Aging kinetics of 14H-LPSO precipitates in Mg-Zn-Y alloy

Alloys with long-period stacking ordered structures(LPSO)have good properties and are highly regarded.Mg-Zn-Y alloy containing LPSO phase was prepared by the traditional casting method,and the aging heat treatment was performed at different temperatures and times.The microstructure and phase constitutions of the alloy were observed by means of optical microscopy and scanning electron microscopy methods.Results show the microstructure of as-cast Mg95.5Zn1.5Y3 mainly consists ofα-Mg,W phases and LPSO phases.During the aging treatment,fine lamellar-shaped 14H-LPSO phase is formed at the grain boundaries and precipitates from the supersaturated magnesium matrix,and the volume fraction increases as the aging time increases.By controlling the aging time,Mg-Zn-Y alloys with different volume fractions of 14H-LPSO phase were prepared.The aging kinetics equation of the 14H-LPSO phase is summarized,that is f=1-exp(-0.2705 t 0.6368).The phase transformation mechanism of 14H-LPSO in Mg95.5Zn1.5Y3 alloy can be described as the change of dislocation energy.     

Ultrasonic Vibration and Rheocasting for Refinement of Mg–Zn–Y Alloy Reinforced with LPSO Structure

In this work, ultrasonic vibration (UV) and rheo-squeeze casting was first applied on the Mg alloy reinforced with long period stacking ordered (LPSO) structure. The semisolid slurry of Mg–Zn–Y alloy was prepared by UV and processed by rheo-squeeze casting in succession. The effects of UV, Zr addition and squeeze pressure on microstructure of semisolid Mg–Zn–Y alloy were studied. The results revealed that the synergic effect of UV and Zr addition generated a finer microstructure than either one alone when preparing the slurries. Rheo-squeeze casting could significantly refine the LPSO structure and α-Mg matrix in Mg_96.9Zn₁Y₂Zr_0.1 alloy without changing the phase compositions or the type of LPSO structure. When the squeeze pressure increased from 0 to 400 MPa, the block LPSO structure was completely eliminated and the average thickness of LPSO structure decreased from 9.8 to 4.3 μm. Under 400 MPa squeeze pressure, the tensile strength and elongation of the rheocast Mg_96.9Zn₁Y₂Zr_0.1 alloy reached the maximum values, which were 234 MPa and 17.6%, respectively, due to its fine α-Mg matrix (α1-Mg and α2-Mg grains) and LPSO structure.     

含长周期堆垛有序结构Mg94Zn2Y4挤压态合金的显微组织与力学性能

采用SEM和TEM等分析方法研究包含长周期堆垛有序结构的挤压态Mg94Zn2Y4合金的显微组织和力学性能。结果表明：铸态Mg94Zn2Y4合金由18R-LPSO和α-Mg两相组成。挤压后,长周期相分层,并形成宽度为50~200 nm的α-Mg 薄片。合金经498 K时效处理36 h后达到时效峰值,在其组织中析出β′相,该析出相的出现显著提高了α-Mg基体的显微硬度,从HV108.9增加到HV129.7;而LPSO结构的显微硬度稳定在HV145左右。TEM分析及其电子衍射花样表明,β′相与α-Mg和LPSO结构具有独特的位相关系,其原子最密排面的堆垛方向垂直于α-Mg和LPSO相最密排面的堆垛方向。由于β′相和18R-LPSO相的共同存在,处于时效峰值态的Mg94Zn2Y4合金的抗拉强度达到410.7 MPa。     

Double-peak ageing behavior of Mg-2Dy-0.5Zn alloy

Ageing behavior of Mg-2Dy-0.5Zn alloy was investigated during isothermal ageing at 180 °C. Two significant ageing peaks were observed at 36 h and 80 h, respectively. Examination of microstructure evolution during ageing revealed that 14H long period stacking ordered (LPSO) phase forms in the α-Mg matrix and its volume fraction increases, (Mg, Zn)_xDy particle phases precipitate and their size, distribution and amount vary, as ageing time increases. The LPSO strengthening and the precipitation strengthening are two main mechanisms responsible for the double-peak ageing behavior observed for the Mg-2Dy-0.5Zn alloy. The first ageing peak is mainly attributed to the precipitation strengthening of a large amount of the fine (Mg, Zn)_xDy particle phases. The second ageing peak arises mainly from the LPSO strengthening of a high volume fraction of the 14H LPSO phase.     

Influence of heat treatment on microstructure and mechanical properties of Mg-10Gd-3Y-1.2Zn-0.4Zr alloy

Microstructure evolution and mechanical properties of the cast Mg-10Gd-3Y-1.2Zn-0.4Zr(mass fraction,%) alloy during annealing at 798 K for different time were investigated.In the as-cast state,the microstructure consists ofα-Mg,Mg_5(Gd,Y,Zn) eutectic compounds and stacking faults(SF) of basal plane distributed from grain boundary to inner grain.During heat treatment at 798 K,the SF and parts of eutectic compounds dissolve into the matrix gradually,simultaneously,a new straight lamellar phase with 14H typ...     

流变挤压铸造制备LPSO结构增强Mg-Zn-Y-Zr合金及其组织性能

采用流变挤压铸造工艺制备了含有LPSO结构的Mg99.9-3xZnxY2xZr0.1(x=0.5、1、2,摩尔分数,%)合金,研究了合金的微观组织特征及力学性能。结果表明,流变挤压铸造能有效细化合金的微观组织。合金的基体组织由尺寸较大的α1-Mg和尺寸较小的α2-Mg晶粒组成,LPSO结构呈细小的网状结构均匀地分布在晶界处,LPSO结构的含量越低,其细化效果越明显。随着挤压压力增大,合金中LPSO结构的厚度越来越小,当压力达到100MPa后,厚度变化趋缓。与常规重力铸造相比,流变挤压铸造能有效提高合金的力学性能,特别是伸长率。在400MPa下的流变挤压铸造Mg96.9Zn1Y2Zr0.1合金的抗拉强度和伸长率较重力铸造下分别提高了19%和170%。     

Comparative Study on Corrosion Behavior and Mechanism of As-Cast Mg-Zn-Y and Mg-Zn-Gd Alloys

Rare earth (RE) elements have large solid solubility in magnesium and are widely used to regulate the microstructure and property of advanced magnesium alloys. However, different kinds of RE elements have different effects on microstructure and property of the alloy. In this study, a Mg-Zn-Y alloy and a Mg-Zn-Gd alloy with alloying elements of the same atomic percentage were designed to clarify the effect of yttrium (Y) and gadolinium (Gd) on the corrosion behavior of as-cast MgZn₂Y_2.66 and MgZn₂Gd_2.66 alloys. The results show that the MgZn₂Y_2.66 alloy is mainly composed of α-Mg phase and long period stacking ordered (LPSO) phase, while MgZn₂Gd_2.66 alloy is mainly composed of α-Mg phase and (Mg, Gd)₃Zn phase (W phase). Generally speaking, the corrosion phenomena of the two alloys in 3.5 wt% NaCl solution are similar. In the early stages of exposure, the alloys underwent uniform corrosion at a relatively low corrosion rate. With prolonged exposure, localized corrosion became dominated and the corrosion rate was greatly increased. However, the corrosion rate of the MgZn₂Y_2.66 alloy, in terms of the corrosion current density, is about one order of magnitude lower than that of the MgZn₂Gd_2.66 alloy. The high corrosion resistance of the MgZn₂Y_2.66 alloy is mainly attributed to the presence of LPSO phase in form of continuous networks and the relatively high corrosion resistance of the corrosion product layer on the alloy.     

稀土Y元素对铸态镁合金组织与力学性能的影响

采用熔炼工艺制备了Mg-2.0Zn-0.2Ca与Mg-2.0Zn-0.2Ca-2Y合金,研究了两种合金的铸态组织及力学性能。结果表明,Y元素的添加细化了Mg-2.0Zn-0.2C合金的铸态组织。Mg-2.0Zn-0.2Ca合金主要由α-Mg与少量Mg₇Zn₃相组成,添加2wt%的Y后,改变了Zn在Mg基体中的固溶度,降低了其固溶强化效果,同时组织中形成了I相和W相。添加Y元素后,合金的规定塑性延伸强度升高,从41.0 MPa升高到50.6 MPa;伸长率降低,从12.6%降低到4.0%。     

Effect of long-period stacking ordered phase on mechanical properties of Mg97Zn1Y2 extruded alloy

The mechanical properties of Mg97Zn1Y2 extruded alloy, composed of Mg matrix phase and a long-period stacking ordered phase, the so-called LPSO phase, with a volume fraction of approximately 24%, were investigated using compression tests at room temperature. The microstructure was varied to a large degree by various heat treatments at high temperatures above 400 °C, and the relationship between the microstructure and mechanical properties was clarified. The plastic behavior of the Mg/LPSO two-phase alloy was compared with that of Mg99.2Zn0.2Y0.6 alloy, composed almost Mg-solid-solution phase, and the strengthening mechanisms at work in the Mg97Zn1Y2 extruded alloy are discussed. The existence of the LPSO phase strongly enhanced the refinement of Mg matrix grains during extrusion, which led to a large increase in yield stress through the Hall–Petch relationship. In addition, the LPSO phases, which were aligned along the direction of extrusion in the Mg97Zn1Y2 extruded alloy, acted as hardening phases, being roughly coordinated with the short-fiber reinforcement mechanism.     

高强Mg-Gd-Y-Zn-Zr合金的微观组织和力学性能

通过金属模铸、热挤压和时效处理(T5)工艺过程制备出高强Mg-7Gd-4Y-1.6Zn-0.5Zr合金,并利用光学显微镜、XRD、SEM及TEM分析研究Mg合金不同状态下的显微组织和力学性能。结果表明:Mg-7Gd-4Y-1.6Zn-0.5Zr合金的铸态组织主要由α-Mg基体和沿晶界分布的片层状第二相Mg12Zn(Gd,Y)组成,经过热挤压变形后,合金晶粒显著细化,时效处理过程中Mg12Zn(Gd,Y)相上析出少量细小的颗粒状Mg3Zn3(Gd,Y)2相。时效态合金的抗拉强度、屈服强度和伸长率分别达到446 MPa、399 MPa和6.1%,其强化方式主要为细晶强化和第二相强化。     

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。     

具有长周期结构的Mg–Gd–Zn–Zr合金的微结构和摩擦学行为

利用传统的熔铸法制备Mg-14.28Gd-2.44Zn-0.54Zr合金,研究铸态和固溶态合金的微结构。利用销-盘装置研究铸态和固溶态合金的室温润滑滑动摩擦磨损行为研究。在外载荷为40 N,滑动速度为30-300 mm/s以及滑行路程为5000 m情况下,测量磨损率和摩擦因数。研究结果表明：铸态合金主要由α-Mg固溶体、分布在基体内的层片状的14H型长周期结构（LPSO）和β-[（Mg,Zn）3Gd]相组成。经过温度为773 K固溶处理35 h后,大量的β相转变成具有14H型X相LPSO结构。由于固溶处理后大量β相转变为热稳定的韧性X-Mg12Gd Zn长周期结构相,固溶合金呈现较低的抗磨损能力。     

Plastic deformation behavior of Mg97Zn1Y2 extruded alloys

The mechanical properties of the Mg97ZnlY2 extruded alloy containing the long-period stacking ordered phase, the so-called LPSO-phase, with a volume fraction of 24%-25%, were examined by compression tests and cyclic tension-compression deformation tests. The plastic behavior of the extruded alloys with compositions of Mg99.2Zn0.2Y0.6 and Mg89Zn4Y7 （molar fraction, %）, which were almost the same compositions of Mg matrix phase and LPSO phase in Mg97Zn1Y2 Mg/LPSO two-phase alloy, respectively, were also prepared. By comparing their mechanical properties, the strengthening mechanisms operating in the Mg97Zn1 Y2 extruded alloy were discussed. Existence of the LPSO-phase strongly enhanced the refinement of Mg matrix grain size during extrusion, which led to a large increment of the strength of alloy. In addition, the LPSO-phases, which were aligned along the extrusion direction in Mg97Zn1Y2 extruded alloy, acted as hardening phases, just like reinforced fibers.     

均匀化处理及冷却方式对Mg-Y-Zn合金显微组织及硬度的影响

利用X射线衍射、光学显微镜、扫描电镜和硬度测试等方法研究了均匀化处理以及冷却方式对Mg-Y-Zn合金显微组织及硬度的影响。结果表明:铸态合金的组织由α-Mg基体、连续网状分布的层片状LPSO相及不规则块状Mg24Y5共晶相构成。均匀化处理过程中,Mg24Y5共晶相逐渐溶解,层片状LPSO相沿平行片层方向向晶粒内部持续生长直至贯穿整个晶粒,并在垂直片层方向发生聚集粗化。相较于水冷态合金,炉冷态合金晶粒内部析出细小弥散的针状相。硬度测试结果表明:均匀化处理初期,合金硬度得到一定程度的提升;进一步延长均匀化处理时间,合金硬度降低。其次,炉冷态合金硬度低于水冷态合金。     

Tensile fracture of as-cast and hot rolled Mg-Zn-Y alloy with long-period stacking phase

An experimental Mg97Zn1Y2(molar fraction,%)alloy was produced by rolling the as-cast alloy.The microstructure of the alloy is composed of theα-Mg(also marked as 2H-Mg with reference to long-period stacking structure according to hexagonal close packed structure)and long-period stacking(LPS)phase.Tensile tests of Mg97Zn1Y2 alloy in comparison with pure Mg were conducted.The fracture morphologies of the tensile specimens were characterized and the microstructures near fracture surface were observed.The results show that the rolled Mg97Zn1Y2 alloy shows a mixed fracture mode including dimples indicating a ductile fracture pattern and a small fraction of cleavage planes indicating a brittle fracture pattern,which is different from the single brittle fracture of the as-cast alloy.In addition,the plastic deformation is mainly from dislocations induced strain with small strengthening effect during plastic deformation in the as-cast Mg97Zn1Y2 alloy,and the strain hardening rate is similar to that of the as-cast pure magnesium.The deformation mechanism of Mg97Zn1Y2 alloy is different from that of the pure magnesium according to a metallographical observation that whether twins are found or not.The strengthening effect hardly exists in the rolled Mg97Zn1Y2 alloy under the same dislocations induced strain,which is different from that of the as-cast alloy with moderate strengthening effect.     

Evaluation of microstructure and mechanical properties of Mg-xCe-0.5Zn alloys by nano-indentation

The microstructure and mechanical properties of Mg-xCe-0.5Zn(x=0.5,1.5,2.5,molar fraction,%) alloys were examined using a nano-indentation technique.The alloys were fabricated using a vacuum induction melting method under an argon atmosphere. The microstructures of Mg-xCe-0.5Zn alloys mainly consist ofα-Mg and eutectic Mg12Ce phase.The volume fraction and size of the eutectic Mg12Ce phase increase with increasing Ce contents.Nano-indentation test results show that the indentation hardness and elastic modulus of the eutectic Mg12Ce phase are higher than those of theα-Mg matrix.In addition,the mean indentation hardness and elastic modulus of the Mg-xCe-0.5Zn alloys increase with the Ce addition amount increasing.     

喷射成形Mg-9Al-xZn合金的微观组织演变

利用喷射成形技术制备AZ91、AZ92和AZ93镁合金沉积柱坯,并对其微观组织演变进行观察.结果表明:铸态普通凝固AZ91合金的晶粒粗大,脆性β-Mg17Al12相连续分布成网状结构;而喷射成形AZ91、AZ92和AZ93合金的组织均匀,晶粒被充分细化,β-Mg17Al12相的网状结构被打破;喷射沉积快速凝固条件下高的冷却速率促进了过饱和α-Mg固溶体组织的形成,使得偏析相减少,形态改善;Zn含量的增加降低了Al在Mg中的溶解极限,促进了β-Mg17Al12相在晶界的析出及α-Mg+β-Mg17Al12离异共晶组织的形成;Zn元素的偏析倾向为激活成分过冷区内的形核质点提供了驱动力,从而阻碍了晶粒长大.     

Investigation of two-phase Mg-Gd-Ni alloys with highly stable long period stacking ordered phases

The effects of the Ni addition on the microstructures of Mg-2Gd-xNi (at.%, x = 0, 0.25, 0.5 and 1) alloys were systematically investigated. A series of two-phase Mg-Gd-Ni alloys consisting of long period stacking ordered (LPSO) phase and α-Mg were obtained. The LPSO phase presented block-like shape and was distributed between α-Mg dendrite arms. Its volume fraction was found to be proportional to the Ni content, ranging from 0 to ∼40% with the Ni content increasing from 0 to 1 at.%. After a heat treatment at 500 °C for 100 h, the LPSO phase experienced a structure transformation from 18R to 14H but was hardly dissolved into the α-Mg matrix, which evidenced a high thermal stability. The mechanism for the effects of the Ni addition on the formation of the LPSO phase was discussed.     

Mg97Y2Zn1合金中18R型长周期有序相的微观结构（英文）

通过第一性原理计算研究Mg97Y2Zn1合金中18R型长周期有序相(LPSO)的微观结构,从理论上确定Zn和Y原子在LPSO相中的排列。结果表明:添加原子首先分布在18R型LPSO相两端的层错层,然后向内部的层错层延伸。计算结果与实验现象非常吻合。同时,也揭示了18R与其他LPSO相之间的微观结构关系;结合能和形成焓表明了18R型LPSO相的稳定性与Y和Zn原子含量之间的关系。计算得到的电子结构揭示了18R型LPSO相微观结构和稳定性潜在的机理。     

Microalloying Effect of Sn on Phase Transformation During Heat Treatment in Mg–Y–Zn–Zr Alloys

The microstructure and mechanical properties of the as-cast and heat-treated Mg-4.6 Y-2.5 Zn-0.6 Zr-x Sn(x = 0, 0.2 and0.5 wt%) alloys were investigated in this work. The results showed that the eutectics have been refined with 0.2% Sn addition and it has no effect on the phase category of the alloys. However, Sn3 Y5 phase was found in 0.5% Sn-added alloy.After heat treatment at 520 °C, the transformation of the long-period stacking ordered(LPSO) phase takes place in the Mg-Y-Zn-Zr alloy, but the transition is not completed in the alloys containing Sn. In addition, during the heat treatment, the mechanical properties of Sn-free alloys are significantly improved, and the strength of alloys containing Sn does not change much. Through observation and analysis of the microstructure and mechanical properties, it is found that Sn addition hinders the process of a0-Mg ? a-Mg ? 14 H and the process is the key to the transition of 18 H-LPSO to 14 H-LPSO.