Study Of Microstructures And Mechanical Behavior Of Mg-Zn-Gd-based Alloys Reinforced By In-situ Quasicrystals

Magnesium alloys have high potential to be utilized as structural materials in such application as vehicles, electric devices and aircrafts etc. A new type of Mg alloys strengthened with in-situ icosahedral quasicrystalline phase (I-phase) is investigated through a novel approach of alloy design: in-situ formation of quasicrystals in Mg alloys by preparing alloys on the basis of Mg-based amorphous compositions.In this thesis, the Mg-Zn-Gd-based alloy series is selected in order to prepare Mg alloys strengthened with in-situ I-phase. Using optical microscopy, scanning electron microscopy, X-ray diffraction, differential thermal analysis and transmission electron microscopy, the effects of alloying elements on the formation of I-phase in Mg-Zn-Gd alloys are studied; the composition range for I-phase of Mg-Zn-Gd alloy in the Mg-rich corner is explored. The effects of I-phase, Laves phase and heat treatment on the microstructures and mechanical properties of Mg-Zn-Gd-based alloys are further investigated. Two Mg-Zn-Gd-based alloys with good comprehensive properties have been developed. Furthermore, the effect of extrusion on microstructure and mechanical properties of the Mg-Zn-Gd-based alloys is studied, and the effects of I-phase and Laves phase on deformation behavior of Mg-Zn-Gd-based alloys are analyzed. The effects of solidification conditions on the formation of I-phase and the mechanical properties of Mg-Zn-Gd-based alloys are investigated. The main conclusions can be summarized as follows:1. The composition range for I-phase in Mg-Zn-Gd-based alloys in the Mg-rich corner is explored according to the Zn/Gd ratio and the contents of Zn and Gd. Under the cast condition, five phase regions can be classified for Mg-Zn-Gd alloy in the Mg-rich corner, i.e. (Ⅰ)α-Mg + W-phase, (Ⅱ)α-Mg + W-phase +I-phase, (Ⅲ)α-Mg + I-phase, (Ⅳ)α-Mg + I-phase + Crystal, (Ⅴ)α-Mg + Crystal, wherein the Crystal includes Mg-Zn, Mg-Gd and Mg-Zn-Gd compounds. The I-phase in as-cast Mg-Zn-Gd alloys has a composition of about Mg: 30±1at.%, Zn: 62at.%, Gd: 8±1at.%. The value of e/a for I-phase is 2.08, which satisfies the e/a of 2.1 for Frank-kasper type icosahedral structure through the Hume-Rothery mechanism.2. Mg-Zn-Gd alloys have a dendritic cast structure as a skeleton in the matrix. Mg-3.5Zn-0.6Gd (at.%) alloy with 8.6vol.% of I-phase shows high mechanical properties. When Cu is added to Mg-Zn-Gd alloys, an MgZnCu Laves phase is formed besidesα-Mg and I-phase. The MgZnCu Laves phase improves significantly the heat resistance of the alloy. When containing 1.5at.% Cu, the Mg-3.5Zn-0.6Gd-1.5Cu alloy has higher mechanical properties at both room and elevated temperatures in comparison with Mg-Zn-Gd alloy. Under creep conditions of 200℃and 50MPa, the total creep strain and steady creep rate of the Mg-3.5Zn-0.6Gd are 0.294 % and 4.72×10-9, whereas those of the Mg-3.5Zn-0.6Gd-1.5Cu alloy are 0.084%and 1.9×10-9, respectively. The creep resistance for Mg-Zn-Gd-Cu alloys exhibited one order of magnitude higher than that of AE42 (the benchmark for commercial heat resistant Mg alloys) under the same creep conditions.3. The heat treatment process of Mg-Zn-Gd alloy is optimized to be solutioning as 440℃/8h water quenching (W.Q.), followed by aging as 200℃/24h. The strengthening effect of heat treatment is low for Mg-Zn-Gd-based alloys due to the small content of solutes, which can be ascribed to the high thermal stability of I-phase and MgZnCu Laves phase.4. The cooling rate has significant influence on the formation of I-phase in Mg-Zn-Gd alloys. For both Mg-3.5Zn-0.6Gd and Mg-3.5Zn-0.6Gd-1.5Cu alloys, the I-phase has a optimum formability at the cooling rate of about 70K/s. When the cooling rate is higher than 70K/s, the volume fraction of I-phase decreases with the increase of cooling rate, since the nucleation and growth of I-phase has been suppressed under the high cooling rate. When the cooling rate is below 70K/s, the volume fraction of I-phase increases with the increase of the cooling rate because the nucleation and growing can be accelerated by the super-cooling effect.5. From the cooling rate of 4.8K/s to 18.5K/s, the strength of the Mg-3.5Zn-0.6Gd alloy can be improved by not only the grain refinement but also the higher strengthening effect of I-phase, due to its higher volume fraction at the higher cooling rate. For Mg-3.5Zn-0.6Gd-1.5Cu alloy with low content of I-phase, the strength of which is improved only by the grain refinement because the cooling rate has little effect on the content of I-phase.6. I-phase and MgZnCu Laves phase of Mg-3.5Zn-0.6Gd and Mg-3.5Zn-0.6Gd -1.5Cu alloy have been broken out and formed a band structure after extrusion. I-phase with nano-size was precipitated during extrusion process, and distributed along the grain boundary and in the matrix. This kind of I-phase precipitate has a composition of Mg42Zn50Gd8(at.%), belonging to F-type icosahedral structure.7. The mechanical properties of as-extruded Mg-3.5Zn-0.6Gd (at.%) alloy is very sensitive to extrusion temperature. The alloy extruded at 573K shows higher mechanical properties than the alloy extruded at 673K, the improvement of ultimate tensile strength (UTS), Yield strength (YS) and elongation is about 3%, 7.5% and 13.7%. However, the mechanical properties of Mg-3.5Zn-0.6Gd-1.5Cu alloy shows little sensitive to extrusion temperature due to the high thermal stability of I-phase and MgZnCu Laves phase, and exhibits similar mechanical properties under extrusion of both 573K and 673K.8. Mg-3.5Zn-0.6Gd alloy with higher content of I-phase exhibits the better deformability than Mg-3.5Zn-0.6Gd-1.5Cu alloy at elevated temperatures. Mg-3.5Zn-0.6Gd-1.5Cu alloy strengthened by I-phase and MgZnCu Laves phase shows higher strength at elevated temperatures as compared with Mg-3.5Zn-0.6Gd alloy. Mg-3.5Zn-0.6Gd alloy extruded at 673K shows higher ductility about 94% at 473K than it extruded at 573K. Low interface energy and quasi-periodic lattice structure allow I-phase to have good bonding properties with matrix, and high symmetry makes I-phase be isotropic in nature, which result in the better plastic compatibility with matrix during deformation and the activity of non-basal slip systems, thus I-phase dispersing in grain boundary would largely improve deformation capability of Mg-3.5Zn-0.6Gd alloy. After addition of Cu element, there formed larger amount of particles dispersed along grain boundary and in the matrix in as-extruded studied alloy, which can increase the yield stress due to particle's strengthening effect. One relative random texture was developed as a result of local lattice rotation due to the presence of I-phase particles, which can greatly improve the ductility of Mg-Zn-Gd alloy containing of I-phase.9. During the deformation at elevated temperatures, Mg-3.5Zn-0.6Gd alloy showed high strain and low strain hardening exponent, while Mg-3.5Zn-0.6Gd-1.5Cu alloy containing Laves phase and I-phase exhibits high stress, stable microstructure (low value of grain growth rate) and high strain hardness exponent. The study of this thesis provides a new approach for developing wrought magnesium alloys: the volume fraction of Laves phase and I-phase can be controlled through adjusting the alloy composition and preparing process, which then results in the variation of the properties of Mg alloys to meet the different demands.