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7 pp. Published in Journal of Alloys and Compounds Copyright © 2007 Elsevier B.V., Nanocrystalline Mg powders of different particle size were obtained by inert gas evaporation and studied during electrochemical and gas-phase hydrogen cycling processes. The samples were compared to dehydrided samples obtained by mechanical milling of MgH2 with and without 2 mol% Nb2O5 as catalyst. The hydrogen overpotential of the pure Mg, which is a measure of the hydrogen evolution at the electrode surface, was observed to be reduced with smaller particle sizes reaching values comparable to samples with Nb2O5 additive. On the other hand gas-phase charging experiments showed the capacity loss with smaller particle sizes due to oxidation effects. These oxidation effects are different depending on the synthesis method used and showed a major influence on the hydrogen sorption kinetics., Authors thank the financial support (project nos. HPRN-CT-2002-00208 and MRTN-CT-2006-035366) from the European Commission

Previous comparative studies on the impact of cryogenic mechanical milling (CMM) of polymers showed significantly changed thermal and mechanical properties and—in the case of some polymer blends (e.g., PMMA/PVDF) an improved miscibility. Those changes were mainly attributed to a reduction of the molecular weight average during the milling process. The crystallization process in macromolecular systems is strongly affected by any small change of the initial chain structure like the chain conformation, the presence of branches and of course a decrease of the molecular weight average. Therefore, the aim of this paper was a comparative study of isothermal crystallization of cryogenic mechanically milled and original (unmilled) polymers and polymer blends. The overall crystal growth rates were determined and the development of the spherulitic morphology was studied in detail by means of hot stage polarized light microscopy. Isothermal crystallization of bulk samples was also investigated using differential scanning calorimetry (DSC). The average spherulite growth rates of the milled homo-polymer as well as the blends were significantly higher than in unmilled samples; even when all experiments started from the molten state. Moreover, the increase in crystal growth was almost independent of the chosen crystallization temperature and thus the degree of undercooling.

Oxidation kinetics of bulk Zr-based metallic glasses seems to be controlled by oxygen diffusion through the scale towards the ZrO 2 /glass interface. Two modes of oxidation were observed: depending on the alloy either: (1) relative slow growth of homogeneous scales consisting of nodules or lamellae of tetragonal ZrO 2 embedding nanocrystals of the late transition metals or (2) formation of fast growing oxide cones exhibiting a lamellar structure of the tetragonal and the less dense monoclinic ZrO 2 thus allowing very fast oxygen diffusion along the interfaces between the lamellae. For both modes an influence of surface preparation on the oxidation reactions has to be mentioned. In addition, the oxidation behavior of these metallic glasses was compared with that of nanocrystalline or coarse crystalline alloys of similar composition. Whereas nanocrystallization improves the oxidation resistance of many Zr-based metallic glasses, coarse crystalline structures promote fast oxidation along grain boundaries, at least in Zr 2 Pd.

Cu–Zr-based bulk metallic glasses are of increasing interest due to their excellent properties. Cu-rich Cu 60 Zr 30 Ti 10 and Zr-rich Zr 69.5 Cu 12 Ni 11 Al 7.5 glasses (numbers indicate at.%) are examples which combine good glass forming ability with excellent mechanical properties making them a material of choice for a variety of applications. Any application, however, requires adequate thermal stability, i.e. resistance against crystallization and oxidation. The aim of this paper is a detailed investigation on the oxidation of Cu 60 Zr 30 Ti 10 metallic glasses. Whereas oxidation in Zr-rich Zr 69.5 Cu 12 Ni 11 Al 7.5 is assumed to be controlled by oxygen diffusion through a homogeneous scale towards the ZrO 2 /glass interface, in Cu-rich Cu 60 Zr 30 Ti 10 metallic glasses multilayered scales were found to develop with an assembly of Cu-oxide needles at the outer surface. Due to developing stresses and the formation of voids the outer oxide layer loses contact at the interface and starts to peel off during ongoing oxidation.

We report on the direct determination of the hydrogen self-diffusion constant D in the hydrogen-storage Zr 69.5 Cu 12 Ni 11 Al 7.5 alloy, prepared in both the icosahedral quasicrystalline and the bulk metallic glass phases, using the technique of NMR diffusion in a static fringe field of a superconducting magnet. The diffusion constant exhibits strong dependence on temperature and hydrogen concentration. D was found to obey classical Arrhenius thermally activated over-barrier hopping form, whereas the significant decrease of D with increasing hydrogen-to-metal concentration ratio H / M is a result of two effects – the increase of the activation energy for hydrogen jumps between interstitial sites due to lattice expansion upon hydrogenation and the decreasing number of available empty interstitials due to site blocking and creation of defects in the lattice during hydrogen loading. The actual alloy structure – the icosahedral, approximant or metallic glass – appears to be less important for the hydrogen diffusivity.

Nanocrystalline Mg powder without and with 2 mol% Nb 2 O 5 catalyst was studied in a 6 M KOH electrolyte as electrode material for electrochemical hydrogen charging processes. Since the hydrogen overpotential of Mg, which is a measure of the hydrogen evolution at the electrode surface, was observed to be reduced by the addition of Nb 2 O 5 , it is assumed that the catalyst influences the electrode reactions. Considering this assumption hydrogenation was studied at different current densities. The storage capacity as well as the kinetic of Mg/Nb 2 O 5 electrodes increased significantly up to 1 wt.% H 2 at a charging time of 30 min with decreasing current density. The storage capacity of nanocrystalline Mg powder showed only minor changes to lower hydrogen contents with decreasing current density.

Nanocrystalline Mg87Ni3Al3M7 (M = Ti, Mn, Mm; Mm = Ce, La-rich mischmetal) hydrides were synthesized by reactive mechanical milling (RMM) in hydrogen atmosphere. Different degrees of the alloys hydrogenation were achieved varying the time of milling at a hydrogen pressure of 5 atm. The influence of the alloying elements (Ti, Mn, Mm) on the amount of the hydrides formed during RMM as well as on the temperature and enthalpy of hydrogen desorption was determined. The amount of hydride formed during RMM was found to be almost the same in the three alloys (∼35 wt.%); for the Ti containing alloy it was slightly higher. The Ti containing alloy reveals the lowest temperature of hydrogen desorption (∼210 °C) and the hydride in the alloy with Mn decomposes at the highest temperature (∼240 °C) among the three alloys studied. The enthalpy of H-desorption is highest for the sample alloyed with Mm (Ce, La), 70–72 kJ/mol; the Ti and Mn containing magnesium alloys have similar enthalpies of hydride decomposition, 56–60 kJ/mol. The alloys studied reveal higher equilibrium pressures of hydrogen absorption compared to nanocrystalline magnesium, indicating some thermodynamic destabilization of the hydride as a result of the alloying. Hydrogen absorption kinetics were studied at isothermal conditions at different temperatures and an influence of the alloying element was observed. The hydrogen absorption in the Ti containing alloy is substantially faster than in the alloys with Mn and Mm.

We present a study of the temperature- and concentration-dependent hydrogen self-diffusion constant D in a Zr69.5Cu12Ni11Al7.5 metallic glass using the technique of nuclear magnetic resonance diffusion in a static fringe field of a superconducting magnet. In the investigated temperature interval between room temperature and 420 K, D is in the range 10−8–10−9 cm2/s and obeys Arrhenius form, D = D 0 exp ( − E a / k B T ) , indicating classical over-barrier-hopping hydrogen diffusion with an activation energy Ea. The concentration dependence of Ea and D was studied for the hydrogen-to-metal ratio H/M between 0 and 1.9. At low hydrogen content, H/M

Diffusion of 6Li in the refractory metals Ta and W has been studied using the nondestructive neutron depth profiling technique. The preliminary results point out the complex behavior of 6Li atoms in W and Ta. The experiment showed that the Fickian diffusion is affected by the presence of traps and radiation defects in the sample surface layer. Further experiments and computer simulations of the diffusion process are in progress.

Due to the high potential of Zirconium and Zr-based alloys regarding corrosion it is of interest to develop even better Zr-based alloys as well as to improve the understanding of their corrosion mechanisms. Production techniques such as rapid quenching and mechanical alloying are used for the design of amorphous or nanocrystalline materials and give the possibility to improve Zr-based alloys, not only by alloying, but also by changing the microstructure. Corrosion of metallic alloys is strongly influenced by the alloying elements as well as by their microstructure. The aim of this paper is to review the current knowledge on the design and corrosion of nanocrystalline Zr-based alloys in comparison to amorphous and crystalline materials. Based on these results, trends for further research and expectation of an improved corrosion behavior of nanocrystalline alloys are outlined.

Prerequisite for nanocrystallization are high nucleation rates combined with slow growth. The resulting mean (quasi)crystal diameter in so-called bulk Zr-based metallic glasses was observed to decrease with the annealing temperature. In a typical metal–metalloid glass (for example in Fe–Ni–B), the finest microstructure was observed at a temperature very close to the maximum in the nucleation rate. The reason for this differences is probably due to the different temperature dependence of the atomic mobility, i.e. the viscosity, as shown in an Angell plot. In fragile glasses (e.g. Fe–Ni–B) viscosity is known to change very strongly with the temperature; strong glasses (e.g. Zr–Cu–Ni–Al) exhibit a temperature dependence of the viscosity very similar to that of the diffusivity below the glass transition. Among other parameters leading to nanocrystallization are very low interfacial energies, e.g. for the quasicrystalline phase in Zr–Cu–Ni–Al, which allows the high nucleation rates necessary for the formation of a nanocrystalline structure, as well as phase separation into two amorphous phases either spinodal or by nucleation and growth prior to any crystallization. The influence of these parameters will be discussed for a number of metallic glasses.

Various nanocrystalline and nano-amorphous Mg–Ni–RE (RE=rare earth or Y) alloys with attractive hydrogen storage properties were prepared by direct quenching of the melt and by crystallization of amorphous precursors. Varying the quenching rate, different microstructures could be obtained by melt-spinning. Having a knowledge of the mechanism and kinetics of the crystallization process, the devitrification of the amorphous samples could be carried out in a controlled way leading to nanocrystalline material. The hydrogenation kinetics and the H-storage capacity of amorphous, partially and completely nanocrystalline alloys were investigated and it was found that they are dependent on the microstructure and on the phase composition of the alloys. Therefore, the crystallization of the amorphous or nano-amorphous alloys was thoroughly studied, applying different experimental methods and approaches. The sequence of the crystal phases formation and the development of the alloys microstructure were investigated. Crystallization kinetics and mechanisms were determined as well.

Glassy Zr 70 Pd 30 as well as Zr 70 Au 30 prepared by melt-spinning exhibit a “catastrophic” oxidation behavior in wet as well as synthetic air. For example, at a temperature of 270 °C, ribbons (about 30 μm in thickness) are fully oxidized within a few hours. Thermogravimetric analysis reveals an oxidation kinetics by orders of magnitude faster than those in comparable Zr–Pt or Zr–Ni, Zr–Cu as well as related ternary or quaternary glasses. Detailed microstructural investigations, in particular cross-sectional microscopy, revealed details of this reaction and indicate for example for Zr–Au glasses at 25 °C an oxygen diffusivity in the developing zirconia scale of about 2×10 −17 m 2 s −1 ; such a diffusivity is known to occur for stabilized ZrO 2 only at about 300 °C. At the as-cast free surface of the ribbon nucleation of the oxidation process starts at defects like scratches or along the grooves at the contact side. The developing “vulcano” like morphology (with diameters in the range of 10 μm or more) can be explained assuming a diffusion controlled thickening of the scale, but linear lateral growth. It is of interest that the occurrence of extreme fast oxidation is correlated with significant changes in the zirconia microstructure as indicated by a decrease in the intensity of the low-indexed diffraction lines; TEM reveals a nanocrystalline structure with ZrO 2 crystal diameters even below 5 nm. The results will be discussed in detail taking into account for example the different redox potentials of the noble metals, their catalytic activity, the Zr-noble metal bond strength as well as the density of states at the Fermi level of the glasses.

Zr-based quasicrystalline alloys can store hydrogen up to a content close to the highest value known for crystalline materials, but there are questions regarding their structural stability during hydrogen charging. Hydrogenation of Zr 69.5 Cu 12 Ni 11 Al 7.5 quasicrystals was observed to increase the (quasi-)lattice constant, but at higher contents also to cause microstructural changes, as indicated by the weakening of the contrast of the quasicrystals in transmission electron microscopy as well as a disappearance of the weak diffraction spots in the corresponding diffraction pattern. A more detailed investigation reveals the formation of approximants at hydrogen contents above H/M=0.4 and a variation in the order of approximants from lower to higher order with increasing amount of hydrogen. In addition, amorphization was observed starting with the nucleation and growth of amorphous hydrides at the interfaces between approximants and the amorphous matrix. The reason for amorphization might be the change in the ratio e/a and/or the expansion of the quasilattice above a critical value. Pd coating can hinder the formation of ZrO 2 layers, thus allowing not only faster hydrogen absorption, but also hydrogen desorption. However, only partial desorption occurred prior to the decomposition of the quasicrystalline phase. Only hydrogenation up to about H/M=0.4, probably below the content necessary for the formation of approximants, seems to be fully reversible.

Glassy Zr–Cu–Ni–Al precursor alloys exhibit quasicrystal forming ability in a narrow concentration range via isothermal heating. The formation of quasicrystals has been explained by various arguments. Often, they are considered as Hume-Rothery phases, atomic size or bond strength dependent structures, or a hybrid structure built from Al 2 Cu- and MoSi 2 -type structural units. The aim of the present investigation is to find out the influence of alloying on quasicrystal formation using transmission electron microscopy and X-ray diffraction. Small addition of elements such as Sn or Mo was found to change the phase selection during the early stages of crystallization significantly. Substitution with Sn, having the same oxidation state, leads to the formation of the icosahedral phase. On the other hand, substitution with Mo, having a higher oxidation state, leads to the formation of the fcc big cube phase. However, in the present investigation substitution of atoms bigger than Zr such as Y or Ce leads to very easy formation of quasicrystals even at lower temperatures. Si, on the other hand, which is smaller than Zr leads to the formation of the tetragonal Zr 2 Ni phase.

The influence of cryogenic mechanical milling on the structure and the thermal behavior of isotactic polypropylene was investigated by means of wide-angle X-ray scattering, differential scanning calorimetry and transmission electron microscopy. The results presented show that structural changes caused by mechanical milling at cryogenic temperatures are only partly reversible. The decrease in the degree of crystallinity as determined by differential scanning calorimetry with increasing milling time is attributed to a reversible transformation of the initial morphology into a “nanostructured” morphology, consisting of small “fragments” of the original isotactic polypropylene lamellae. During recrystallization from the molten state, there still exists an influence of the previous milling treatment, leading to significantly changed crystallization behavior. The reason for this behavior might be a decrease in the molecular weight or a change in the molecular weight distribution or the formation of long-chain branched polypropylene molecules induced by cryogenic mechanical milling.

Palladium (Pd) coating was found to enhance significantly the absorption/desorption behavior of hydrogen in amorphous Zr69.5Cu12Ni11Al7.5, thereby improving the applicability of this alloy to hydrogen storage systems. This paper demonstrates the different characteristics of deuterium/hydrogen absorption/desorption in this Pd-coated metallic glass. Pd-coated amorphous ribbons were electrochemically charged with hydrogen or deuterium to various concentrations. Absorption and desorption characteristics were determined by means of a hydrogen determinator and thermal desorption spectroscopy (TDS), respectively. Hydrogen effects on the thermal stability and crystallization characteristics were studied using differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The hydrogen uptake was found to be twice faster than that of deuterium. Hydrogen-induced phase transformations, however, might prevent the use of this alloy for hydrogen storage applications.

Zr-based metallic glasses and quasicrystals might be excellent materials for hydrogen storage because of their high number of tetrahedrally coordinated sites suitable for interstitial hydrogen and their favorable hydrogen chemistry. However, hydrogen desorption at low temperatures of the amorphous as well as quasicrystalline Zr–Cu–Ni–Al alloys is hindered by a thin ZrO2 barrier and proceeds only at higher temperatures together or even after phase transformations of the metastable alloys. Pd coating was observed to improve the absorption and in particular the desorption kinetics significantly. Hydrogen charging was performed electrochemically in a 2:1 glycerin–phosphoric acid electrolyte. Hydrogen desorption was studied by means of DSC as well as TDA, the microstructure by X-ray diffraction and TEM. In Pd-coated alloys full desorption is observed even below 300 °C, but only after hydrogenation up to about H/M=0.4; higher hydrogen contents led to irreversible microstructural changes, in amorphous as well as quasicrystalline materials. The characteristics of hydrogen desorption and the micromechanism of decomposition during annealing of the hydrogenated Pd-coated metastable alloys at higher temperatures are discussed in detail.

Oxidation of a rapidly solidified Mg–Ni–Y alloy was studied in a wide temperature range in air by means of TG, DSC, XRD, TEM, SEM/EDAX. The alloy was in different microstructural states and with a different phase composition depending on the temperature of oxidation. The oxidation kinetics have been found to obey different laws (linear, logarithmic, parabolic) at the different temperatures. At low temperatures of oxidation (200–350°C) the alloy oxidizes slowly with linear kinetic law. In the temperature range of 350–430°C generally a logarithmic model describes best the experimental kinetic data, as the initial stage of oxidation follows also a linear kinetic equation. At temperatures above 430°C a parabolic law is valid. During further increase of the temperature, again a linear kinetic law was followed due to partial cracking of the oxide film. The rate constants, activation energies and pre-exponential factors were also derived. The products of oxidation, the morphology and microstructure of the oxide film, alloy matrix and oxide/alloy matrix interface were investigated. The phase composition and microstructure of the metal matrix do not differ very much from those resulting after annealing in a protective argon atmosphere. Grain growth proceeds with a low rate even after long term annealing at temperatures of about 400–450°C. It was found that the oxide film consists of MgO and MgNiO2 and traces of yttrium oxides. The outer layer of the scale is nanocrystalline. At the oxide/alloy matrix interface dense coarse crystals with most probable composition Mg24Y5 were observed. The microscopic observations as well as the kinetic results indicate that the oxide film in Mg87Ni12Y1 works successfully as a protective layer at temperatures up to 500–520°C, retarding the oxygen transport in the scale and to the oxide/metal interface.

Zr–Cu–Ni–Al alloys belong to the best glass forming systems known. Zr 68.5 Cu 13 Ni 11 Al 7.5 metallic glasses [1] are known to transform by primary crystallization into a quasicrystalline structure. Since small Hf-addition were found to stabilize the formation of quasicrystals, i.e. to reduce the influence of oxygen on the crystallization, a glass with 5 at.% Hf as a replacement of Zr was chosen to compare with the transformation kinetic of Zr 69.5 Cu 12 Ni 11 Al 7.5 . From crystallization statistics nucleation was found to be homogeneous in the temperature regime between 653 and 713 K with transient rates. In order to understand the observed crystallization process in more detail a new virtual space algorithm is presented. It is capable of calculating the real projected distribution of crystallite sizes in a thin TEM transparent layer including hard impingement up to a crystalline volume fraction of 100%. Another improvement as compared to the standard Kolmogorov–Johnson–Mehl–Avrami (KJMA) procedure [J. Chem. Phys. 7 (1939) 1103; J. Chem. Phys. 8 (1940) 212; J. Chem. Phys. 9 (1941) 177] is the parallel fit of several experimental distributions with large ensembles of crystallites for one temperature and different annealing times. A least sum of error squares optimization method is implemented, yielding the set of relevant parameters, i.e. the stationary nucleation rate, the incubation time and the effective diffusion coefficient for the primary crystallization. By fitting the nucleation rates with a diffusion controlled classical model, the interface energies and the metastable melting temperature of both alloys are estimated. Based on these data is the evaluation of a TTT diagram for the crystallization of the icosahedral phase in Zr 64.5 Cu 12 Ni 11 Al 7.5 Hf 5 and Zr 69.5 Cu 12 Ni 11 Al 7.5 .

Zr-Cu-Ni-Al belongs to the best glass forming systems known; these glasses are suitable as precursor material for nanocrystalline alloys. For an application as hydrogen storage materials for example it is of great interest to know more about these metastable materials in regard to their environmental properties. Corrosion as studied by a salt spray test or anodic polarization in aqueous solutions exhibit a rather high sensitivity with no significant differences between the amorphous and nanocrystalline state. Hydrogen charging was performed electrochemically in a glycerine-phosphoric acid electrolyte. In Zr-Cu-Ni-Al alloys absorption kinetics and storage capacity were found to be very similar for the amorphous and the nanocrystalline phase. In the nanocrystalline alloy consisting mainly of a fcc (big cube) phase with a NiTi 2 type structure a hydrogen induced amorphization was observed. Oxidation of metastable Zr-based materials in air was studied below the glass transition temperature at 360°C by thermogravimetry. Oxidation resistance was found to improve very significantly from the amorphous to the nanocrystalline microstructure. The scales formed on both materials consist mainly of columnar ZrO 2 with diameter in the nanometer range; Probably including the other metals as a nanocrystalline solid solution.

Hydrogenation of glassy Zr–Cu–(Ni, Pd)–Al alloys was performed electrochemically in a 2:1 glycerin–phosphoric acid electrolyte. In comparison to Zr–Cu–Ni–Al quasicrystals the absorption kinetics in amorphous Zr–Cu–Ni–Al were found to be slower, but the storage capacity is similar. Desorption is probably hindered by the formation of thin ZrO 2 layers. Partial replacement of Ni by Pd in an amorphous Zr 68.5 Cu 13 Ni 11 Al 7.5 alloy was found to accelerate absorption kinetics. The observed increase in length during hydrogenation indicates an interesting new result: instead of a continuous increase of the specific volume per hydrogen atom, a decrease from rather high value at very low hydrogen content is observed. This means that not only the site for the hydrogen atom is expanded, but also the neighboring still empty ones. Thermal stability was studied by X-ray diffraction and TEM. Amorphous Zr 68.5 Cu 13 Ni 10 Pd 1 Al 7.5 and Zr 68.5 Cu 13 Ni 5.5 Pd 5.5 Al 7.5 was found to transform into a nanocrystalline microstructure of an icosahedral quasicrystalline phase. Zr 68.5 Cu 13 Pd 11 Al 7.5 transform mainly into tetragonal Zr 2 (Cu, Pd) and an unidentified phase. Above a hydrogen content of H/M=0.05, the formation of quasicrystals is replaced by a tetragonal phase with lattice parameters close to those of Zr 2 Ni. At high hydrogen contents (about H/M=1.0) phase separation is assumed to take place followed by the formation of nanocrystalline ZrH 2 .

(2001). Environmental Behavior of Magnesium and Magnesium Alloysd. Materials Technology: Vol. 16, No. 2, pp. 110-126.

Hydrogenation of quasicrystalline Zr69:5Cu12Ni11Al7:5 was performed electrochemically in a 2:1 glycerine‐phosphoric acid electrolyte. Hydrogen absorption/desorption as well as the influence of hydrogen on the formation and stability was studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and thermal desorption spectroscopy (TDS). In comparison to amorphous Zr‐Cu‐Ni‐Al the absorption kinetics in quasicrystalline Zr‐Cu‐Ni‐Al were found to be faster. Desorption is hindered in both materials probably due to the formation of thin ZrO2 layers. Only partial desorption of hydrogen was observed by means of TDS to occur prior to the decomposition of the quasicrystalline phase. The observed increase in length during hydrogen charging indicates an interesting new result. Instead of the continuous increase of the specific volume per hydrogen atom at very low hydrogen concentration typical for the filling up of larger trapping sites, a decrease from a rather high value is observed. This means that not only the site for the hydrogen atom is expanded, but also the neighboring still empty ones. Above a hydrogen content of H/MD0.05 the formation of Zr‐Cu‐Ni‐Al quasicrystals is replaced by tetragonal Zr2Cu, tetragonal Zr2Ni and hexagonal Zr6NiAl2. At high hydrogen contents (about H/MD1.0) phase separation is assumed to take place followed by the formation of nanocrystalline ZrH2. Icosahedral Zr69:5Cu12Ni11Al7:5 was found to decompose through a discontinuous transformation by complex precipitation reactions; it transforms mainly into tetragonal Zr 2Cu, tetragonal Zr2Ni and hexagonal Zr6NiAl2. Hydrogenation of quasicrystals at low concentrations does not change the phases formed during decomposition. At hydrogen contents higher than H/MD1.1 decomposition of quasicrystals starts with the precipitation of tetragonal ZrH 2 x . © 2000 Elsevier Science B.V. All rights reserved.

Oxidation of Al–Cu–Fe and Al–Pd–Mn quasicrystals as compared to related crystalline phases was studied in air at high temperatures. Parabolic rate constants at 840°C were about 6×10 −12 g 2 cm −4 s −1 for icosahedral Al–Cu–Fe, but only 3×10 −13 g 2 cm −4 s −1 for cubic AlFe. The oxide layer formed on the icosahedral alloy was inhomogeneous with respect to the oxide growth morphology. Oxide compositions were studied by Auger electron spectroscopy (AES) and X-ray induced photo electron spectroscopy (XPS). The θ-Al 2 O 3 layer on Al–Cu–Fe did not contain any quantities of Cu or Fe that were significant according to the analyses by AES and XPS. In an advanced oxidation stage the θ-Al 2 O 3 layer was penetrated by α-Al 2 O 3 nodules, which contained about 5% Cu. Oxidation of Al–Pd–Mn quasicrystals was found to be strongly influenced by evaporation of Mn. Whereas the formation of δ-Al 2 O 3 was observed at 800°C on Al–Pd–Mn quasicrystals followed by growth of needle-like θ-Al 2 O 3 , oxidation of hot isostatically pressed (HIPed) specimens at 850°C led to the formation of a θ-Al 2 O 3 layer containing also extremely thin areas. The metallic Al(LMM) Auger peak observed in these areas was four times the oxidic Al(LMM) peak. Neither in the thick nor the thin oxide areas were Pd or Mn observed.

The combination of local tetrahedral order and favorable chemical composition makes amorphous and quasicrystalline Zr–Cu–Ni–Al alloys good candidates for hydrogen storage applications. Hydrogen evolution from these alloys has been studied by means of thermal desorption spectroscopy (TDS). The characteristics of hydrogen desorption from an initially quasicrystalline phase are analyzed in detail for the first time. Spectra analysis is supported by data from variety of other experimental techniques, such as differential scanning calorimetry (DSC), Auger electron spectroscopy (AES), elastic recoil detection analysis (ERDA) and transmission electron microscopy (TEM). Hydrogen desorption from both alloys is found to be a very complex process, being affected by phase transformations that take place during annealing, by hydride formation, and by the formation of a thin zirconium oxide on the surface.

The present experiment tested the hypothesis that phantom limb pain amputees show a different pattern of psychophysiological reactivity to stress compared with painfree amputees. Six phantom limb pain (PLP) and five painfree upper-extremity amputees were administered two groups of tasks: stressful and relaxing. The measured dependent variables were skin temperature recorded at both stump and intact site, heart rate, blood pressure, subjective pain and stress ratings. Phantom limb pain patients were characterized by higher stump temperature compared with phantom limb painfree patients. This effect was observed during the whole recording. Consistently with the between-subjects effect, when PLP patients exhibited higher skin temperature (during relaxation, compared with during stressful tasks), they perceived more pain. The experiment showed higher cardiovascular reactivity in PLP patients, specifically to the stressful free-speech task, which focused on recollection of the amputation event. As compared with the painfree patients, during the personal stressor, PLP patients' heart rate and systolic blood pressure increased, indicating a greater sympathetic response. Results indicate that peripheral factors, such as stump temperature reliably differentiate PLP from painfree patients. Moreover, the clear cardiovascular hyperreactivity observed in PLP patients during their report of amputation suggests that PLP is associated with a long-term emotional memory for the painful experience of that event. These results are consistent with most reports in the literature relating phantom pain development to the experience of preamputation pain.

The oxidation of a quasicrystal with the nominal compositionAl63Cu25Fe12 was studied around 800°Cin environmental and synthetic air by means of thermogravimetric analysis,electron microscopy, and analytical electron spectroscopy. In an earlyoxidation stage, γ-Al2O3 formed with an orientational relationship tothe quasicrystal. At the oxide–metal interface, γ-Al2O3transformed into large hexagonal shaped α-Al2O3grains. The change in surface morphology indicated that at theoxide–gas interface γ-Al2O3 continued togrow as Θ-Al2O3. Locally the metastable aluminalayer was transformed thoroughly into α-Al2O3,which then continued to grow with a nodular morphology. On top of the oxidenodules, several at.% of Cu2+ were detected.

Thermal barrier coating (TBC) systems protect turbine blades against high-temperature corrosion. They consist of a MCrAlY bond coating and a ceramic top layer. The oxidation resistance of MCrAlY coating is based on the formation of α-Al2O3 in the steady-state stage of oxidation. The α-Al2O3 grows very slowly between the metallic bond coating and the ceramic top coating and is thermodynamically stable. High-temperature oxidation experiments with vacuum-plasma-sprayed (VPS) and high-velocity-oxygen-fuel (HVOF)-sprayed MCrAlY coatings showed that the transient stage of oxidation for HVOF coatings is very short. Moreover, in contrast to the VPS coatings, where also Cr2O3, NiO or CoO are identified, on HVOF coatings, only metastable modifications of alumina were observed. The transformation of metastable alumina modifications to α-Al2O3 is very fast. Oxidation experiments in a high-temperature chamber attached to a X-ray diffractometer were carried out. The morphology and the structure of the formed oxide scale were characterized by X-ray diffraction and scanning electron microscopy (SEM).

Oxidation of Zr-based metallic glasses in air below the glass transition temperature was studied in the temperature range between 330°C and 370°C by thermogravimetric analysis, X-ray diffraction, scanning and transmission electron microscopy (SEM and TEM). Oxidation kinetics were observed to follow a parabolic law except for Zr65Cu25Al10. Whereas Zr70Ni20Al10 is oxidation resistant, the oxidation rate increases from Zr 75.6 Cu 12.2 Ni 12.2 to Zr 65 Cu 25 Al 10 and Zr 69.5 Cu 12 Ni 11 Al 7.5 ; Zr 65 Cu 27.5 Al 7.5 has the fastest oxidation of the glasses investigated. Oxidation resistance was found to deteriorate by addition of Cu, but improved by Al additions. The scales formed during the oxidation have a nanocrystalline microstructure consisting mainly of tetragonal and monoclinic ZrO2 probably incorporating the other metals; no Zr-depleted zone could be observed in the remaining glassy or crystallized matrix. Crystallization was found to compete with the oxidation process of most glasses; only Zr65Cu25Al10 was still nearly amorphous after oxidation annealing for 4 h at 360°C. Depending on the glass tetragonal Zr2Ni and Zr2Cu as well as hexagonal Zr6NiAl2 were identified in the matrix.

Zr-based metallic glasses and quasicrystals were electrochemically charged with hydrogen. Hydrogen absorption/desorption as well as the influence of hydrogen on the formation and stability of quasicrystals were studied by means of X-ray diffraction, electron microscopy, differential scanning calorimetry (DSC) and thermal desorption analysis (TDA). In Zr69.5Cu12Ni11Al7.5 absorption kinetics and storage capacity were found to be better for the quasicrystalline than for the associated amorphous phase. Dissolved hydrogen was observed to play an important role not only in the stability of the quasicrystals, but also in their formation from glassy precursor material. Even hydrogen contents as low as 5 at.% changed the formation as well as the decomposition of the icosahedral quasicrystals: the formation was shifted to higher temperatures whereas decomposition temperatures were lowered. At larger hydrogen contents, changes in both transformations were observed, yet not fully understood.

The aim of the present work was to prepare a three-phase nanocomposite containing high saturation {alpha}-Fe and Fe{sub 3}B soft phases and the Nd{sub 2}Fe{sub 14}B hard magnetic phase. The requirements for the microstructure of an exchange coupled material are a very fine and homogeneous grain size distribution and substantial absence of any intergranular phase. Since the crystallization of amorphous alloy is a multistage process, coarsening of the phase crystallizing in the first stage may occur till completion of the crystallization process, leading to deterioration in coercivity. The effect of annealing treatment on the intrinsic coercivity was therefore determined. Results of these investigations are reported in this paper.

The Zr57Nb5Al10Cu15.4Ni12.6 bulk metallic glass forming liquid is reinforced with WC, SiC, W, or Ta particles. Structure, microstructure and thermal stability of the composites are studied by X-ray diffraction, optical microscopy and differential scanning calorimetry. The metallic glass matrix remains amorphous after adding up to 20 vol.% of particles. The reactions at the interfaces between the matrix and the different reinforcing materials are investigated with scanning electron microscopy, transmission electron microscopy and electron microprobe. The mechanical properties of the composites are studied in compression and tension. The influence of the introduced particles on the thermal stability of the matrix as well as on the mechanical properties is discussed.

Amorphous and nanocrystalline Mg-based alloys were produced by rapid quenching (melt-spinning) and their hydrogenation properties were studied. The thermal stability and crystallization behaviour of the as-quenched and hydrogenated alloys were investigated as well. It was found that the as-cast nanocrystalline/amorphous Mg 75 Ni 20 Mm 5 (Mm=Ce, La-rich mischmetal) alloy possesses the best hydriding properties (hydrogenation kinetics and hydrogen absorption capacity) with maximum hydrogen capacity of 4.0 wt.% H. The difference in the hydriding properties of the as-quenched nanocrystalline and completely crystallized (with grain size in the range of 100–150 nm) Mg 75 Ni 20 Mm 5 alloys was found to be insignificant. The amorphous and crystallized (after heat treatment) Mg 87 Ni 12 Y 1 alloys show slower hydriding kinetics and lower hydrogen absorption capacity compared to the other Mg-based alloys studied. The amorphous Mg 87 Ni 12 Y 1 exhibits faster initial hydrogenation kinetics than the partially and fully crystallized alloys with the same composition, due to faster hydrogen diffusion in the amorphous phase, but the hydrogen absorption capacity of all Mg 87 Ni 12 Y 1 alloys having different microstructure is practically the same. The crystallization of melt-spun Mg 75 Ni 20 Mm 5 and Mg 87 Ni 12 Y 1 alloys is a two-step process. The primary crystallization of α-Mg (for Mg 87 Ni 12 Y 1 ) takes place at about 160°C, followed by transformation of the residual amorphous matrix into a metastable phase, assigned as fcc Mg 6 Ni (isomorphic with fcc Mg 6 Pd ( a o =2.0108 nm)). This intermediate metastable phase decomposes during further annealing (at about 300°C) into the equilibrium phases Mg 2 Ni and Mg. The product of the first crystallization reaction for the as-cast Mg 75 Ni 20 Mm 5 alloy is Mg 2 Ni, most probably realized by growth of the quenched-in Mg 2 Ni nanocrystals. The second reaction corresponds to transformation of the residual amorphous matrix into Mg 17 Mm 2 .