Study On Microstructure Refinement & Mechanical Properties Of Wrought Magnesium Alloy

Magnesium alloys have been widely used as the light structure materials in the aviation, automotive and electronic industry due to their low density, high specific strength,specific stiffness, good damping capacity, good shielding properties. On the other hand, wrought magnesium alloy based on microstructure refinement is the chief orientation in enhancing the strength and deformability of Mg alloys. The microstructures and mechanical properties of Mg-3Zn-(Ca, Sr, Sb) alloys and Mg-3Al-(Ca, Sr) have been investigated by using optical microscope(OM), scanning electron microscope(SEM), Energy Dispersive Spectroscopy(EDS), X-ray diffraction(XRD) and inductively coupled plasma(ICP).The results indicate that a small amount of Ca, Sr, and Sb refined the as-cast microstructure remarkably of Mg and Mg-3Zn alloys. Ca dissolved in pure magnesium completely and formed a ternary phase of Ca₂Mg₆Zn₃, which did not match the crystal structure ofα-Mg, distributed in the grain boundaries and among the dendrites, and restrained the growth of grains. Sr and Sb formed Mg₁₇Sr₂å’ŒMg₃Sb₂ in Mg-3Zn, but did not combine with Zn. The two binary phases matched the matrix in some degree, refined the alloys'microstructure more efficiently as the non-spontaneous nucleation sites.Moderate amount of refining elements addition observably elevated the strength and ductility at room temperature of worked and annealed pure Mg and Mg-3Zn. But excess Ca addition didn't result in further refinement but decline of strength as brittle Ca₂Mg₆Zn₃ was manifold to a network. Excess Sr and Sb would also enlarge the scale of second-phase, which is harmful to the ductility and strength of alloys.Sr refined the microstructure of Mg-3Al matrix by reducing the condensate depression of alloys and baffling the growth of grains. Within the range of 0.05wt%～1.0wt%, the dosage of 0.5wt% Sr refined the alloy most efficiently which is superior to Ca+Sr compound addition or simply Ca. Massive second-phase, Al₃Mg₁₃Sr was found when the content of Sr increased to 0.5wt%, laminar Al₄Sr was found within a very small amount, and both of them were brittle and resistant to elevated temperature. Excess Sr addition would enlarge the scale of the two second-phases and depress the strength and ductility of alloys.Mg-3Al ingot dynamic recrystallized during extrusion which resulted in refinement of microstructure and enhanced ductility and strength. Rolling on extruded stock work-hardened it and further elevated its strength, thus declined the ductility. The rolled alloy was composed of raff twin crystals and anomalous deformed grains which recrystallized after anneal at 300℃for one hour. The rule through out the range of 0.05wt%～1.0wt% was that the increase of Sr always resulted in the enhancement of alloy's strength and delay of its recrystallization.