| Mg-Sn based alloys have been paid much attention as new heat-resistant Mg alloys because of the larger solubility of Sn in α-Mg matrix and Mg2Sn intermetallic compound with higher thermal stability. As one group of the important alloying elements, RE can not only increase the room temperature strength and the creep resistance at elevated temperatures, but also can improve the room temperature plasticity and toughness. Unfortunately, as the alloying fundamental, little work has been done on the Mg-Sn-X system phase diagrams, which makes the development of new Mg-Sn based alloys only by conventional trial-and-error method. Recently, MgGdsMn1Sco.3alloy with the lower content Sc has been successful designed through the CALPHAD method, and the content of expensive Sc decreases from6-15wt.%to0.3-lwt.%. At the same time, the creep resistantce of the alloy at350℃still remains well. It means that it is feasible to design high performance Mg alloys on the basis of phase diagram and thermodynamics. In this thesis,therefore, the phase equilibrium of the Mg-RE(Y, Nd, Gd) binary systems and Mg-Sn-Y(Si) ternary systems have been determined through both diffusion couple technique and alloy method by means of EPMA, XRD and SEM-EDS. On the basis of determined Mg-Sn-Y ternary phase diagram, a series of Mg-Sn-Y alloys are designed, and their microstructures, room-and elevated-temperature mechanical properties and creep resistance properties of as-extruded Mg-Sn-Y ternary alloys are systematically studied.The solubility of RE(Y, Nd, Gd) in the a-Mg solid solution had been obtained at300-500℃. Subsequently, the solvus is extended to the eutectic temperature, and the maximum solubility is obtained, i.e.4.7at.%Y,3.2at.%Nd and7.1at.%Gd, respectively, which is higher than presently accepted ones. The composition homogeneity ranges of Mg24Y5-x and Mg2Y1-x phases are determined at300-500℃, whose regions obviously shift to the Mg-rich corner and much wider than that in the current Mg-Y binary phase diagram. The composition homogeneity range of Mg41Nd5has no apparent change compared with the presently accepted Mg-Nd phase diagram, but the composition homogeneity ranges of Mg3Nd and MgNd obviously become wider over400℃. Meanwhile, the phase equilibrium boundary of Mg3Nd/MgNd and MgNd/(MgNd+a-Nd) shift to the Nd end. Both Mg5Gd and Mg3Gd phases have a certain composition homogeneity range, but are not the stoichiometric compounds as early reported in the Mg-Gd binary system. The composition homogeneity ranges of Mg2Gd and MgGd shift to the Gd end and much wider than that in the presently accepted Mg-Gd phase diagram. At the same time, the maximum solubility of Mg in the a-Nd and a-Gd solid solution are10.0at.%and15.4at.%, respectively, which is higher than that the presently accepted results. However, it is necessary to in detail and completely study the phase equilibrium relationships about Mg-50at.%RE(Y, Nd, Gd) over500℃in details in the near future, so that the Mg-RE binary phase diagrams can be reliably re-constructed. Thus, it can provide the valuable information to establish of accurate and reliable Mg-RE based thermodynamics data, and it is feasible for alloy design, heat treatment and formation processing of Mg-RE based Mg alloys.The isothermal sections of Mg-Sn-Si(Y) ternary systems have been constructed at300-500℃, and the phase equilibria have no change with increasing temperature. There are only Mg2Si and Mg2Sn intermetallic compounds in equilibrium with the α-Mg solid solution in the Mg-Sn-Si ternary system, and no ternary compounds have been found. Meanwhile the phase equilibrium boundary of Mg2Sn/Mg2Si has obtained. In the Mg-Sn-Y ternary system, there are four intermetallic compounds in equilibrium with the α-Mg solid solution, i.e. Mg2Sn, MgSnY, Sn3Ys and Mg24+xY5.It is interesting to find that either Y or Sn is not simultaneously soluble in the α-Mg solid solution. The Sn is soluble in the a-Mg solid solution when the α-Mg phase is in equilibrium with Mg2Sn and MgSnY phases. The Y is soluble in the a-Mg solid solution when the α-Mg matrix is in equilibrium with Sn3Y5and Mg24Y5or Sn3Ys and MgSnY phases.The Mg-1.0Sn-1.5(3.0,3.5) Y (at.%) ternary alloys had been designed according to the results of Mg-Sn-Y ternary phase diagrams as mentioned above, and then the metal die casting alloys are indirectly extruded at300℃. The grains of the a-Mg matrix were remarkably refined in the indirectly extruded Mg-1.OSn-1.5(3.0,3.5) Y (at.%) ternary alloys, i.e., about3,7and9μm, respectively. The coarse MgSnY, Sn3Ys+MgSnY and Sn3Y5compounds, inherited from the solidification, are mainly distributed along grain boundaries of the a-Mg matrix. Meanwhile, the fine and disperse phases precipitate inside the a-Mg matrix. The three as-extruded alloys all have the excellent ductility and a high R value of compression/tension yield asymmetry. The ductility of Mg-1.OSn-1.5Y (at.%) alloy reaches24%at room temperature, and the R value of these alloys were0.82,0.85and1.08, respectively. The outstanding elevated temperatures properties have been identified in three alloys at150-250℃. The tensile tests of those extruded alloys at150℃show that the yield strentgth (TYS) are118MPa,155MPa and162MPa with incressing the Y content, respectively, slightly decreasing with increase temperature. The remarkable creep resistance properties have been also identified in the three alloys. The secondary creep rates of the as-extruded Mg-1.0Sn-1.5(3.0,3.5) Y alloys are1.39×10-9s-1,7.21×10-10s-1and6.84×10-10s-1at150℃/75MPa,9.26×10-8s-1，3.43×10-8s-1and3.65×10-8s-1at200℃/75MPa, respectively. At the same time, the activation energies of as-extruded Mg-1.0Sn-1.5(3.0,3.5) Y alloys have been approximately calculated, which indicate that the creep deformation of those alloys happens by dislocation climb controlled mechanism. |
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