| Magnesium and its alloys are the lightest structural metal up to now. However, textures are easily developed in the fabrication of Mg-based products, resulting in an anisotropy to the microstructures and the mechanical properties. Alloying, which is an effective method to modify microstructures and mechanical properties of magnesium alloys, is attracting increasing attention.Alloying with rare earth(RE), which is of unique properties, is a hot research area these years. The addition of RE elements is widely found to modify textures and mechanical properties of magnesium-based alloys. As alloying elements, REs might interact with intrinsic alloying elements in magnesium alloys, bringing about the variation of the existing forms(i.e. solution or compounds) of relevant elements, which influences the microstructural evolution during the hot forming process and consequent microstructures and mechanical properties. However, there have been few researches focusing on the interaction of REs and other alloying elements. Consequently, the effects of the RE addition on microstructures and mechanical properties of magnesium alloys need to be further clarified. In this study the RE Er was selected and added into the common commercial magnesium alloy ZK60 for study in order to theoretically guide the modification of textures of magnesium alloys and the design of magnesium alloys with excellent properties. In this study the effect of the RE addition on the secondary phases and the dissolution was investigated. And then further the mechanical properties and the evolvement of microstructures in the process of hot deformation were systematically investigated. The effects of the RE addition on the dynamic recrystallization(DRX), the texture and mechanical properties were obtained. The major conclusions can be summarized as follows.1) The existing form of Er in alloys was Mg–Zn–Er ternary compounds. I-phase(quasicrystalline) was the main Er-bearing phase in alloys with Zn/Er mass ratio between 6 and 12. W-phase(face-centered cubic) and I-phase coexisted in alloys with Zn/Er mass ratio ranging from 1.5 to 3. W-phase is an ordered phase and is coherent with the a-Mg matrix. The orientation relationship between W-phase and a-Mg can be described as w Mg<001 >// <1120 >, ( )wMg022 / /(0002). Zn existed in the form of both compounds and solid solution. In homogenized billets both the volume fraction of Mg–Zn particles and the dissolution of Zn solutes in a-Mg generally decreased with increasing Er modification.2) The addition of Er affected on the distribution of particles, which influenced the morphology of twins occurring in deformation. The much more Mg–Zn particles inside grains made twins need-like in the Er-free alloy. In contrast, most twins in Er-modified alloys had lens appearance. With the addition of Er the twinning rate decreased noticeably as a result of the variation of the particle distribution, the initial grain size and the dissolution.3) During hot forming process several DRX mechanisms were operated around the peak strain. In spite of the different particle sorts, particle-stimulated nucleation(PSN) promoted randomly oriented grains. With the increasing strain, discontinuous DRX(DDRX) and twinning induced DRX(TDRX) became weaker while continuous DRX(CDRX) prevalent. The higher the volume fraction of particles, the greater contribution made by PSN. With the ever-being broken of compounds in straining process, the impact of PSN was gradually exhausted. A basal texture was gradually developed in the deforming process among all alloys. The grain refinement of cast alloys or the introduction of a good few homogenously-populated hard secondary particles into the cast microstructure, is favorable for obtaining weak deformation textures in magnesium alloys.4) Ascribed to the interaction between Er and Zn, the Er addition in the experimental range(0~4 wt%) slightly decreased the elongation and scarcely improved the tensile strengths of the extruded alloys. The heat treatment could be used to further control mechanical properties of deformed magnesium alloys. In the process of solutionization at 400℃, Mg-Zn compounds gradually dissolved in the matrix and defects in the a-Mg matrix gradually vanished. Mechanical strengths decreased but the plasticity increased continually by increasing the solutinization time(1.5~12h). Upon further aging at 200℃, Mg-Zn compounds including particles in rod(β1′) and plate(β2′) shape statically precipitated in the microstrutures, which finally improved the strengths of extrusion bars. The plasticities could also be enhanced by the aging treatment because defects reserved in the matrix of solutionized bars could be consumed by the static precipitation in the aging treatment. |
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