effect with mechanical properties of the alloy.