Investigation On Wrought Magnesium Alloys Containing Rare Earth Elements With High Plasticity

Wrought magnesium alloys have low ductility compared with aluminum alloys because of their hexagonal close pack crystal structure which has limited slip systems. Wrought magnesium alloys take 10% only in total magnesium alloys that is less than the cast magnesium alloy. To develop the plastic working technology of magnesium alloy is an important positive factor, and another important positive factor for accelerating the application of wrought magnesium alloy in industries is to develop new alloys. Therefore, it is very important to develop new magnesium alloys with perfect ductility to meet the needs of their deformation and application. China has the most plentiful magnesium and rare earth resources in the world, which provides advantages in researching and developing high properties magnesium alloys containing rare earth elements with the acceptable cost. In this work,magnesium alloys containing rare earth elements were developed for the increase of ductility and deformation ability of alloys.In the present work, the microstructures, mechanical properties of as-cast and as-extruded ZK20+xNd alloys, ZK20+xCe alloys and ZM21+xNd alloys had been investigated systematically by optical microscope (OM), scanning electron microscope (SEM), X-ray diffractometer (XRD), mechanical properties testing, etc.Results showed that the microstructure of as-cast alloys gradually defined with the increase of the Nd or Ce addition, and Mg-Zn-Re ternary phases appeared. The morphology of ternary Mg-Zn-Re phases at intragranular changed from partical,to broken-network,and then to network with the increase of the Nd or Ce addition. Broken-network or network Mg-Zn-Re phase caused the decrease of alloy properties and a change in the fracture mechanism from cleavage fracture to intergranular fracture.Results showed that Mg-Zn-Nd ternary phases T1(Mg 27.0-33.4, Zn 60.2-66.4, Nd 6.1-7.4)and T2((Mg,Zn)11.5Nd)appeared in ZK20+x Nd alloys; Mg-Zn-Ce ternary phasesτ1 ( CeMg7Zn12 ) andτ2 (Ce(Mg,Zn)10.1) appeared in ZK20+xCe alloys; Mg-Zn-Nd ternary phases T1 and T2 appeared as well as in ZK 20+x Nd alloys.The grain size of as-extruded alloys gradually decreased with the increase of Nd or Ce addition from 0 to 0.7 wt. %. The average grain size of ZK20 alloy was 23.9μm after extrusion,but it was 238.8μm before extrusion; the average grains size of ZK20+0.5Nd alloy was 4.9μm after extrusion,but it was 78.2μm before extrusion.In as-extruded ZK20+xNd alloys,ZK20+xCe alloys and ZM21+xNd alloys, Nd or Ce addition caused the slight increase of alloy strength, but it caused alloy ductility to increase significantly. It led to a little increase of alloy strength by the Nd or Ce addition that Zn amount inα-Mg solid solutions decreased because of the formation of Mg-Zn-Re ternary phases.The as-extruded alloys ZK20+0.5Nd, ZK20+0.7Ce and ZM21+0.5Nd showed higher properties respectively. The UTS and El. of as-extruded ZK20+0.5Nd alloy were 237 MPa and 32.8% respectively, with the increase of 5% and 50% on those of ZK20 alloy respectively. The UTS and El. of as-extruded ZK20+0.7Ce alloy were 261MPa and 24.6% respectively, with the increase of 8% and 44% on those of ZK20 alloy. The UTS and El. of as-extruded ZM21+0.5Nd alloy were 232.1 MPa and 32.2%, of which the elongation increased significantly.The effects of ingot homogenizing annealing at different temperatures on properties of as-extruded ZK20 and ZK20+0.5Nd magnesium alloy were investigated. The results showed Mg-Zn and Mg-Nd-Zn compounds which were observed in the cast ingot disappeared gradually with the increase of annealing temperature. The strength of as-extruded ZK20 alloy after homogenizing annealing decreased than that of the alloy without annealing, while the strength of ZK20+0.5Nd remained unchanged. It can be concluded that the optimal homogenizing annealing process were 390℃×10 h for ZK20, and 420℃×10 h for ZK20+0.5Nd.Incomplete dynamic recrystallization had occurred in alloys at compression deformation at 400℃,0.5S-1. Peak stress and peak strain increased with the increase of Nd or Ce addition.ZK20+0.5Nd alloy and ZM21+0.5Nd alloy showed the advantage in deformation with the better ductility. ZK20+0.7Ce showed the advantage in application, since its strength hit the level of commercial magnesium alloys and its ductility was better than common commercial magnesium alloys.