Phase Stability in Mechanically Alloyed Mg–Ni System Studied by Experiments and Thermodynamic Calculations

In this study, microstructural evolution of Mg–Ni alloy during mechanical alloying(MA) was investigated.Also, a thermodynamic approach was utilized to predict the most stable phases formed in Mg–Ni alloy after MA. The phase composition and microstructural properties of Mg–Ni alloy were assessed by X-ray diffractometry, high-resolution field emission scanning electron microscopy and high-resolution transmission electron microscopy. The results showed that ball milling of magnesium and nickel powder mixture for 70 h yields nanostructural Mg2Ni compound with an average grain size of ~20 nm. Thermodynamic calculations revealed that in the composition ranges of 0.0 \ XMg\ 0.03(at.%)and 0.97 \ XMg\ 1, there is no driving force for amorphous phase formation. In the composition range of 0.07 \ XMg\ 0.93, the change of Gibbs free energy for amorphous phase formation was more negative than solid solution.While for XMg= 0.66(nominal composition of Mg2Ni intermetallic phase), the change of Gibbs free energy for intermetallic phase was found to be more negative than both amorphous and solid solution phases indicating that Mg2Ni intermetallic compound is the most stable phase, in agreement with the experimental observations.     

Microstructural evolution during controlled ball milling of (Mg2Ni+MgNi2) intermetallic alloy

Nearly dual-phase Mg–Ni alloy fabricated by ingot metallurgy (IM) and comprising 30 vol% Mg₂Ni and 61 vol% MgNi₂ intermetallic compounds (remaining 9 vol% of unreacted Mg) was mechanically (ball) milled under controlled shearing for 10, 30, 70 and 100 h. The majority of the medium- and small-sized powder particles exhibited a relatively homogeneous microstructure of milled Mg₂Ni and MgNi₂. A fraction of large-sized particles developed the ‘core and mantel’ microstructure after milling for 70 and 100 h. The ‘core’ contains poorly milled MgNi₂ particles and the ‘mantel’ is a thoroughly milled mixture of Mg₂Ni, MgNi₂ and, possibly, residual Mg. X-ray diffraction provides evidence of nanostructurization and eventual amorphization of a fraction of a heavily ball milled Mg₂Ni phase. The remnant Mg₂Ni developed a nanocrystalline/submicrocrystalline structure. The co-existing MgNi₂ phase developed a submicrocrystalline structure within the powder particles. The results are rationalized in terms of enthalpy effects by the application of Miedema’s semi-empirical model to the phase changes in ball milled intermetallics.     

The structural and kinetic characteristics of Mg1.9Al0.1Ni alloy synthesized by mechanical alloying

The structural and kinetic characteristics of the mechanically alloyed Mg_1.9Al_0.1Ni were investigated. It was found that Mg_1.9Al_0.1Ni can absorb/desorb about 3.55/3.44 mass% H at a high rate and it has a hexagonal crystal structure as Mg₂Ni. The hydriding/dehydriding (H/D) rates in the two-phase (–β) region of Mg_1.9Al_0.1Ni were measured and studied at temperatures ranging from 553 to 623 K under an approximately isobaric condition. The obtained data of H/D rates indicated that hydrogen diffusion was the rate-controlling step through the hydride phase. A new model was successfully used to calculate the kinetic experimental results. It can be seen that theoretical calculation agrees well with experimental data. The corresponding activation energies are 47 600 and 54 500 J/mol H₂ for H/D processes, respectively.     

Synthesis and electrode characteristics of the new composite alloys Mg2Ni-xwt.% Ti2Ni

A new composite alloy Mg₂Ni-xwt.% Ti₂Ni has been successfully synthesised using a ‘particle inlaying’ method. Scanning electron microscopy and energy dispersive spectroscopy revealed that very fine Ti₂Ni particles were inlaid onto the surface of Mg₂Ni particles by mechanical treatment and sintering. XRD showed the composite alloys were composed of primary alloys Mg₂Ni, Ti₂Ni and new phases TiNi, Ti---Mg formed in the composite procedure. The electrode characteristics of Mg₂Ni-xwt.% Ti₂Ni alloys in an alkaline solution have been investigated and compared with those of Mg₂Ni. The discharge capacity of the alloy electrode was effectively improved from 8 mA h g¹ of Mg₂Ni to 165 mA h g¹ of Mg₂Ni-40wt.% Ti₂Ni at ambient temperature, which is almost comparable with that of Ti₂Ni electrode (170 mA h g¹). It is believed that the fine Ti₂Ni particles inlaid on the surface of Mg₂Ni particles play a two-fold role: firstly, they hydride-dehydride as hydrogen storage materials themselves: secondly, they provide active sites and pathways for Mg₂Ni hydriding-dehydriding. This is supported by analysis of discharge behaviour and electrochemical impedance spectra studies.     

Influence of tin on the electrochemical and gas phase hydrogen sorption in Mg2−xSnxNi (x = 0, 0.1, 0.3)

Mg_2−xSn_xNi (x = 0, 0.1, 0.3) alloys were synthesized by reactive ball milling under protective Ar atmosphere and liquid n-heptane. The microstructure and the morphology of the powders were determined by X-ray diffraction and scanning electron microscopy. The as-milled alloys consist of Mg₂Ni nanocrystals with an average grain size in the range 3–7 nm, depending on the alloy composition. Sn containing phases were not detected even in the Sn-rich alloy. Obviously, Sn is dissolved in the Mg₂Ni intermetallic compound. Gas phase sorption of hydrogen was not observed in the alloys containing Sn (Mg_2−xSn_xNi; x = 0.1, 0.3). It was suggested that Sn impedes the process of hydrogen molecules decomposition. The as-milled alloys absorbed reversibly hydrogen electrochemically. Mg₂Ni alloy showed the highest discharge capacity of 300 mAh/g. The capacity of Mg_1.9Sn_0.1Ni and Mg_1.7Sn_0.3Ni was about 260 mAh/g. It was found that Sn improved the cycle life of the electrode.     

Synthesis of fine composite particles for hydrogen storage, starting from Mg-YNi2 mixture

We synthesized new composite particles for hydrogen storage on the basis of an idea of “particle designing”. As starting materials, powders of Mg and YNi₂ were selected. Fine composite particles containing mainly Mg₂Ni could be designed by repetitive hydriding and dehydriding cycles at 673 K. In the synthesis process of the composite particles, the following two points were found to be essential for this technique. The first point is that, after being activated by the sequential processes of hydrogenation, amorphization and disproportionation, YNi₂ reacts effectively with Mg. The second point is that evaporated Mg, which occurs during dehydriding, adheres to the surface of the activated YNi₂ and accelerates a diffusion reaction to form Mg₂Ni at the interface. In these composite particles, Mg₂NiH₄ is formed, even at 373 K, under a hydrogen pressure of 5 MPa.     

Hydriding combustion synthesis of Mg–CaNi5 composites

The composites of Mg–x wt.% CaNi₅ (x = 20, 30 and 50) were prepared by hydriding combustion synthesis (HCS) and the phase evolution during HCS as well as the hydriding properties of the products were investigated. It was found that Mg reacted with CaNi₅ forming Mg₂Ni and Ca during the heating period of HCS. Afterwards, the resultant Mg₂Ni and Ca as well as the remnant Mg reacted with hydrogen during the cooling period. The lower platform in the P–C isotherms corresponds to the hydriding of Mg, and the higher one corresponds to Mg₂Ni. With the increase of the content of CaNi₅ from 20 to 50 wt.%, the hydrogen content of the HCS products increases at first and then decreases. The Mg–30 wt.% CaNi₅ composite has the maximum hydrogen capacity of 4.74 wt.%, and it can absorb 3.51 wt.% of hydrogen in the first hydriding process without activation.     

Crystal structures of VSn2, NbSn2 and CrSn2 with Mg2Cu-type structure and NbSnSb with CuAl2-type structure

Several binary stannides of the early transition metals T have been reported with the composition T₂Sn₃ previously. However, the present structure refinements from single-crystal X-ray data show that they have the compositions VSn₂, NbSn₂ and CrSn₂ (R = 0.028, R = 0.018 and R = 0.021 with 17 variable parameters and 828, 512 and 440 structure factors respectively). Their orthorhombic Mg₂Cu-type structure is closely related to the structures of MoSn₂ (Mg₂Ni type) and CuAl₂. The latter structure type was confirmed for NbSnSb by a structure refinement from single-crystal data (R = 0.010 for eight variables and 254 F values). Electrical conductivity measurements show CrSn₂ and MoSn₂ to be metallic conductors.     

不同激光功率下镁合金表面激光熔覆Ni60合金涂层的显微组织和磨损性能

为提高镁合金表面的耐磨性,利用5kW横流连续CO2激光器在AZ31B镁合金表面熔覆Ni60合金粉末,制备了无裂纹、气孔等缺陷的熔覆层。分析讨论了不同激光功率下熔覆层的显微组织和磨损性能。结果表明:熔覆层的显微组织为典型的枝晶状态,且随着激光功率的增加,枝晶尺寸增加;不同的激光功率下,熔覆层都由Mg、MgNi2、Mg2Ni3Si、Mg2Ni、Mg2Si和FeNi组成,但当激光功率增加时,Mg相含量逐渐减小,其它相含量逐渐增多。在枝晶细化和各种金属间化合物的共同作用下,熔覆层的显微硬度和耐磨性能都得到提高,且激光功率P=3 000W时,提高程度最大,即显微硬度提高了840%～1 102%,磨损失量是原始AZ31B镁合金的8.57%。     

Effects of sintering on composite metal hydride alloy of Mg2Ni and TiNi synthesized by mechanical alloying

As-milled composite metal hydrides composed of Mg₂Ni and TiNi phases were cold-pressed under a pressure of 490 MPa and sintered for 1 h at 5×10⁻⁶ Torr and 300 °C. Electrochemical characteristics of the sintered composite metal hydride electrode were investigated. The maximum discharge capacity of the sintered composite alloy electrode was 125 mAh/g at a discharge current density of 100 mA/g. This value was similar to that of the as-milled one before sintering. However, the sintered electrode retained 80% of the maximum discharge capacity after 150 cycles, while the as-milled electrode retained only 55%. This is because after the sintering process an interface between Mg₂Ni and TiNi plays a role similar to a diffusion layer of hydrogen. In the sintered composite electrode, when a discharging step proceeds, hydrogen absorbed in a Mg₂Ni particle can move into a TiNi phase through the bonded-interface between Mg₂Ni and TiNi, then discharges at the interface between TiNi and the electrolyte. Also, the electrochemical impedance spectroscopy (EIS) tests showed that the composite alloy electrodes had a lower charge-transfer resistance and a higher hydrogen diffusion coefficient than those in single-phase Mg₂Ni. This indicates that TiNi particles in the composite are the active sites for redox reaction of hydrogen and the pathway for the diffusion of hydrogen     

Electrochemical characteristics of Mg2−xZrxNi (x = 0–0.6) electrode alloys prepared by mechanical alloying

The electrode alloys Mg_2−xZr_xNi (x = 0, 0.15, 0.3, 0.45 and 0.6) were prepared by mechanical alloying (MA). Mg in the alloy was partially substituted with Zr in order to improve the electrochemical characteristics of the Mg₂Ni-type alloy. The microstructures and the electrochemical characteristics of the experimental alloys were measured systemically. The effects of substituting Mg with Zr and MA technique on the microstructures and electrochemical performances of the alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the substitution of Zr is favourable for the formation of an amorphous phase. For a fixed milling time, the amorphous phase in the alloy grows with increasing Zr content. The electrochemical measurement indicates that the substitution of Zr can dramatically enhance the discharge capacity with preferable cycle stability, and it markedly improves the discharge voltage characteristic of the alloys. For x ≤ 0.3, the discharge capacity of the alloys monotonically increases with milling time. But for x > 0.3, it has a maximum value with the change of milling time.     

AZ91D镁合金激光熔覆Al-Si涂层微观组织及热力学分析

以Al-Si共晶成分合金粉末为熔覆材料在AZ91D镁合金表面进行了激光熔覆试验.采用光学显微镜、扫描电镜、能谱仪、X射线衍射仪分析了涂层的微观组织,并利用Thermo-Calc软件分析了涂层的相组成、相成分及结晶转变过程.结果表明,涂层微观组织分为两层,上半层为Al₁₂Mg₁₇基体上均匀分布着Mg₂Si树枝晶和细小的Al₃Mg₂针状相,其结晶过程为液相→液相+Mg₂Si→Mg₂Si+Al₁₂Mg₁₇→Mg₂Si+Al₁₂Mg₁₇+Al₃Mg₂;下半层由Mg₂Si颗粒、α-Mg树枝晶和(α-Mg+Al₁₂Mg₁₇)共晶组织组成,其结晶过程为液相→液相+Mg₂Si→液相+Mg₂Si+α-Mg→Mg₂Si+α-Mg+(α-Mg+ Al₁₂Mg₁₇)共晶组织.研究结果对AZ91D合金表面激光熔覆Al-Si合金涂层微观组织及其转变过程分析具有指导意义.     

Study of amorphous Ni–S(La) alloy used as HER cathode in alkaline medium

The hydriding combustion synthesis (HCS) of Mg₂FeH₆ and Mg₂Ni_1−xFe_x hydrides was systematically studied by changing the value of x from 0.25 to 1.0. This study aimed at improving the Mg₂FeH₆ yield in production and examining the effect of the addition of nickel to the hydrogen storage properties of the Mg₂FeH₆ hydride. In synthesizing metallic hydrides, the raw materials in metallic powders were mechanically activated by a ball mill before the HCS treatment. As a result, the ball-milled 2Mg + Fe recorded as much as 5 mass% in hydrogen storage capacity during the HCS treatment, and the final product successfully indicated a high purity Mg₂FeH₆. Interestingly, the deformation enthalpies of the Mg₂Ni_1−xFe_x hydrides were larger by 10% or more than Mg₂NiH₄ and Mg₂FeH₆, taken individually. This was collateral evidence to prove that the HCS of Mg₂FeH₆ and Mg₂Ni_1−xFe_x hydrides with ball milling (BM) produced a new structure of the Mg–Ni–Fe–H system due to the synergy effect.     

Mg6Ir2H11, a new metal hydride containing saddle-like [IrH4] and square-pyramidal [IrH5] hydrido complexes

Mg₆Ir₂H₁₁ has been synthesised by both hydrogenation of the intermetallic compound Mg₃Ir at 20 bar and 300 °C, and sintering of the elements at 500 °C under 50 bar hydrogen pressure. Neutron powder diffraction on the deuteride indicates a monoclinic structure (space group P2₁/c, Mg₆Ir₂D₁₁: a=10.226(1), b=19.234(2), c=8.3345(9) Å, β=91.00(1)°, T=20 °C) that is closely related to orthorhombic Mg₆Co₂H₁₁. It contains a square-pyramidal [IrH₅]⁴⁻ complex and three saddle-like [IrH₄]⁵⁻ complexes of which one is ordered and two are disordered. Five hydride anions H⁻ are exclusively bonded to magnesium. The compound has a red colour, is presumably non-metallic and decomposes under 3 bar argon at 500 °C into Mg₃Ir, iridium and a previously unreported intermetallic compound of composition Mg₅Ir₂.     

连接温度对TiAl/Ti3AlC2扩散焊接头界面结构及性能的影响

采用Ti/Ni复合中间层实现了TiAl合金和Ti₃AlC₂陶瓷的扩散连接,利用SEM,XRD等分析方法对接头界面结构进行了分析.结果表明,TiAl/Ti₃AlC₂接头典型界面结构为TiAl/Ti₃Al+Al₃NiTi₂/Ti₃Al/α-Ti+Ti₂Ni/Ti₂Ni/TiNi/Ni₃Ti/Ni/Ni₃(Ti,Al)/Ni₃Al+TiC_x+Ti₃AlC₂/Ti₃AlC₂.随着连接温度的升高,TiAl/Ti界面处的Ti_ss层逐渐减小,Ti₃Al化合物层逐渐变厚;TiNi化合物层厚度显著增加,Ti₂Ni和Ni₃Ti层厚度基本保持不变.接头抗剪强度随连接温度升高先增加后减小,当连接温度为850℃时,接头的抗剪强度最高可达到85.3 MPa.接头主要在Ni/Ti₃AlC₂界面及Ti₃AlC₂基体处发生断裂.     

Mechanically milled Mg composites for hydrogen storage the transition to a steady state composition

Magnesium alloys are potentially the best materials for gaseous hydrogen storage. However, their practical use is limited by poor hydrogen absorption and desorption kinetics. This problem can be resolved by mixing Mg alloys with other materials to form composites. We present an investigation of the initial hydriding characteristics, as well as the compositional transformation of composites made of La₂Mg₁₇ + LaNi₅ mechanically milled in a 2:1 weight ratio. Composites produced with varying durations and intensities of milling were tested. Those milled to the greatest extent proved to have the best initial hydrogen absorption and desorption kinetics. The kinetics of the most heavily milled composite were superior to those of La₂Mg₁₇. This composite absorbed 90% of its full hydrogen capacity (3.5 wt.% H₂) in less than 1 min at 250°C and desorbed the same quantity of hydrogen in 6 min. Under the same conditions pure La₂Mg₁₇ took 2.5 h to absorb and 3 h to desorb 90% of its full hydrogen capacity (4.9 wt.% H₂). Scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction were used to characterize the mechanically milled powders before and after hydriding. The unhydrided powders consisted of LaNi₅ grains surrounded by a fractured LaMg₁₇ matrix. Hydrogen cycling, at temperatures up to 350°C, induced phase changes, segregation, and disintegration of the composites. The resulting fine powder (less than 1 μm) consisted primarily of Mg, Mg₂Ni, and La phases.     

镍中间层对铝/镁异种金属搅拌摩擦焊接头微观组织的影响

采用搅拌摩擦焊(friction stir welding, FSW),引入厚度为0.05 mm镍箔作为中间层,在焊接速度不变条件下,采用不同转速对厚度为4 mm的6061铝合金和AZ31镁合金进行平板对接,对接头进行系列微观组织表征及力学性能测试,探讨转速对接头中镍颗粒分布状态,金属间化合物(intermetallic compounds, IMCs)种类与分布及接头强度的影响规律. 研究结果表明:与未引入中间层接头相比,引入镍改变了铝/镁异种金属FSW接头焊核区(weld nugget zone, WNZ)中IMCs种类及分布,WNZ存在明显的镁合金与铝合金相间的带状组织,其上分布着絮状Al₁₂Mg₁₇、颗粒状Mg₂Ni、层状Al₃Mg₂及大小不一的镍箔颗粒;随着转速增加,镍箔颗粒分布愈加均匀,Al₃Mg₂数量相对减少,且脆性Al₃Mg₂由连续分布逐渐演变为断续分布;当转速为750 r/min时,接头抗拉强度达到最大值,与未引入中间层接头相比,引入镍中间层接头抗拉强度提高了56 MPa,达到镁合金的56.9%.     

Dielectric properties of sintered aluminum nitride

In the present work, the effects of sintering additives on dielectric loss tangent (tan δ) of AlN ceramics were explored. Different amounts of Y₂O₃ and Mg₃N₂ were respectively added as sintering additives to AlN powders, and pressureless-sintering was performed at 1900 °C for 2 h in a nitrogen flow atmosphere. The resulted AlN ceramics became denser due to the addition of Mg₃N₂, and nearly fully dense sample was obtained with a relative density of 0.998. tan δ decreased with increasing Mg₃N₂ amount and it was found to depend on the density of the AlN ceramic. The best tan δ value of 3.9 × 10⁻⁴ was obtained by adding 1 mol% of Y₂O₃ and Mg₃N₂ together.     

Ti600和Ni-25%Si合金钎焊接头性能及失效机理分析

采用Ti-Zr-Ni-Cu非晶钎料对高温钛合金Ti600和Ni-25%Si (原子分数,%)合金进行钎焊试验,重点研究了钎焊温度对镍硅与钛合金接头组织及性能的影响,结合接头组织特征及断口结构分析阐明了Ti600和Ni-25%Si合金钎焊接头的失效机理. 结果表明,钎缝内部包含多个区域,随着连接温度从900 ℃上升至980 ℃,包含(Ti,Zr)₂Si和Ti₂Ni相的区域逐渐消失,包含Ti₅Si₃和Ti₂Ni相的区域逐渐变厚,最终占据全部钎缝. 力学性能分析表明,随着钎焊温度的升高,接头抗剪强度先增大后降低. 当钎焊温度为960 ℃时,接头的抗剪强度能够达到峰值177 MPa. 在脆性Ti₂Ni相基体上弥散分布的Ti₅Si₃相颗粒破坏了Ti₂Ni相的连续性,阻碍了裂纹在钎缝内部的扩展是钎焊接头抗剪强度提升的根本原因.     

Experimental Investigation of the Mg–Zn–Zr Isothermal Section at 400 °C

Mg-Zn-Zr series Mg alloys(ZK) are one of the most important commercial Mg alloys due to their good comprehensive mechanical properties. The phase equilibria of the Mg-Zn-Zr system at 400 ℃ covering the overall composition range were investigated by X-ray diffraction and electron probe microanalyses on thirteen ternary alloys. Three ternary compounds, τ_1, τ_2 and τ_3, were detected to be thermodynamically stable at 400 ℃, and their homogeneity range was determined to be Mg_((7-17))Zn_((80-88))Zr_((4-6)), Mg_((15-22))Zn_((66-65))Zr_((9-16)) and Mg_9 Zn_(68)Zr_(23)(in at.%), respectively. Eight three-phase regions and four two-phase regions were observed. The maximum solubility of Mg in Zn_(22) Zr, Zn_(39) Zr_5 and Zn_3 Zr phases was measured to be 0.52, 0.37 and 0.99 at.%, respectively, while the solubility of Zr in MgZn_2 and Mg_2 Zn_3 phases is negligible. The isothermal section of the Mg-Zn-Zr system at 400 ℃ was then constructed based on the present experimental data.