Study On Microstructures And Properties Of Mg-xGd-0.6Zr Alloys

Compared with other metals, magnesium and magnesium alloys have attracted increasing interest for potential application due to their low density, and excellent damping capacity, electromagnetic shielding, and thermal conductivity. Magnesium and magnesium alloys are one of ideal metals exhibited simultaneously with structure and function, but magnesium with close-packed hexagonal structure has less slip systems, which lead to be weak workable, and pure magnesium has low strength, ultimate tensile strength(UTS) only 100MPa,elastic modulus only 45GPa. So low strength and weak workability are two bottlenecks that effect the application of magnesium and magnesium alloys in industrialization. Rare-earth magnesium alloys have been the focus of intense investigation for the past few decades due to those outstanding mechanical properties at room and elevated temperature, especially Mg-Gd series alloys, meeting the need of new metal materials with low density, excellent cast performance, mechanical properties and resistance corrupt in the automotive, architectural and aerospace industry.The elements xGd(x=2,4,6%) and 0.6%Zr were added to pure magnesium in this paper. The effect of Gd contents on the microstructure and properties of Mg-xGd-0.6Zr alloys in different heat treatment and extruded processes were studied by optical microscopic, scanning electron microscopic equipped with energy dispersive spectroscopy, transmission electron microscopy and micro-hardness and tensile testers.The results show that the as-cast Mg-xGd-0.6Zr (x=2, 4, 6%) alloys display grain structure and no Gd element segregation appears in the microstructure. With the increasing of Gd content, the grain size of as-quenched Mg-xGd-0.6Zr (x=2,4,6%) decrease and the second phases Mg5.05Gd contents increase. The morphology characters of the second phases Mg5.05Gd in as-annealed alloys included two kinds, one of them is regular shape, about 5μm×5μm×1μm, one of them is irregular shape. The weight percent of the second phases Mg5.05Gd in as-annealed GK4 and GK6 alloys are 2.787wt.%,5.340wt.%, respectively.For as-cast and as-annealed Mg-xGd-0.6Zr alloys, the seconde phases could make the hardness of unit Gd content obviously increase. There is a line increasing relationship between Gd atom percent and micro-hardness of as-quenched alloys and compared with the Mg-Gd seris alloys, the addition of Zr weakens the solid solution strengthen effects. The grain size of alloys after different temperature treated increase with solution temperature increase. The second phase Mg2Gd, Mg3Gd brought in due to non-equilibrium solidification during the casting process could be transformed into equilibrium phase Mg5.05Gd which could be dissolved intoα-Mg solid solution phase at 460°C solution temperature. The optimization of solid solution treatment process is 300°C×6h + 460°C×10h. Micro-hardness value of alloy treated under 420°C is highest due to interaction of solid solution strengthening and second strengthening.After alloys in different states extruded, the strength of Mg-2Gd-0.6Zr and Mg-4Gd-0.6Zr alloys are almost the same and the strength of Mg-6Gd-0.6Zr is highest. However, the elongation of all extruded alloys is over 30%. The ultimate and yield tensile strength and the elongation of Mg-xGd-0.6Zr(x=2, 4, 6%)-Ext alloys are 206,207,237MPa, 150,145,168MPa and36.8%,43.4%,33.4% respectively. After aged 10h, the ultimate and yield tensile strength of extruded Mg-6Gd-0.6Zr alloys increases slightly, to be 243,175MPa respectively, at the same time, the elongation of alloys rarely decreases, to be 31.7%. The fracture mechanism of all studied alloys is ductile fracture feature.Refinement strengthen, solid solution strengthen, precipitation strengthen and resident compression stress are contributive to the strength of Mg-xGd-0.6Zr (x=2,4,6%) alloys and yield tensile strength relates with the line relationship of four kinds of strengthen ways. The decreased c/a value and the grain refinement are helpful to high plasticity for Mg-xGd-0.6Zr alloys. The dislocation proliferation and reaction between solute atoms and dislocation can be used to explain the yield phenomenon with obvious low and upper yield points and yield flat of Mg-xGd-0.6Zr alloys.