An Overview of Hydrogen Interaction with Amorphous Alloys

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Theories, experimental results and applications associated with hydrogen behavior in amorphous metals and alloys are reviewed. An emphasis is made on the potential use of these advanced materials for hydrogen storage technology. Therefore, several
  Advanced Performance Materials 6, 5–31 (1999)c  1999 Kluwer Academic Publishers. Manufactured in The Netherlands. An Overview of Hydrogen Interactionwith Amorphous Alloys N. ELIAZ AND D. ELIEZER  Department of Materials Engineering, Ben-Gurion University of the Negev, Beer-Sheva 84105, Israel Abstract.  Theories, experimental results and applications associated with hydrogen behavior in amorphousmetalsandalloysarereviewed.Anemphasisismadeonthepotentialuseoftheseadvancedmaterialsforhydrogenstorage technology. Therefore, several properties that are especially relevant for such applications are assessed.These include structural models for hydrogen occupancy, sorption characteristics, solubility, diffusion behaviorand thermal stabilities. Hydrogen effects on the mechanical properties and fracture modes of glassy metals arealso presented, and possible mechanisms of hydrogen embrittlement are discussed. Similarities and differencesbetween hydrogen behavior in amorphous and crystalline metals and alloys are discussed in detail. Keywords:  amorphous, hydrogen diffusivity, hydrogen embrittlement, hydrogen storage, metallic glasses 1. Introduction Amorphous metals and alloys can be produced [1–5] by various techniques, such as: rapidquenching of a melt, thermal evaporation, sputtering, electrodeposition and ion implanta-tion. In all these methods, samples are usually obtained in the form of thin film or ribbon.Amorphousalloyscanalsobefabricatedbymechanicalalloying,orinsomecasesbysimplyhydrogenating the crystalline alloys, to obtain powder samples.The interaction of hydrogen with amorphous metals has been studied extensively duringthe last two decades. These studies were motivated by both scientific and technologicalinterests, mainly the potential use of amorphous hydrides in hydrogen storage technology.Many papers dealing with specific systems [6–10] as well as some reviews [11–17] havealready been published on this subject.The present paper is aimed to summarize our recent progress and to review our currentunderstanding of hydrogen interaction with amorphous metals and alloys. Various aspectsof this subject, including structural models for hydrogen occupancy, absorption/desorptioncharacteristics,hydrogendiffusivityandsolubility,hydrogeneffectsonthethermalstability,andpossiblemechanismsofhydrogenembrittlementinamorphousmaterials, arereviewed.Hydrogen-induced amorphization, however, will be reviewed in detail elsewhere [18]. Inaddition to analytical discussions, an attempt is made to identify topics that warrant furtherresearchandtodescribepotentialapplicationsofhydrogeninamorphousmetalsandalloys. 2. Structural models for hydrogen occupancy Various structural models [19–29] have been suggested in order to explain hydrogen ab- sorption and diffusion in amorphous alloys. Kirchheim et al. [19–22] propose that since  6  ELIAZ AND ELIEZER Figure 1 . Potential trace for hydrogen in an amorphous metal, where dissolution from a reference state requiresthe free enthalpy  G . The distribution  n ( G ) of the free enthalpy is shown below. The occupation  o ( G ) of theequilibrium sites  e  is governed by Fermi-Dirac statistics [21]. amorphous alloys lack long-range order (LRO), hydrogen atoms might occupy a wide va-riety of interstitial sites, resulting from a distribution of both chemical and geometricalconfigurations in the amorphous structure. Hence, a broad continuous distribution of inter-stitialsiteenergiesisusedtoexplaintheconcentrationdependenceofthechemicalpotentialof hydrogen and its diffusivity in amorphous metals. Due to theoretical considerations [30],the shape of this energy distribution can be related to the shape of the first peak of theradial distribution function (RDF), which is usually assumed to be of a Gaussian form foramorphous metals. Thus, assuming that sites for hydrogen atoms in amorphous alloys havedifferent potential energies (see figure 1), the density of sites ( n ) is expressed [19–21] bythe Gaussian function: n ( G ) =  dN dG =  1 σ  √  π exp  −  G − G o σ   2   (1)where  dN   is the number of sites available for hydrogen at energy  G  within the interval dG ,  G o is the mean energy related to a standard state, and  σ   is the width of the Gaussianfunction. Applying Fermi-Dirac statistics, the occupancy is obtained: o ( G ) =  n ( G ) 1 + exp  G − µ  RT    (2)  AN OVERVIEW OF HYDROGEN INTERACTION  7where  µ  is the chemical potential of hydrogen. The concentration of hydrogen atoms isthen obtained by integration of Eq. (2): c =    ∞−∞ o ( G ) dG  (3)where the Fermi-Dirac distribution may be approximated by a step function, yielding:2 c = 1 ± erf  G o − µσ  + for  G o < µ  and  − for  G o > µ  (4)Harris and co-workers [23, 24] suggest a model for the general shape of the site-energydistribution and hydrogen absorption capacities of binary ETM-LTM amorphous alloys.This model is based on accurate measurements of the hydrogen chemical potential in amor-phousZr-Nialloysbytheelectrochemicalmethod. Anassumptionismadethatthestructureoftheseamorphousalloysiscomposedofpacked, distortedtetrahedra, formedbyarandomdistribution of the two kinds of atoms. Calculations are made of the probability of findinga particular type of tetrahedra and of the energy of a hydrogen atom in a tetrahedron. Abell-shaped function is chosen to describe the distribution of site energies about these meanvalues, due to the range of distortions about ideal tetrahedral structures. The maximumhydrogen content in amorphous Zr-Ni alloys, as predicted by this model, agrees excellentlywith the experimental data. Harris and Curtin thus proceed to apply this model to othersystems, including Zr-Ni alloys prepared by mechanical alloying [25] and ternary Zr-Ni-Band Zr-Nb-Ni amorphous alloys [26]. A comprehensive review of their works is given inreference [27].Results of neutron diffraction and scattering experiments [15, 31, 32], however, suggesttheoccupancyofothertypesofsitesthanthat,onwhichthemodelofHarrisetal. iscriticallybased. This apparent contradiction may result from the fact that while the electrochemicalmeasurementprobesthesiteoccupancyinaparticularrangeofchemicalpotentials,neutrondiffraction and scattering experiments give information on the average configuration of alloccupied sites [33].Rushetal. [34]werethefirstonestousehydrogenvibrationsasaprobeforinvestigatingthe local topology in the amorphous structure. In their work, they compare the vibrationalspectraofglassyandcrystallineTiCuH(figure2). Largedifferencesbetweenthedensityof statesdistributionsforthetwosamplesareevident. Intheamorphousalloy, thepeakoccursat the same frequency as in the crystalline alloy, but with a strongly increased width. Thus,the hydrogen sites occupied in the amorphous and crystalline structures may be regarded,on the average, to be very similar (short-range order). The large width indicates that thetetrahedron may be heavily distorted and fluctuates in its chemical composition. The wingtowards frequencies below 100 meV may also show that octahedral site occupation occursaswell. Theobservationofthebroaderdensityofstatesdisagreeswiththemicrocrystallineor microcluster model for amorphous metals, in which the local environment would notdiffer from that of the crystal [35]. Later experiments on other glasses (e.g., ZrNiH [36–38]andZr 2 Pd[39])qualitativelyallrevealedthesameresult—thelocalhydrogenspectraintheamorphous substance center around similar frequencies to those in the crystalline state, butthe frequency distributions are generally much broader and washed out. Thus, on the basis  8  ELIAZ AND ELIEZER Figure 2 . Neutron spectra measured at 78 K for crystalline TiCuH 0 . 93  and amorphous TiCuH 1 . 3 . The energyresolution (FWHM) near the peak is indicated by the horizontal bar [34]. of these measurements, we may conclude that the hydrogen atoms remain in amorphousmetals essentially at the same polyhedral sites as in the crystalline counterparts; changes intopology are therefore rather restricted [35].The idea that hydrogen might occupy in amorphous alloys interstitial sites, which aresimilar to the octahedral and tetrahedral interstitial sites in crystalline alloys, is suggestedon the basis of other experiments as well (see, for example, [20, 40]). This idea is basedin these works on measurements of similar values of hydrogen solubility [40], frequencyfactor (  D 0 ) and activation energy for diffusion ( Q ) [20] in amorphous and in fcc structures. 3. Absorption-desorption characteristics Amorphous alloys are thermodynamically metastable, and decompose into multiple crys-tallinephaseswhenheatedtothecrystallizationtemperature, T  cryst ∼ 300–500 ◦ C,depending  AN OVERVIEW OF HYDROGEN INTERACTION  9on the chemical composition of the alloy. Thus, in charging amorphous alloys with hydro-gen, either electrochemically or from the gaseous phase, care must be taken not to raise thetemperature above  T  cryst . We cannot activate the surface by heating in vacuum or in H 2  gas,as is usually done in the case of crystalline samples, but generally circumscribe ourselvesto abrading the surface with emery paper and cleaning ultrasonically in acetone and ether.Coating with a Pd overlayer after cleaning by argon-ion sputtering has also been known tobe effective [33].In crystalline alloys, if a phase transformation occurs in the metal-hydrogen system, thechemical potential of hydrogen (or its pressure) remains constant within the two-phaseregion, although the total hydrogen concentration increases. However, one of the mostcharacteristic changes in going from crystalline to amorphous alloys is the disappearanceof the plateau in the pressure-concentration isotherms [12, 15, 22, 33], as evident fromfigure 3 for a Zr 50 Ni 50  alloy [41]. Spit et al. [42, 43] as well report the absence of plateau inthe  p - c - T   curvesof Ni(Zr,Ti)amorphousalloys. Theyrelateiteithertoaratherlowcritical Figure 3 . Pressure-composition isotherms for hydrogen in amorphous and crystalline Zr 50 Ni 50  [41].
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