Hydrogen sorption in the LaNi5-xAlx-H system (0 ≤ x ≤ 1)

A thermodynamic assessment of hydrogen sorption in LaNi_5-xAl_x-H (0 ≤ x ≤ 1) alloys was performed combining experimental and theoretical investigations. The occurrence of sloped plateaux in Pressure Composition Isotherms (PCI) is discussed on the basis of compositional inhomogeneities and of possible re-distribution of metallic elements between the hydrogen rich and the hydrogen poor phases.In order to provide input for the thermodynamic assessment, literature data were reviewed and PCI experiments were performed at increasing temperatures on alloys with different Al contents. The presence of a compositional distribution was investigated using Rietveld refinement of X-Ray diffraction patterns, volumetric and calorimetric measurements, both on as received and annealed samples with LaNi_4.8Al_0.2 nominal composition. Moreover, ab initio energies of formation for LaNi₅, LaAl₅ and the respective hydrides were calculated. A full CALPHAD assessment of the Gibbs energy for LaNi_5-xAl_xH_y (0 ≤ x ≤ 1, 0 ≤ y ≤ 7) phase was obtained, showing a good agreement between experimental and calculated results.The presence of sloped and flat plateaux has been related to the occurrence of ortho- and para-equilibrium conditions.     

Concepts derived from phase diagram studies for the strengthening of Fe–Al-based alloys

Fe–Al-based alloys, i.e. alloys which contain either disordered A2 -(Fe,Al), B2-ordered FeAl or D0₃-ordered Fe₃Al as majority phase, have a considerable potential for developing materials for structural applications, but insufficient strength and creep resistance have been identified as obstacles for the use of Fe–Al-based alloys at high temperatures. At the ‘Discussion Meeting on the Development of Innovative Iron Aluminium Alloys’ held at the Max-Planck-Institut für Eisenforschung GmbH, Düsseldorf on March, 9th 2004 a couple of presentations were made with emphasis on improving these properties at high temperatures. In the current article those strengthening mechanisms which are provided by the phase diagram—solid-solution hardening, strengthening by precipitates, or ordering—are reviewed. Besides results obtained for the binary Fe–Al system special emphasis is put forward to those ternary systems for which results have been presented at the ‘Discussion Meeting’.     

In situ electrochemical nanoindentation: A technique for local examination of hydrogen embrittlement

Nanoindentation combined with AFM (NI-AFM) has been used to study the effect of electrochemically in situ charged hydrogen on the deformation of small volumes of nickel and copper single crystals. Hydrogen reduces the unstable elastic plastic transition load (pop-in) in nickel, but does not have any effect on copper. It has been shown that the activation energy for the onset of plasticity (dislocation nucleation) is reduced by dissolved hydrogen. This is because hydrogen reduces shear modulus and stacking fault energy in nickel, whereby the former results in hydrogen-enhanced decohesion (HEDE) and the latter in the hydrogen-enhanced plasticity (HELP) mechanism.     

Review of Hydrogen Embrittlement in Metals: Hydrogen Diffusion,Hydrogen Characterization,Hydrogen Embrittlement Mechanism and Prevention

Hydrogen dissolved in metals as a result of internal and external hydrogen can affect the mechanical properties of the metals, principally through the interactions between hydrogen and material defects. Multiple phenomena such as hydrogen dissolution, hydrogen diffusion, hydrogen redistribution and hydrogen interactions with vacancies, dislocations, grain boundaries and other phase interfaces are involved in this process. Consequently, several hydrogen embrittlement(HE) mechanisms have been successively proposed to explain the HE phenomena, with the hydrogen-enhanced decohesion mechanism, hydrogenenhanced localized plasticity mechanism and hydrogen-enhanced strain-induced vacancies being some of the most important. Additionally, to reduce the risk of HE for engineering structural materials in service, surface treatments and microstructural optimization of the alloys have been suggested. In this review, we report on the progress of the studies on HE in metals, with a particular focus on steels. It focuses on four aspects:(1) hydrogen diffusion behavior;(2) hydrogen characterization methods;(3) HE mechanisms; and(4) the prevention of HE. The strengths and weaknesses of the current HE mechanisms and HE prevention methods are discussed, and specific research directions for further investigation of fundamental HE mechanisms and methods for preventing HE failure are identified.     

添加Si对A_2B_7型电极合金结构及电化学贮氢性能的影响(英文)

为改善La–Mg–Ni系A2B7型合金的电化学贮氢性能,在合金中添加一定量的Si元素,通过真空熔炼及退火处理的方法制备La0.8Mg0.2Ni3.3Co0.2Six(x=0-0.2)电极合金。研究Si元素的添加对合金结构及电化学贮氢性能的影响。结果表明,铸态及退火态合金均为多相结构,分别为Ce2Ni7型的(La,Mg)2Ni7相和CaCu5型的LaNi5相以及少量的残余相LaNi3。Si元素的添加没有改变合金的主相,但使得合金中的(La,Mg)2Ni7相减少而LaNi5相增加。添加Si显著地影响了合金的电化学性能。随着Si含量的增加,铸态及退火态合金的放电容量逐步降低,但循环稳定性却随着Si含量的增加而增强。此外,合金电极的高倍率放电性能、极限电流密度、氢扩散系数以及电化学交流阻抗谱的测试均表明合金的电化学动力学性能随着Si含量的增加先增加而后减小。     

Electrochemical Hydrogen Storage Characteristics of the as-Cast and Annealed La0.8-xPrxMg0.2Ni3.35Al0.1Si0.05 (x=0~0.4) Electrode Alloys

采用铸造及退火工艺制备了La0.8-xPrxMg0.2Ni3.35Al0.1Si0.05 (x=0, 0.1, 0.2, 0.3, 0.4)电极合金。系统研究了Pr的替代对合金的结构与电化学储氢性质的影响,结果表明除少量残余LaNi3相外,铸造及退火合金是由六方Ce2Ni7型(La, Mg)2Ni7相与六方CaCu5型LaNi5相构成的。Pr对La的置换对合金的电化学储氢性质产生明显影响,铸造及退火合金的放电容量和高倍率放电能力随Pr含量的增加先升后降。当Pr含量由0增加至0.4时,铸造及退火合金的100次充放电循环后容积保持率S100从64.96%和72.82%分别增加至77.94%和91.81%     

Effect of Pr on the Structure and Magnetic Properties of CoPt Alloys

对PrxCo50-xPt50(x=0,0.2,0.4,0.6,0.8,1.0)合金的显微结构和磁性能进行了研究。X射线衍射结果表明:经1000℃均匀化后合金为单一的面心立方相(fcc),而经675~750℃退火后合金由硬磁相(面心四方结构,fct)和软磁相组成。随着Pr含量增加,fct的(111)衍射峰向高角度偏移,并且c/a增大导致有序度S减小。合金Pr0.4Co49.6Pt50经675°C退火60min后,矫顽力和剩磁比达到最大值。PrxCo50-xPt50(x=0,0.2,0.4,0.6,0.8,1.0)经675°C退火60min的样品,矫顽力随着Pr含量的增加而单调减小,但剩磁比先增大后减小。     

退火处理对La1．5Mg0．5Ni7．0贮氢合金相结构和电化学性能的影响

研究了退火温度对A287型La1.5Mg0.5Ni7.0合金的相结构和电化学性能的影响。结果表明：铸态合金由LaNi，相、LaMgNi4相、（La，Mg）Ni3相以及Gd2Co7型相组成，退火处理后，合金由Gd2Co7型相、Ce2Ni7型相和PuNi3型（La，Mg）Ni3相组成：随着退火温度升高，PuNi3型相的丰度减小，ce2Ni7型相的丰度增加，（La，Mg）Ni3相的a轴参数、c轴参数和晶胞体积均增大；经1073K保温24h退火后，合金电极具有最高的放电容量（391．2mAh／g），退火温度升高，合金的最大放电容量略有降低：合金电极的循环稳定性随着退火温度的升高不断提高，在1173K时合金电极经150次循环后其电极容量保持率C150／Cmax=82％；合金的高倍率放电性能（HRD）随退火温度升高略有增加，在1173K时，合金电极的HRD最好（HRD900=89．0％）；交换电流密度I0、极限电流密度I1及氢扩散系数D随着退火温度的升高而增大。     

Wear behaviour of an FeAl intermetallic alloy containing carbon and titanium

FeAl based alloys with carbon and titanium additions were prepared using arc induction melting and their effect on wear behaviour was investigated using ball-on-disk technique. The experimental results showed that carbon addition to FeAl alloys results in formation of perovskite-type Fe₃AlC_0.5 carbide phase and graphite. Addition of Ti promotes the formation of TiC and Fe₃AlC_0.5 and prevents the formation of graphite in the alloy. Hardness and wear resistance of FeAl based alloys increase with increase in the volume fraction of carbides. The FeAl alloys containing Ti exhibited low wear rate and coefficient of friction. Examination of wear tracks revealed micro ploughing at a lower load of 5N. Thin surface flakes with traces of their detachment were observed at a higher load of 10N. It was also observed that presence of graphite in localized regions reduce the wear resistance of the alloy. The results are correlated with observed microstructure and hardness.     

Synthesis and grain growth kinetics of in-situ FeAl matrix nanocomposites （ Ⅱ ）： Structural evolution and grain growth kinetics of mechanically alloyed Fe-Al-Ti-B composite powder during heat treatment

Morphological changes, structural evolutions and grain growth kinetics of mechanically alloyed（MAed） Fe50Al50, Fe42.5Al42.5Ti5B10 and Fe35Al35Ti10B20 （mole fraction, %） powders were investigated by XRD and SEM, when being isothermally annealed at 1 073-1 373 K. The effect of different Ti and B addition on the grain growth of FeAI phase was also discussed. The results show that the nanocrystalline FeAI and in-situ TiB2/FeAl nanocomposite powders can be synthesized by subsequent heat treatment. Besides the relaxation of crystal defects and lattice stress, the transformation from Fe-based solid solution into B2-FeAl and TiB2 occurs upon heating of the MA-processed alloys. Although the grain growth takes place, the grain sizes of both FeAl and TiB2 are still in nanometer scale. The activation energies for the nanocrystalline FeAl growth in the three alloys are calculated to be 534.9, 525.6 and 1 069.6 kJ/mol respectively, according to kinetics theory of nanocrystalline growth. Alloys with different TiB2 contents exhibit unequal thermal stability. The presence of higher content TiB2 plays significant role in the impediment of grain growth.     

Wet oxidation of stainless steels: New insights into hydrogen ingress

It is well established that hydrogen derived from water vapor can penetrate oxidizing alloys with detrimental effect. However, the complexities of tracking hydrogen in these materials have prevented the direct profiling of hydrogen ingress needed to understand these phenomena. Here we report hydrogen profiles in industrially-relevant alumina- and chromia-forming steels correlated with the local oxide-metal nano/microstructure by use of SIMS D₂O tracer studies and experimental protocols to optimize D retention. The D profiles unexpectedly varied markedly among the alloys examined, which indicates mechanistic complexity but also the potential to mitigate detrimental water vapor effects by manipulation of alloy chemistry.     

Measurement of the corrosion rate of magnesium alloys using Tafel extrapolation

The hypothesis that the corrosion of Mg alloys can be adequately estimated using Tafel extrapolation of the polarisation curve is termed herein the electrochemical measurement hypothesis for Mg. In principle, such a hypothesis can be disproved by a single valid counter example. The critical review of Mg corrosion by Song and Atrens in 2003 indicated that, for Mg alloys, Tafel extrapolation had not estimated the corrosion rate reliably. This paper examines the recent literature to further examine the electrochemical measurement hypothesis for Mg. The literature shows that, for Mg alloys, corrosion rates evaluated by Tafel extrapolation from polarisation curves have not agreed with corrosion rates evaluated from weight loss and hydrogen evolution. Typical deviations have been ∼50-90%. These were much larger than the precision of the measurement methods and indicate a need for careful examination of the use of Tafel extrapolation for Mg. For research that nevertheless does intend to use Tafel extrapolation to elucidate corrosion of Mg associated with service, it is strongly recommended that these measurements be complemented by the use of at least two of the three other simple measurement methods: (i) weight loss rate, (ii) hydrogen evolution rate, and (iii) rate of Mg²⁺ leaving the metal surface. There is much better insight for little additional effort.     

Synthesis and grain growth kinetics of in-situ FeAl matrix nanocomposites (Ⅱ): Structural evolution and grain growth kinetics of mechanically alloyed Fe-Al-Ti-B composite powder during heat treatment

Morphological changes, structural evolutions and grain growth kinetics of mechanically alloyed(MAed) Fe50Al50, Fe42.5Al42.5Ti5B10 and Fe35Al35Ti10B20 (mole fraction, %) powders were investigated by XRD and SEM, when being isothermally annealed at 1 073-1 373 K. The effect of different Ti and B addition on the grain growth of FeAl phase was also discussed. The results show that the nanocrystalline FeAl and in-situ TiB2/FeAl nanocomposite powders can be synthesized by subsequent heat treatment. Besides the relaxation of crystal defects and lattice stress, the transformation from Fe-based solid solution into B2-FeAl and TiB2 occurs upon heating of the MA-processed alloys. Although the grain growth takes place, the grain sizes of both FeAl and TiB2 are still in nanometer scale. The activation energies for the nanocrystalline FeAl growth in the three alloys are calculated to be 534.9, 525.6 and 1 069.6 kJ/mol respectively, according to kinetics theory of nanocrystalline growth. Alloys with different TiB2 contents exhibit unequal thermal stability. The presence of higher content TiB2 plays significant role in the impediment of grain growth.     

Deformation behavior of an ordered B2 phase in Ti–25Al–25Zr alloy

The mechanical behavior of the B2 phase in alloy Ti–25Al–25Zr has been studied under compression. True stress–strain curve exhibits similar behavior to those of typical B2 intermetallics such as NiAl and FeAl. The alloy exhibits highest yield strength in comparison to those reported in other titanium based B2 alloys with around 2% plastic strain. The microstructural characterization of specimen after compression reveals that the B2 phase transforms to an orthorhombic martensitic phase during compression.     

Metastable Ti---Ni---Fe---Si alloys prepared by mechanical alloying

The phase formation and physical properties of mechanically alloyed Ti₅₆Ni₁₈Fe₁₀Si₁₆ have been investigated. The as-milled samples are amorphous and undergo a transition to the icosahedral quasi-crystalline phase on annealing at about 1025 K. Mechanical alloying in the presence of an additive of 1% quasi-crystalline phase yielded the same phase directly. Alloys have been studied by X-ray diffraction. Mössbauer spectroscopy and magnetic susceptibility methods. These results may be compared with those in the literature for amorphous and quasi-crystalline alloys of similar composition prepared by rapid solidification from the melt. In all cases the alloys produced by mechanical alloying show a greater concentration of open volume defects and in the icosahedral phase, a greater degree of disorder and largerphason strains. Hydrogen diffusion studies of these alloys have shown that the mean interatomic distance increases for short hydrogenation times, but that for longer hydrogenation times the hydrogen increases local atomic order which results in a reduction of mean interatomic distances.     

Electrodeposition of Zn–Ni coatings as Cd replacement for corrosion protection of high strength steel

Electrodeposition of Zn–Ni coatings performed in acidic baths are not suitable for high strength steels due to their high susceptibility to hydrogen embrittlement.In this work, Zn–Ni coatings were deposited on a high strength steel (4340) upon stirring conditions from an alkaline bath. A complete characterisation of the coatings (corrosion, morphology and composition) has been accomplished, correlating the electrodeposition conditions with these features. The best protective properties of the grown coatings were achieved for the alloys with a single phase structure of γ-Ni₅Zn₂₁ and a denser morphology. Additionally, the hydrogen content incorporated is lower than even cadmium-coated 4340 steel which has undergone a postbaking dehydrogenation treatment.     

Effect of annealing treatment on electrochemical properties of Mm-based hydrogen storage alloys for Ni/MH batteries

Several multi-component Mm-based hydrogen storage alloys with cobalt content from 0.8–1.0 have been prepared. The hydrogen absorption–desorption characteristics in gas–solid reactions and the electrochemical properties as MH electrodes have been investigated. The addition of small amounts of Al effectively lowers the hydrogen equilibrium pressure and improves the cycling stability of the alloys. Electrochemical measurements show that the MmNi_3.4Co_1.0Mn_0.5Al_0.1 alloy exhibits a maximum electrochemical capacity of 322 mA h g⁻¹ with a capacity decay of about 19.5% after 100 cycles. Annealing treatments flatten the plateau region and lower the hydrogen equilibrium pressure, which results in an increase of hydrogen uptake below 1 atm. The increasing trend of hydrogen storage capacity from the as-cast sample to the annealed sample in the gas–solid reactions is in good agreement with the electrochemical results. The electrochemical discharge capacity of the MmNi_3.4Co_1.0Mn_0.5Al_0.1 alloy increases to 334 mA h g⁻¹ and 340 mA h g⁻¹ after annealing for 3 h and 28 h, respectively, from 322 mA h g⁻¹ in the as-cast condition. The electrochemical cycling stability of the annealed samples was also greatly improved. The capacity decay for both annealed samples is about 8.3% and 6.8%, respectively, after 100 charge–discharge cycles. It was suggested that annealing treatments enhance the compositional homogeneity and cause the secondary phase (separated phase) to dissolve in main phase, which result in the improvement of electrochemical cycling stability of the alloy electrodes.     

Temporal evolution of non-equilibrium γ' precipitates in a rapidly quenched nickel base superalloy

The temporal evolution of non-equilibrium γ' precipitates in a rapidly quenched and isothermally annealed commercial nickel base superalloy has been investigated by coupling transmission electron microscopy and atom probe tomography. When subjected to rapid quenching from above the γ' solvus temperature, the supersaturated single phase γ matrix appears to undergo compositional phase separation possibly via spinodal decomposition to form solute-rich and solute-depleted regions. The regions that have a depletion in Cr and Co undergo an ordering process resulting in the γ' domains which exhibit a far–from equilibrium composition. Upon isothermal annealing, the γ/γ' interface sharpens and the compositions of both γ and γ' phases approach equilibrium. The influence of a non-classical mechanism of γ' precipitation on the size distribution of precipitates as well as the precipitate and matrix compositions, and its subsequent evolution during isothermal annealing has been discussed.     

Nanoscale characterization of FeAl-HVOF coatings

FeAl intermetallic is one of the highly attractive aluminides due to its high specific properties and good oxidation resistance. The present work investigates the production of iron aluminide coatings obtained by means of the High Velocity Oxygen Fuel process. Two well differentiated structures have been achieved by the modification of particle size distribution, reaching lower oxidation at splat boundaries with particle sizes in the range of 40-60 μm. Such features have been studied by using Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) as well. TEM has also been employed to illustrate the grain size and to reveal the degree of the compound ordering in the as-sprayed form; despite rapid solidification being a way to induce disordering in intermetallics, since the feedstock is a Fe-40 at.% Al milled powder, it becomes slightly ordered at the high temperatures during in-flight time. Moreover, a further attempt to study how the order affects to the coating structure, the initial powder was annealed in order to induce such ordering before spraying. While the ordering observed in the as-sprayed milled powder came from the thermal history of particles during the spraying process, the ordering of the as-annealed powder is much more evidenced by the visible superlattice spots.     

Effect Of Si Content On Ordering Degree And Electromagnetic Characteristics In Fesial Alloys

FeSiAl alloys ribbons synthesized by melt-quench were annealed in vacuum at 873 K for 60 rain. The flaky powders were prepared by milling the annealed ribbons for 70 h. After milling, the powders were heat treated at 573 K for 90 rain. The ordering degree of the powders lattice structure was analyzed by X-ray diffraction （XRD）. The measurement of specific saturation magnetization was carried out by vibrating samples magnetometer （VSM）. Complex permittivity and complex permeability in the frequency band of 0.5-18 GHz were measured with the vector network analyzer. The ordering degree of the superlattice structure increased from 0.2＇7 to 0.49. Complex permittivity and complex permeability decreased with increasing Si content. After ordering, the specific saturation magnetization decreased from 134.2 to 85.0 A.m2.kg-1. For use in anti-EMI material, the total contents of Si and Al in FeSiAl alloys should be controlled at a low level.