Microstructure Formation and Mechanical Property Involving Icosahedral Quasicrystal Phase of Y Rich Mg-Zn-Y Quasicrystal Alloy

The microstructure formation and mechanical property involving icosahedral quasicrystal (I-phase) in the Y-rich Mg-Zn-Y alloy have been studied. The equilibrium formation of I-phase from the Y-rich Mg-Zn-Y melt is through a peritectic reaction between the Y-rich melt and the primary W-phase, which is discussed in detail. The independent nucleation and coupling growth mechanism between the W-phase and the I-phase, from the melt, are revealed, which is significant for understanding the peritectic reaction process involving icosahedral quasicrystal in the Mg-Zn-Y alloy. The mechanism of the quasicrystal phase strengthened magnesium alloys is also discussed here.     

Effect of Ca on crystallization of Mg-based master alloy containing spherical quasicrystal

Spherical icosahedral quasicrystalline phase （I-phase） was obtained by introducing Ca into Mg-Zn-Y alloy under conventional casting conditions. Due to the addition of Ca, Mg45Zn50Yn4.5Ca0.5 primary I-phase, which is thermodynamically stable and homogeneously distributed, was generated instead of decahedral quasicrystalline phase during the solidification process; the morphology of primary I-phase in the solidification microstructure changed from petal-like one （60-80 μm） to spherical one （≤ 15 μm）. When the mass fraction of Ca reaches 0.05%, sphericalI-phase with the largest quantity, highest spheroidization rate and highest circular degree can be obtained. Meanwhile, due to the changed morphology and the decreased size of primaryI-phase, the hardness of Mg-Zn-Y-Ca master alloy is reduced. The application of spherical I-phase as particulate reinforced phase provides great opporttmities for the improvement of strength and toughness of magnesium alloys.     

Investigation on microstructure characterizations and phase compositions of rapidly solidification/powder metallurgy Mg-6 wt.% Zn-5 wt.% Ce-1.5 wt.% Ca alloy

Rapidly solidification/powder metallurgy (RS/PM) Mg-6Zn-5Ce-1.5Ca (wt.%) alloy in form of rods has been prepared by hot extrusion with RS powders, produced via atomizing the alloy melt and subsequent splat-quenching on the water-cooled copper twin-rollers. Microstructure characterizations and phase compositions of the alloy have been investigated. The results showed that the RS/PM Mg-6Zn-5Ce-1.5Ca alloy was the characteristics of very fine grains with the size ranging from 200 to 650 nm and was composed of α-Mg, Mg_xZn_yCe_z phase with a few Ca (about 1 at.%) shortened as the Mg_xZn_yCe_z-(Ca) phase, and a small quantity of Mg₅₁Zn₂₀ phases. The melting point of the Mg_xZn_yCe_z-(Ca) phase in the alloy was a little higher than that of the Mg_xZn_yCe_z phase in RS/PM Mg-6Zn-5Ce alloy, which may be due to the dissolving of Ca in it. Moreover, the atomic percentage ratio of Zn to Ce in the Mg_xZn_yCe_z-(Ca) phase around the grain boundary was close to 1.5.     

Microstructure evolution and mechanical properties of Mg-12Zn-2Y alloy containing quasicrystal phase fabricated by different casting processes

Although icosahedral quasicrystal phase (denoted as I-phase) has been verified as an outstanding reinforcing phase, the mechanical properties of quasicrystal-reinforced Mg-Zn-Y alloys fabricated by traditional casting processes are still unsatisfactory due to the serious segregation of intermetallic compounds. In this study, the microstructure and mechanical properties of Mg-12Zn-2Y alloy fabricated by different casting processes, including permanent mold casting, squeeze casting and rheo-squeeze casting with ultrasonic vibration, were systematically investigated and compared. The results show that massive, large-sized I-phase and Mg₇Zn₃ phase gather together in the permanent mold cast sample, while the squeeze casting process leads to the transformation of I-phase into fine lamellar morphology and the amount of Mg₇Zn₃ decreases. As to the rheo-squeeze casting process, when the ultrasonic vibration is exerted with power from 800 W to 1,600 W, the α-Mg grains are refined and spheroidized to a large extent, and the lamellar spacing of the eutectic structure is significantly reduced, accompanied by some tiny granular I-phase scattering in the α-Mg matrix. However, when the ultrasonic power continuously increases to 2,400 W, the eutectic structure becomes coarse. The best mechanical properties of the rheo-squeeze cast alloy are obtained when the ultrasonic power is 1,600 W. The microhardness, yield strength, ultimate tensile strength and elongation are 79.9 HV, 140 MPa, 236 MPa, and 3.25%, which are 44.1%, 26.1%, 25.5%, 132.1% respectively higher than the corresponding values of the squeeze casting sample, and are 47.6%, 44.3%, 69.8%, and 253.3% respectively higher than the corresponding values of the permanent mold casting sample.

     

Corrosion behavior of as-cast Mg68Zn28Y4 alloy with/-phase

Mg₆₈Zn₂₈Y₄ alloys with stable icosahedral quasicrystals (Zn₆₀Mg₃₀Y₁₀) were prepared by cast method. By simulating the environment of ocean, the alloy was eroded in 3.5% (mass fraction) NaCl for 2, 4 and 30 h. The microstructures of the samples and eroded alloys were analyzed by OM and SEM. The compositions and the quasiperiodic structures were identified respectively by EDS and TEM. And the corrosion potential and corrosion current density before and after immersion were measured by potentiodynamic polarization measurements in 3.5% NaCl. The results show that I-phases grow in the mode of conglomeration, piling and transfixion. The Mg₇Zn₃ matrix and α(Mg) solid solution are eroded badly, while W-phase is eroded partially. At the same time, the I-phases exhibit excellent corrosion resistance property. The resistance to corrosion of Mg₆₈Zn₂₈Y₄ alloy is improved by increasing exposed I-phases. With adding element Y to Mg₆₈Zn₃₂ alloy, the corrosion current is decreased by one order of magnitude. And after the immersion of as-cast Mg₆₈Zn₂₈Y₄ alloy for 30 h, the corrosion current density is reduced by two orders of magnitude compared with that of uneroded Mg₆₈Zn₃₂ alloy.     

Stability of remelting and solidification interfaces of triple-phase region during peritectic reaction at lower speed

Peritectic reaction was studied by directional solidification of Cu-Ge alloys. A larger triple junction region of peritectic reaction was used to analyze the interface stability of the triple junction region during peritectic reaction. Under different growth conditions and compositions, different growth morphologies of triple junction region are presented. For the hypoperitectic Cu-13.5%Ge alloy, as the pulling velocity (v) increases from 2 to 5 μm/s, the morphological instability of the peritectic phase occurs during the peritectic reaction and the remelting interface of the primary phase is relatively stable. However, for the hyperperitectic Cu-15.6%Ge alloy with v=5 μm/s, the nonplanar remelting interface near the trijunction is presented. The morphological stabilities of the solidifying peritectic phase and the remelting primary phase are analyzed in terms of the constitutional undercooling criterion.     

Al-Cu-Fe准晶凝固过程的动力学

根据经典的形核理论,对准晶J相及其晶体近似相R相,采用所提出的自由能计算模型,计算并比较了非均质形核方式下的准晶I相及其晶体近似相R相的形核功及形核率.分析表明,从小的过冷直到准晶I相平衡液相面温度(TL=1 130K)下150K的过冷范围,准晶I相都会首先形核,而其晶体近似相尺相不具备从液相中初生形核的动力学条件.     

Icosahedral quasicrystalline phase in an as-cast Mg-Zn-Er alloy

The microstructure of an as-cast Mg-Zn-Er alloy was investigated through scanning electron microscopy (SEM) and transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS). The results indicate that two different second phases, one with eutectoid-lamellar morphology and the other with granular shape, distribute in the α-Mg matrix. The coexistence of the face-centered icosahedral quasicrystalline phase (I-phase) and W-phase with the face-centered cubic structure is found in the as-cast alloy. The coexistence of I-phase and W-phasc in the Mg-Zn-Er alloy is because the W-phase is the primary phase and the I-phase forms by peritectic reaction during solidification.     

Effect of cooling rate on microstructure and glass-forming ability of a (Ti33Zr33Hf33)70(Ni50Cu50)20Al10 alloy

The alloy (Ti₃₃Zr₃₃Hf₃₃)₇₀(Ni₅₀Cu₅₀)₂₀Al₁₀ developed by equiatomic substitution was prepared by using different quenching rates, i.e. rapid quenching by melt-spinning and relatively slow cooling using suction casting. The microstructural comparison between the as-spun ribbons and the as-cast bulk specimens reveals a different length scale of an icosahedral phase that formed upon solidification. The as-spun sample exhibits a mixture of nano-scale icosahedral, Zr₂Cu-type and amorphous phases after complete crystallization at 973 K indicating a high stability of the nano-scale icosahedral phase. Due to its small size (30–50 nm) there is no significant strain in the icosahedral phase. On the other hand, in the as-cast bulk sample Zr₂Cu-type and amorphous phases form in the micro-scale modulated icosahedral phase without any discernible exothermic reaction. The structural modulation of the icosahedral phase indicates the occurrence of strains during the growth. The different local accumulation of strain during the growth of the icosahedral phase induces a phase selection between the Zr₂Cu-type or amorphous phases in this alloy.     

包晶合金的定向凝固与生长

综述了包晶合金定向凝固中包晶相的形核与生长机制，讨论了利用最高界面生长温度假设以及成分过冷和充分形核判据下包晶合金的微观组织与相选择规律，对包晶相的3种生长机制进行了分析，并在Sn-Cu包晶合金定向凝固实验中发现其包晶反应不仅存在于凝固界面处，在后续冷却过程中也能进行，最后针对国内外包晶合金定向凝固研究的现状，提出了其需要进一步研究的问题。     

On the solidification microstructure of Mg–30Zn–2.5Y metal–intermetallic alloy

The solidification microstructures of the Mg–30Zn–2.5Y alloy have been studied in detail using both optical and analytical electron microscopy. CALPHAD simulation has been carried out for in-depth understanding of the solidification process. The work clarifies phase equilibrium in the vicinity of the Mg corner of the Mg–Zn–Y ternary system, showing that a ternary icosahedral (I) quasicrystal phase is in equilibrium with the (Mg) solid solution phase. The suppression of the Z-phase during solidification is attributed to low thermodynamic driving force and sluggish transformation kinetics for ordering icosahedral clusters into the crystalline structure. The current work lays the foundation for developing (Mg)–matrix composites with a ternary icosahedral phase as well as other intermetallic compounds. The work presents a case study of solidification process to demonstrate the feasibility to cast (Mg)–matrix composite containing the I-phase from the $L+I$ semi-solid phase region, as long as the equilibrium volume fraction of the I-phase is not too high to guarantee practical slurry fluidity before casting. This would be useful for making use of primary I-phase particles as effective nucleation sites to improve structural refinement and thus maximize the benefit from the I-phase reinforcement.     

Effect of Li on electrochemical properties of Ti1.4V0.6Ni quasicrystal alloy produced by rapid quenching

The effect of Li on the electrochemical performance of electrodes consisting of Ti_1.4V_0.6Ni quasicrystal was investigated at room temperature in three-electrode cell set-up. The quasicrystal sample was initially synthesized by arc melting, followed by melt-spinning, and then infiltrated with Li atoms by electroosmosis. According to X-ray diffraction, all ribbon samples were determined to be icosahedral quasicrystal phase (I-phase), V-based solid solution phase with BCC structure and face centered cubic (FCC) phase with Ti₂Ni-type structure. After infiltrating some Li atoms into the Ti_1.4V_0.6Ni quasicrystal lattice under the condition of an electroosmosis current of 0.6 A, we could observe the appearance of Li diffraction peaks. Importantly, the slight shift to the left observed in the diffraction peaks indicated that lattice expansion was caused by the infiltration of Li. The discharge capacity of Ti_1.4V_0.6Ni–Li material was higher than that of Ti_1.4V_0.6Ni.The maximum discharge capacity of 307.1 mAh/g was recorded for Ti_1.4V_0.6Ni–Li at a current density of 30 mAh/g. Both high-rate dischargeability and cycling stability were enhanced as a result of infiltrating Li. The lithium could get into the lattice, which resulted in the formation of microspores on surface of alloy, thus improving electrochemical activity of the alloy electrode. At the same time, the electrochemical reaction kinetics of alloy electrodes was also researched.     

Mg-Zn-Y三元合金中二十面体准晶相生长形貌及其演化

采用常规凝固方法在Mg-Zn-Y三元合金系中获得了大体积分数的二十面体准晶。二十面体准晶相有2种典型形貌特征：一种为花瓣状，另一种为多边形形状。这是准晶在生长过程中形貌演化的结果。合金的成分和冷却速率是影响准晶形貌的主要岗素。准晶合金中生成的低温相越多，冷却速率越慢，准晶相生长的时间和空间条件越好，准晶相越容易由花瓣状演化成多边形状。准晶的形成有包晶反应参与，包晶反应是一种由初生固态相扩散控制的反应，因此，准晶在生长初期和最终形态其元素含量有明显差别，分析表明，准晶生长初期Mg元素含量偏低，Zn和Y元素含量偏高。     

Li对快速凝固Mg96.5Gd2.5Zn1合金微观组织和力学性能的影响

本文考察了快速凝固条件下不同含量Li元素添加对长周期有序结构相增强Mg-Gd-Zn合金微观组织和力学性能的影响。结果表明,随着Li元素的添加,铸态合金中Gd、Zn溶质原子在镁基体晶粒中的过饱和度降低、(Mg,Zn)₃Gd晶界析出相增加、镁基体晶粒尺寸减小。而固溶处理后,随着Li含量的增加,合金中14H型长周期堆垛有序结构相(LPSO)的形成受到抑制,同时纳米/亚微米(Mg,Zn)₃Gd颗粒相大量析出,当Li为7.6at. %时合金中无LPSO形成。经热挤压变形后,合金中块状14H相发生扭着分层开裂、层片状14H相发生不同程度溶解、(Mg,Zn)₃Gd相破碎细化、基体发生不同程度再结晶;不加Li的Mg_96.5Gd_2.5Zn₁表现出最佳的力学性能(UTS=325,δ=9.5%),而含Li合金则随Li含量的增加,力学性能逐步下降。合金在不同条件下的组织转变机理及力学行为变化被进行了分析。     

3D Morphology and Formation Process of the Icosahedral Quasicrystalline Phase in Rapidly Solidified Al-Mn Alloy

Three-dimensional morphology and formation process of icosahedral quasicrystal phase have been investigated in a melt-spun Al-18 Mn alloy(in wt%).Three distinct layers corresponding to varying temperature gradient have been observed on the cross section of the ribbons.3D morphologies of cellular and dendritic icosahedral phase have been obtained through electro-etching.A model has been proposed to describe the formation process of the icosahedral phase and α-Al during the rapid solidification.The icosahedral phases are primarily precipitated from the melt into fine cellular and dendritic particles,and subsequently engulfed by the α-Al which propagates in a planar morphology.     

Quasicrystalline phases and their approximants in AlMnZn alloys

Two AlMnZn alloys in melt-spun condition have been studied by transmission electron microscopy. The Al₂₄Mn₅Zn alloy was found to be fully icosahedral, while the Al₁₂Mn_2.9Zn alloy gave rise to decagonal quasicrystal. The decagonal phase grew in clusters with an orientation relationship between the grains suggesting nucleation on an icosahedral seed. On annealing at 500–600°C, the quasicrystalline phase transformed to a body centered orthorhombic phase L (a = 1.24, b = 1.26 andc = 3.05nm) with a high density of planar defects. This phase transforms to an ordered and defect free monoclinic phase M, a superlattice structure of L (a = c = 1.77, b = 3.05nm and β = 89.1°). The L phase is shown to be a rational approximant of the icosahedral phase. The interrelationship among quasicrystalline phases and their rational approximants in AlMnZn system are highlighted.     

Microstructure and corrosion behavior of rapidly solidified Mg-Zn-Y alloys

The effect of a minor change in alloy composition on the microstructure and corrosion properties of melt spun Mg_98.3?xZn_xY_1.7 ribbons with x=9–12 is studied by X-ray diffractometry, differential scanning calorimetry, transmission electron microscopy and a dynamic polarization test. The ribbon specimens with x=9–10 revealed an in-situ composite microstructure consisting of icosahedral quasicrystalline phase (I-phase) particles distributed in an α-Mg matrix. The ribbon specimens with x=11 and 12 contained a minor MgZn₂ phase together with an α-Mg phase and I-phase. With increasing Zn content, the corrosion potential increased because of a mixed potential effect, but the formation of a MgZn₂ phase deteriorated the corrosion property through preferential attack, causing an irregular boundary between the corrosion product and the substrate. These results indicate that it is important to control alloy chemistry not to form the MgZn₂ phase in developing an I-phase strengthened Mg-Zn-Y alloy for structural applications.     

Mg-6Zn-3Sn合金的均匀化热处理及其对显微组织和力学性能的影响（英文）

利用DSC、OM、XRD、SEM、TEM、维氏硬度计和万能试验机，研究Mg-6Zn-3Sn合金均匀化过程中的显微组织和力学性能演变，分析合金过烧的原因和扩散动力学，确定合金的均匀化制度。结果表明，单级均匀化处理后，Mg₂Zn₃相分解，同时伴随着Mg₂Sn相的析出。当均匀化温度上升到350℃时，Mg₂Zn₃相导致合金的过烧。合金合适的均匀化制度为(335℃，24 h)+(400℃，6 h)。在双级均匀化过程中，合金的硬度持续下降，力学性能呈现先升后降的趋势，这与Zn和Sn原子的固溶和Mg₂Sn相的析出有关。     

Microstructural control and hardening response of Mg–6Zn–0.5Er–0.5Ca alloy

The effects of heat treatment on microstructures and hardening response of Mg–6Zn–0.5Er–0.5Ca(wt%) alloy were investigated by optical microscope(OM), scanning electron microscope(SEM), and transmission electron microscope(TEM) in this paper. The results show that the Mg–6Zn–0.5Er–0.5Ca alloy contains Mg_3Zn_6Er_1 quasicrystalline phase(Iphase) and Ca_2Mg_6Zn_3 phase under as-cast condition. Most of the Ca_2Mg_6Zn_3 phases and I-phases dissolve into matrix during heat treatment at 475 ℃ for 5 h. After the as-solution alloy was aged at 175 ℃ for 36 h, a large amount of MgZn_2 precipitate with several nanometers precipitate. It is suggested that the trace addition of Ca results in refining the size of the precipitate, and the presence of the nanoscale MgZn_2 phase is the main factor to improve the peak-aged hardness greatly to 87 HV, which increases about 40 % compared with that of as-cast alloy.     

Phase formation and structures of quasicrystals and approximants in the Zn–Mg–(Ti, Zr, Hf) system

The compositional regions for primitive (P) and face-centered (F) icosahedral quasicrystals (iQc) have been determined to be around Zn₈₄Mg₉Zr₇ and Zn₇₅Mg₁₈Zr₆, respectively. A 1/1 approximant of the F-type iQc was found to have a composition around Zn₇₇Mg₁₈Zr₅. A similar tendency has been verified for the Zn–Mg–Hf system. No stable iQc was observed in the Zn–Mg–Ti system. High-resolution X-ray measurements performed with synchrotron radiation showed that the stable iQcs are highly ordered and contain less phason disorder. High-angle annular dark field (HAADF) scanning transmission electron microscopy observation of the 1/1 Zn–Mg–Hf approximant effectively revealed the Hf positions in the structure, whose local contrasts can be reasonably interpreted by a structural model where icosahedral and dodecahedral Hf clusters are mutually interpenetrated. Similar appearances of local contrasts were frequently observed in HAADF images of the F-type iQc, indicating that the iQc structure is also build up of icosahedral clusters that are almost identical to those in the 1/1 approximant.