Nanostructured electrode materials for Ni-MH x batteries prepared by mechanical alloying

Nanocrystalline TiFe- and Mg₂Ni-type alloys were prepared by mechanical alloying followed by annealing. The structure and electrochemical properties of these materials were studied. The properties of hydrogen host materials can be modified substantially by alloying to obtain the desired storage characteristics. It was found that the respective replacement of Fe in TiFe by Ni and Mn improved not only the discharge capacity but also the cycle life of these electrodes. On the other hand, a partial substitution of Mg by Mn in Mg_2?x M _x Ni alloy leads to an increase in discharge capacity, at room temperature. Furthermore, the effect of the nickel and graphite coating on the structure of the nanocrystalline alloys and the electrodes characteristics were investigated. In Mg₂Ni-type alloy mechanical coating with graphite effectively reduced the degradation rate of the studied electrode materials.     

New materials produced by mechanical alloying

The application of mechanical alloying (MA) to alloys based on Fe, Cu, Al, Ti, Co, Ni, Mg, and Nb is reviewed. Enhancement in physical and mechanical behavior, beyond ingot metallurgy and rapid solidification levels, can be achieved by MA, and should lead to commercialization of a number of MA alloys.Conducted under the joint Moscow-Moscow program on Synthesis of Advanced Materials (SAM).     

Development of new Al-based nanocomposites by mechanical alloying

Al-based binary (Al–Mg) and ternary (Al–Mg–Zr) elemental powder mixtures were mechanically alloyed to develop new Al–Mg–Zr nanocomposite materials. The phase evolution was studied in the as-milled and heat-treated powders by XRD and TEM/EDS analyses. For the binary Al–Mg alloy, the predominant phase was an Al(Mg) solid solution (SS) and an amorphous phase was not possible to be synthesized. Upon adding 5 at.% Zr to the Al–10Mg blended powder, some free Mg was present in addition to the formation of an Al(Zr,Mg)SS, which transformed to the Al₃Zr intermetallic after annealing. When the Zr content was increased a nanocomposite of a solid solution and an intermetallic was obtained with considerable improvement in terms of structural stability and hardness. The presence of an oxide phase at 35% Zr might be responsible for the increased hardness in this particular alloy.     

Enhanced mechanical property by introducing bimodal grains structures in Cu-Ta alloys fabricated by mechanical alloying

Dispersion-strengthened copper alloys can achieve ultra-high strength,but usually at the expense of duc-tility.In this study,a strategy for overcoming strength-ductility tradeoff of Cu alloys is realized through the introduction of bimodal grains structures.Cu-Ta alloys with only 0.5 at.％Ta content were successfully prepared by mechanical alloying combined with spark plasm sintering.The samples prepared by one-step and two-step ball milling methods are named as Cu-Ta(Ⅰ)and Cu-Ta(Ⅱ),respectively.The microstructural characterizations revealed that ultra-fine equiaxed grains with uniformly dispersed Ta precipitates were obtained in the Cu-Ta alloys.High strength of 377 MPa for yield strength together with elongation of～8％was obtained in Cu-Ta(Ⅰ).Bimodal grains structures composed of fine-grain zones and coarse-grain zones were successfully introduced into Cu-Ta(Ⅱ)by a two-step ball milling approach,and both yield strength(463 MPa)and elongation(～15％)were significantly synergistic enhanced.The hardness values of both Cu-Ta(Ⅰ)and Cu-Ta(Ⅱ)were almost kept nearly constant with the increase of annealing time,and the softening temperatures of Cu-Ta(Ⅰ)and Cu-Ta(Ⅱ)are 1018 and 1013 ℃,reaching 93.9％and 93.5％Tm of pure Cu(1083 ℃),respectively.It reveals that the Cu-0.5 at.％Ta alloys exhibit excellent thermal stability and exceptional softening resistance.Ta nanoclusters with semi-coherent structures play an essential role in enhancing the strength and microstructural stability of alloys.Bimodal structures are beneficial to the activation of back stress strengthening and the initiation and propagation of microcracks,thus obtaining the extraordinary combination of strength and elongation.This study provides a new way to fabricate dispersion-strengthened Cu alloys with high strength,high elongation,excellent thermal stability and softening resistance,which have potential application value in the field of the future fusion reactor.     

Alloying behaviour in nanocrystalline materials during mechanical alloying

The alloying behaviour in a number of systems such as Cu-Ni, Cu-Zn, Cu-Al, Ni-Al, Nb-Al has been studied to understand the mechanism as well as the kinetics of alloying during mechanical alloying (MA). The results show that nanocrystallization is a prerequisite for alloying in all the systems during MA. The mechanism of alloying appears to be a strong function of the enthalpy of formation of the phase and the energy of ordering in case of intermetallic compounds. Solid solutions (Cu-Ni), intermetallic compounds with low ordering energies (such as Ni₃Al which forms in a disordered state during MA) and compounds with low enthalpy of formation (Cu-Zn, Al₃Nb) form by continuous diffusive mixing. Compounds with high enthalpy of formation and high ordering energies form by a new mechanism christened as discontinuous additive mixing. When the intermetallic gets disordered, its formation mechanism changes from discontinuous additive mixing to continuous diffusive one. A rigorous mathematical model, based on iso-concentration contour migration method, has been developed to predict the kinetics of diffusive intermixing in binary systems during MA. Based on the results of Cu-Ni, Cu-Zn and Cu-Al systems, an effective temperature (T _eff) has been proposed that can simulate the observed alloying kinetics. TheT _eff for the systems studied is found to lie between 0·42–0·52T ₁.     

Nanostructural materials formation by mechanical alloying: Morphologic analysis based on transmission and scanning electron microscopic observations

The development of nanostructured materials offers new scientific and technological perspectives due to the specific interesting physical properties of these materials. These properties derive either from their reduced grain size or from the structure and properties of the grain boundaries, which constitute a significant volume fraction. Mechanical alloying, widely used to produce dispersion-strengthened and amorphous alloys, has been employed in recent years to synthesize nanocrystalline metallic, semiconductors, and covalent component-based materials. Based on statistical analysis of transmission and scanning electron microscopic images, the distribution and spatial repartition of the nanostructural material prepared by mechanical alloying and/or attrition are presented for some specific cases.     

基于外场辅助的机械合金化研究

机械合金化（MA）技术作为一种制备纳米材料的有效方法已获得广泛的应用。把机械合金化过程中的磨球机械能与其它物理能有机地结合起来,能够增强对粉末的作用,有效提高机械合金化效率。本文简单回顾了机械合金化的发展,对外加物理场辅助作用下的几种高能球磨工艺进行了详细分析。采用物理能辅助机械球磨,从而使粉末得到复合作用或活性激活,是机械合金化效率提高的原因。     

Fabrication of Mg2Si thermoelectric materials by mechanical alloying and spark-plasma sintering process

A mixture of pure Mg and Si powders with an atomic ratio 2:1 has been subjected to mechanical alloying (MA) at room temperature to prepare the Mg2Si thermoelectric material. Mg2Si intermetallic compound with a grain size of 50 nm can be obtained by MA of Mg66.7Si33.3 powders for 60 hours and subsequently annealed at 620 degrees C. Consolidation of the MA powders was performed in a spark plasma sintering (SPS) machine using graphite dies up to 800-900 degrees C under 50 MPa. The shrinkage of consolidated samples during SPS was significant at about 250 degrees and 620 degrees C. X-ray diffraction data shows that the SPS compact from 60 h MA powders consolidated up to 800 degrees C consists of only nanocrystalline Mg2Si compound with a grain size of 100 nm.     

机械合金化制备Cu-Cr-C复合粉体

以Cu、Cr、C粉末为原料,采用机械合金化方法制备CuCr-C复合粉体,其中Cr、C的添加量按照Cr_3C_2质量分数为5%来计算;利用X射线衍射(XRD)和扫描电镜(SEM)研究机械合金化过程中粉末的物相和微观形貌,并结合能谱仪(EDS)面扫描得到粉末的元素微观分布。结果表明:随着机械合金化的进行,C、Cr和Cu形成Cu-Cr-C过饱和固溶体,随着球磨时间的延长,粉末粒径细化,颗粒形态由片状向球状发展;球磨30 h后,生成Cr_3C_2增强相,粉末细化趋势变缓并逐渐产生团聚,故原位生成Cr_3C_2的最佳球磨时间为30 h。     

Synthesis of Mg-Ti alloy by mechanical alloying

Mg-based Mg-Ti binary alloys have been synthesized by mechanical alloying of Mg and Ti powder blends. It was found that mechanical alloying of Mg and Ti results in a nanocrystalline Mg-Ti alloy and an extended solubility of Ti in Mg, due to the favorable size factor and the isomorphous structure of Mg and Ti. In the case of Mg-20at.%Ti, about 12.5% Ti is dissolved in the Mg lattice when the mechanical alloying process reaches a stable state. The rest (about 7.5 at.%) remains as fine particles in the size of 50–150 nm in diameter. Dissolution of 12.5 at.% Ti in the Mg lattice causes a contraction of the unit cell volume from 0.0464 to 0.0442 nm³ and a decrease of the c/a ratio from 1.624 to 1.612 of the hexagonal structure. The supersaturated solid solution Mg-Ti alloy decomposes upon thermal annealing at temperatures above 200°C. Hydrogenation enhances the decomposition process at lower temperatures.     

Metastable phases formation induced by mechanical alloying

Elemental aluminium, titanium and iron powders with compositions of Al₉₀Ti₁₀, Al₅₅Ti₄₅, Al₆₅Ti₂₅Fe₁₀, respectively, were mechanically alloyed in a planetary ball mill. The sequence of phase formation was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). Various metastable phases were experimentally observed: supersaturated solid solution Al(Ti) for Al₉₀Ti₁₀, amorphous phase and L1₂-Al₃Ti compound for Al₅₅Ti₄₅, amorphous phase and supersaturated solid solution Al(Ti,Fe) for Al₆₅Ti₂₅Fe₁₀, and an fcc crystalline phase was inevitably found in those alloys. The formation of the fcc crystalline phase has been critically assessed. The results suggest that the fcc crystalline phase seems to be metastable and it cannot be solely attributed to the contamination from the milling atmosphere underthe present experimental conditions.     

采用机械合金化方法制备Ti3Ni2合金的结构及电化学储氢性能

采用机械合金化方法用Ni粉和Ti粉得到了Ti3Ni2非晶合金。晶态Ti3Ni2合金初始容量比非晶合金要高。晶态合金初始容量可以达到240mAh／g。而非晶合金容量为173mAh／g。随着循环次数的增加，晶态合金放电容量呈线性下降趋势。而对于非晶电极材料来说，随着循环的进行，初始容量下降，但是达到一定循环次数以后，电极的放电容量达到基本稳定。     

An attempt to extract common behaviour from heterogeneous results about mechanical alloying

The first stages of the mechanical alloying process, fragmentation and welding, are analysed. In order to clarify the mechanisms of milling-induced diffusion, different interstitial solid solutions are examined. It is shown that diffusion of interstitial is mainly controlled by the stress field. In case of a substitutional solid solution, the mechanism of atomic transport is controlled by shear deformation. The consequence of the corresponding diffusion models are discussed.     

Nitrogen addition to iron powder by mechanical alloying

Nitrogen was alloyed into iron (a) by mechanical processing in a nitrogen gas environment, and (b) by mechanically alloying with iron-nitride powders to characterize resulting nano-structure and nitrogen distribution. Although the infused nitrogen concentration was significantly greater than the thermodynamic equilibrium solubility of iron, no nitrides formed, even for nitrogen concentrations as high as 4.1 wt.% However, a bctFe phase did form. Lattice expansion calculations indicate that the sum of the interstitial bcc-Fe and bctFe nitrogen concentrations was significantly less than the total measured nitrogen concentration. A considerable portion of the mechanically infused nitrogen was determined to be associated with nanograin boundaries.     

Formation mechanism of titanium silicide by mechanical alloying

The syntheses of five titanium silicides (Ti₃Si, TiSi₂, Ti₅Si₄, Ti₅Si₃, and TiSi) by mechanical alloying (MA) have been investigated. Rapid, self-propagating high temperature synthesis (SHS) reactions were involved in producing the last three materials during room temperature high-energy ball-milling of elemental powders. Such reactions appeared to occur through ignition by mechanical impact in the fine powder mixture formed after a critical milling period. From in-situ thermal analyses, each critical milling period for the formation of Ti₅Si₄, Ti₅Si₃, and TiSi was observed to be 22, 35.5 and 53.5 minutes, respectively. However, the formation of Ti₃Si and TiSi₂ did not occur even after 360 minutes of milling of as-received Ti and Si powder mixture, due to the lack of homogeneity of the powder mixture. Other ball-milling procedures were employed for the syntheses of Ti₃Si and TiSi₂ using different sizes of Si powder and milling medium materials. Ti₃Si was synthesized by milling a Ti and 60 minutes premilled Si powder mixture for 240 minutes. -TiSi₂ and TiSi₂ were produced by high energy partially stabilized zirconia (PSZ) ball-milling for 360 minutes in a steel vial followed by jar-milling of a Ti and 60 min premilled Si powder mixture for 48 hr. The formation of Ti₃Si and TiSi₂ occurs through a slow solid state diffusion reaction, and the product(s) and reactants coexist for a certain period of time. The formation of titanium silicides by MA and the reaction rate appeared to depend on the homogeneity of the powder mixture, milling medium materials, and heat of formation of the product involved.