Study On Microstructures And Properties Of Mg-2Zn-Mn Alloy With Small Addition Of Nd

As the lightweight structural materials, magnesium alloys are increasingly widespread used in automobile, telecommunication and aerospace industry. However, the poor plastic deformability of magnesium alloy due to the hexagonal structure greatly limits its application. The properties of wrought magnesium alloy can be modified by the alloying. Specially, the rare earth elements as the important micro-alloying elements, can significantly affect the microstructure and properties of magnesium alloys, which has been become the hot research in the wrought magnesium alloys. In order to improve the production efficiency, reduce the cost and expand the application, it needs to develop the rare earth wrought magnesium alloys with the high hot-workability and room temperature ductility.The paper focus on obtaining the optimal composition alloy which has a high ductility by adding the different contents of rare earth elements Nd into the good hot-workability ZM21 alloy. Furthermore, the effects of Nd on the microstructures and mechanical properties of ZM21 alloy were studied. The cast ingots were prepared by semi-continuous casting. The effects of different contents Nd on the microstructures, alloy phases and mechanical properties of the as-cast ZM21 alloy were investigated. In addition, the effects of the homogenization annealing on the microstructures and second phases, and the different contents Nd on the microstructures and mechanical properties of the as-extruded ZM21 alloy were also respectively researched. The results show that the addition of Nd to the as-cast ZM21 alloy results in the average grain size increases, and the second phases segregated among the dendrites increase, coarsen and become more and more continuous. With the the addition of Nd, the MgZn phases in the as-cast ZM21 alloy were replaced with a large number of Mg-Zn-Nd ternary compounds T2 phase ((Mg, Zn)_11.5Nd) and a few Mg₁₂Nd phase.When the Nd content increased to 0.7%, a small amount of Mg-Zn-Nd ternary compounds T₃ phase ((Mg, Zn)₃Nd) occurred besides the T₂ phase. The different Nd contents had less effect on the tensile strength and yield strength of the as-cast ZM21 alloy that the values were respectively in 174¹85MPa and 54⁵8MPa. However, the elongation decreased with the increase of Nd content. When the Nd content varied from 0 to 0.5%, the elongation decreased from 17.3% to 13.0%. With the increasing of Nd content, the hot compression flow stress of the as-cast ZM21 alloys gradually increased from 51MPa to 93MPa.With the 420℃×10h homogenization annealing, the dendrite segregation in the as-cast ZM21 alloy with no Nd content had been basically eliminated. With the Nd content increasing, the degree of eliminating dendritic segregation decreased. Specially, the ternary compounds T3 phase appeared in addition to the incompletely dissolved T2 phase in the alloys of adding 0.1%, 0.3% and 0.5% Nd. After the hot extrusion deformation at 400℃,the grains were gradually refined from 37μm to 21μm with the Nd content increasing. The phase compositions of the as-extruded alloys are constant with the alloys after the homogenization annealing. It was obvious that the basal texture paralleled to the extrusion direction were weakened at the Nd content reached to 0.3%. Compared to the as-cast alloys, the tensile strengths, yield strengths and elongations of the as-extruded alloys were greatly improved, respectively reached to 232²53MPa, 141¹73MPa and 19.1%³2.2%. For the as-extruded alloys, when the Nd content increased from 0 to 0.3%, the tensile strength decreased from 248MPa to 235MPa and the yield strength decreased from 167MPa to 142MPa. The elongation was improved with the increasing of Nd content, and with the 0.5% Nd, the elongation had the highest value of 32.2%, remarkably improved the ductility of the as-extruded ZM21 alloy.