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.     

热处理对Mg94Zn2Y4合金微观组织与力学性能的影响

研究了在773 K、48h 条件下热处理对Mg94Zn2Y4合金的微观组织与力学性能的影响。研究结果表明，块形和板条结构的18R 长周期堆垛结构相可直接从熔体凝固过程中形成。热处理后，绝大多数的块形和板条结构相转变为细片状或针状的14H相。在热处理过程中，有相当体积分数的 LPSO（长周期堆垛结构）相由 18R 转变为 14H。结果还表明，经过热处理，块形和板条结构相与针状相可以在 α-Mg 基体中共存，并作为影响因素，使合金晶粒得到细化，晶粒尺寸为14-24 μm（平均晶粒尺寸为19 μm），使极限拉伸强度、屈服强度以及伸长率分别由铸态时的 182 MPa、135 Mpa 和 10.2% 提高至 245 MPa、157 MPa 和 13.8%.     

Mg-Zn-Y三元合金富Mg区凝固组织及二十面体准晶相形貌

通过常规凝固方法在Mg28Zn2Y三元合金中的富Mg区获得了二十面体稳定准晶相。Mg28Zn2Y三元合金铸态组织中存在α-Mg相，Mg7Zn3相和二十面体准晶相。该合金的准晶形貌有两种：一种为尺寸较大、具有五次对称性的花瓣状；另一种为尺寸稍小、对称性不太明显的团状。五次对称性的花瓣状形貌是二十面体准晶，是按其特有的五次对称轴的择优方向自由生长所致。冷却速率直接影响合金凝固过程中准晶熟化时间。熟化时间越长，越容易使准晶花瓣端部粗化以及在端部发生分又。端部的分叉容易使得花瓣准晶破碎，破碎的准晶游离到低温相中，在界面能的作用下形成多边形形貌。     

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。     

Characterization of planar features in Mg–Y–Zn alloys

The planar features in a Mg–8Y–2Zn–0.6Zr (wt.%) alloy solution-treated at 500 °C for 1 h have been examined using conventional transmission electron microscopy and atomic-resolution high-angle annular dark-field scanning transmission electron microscopy. Three types of planar features are detected in the microstructure. The first type, which was previously reported to be an intrinsic stacking fault I₁ bounded by a Frank partial dislocation, is shown to be the 14H precipitate phase that is associated with Shockley partial dislocations. The second type is also a precipitate phase that has a single unit cell height and is associated with Shockley partial dislocations. The third type of planar feature comprises small ribbon-like stacking faults. These stacking faults are determined as intrinsic I₂ type bounded by two Shockley partial dislocations, which is further confirmed by computer simulation. The stacking fault energy associated with the faults is much smaller than that of pure magnesium.     

多孔Mg97Zn1Y2材料的力学性能与阻尼性能

多孔材料是一种新型轻质的结构-功能材料，具有较高的阻尼性能，同时兼具密度小、质量轻等特点，使其具有广阔的应用价值。基于此，选取含有长周期特殊结构相（LPSO相）的Mg97Zn1Y2为基体，以MgCO₃为发泡剂，SiC为增粘剂，通过熔体发泡法制备出镁基多孔材料，通过OM、SEM、TEM、XRD及DMA等技术手段，研究多孔Mg97Zn1Y2材料的力学性能、阻尼性能等。结果表明，多孔Mg97Zn1Y2材料主要由镁基体（α-Mg）、LPSO相和SiC相组成，当应变幅值较小时，多孔Mg97Zn1Y2复合材料的阻尼值明显优于Mg97Zn1Y2合金，随着孔隙率的增加，阻尼性能得到改善。孔隙率的增加降低了多孔Mg97Zn1Y2复合材料的抗压缩性能。此外，除了Mg97Zn1Y2合金本身的位错阻尼、界面阻尼和晶界阻尼之外，还有“气相”阻尼，共同影响着多孔Mg97Zn1Y2材料的阻尼性能。     

快速凝固Mg94.6Zn4.8Y0.6镁合金薄带的组织与性能

采用单辊快速凝固技术制备了Mg94.6Zn4.8Y0.6合金薄带,研究了薄带的组织及性能特征。结果表明:Mg94.6Zn4.8Y0.6镁合金快速凝固薄带组织由过饱和的单相-αMg固溶体组成,沿厚度方向分为两个晶区:近辊面粗大等轴晶区和自由面细小等轴晶区。Mg94.6Zn4.8Y0.6合金快速凝固薄带的显微硬度为85.95HV,在250℃左右保温2 h的显微硬度最高,超过该温度(250~300℃)显微硬度显著下降。     

Single and double aging treatments on Mg97Zn1Y2 alloy

The development of magnesium alloys was limited due to the low absolute strength and poor corrosion resistance. It was found that the optimal performance could not be achieved in some alloys by a single quenching and aging treatment, but could be achieved after a graded aging or multiple-stage aging heat treatment. The Mg97Zn1Y2 alloy was prepared and subjected to single and double aging treatments. Single aging was carried out at 250 ℃ for 6 to 15 h. For double aging, the first step was performed the same as the single aging. The second step was performed at 350 ℃ for 12 h. The microstructure and properties of the alloy with single and double aging were analyzed by means of hardness measurement, optical microscopy, scanning electron microscopy, X-ray diffraction, and polarization curve measurements. Results show that the precipitated nanoscale phases are formed during aging, and evenly distributed in the matrix. Compared with the single aging treatment, the hardness and corrosion resistance of the alloy are further improved due to the double aging treatment.     

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.     

Thermodynamic stability of Mg–Y–Zn ternary alloys through first-principles

To clarify the thermodynamic stability of a Mg-based long-period stacking ordered (LPSO) structure, we systematically study the energetic preference for alloys on multiple stacking sequences with different compositions for random mixing of constituent elements, Mg, Y, and Zn, based on special quasirandom structure (SQS). Through calculation of the formation free energy of SQSs, it was found that the Mg–Y–Zn alloy exhibits phase separation into Mg- and Y–Zn- rich phases, which is consistent with previous theoretical studies. The bulk modulus of SQSs for various compositions, stacking sequences, and atomic configurations is approximately 35 GPa, i.e., it does not show significant dependence on the atomic arrangements, which therefore means that there are not significant differences among the effects of phonon on the stability of each structure in the LPSO structure. Introducing a stacking fault into hcp stacking sequence results in the acquisition of a “negative” energy, which indicates the profound relationship between the introduction of stacking faults and the formation of an LPSO structure.     

含LPSO相Mg_(94)Y_4Zn_1Ni_1合金的组织和力学性能

采用OM、SEM、TEM和电子万能试验机研究了Mg_(94)Y_4Zn_1Ni_1(at%)合金在铸态、退火、挤压和时效态的显微组织与力学性能。结果表明:铸态合金组织由胞状α-Mg相、网状18R LPSO相和块状Mg_(24)(Y,Zn,Ni)_5相组成。退火后,合金中未析出14H LPSO相。经挤压变形,18R LPSO相转变为长条状并沿挤压方向排列,挤压态合金的抗拉强度达到417 MPa,显著高于铸态和退火态合金。经过T5和T6时效处理,在合金的基体中析出大量细小的共格β'沉淀相,合金得到进一步强化。T5态和T6态合金的抗拉强度分别为434和432 MPa,屈服强度均高于300 MPa。     

Mg30Zn60Y10合金中二十面体准晶相的形成和生长

通过常规凝固方法在Mg30Zn60Y10三元合金中获得大体积分数准晶。准晶相由包晶反应生成。能谱分析结果表明包晶反应中初生相的成分近似为Mg16.32Zn70.60Y13.08，准晶相成分为Mg36.94Zn56.21Y6.63。差热分析显示初生相在723℃形成，准晶相生成温度为648℃。利用快淬方法使准晶相生长初期形貌得以保留，发现准晶相初期形貌受初生相形貌直接影响，但随后则接其晶体学优先方向生长，最终长成花瓣状准晶。准晶合金凝固过程中生成的低温相越多，准晶相熟化时间越长，越容易使准晶花瓣端部粗化以及在端部发生分叉。端部的分叉使得花瓣准晶破碎，这些破碎的准晶游离到低温相中在界面能的作用下形成多边形结构。     

Mg97Zn1Y2合金的摩擦磨损性能研究

研究了铸态Mg97Zn1Y2合金的室温、高温力学性能和干摩擦条件下摩擦磨损行为,并与AZ91合金进行了对比.结果表明:室温下AZ91合金的屈服强度要高于Mg97Zn1Y2合金,但高温下Mg97Zn1Y2合金较AZ91合金表现出更好的热强性,当温度超过150 ℃时,AZ91合金的屈服强度急剧下降,Mg97Zn1Y2合金则下降较少.Mg97Zn1Y2合金由轻微磨损向严重磨损的转变点明显滞后于AZ91合金,相同载荷范围内Mg97Zn1Y2合金磨损表面的温升要低于AZ91合金,其原因是Mg97Zn1Y2合金中的金属间化合物Mg12YZn较AZ91合金中的Mg17Al12相具有更佳的热稳定性.     

快速凝固Mg94.6Zn4.8Y0.6合金薄带微观结构及显微偏析

采用单辊快速凝固技术制备了Mg94．6 Zn4．8 Y0.6合金快速凝固薄带，并通过扫描电镜的背散射及线扫描研究了快速凝固Mg94．6 Zn4．8 Y0.6合金薄带的显微偏析。结菜表明，由于溶质截流，单辊快速凝固Mg94．6 Zn4．8 Y0.6合金薄带为过饱和单相α—Mg固溶体的蜂窝状组织，Mg、Zn和Y宏观分布比较均匀。但在薄带厚度方向上存在偏析，在晶内也存在显微偏析，偏析形成是由于从辊面到自由面凝固速度的差异及溶质传输和晶格结构造成的，其中Zn元素偏析最大，Y元素次之，Mg元素最小。     

Recrystallization and microstructural evolution during hot extrusion of Mg97Y2Zn1 alloy

This study revealed that the extrusion temperature has a great influence on microstructure and mechanical properties of the Mg₉₇Y₂Zn₁ alloy. The average grain sizes increased from 3 μm to 8 μm with increasing extrusion temperatures from 623K to 773 K. Both dynamic recrystallization (DRX) and static recrystallization (SRX), which occur during and after deformation, respectively, were observed. The alloy, which extruded at a relatively high temperature, exhibited lower strength because the strain strengthening was balanced by the softening that originated from DRX. Three types of morphologies, namely, big recrystallized grains, fine recrystallized grains, and non-recrystallized grains, were observed in the extruded microstructures obtained at 623 K. The dislocation density was quite high in the fully recrystallized grain. The extruded microstructures obtained at 773 K were composed of large grains with more uniform size. Their degree of recrystallization was higher and the dislocation density also declined. All dislocation in the grain were distinguished as 〈c+a〉 dislocations. Submicron scale precipitates were distributed along the newly formed recrystallized grain boundaries and had a remarkable pinning effect on the recrystallized grain growth after extrusion at 773 K. The precipitates can be divided into two main types: mixed type and single type.