Effects Of Erbium On The Microstructures And Mechanical Properties Of ZK21 Wrought Magnesium Alloys

In the present paper, Based on the ZK21(Mg-1.5%Zn-0.6%Zr, wt%, the same below) wrought magnesium alloy, and modified with rare-earth Er. Optical microscope, X-rays diffraction analysis(XRD), differential scanning calorimeter(DSC), scanning electron microscopy(SEM) and energy dispersive spectroscopy(EDS) was used to study the microstructure of as-cast, heat-treatment state and deformation processing state. Therm-mechanical simulation experiment of the investigated alloy was performed before rolling or extrusion plastic deformation. Er existing forms and microstructure and mechanical properties were studied under different conditions.The result indicate that the microstructure of semi-continuous casting ZK21 magnesium alloy was mainly consisted of by theα-Mg and a small amount of Mg-Zn eutectic phase was formed in non-equilibrium solidification on grain boundaries. The addition of rare-earth Er was formed (Mg,Zn)4Er phase with Mg,Zn. After homogenization, existing in the form of (Mg,Zn)6Er phase. Er solution to the magnesium alloy after heat-treatment, and with the increase of Er content, the solution degree of Er inα-Mg increased. A certain amount of Er could refine the microstructure of semi-continuous casting ZK21 magnesium alloy, the average grain size of as-cast alloy is about 94μm, when the content of Er reached 2.0%, the average grain size is about 62μm.Dynamic recrystallization occurred obviously during hot rolling in magnesium alloys. When hot-rolled to 2mm, the average grain size was refined form 200μm to 30μm of after homogenized; when cold-rolled to 1.5mm, the average grain size was about 15μm, and with tensile strength, yield strength and elongation are 285MPa, 270MPa, 16% respectively. After 260℃,1h annealed, the elongation further increased to 21%.The addition of rare-earth Er could promote homogeneous deformation and homogeneous recrystallization during hot deformation. Inadequate recrystallization occurred on 350℃hot extrusion, a mount of original microstructure was found in the ZK21 alloy. The original microstructure reduced with Er contents increase, when the content of Er reached 1.0%, the original microstructure was the smallest proportion, with homogeneous microstructure, showing good ductility. After 420℃hot extrusion, dynamic recrystallization occur completely on the alloy matrix, but the size of recrystallized grain is inhomogeneous. The addition of rare-earth Er could hinder the recrystallization grain growth, the average recrystallization grain size is about 4~6μm, and the elongation improved accordingly with Er content increased. After hot extrusion, when the content of Er reached 4.0%, tensile strength, yield strength and elongation could achieve 230MPa, 270MPa and 37% respectively.