Effects Of Y And Gd On Microstructure And Properties Of AZ61Magnesium Alloy

Magnesium alloy is the most lightweight material with its high specific stiffness, high specific strength, good casting abilities, good recovery properties, good biological properties and other good properties. Magnesium alloys have been widely-used in transportation, communications, advanced equipment and other related fields. At elevated temperature, the mechanical and creep properties of magnesium alloys are so poor, which is the main factor to hinder their future applications.The effects of rare earth Y and Gd on microstructures, room and elevated temperature mechanical properties, and creep properties are studied through preparation of alloy, analysis of microstructure, tensile properties test, and creep test.The results show that, appropriate rare earth element Y and Gd can refine grains of Mg-6Al-1Zn-mY-nGd(m=0.3-1.2, n=0.6-2.4, n/m=2) as-cast and aged magnesium alloy, increase mechanical properties. For as-cast alloy, with the increase of Yand Gd, the bone shape-like Mg17Al12phase decrease, the block shape-like Al2Y and Al2Gd secondary phase increase, the grain sizes first decrease and then increase. When the content of Y and Gd is2.7%, the grain size is smallest, and the tensile strength of alloy reaches its peak value236.2MPa at room temperature. After solution and aging treatment, with the increase of Yand Gd, the point and small-block shape-like Mg17Al12phase decrease, the block shape-like Al2Y and Al2Gd secondary phase increase, the grain sizes first decrease and then increase. When the content of Y and Gd is2.7%, the grain size is smallest, Mg17Al12phase disappear, the tensile strength of alloy reaches its peak value at room temperature and elevated temperature, respectively,254.8MPa at room temperature,164.4MPa at175℃. It is considered that, with the increase of Y and Gd, the mechanical properties of Mg-6Al-1Zn-mY-nGd(m=0.3-0.9, n=0.6-1.8, n/m=2) as-cast and aged magnesium alloys are improved in various degrees by the effects of solid-solution strengthening, precipitation strengthening and fine-grained strengthening.The tensile fracture morphology of Mg-6Al-1Zn-0.9Y-1.8Gd aged alloy shows obvious cleavage platform appeared brittle fracture under room temperature, as the temperature increases, it shows dimple and tear ridges, which transforms from brittle fracture to ductile fracture. The elongation of alloy is23.6%at175℃.Refining the grain size is reason that the calculation values of planar mismatch between α-Mg matrix and Al2Y and Al2Gd are more than15%, which is not as heterogeneous nucleus but as Y and Gd enrichment at the grain boundaries during solidification process to hinder Mg17Al12phase and high melting point compounds are gathered to prevent grain growthWith the stress and temperature increase, the steady creep rate of Mg-6Al-1Zn-0.9Y-1.8Gd magneisum alloy decreases in the temperature range of150℃-200℃and at different stresses50/70MPa. The creep activation energy of the alloy is closer to the activation energy of grain boundary sliding, which may be controlled by grain boundary sliding diffusion.