Laves Phase C15 Research Articles

Internal friction studies by resonant ultrasound spectroscopy

Abstract A description is given of the use of resonant ultrasound spectroscopy (RUS) to study internal friction in solids. The technique is applicable to a wide range of sample sizes and shapes. A method to extract the internal friction from the RUS spectrum in the presence of a coherent background signal is discussed. RUS has been used to study the internal friction due to hydrogen motion in hydrogen–metal materials. A comparison is made of internal friction due to hydrogen motion in a C15 Laves-phase intermetallic compound with that in a quasicrystalline material. The internal friction peak in the quasicrystalline material is much broader, probably reflecting a distribution of local environments for the hydrogen in this material. A low-temperature, internal friction peak due to deuterium motion in TaV 2 D 0.17 is discussed. This peak was shown previously to result from a rapid local motion of D on a hexagon of interstitial g sites. A novel explanation is proposed for the temperature dependence of this peak.

Elastic moduli of polycrystalline TiCr 1.8 over the temperature range of 3–410 K

The elastic moduli of polycrystalline TiCr 1.8 have been measured over the temperature range of 3–410 K using the technique of resonant ultrasound spectroscopy (RUS). The reported results are for the C15 Laves-phase structure. The measured moduli display a normal temperature dependence, approaching 0 K with zero slope and decreasing linearly with increasing temperature at the higher temperatures. The Debye temperature, calculated from the measured moduli, was found to be 500 K. This material can absorb considerable amounts of hydrogen, but is unstable at ambient temperatures. The TiCr 1.8–H 2 system has a relatively low critical temperature, T c, which has previously enabled thermodynamic measurements to be made above T c. An elastic energy contribution to the enthalpy of formation has been calculated from the present measurements and compared to the thermodynamic results. The comparison suggests that the electronic contribution to the enthalpy of formation is comparable in magnitude to the elastic contribution, but opposite in sign.

A kinetic neutron diffraction study of the crystallisation of a -Er 7 Fe 3

We report a kinetic neutron diffraction study of the initial crystallisation, phase formation and subsequent transformation of amorphous Er7Fe3. The final product of the crystallisation process is the C15 Laves-phase compound ErFe2; however a novel and previously unreported intermetallic ErFe compound is formed during the initial crystallisation process. The neutron data are complemented by Mossbauer-spectroscopy measurements of the precursor material and of the final ErFe2 phase.

A neutron polarisation analysis study of the spin-glass phase of Y(Al 1-x Fe x ) 2

Diffuse neutron scattering with full polarisation analysis has been used to characterise the spin correlations in the extended spin-glass phase of the C15 Laves-phase compound, Y(Al1-xFex)2, at concentrations of x=0.25 and x=0.65. Whilst the nuclear diffuse scattering indicates weak anticlustering of Fe atoms on the Al sublattice, the magnetic scattering can be interpreted as arising from small ferromagnetic clusters of less than 10 A in diameter, with these continuing to exist at much higher temperatures than the spin-glass temperature. The ferromagnetic correlations which give rise to the clusters are short-range, and some evidence is found for weak antiferromagnetic correlations.

Magnetic correlations in Nb 1-y Fe 2+y

The evolution of spin correlations with temperature in the ferromagnetic C14 Laves-phase compound Nb1-yFe2+y with y=-0.03 and 0.01 has been studied by small-angle neutron scattering. The Q-dependence of the scattering is squared-Lorentzian in form for y=-0.03 but is predominantly Lorentzian for y=0.01. The magnetic correlation range remains close to 30 A, showing no evidence of any critical divergence at the reported Curie points (TC∼20 K) of the respective compounds.

Temperature dependence of the elastic shear modulus of the Laves-phase materials TaV 2 and TaV 2H x

Resonant ultrasound spectroscopy has been used to measure the shear modulus of polycrystalline TaV 2H x for x=0, 0.06, 0.10, 0.18, 0.34, and 0.53 over the temperature range of 20–300 K. The results show a strong influence of hydrogen on both the magnitude and the temperature dependence of the shear modulus of these materials. These results are in qualitative agreement with a model involving the electronic structure of these C15 Laves-phase materials. The symmetry of the C15 structure results in doubly-degenerate electronic energy levels at the X point of the Brillouin zone. These levels couple to the shear strain e 4 with resulting effects on the single-crystal elastic constant c 44. The strong temperature and concentration dependence of c 44 contributes to the measured shear moduli of these polycrystalline materials.

Fluctuation effects and scaling behaviour in C15 Laves-phase superconductor CeRu 2

The temperature dependence of the resistance in the presence of magnetic field showing evidence of fluctuation effects is reported in a single crystal C15 Laves-phase superconductor CeRu 2. Measurements performed in different magnetic field strengths indicate that the observed fluctuation effects in this low T C superconductor can be scaled using three-dimensional-scaling functions.

Ultrasonic attenuation and dispersion due to hydrogen motion in the C15Laves-phase compound TaV2Hx

Hydrogen in the C15 Laves-phase material TaV2Hx has been studied bymeans of resonant ultrasound spectroscopy over the temperature range of15-345 K for a series of hydrogen concentrations (x = 0.00-0.53). Ultrasonicloss peaks and frequency shifts (dispersion) associated with the hydrogenmotion were observed, yielding parameters for the hydrogen motion. Hydrogen inthese materials is known to occupy the tetrahedral g sites which form aseries of interlinked hexagons. The ultrasonic results were associated with Hhopping between g-site hexagons. The relaxation rates for x⩽0.18 werebest described as a sum of two Arrhenius processes. For x = 0.34 and 0.53 onlya single Arrhenius process was needed to fit the results, although thepresence of a second Arrhenius mechanism could not be over-ruled. A singlerelaxation rate was sufficient to fit the data; a distribution of rates wasnot required. The magnitudes of the attenuation and dispersion dependedlinearly on the hydrogen concentration implying that it is the relaxation ofisolated H atoms (the Snoek effect) that is responsible for the mechanicaldamping. The faster local motion of H reported from nuclear magnetic resonancemeasurements for motion within g-site hexagons was not observed in thepresent study. This suggests that the H hopping rate for the local motionremains above the ultrasonic frequencies over the temperature range of study,or perhaps that too few H atoms participate in the local motion.

Study of the hydrogenation/dehydrogenation processes of ZrCr 0.7Ni 1.3, a Laves phase-rich multi-component system, by in-situ neutron diffraction under hydrogen gas pressure

Powder neutron diffraction phase analyses were performed on a ZrCr 0.7Ni 1.3 sample by using the Rietveld method. The average phase proportions are 59% C15-Laves phase, 36% Zr 7Ni 10 and 5% elemental free chromium. In-situ neutron diffraction experiments were performed at 323 K under deuterium pressure. Cyclic refinements were carried out on the data recorded during both the absorption and desorption cycles. A non-reversible amount of absorbed hydrogen was found to be stored by the minor Zr 7Ni 10 phase. After complete hydrogenation, the Zr 7Ni 10 phase was found to transform into an amorphous phase, accompanying the overall reduction of reversible hydrogen capacity of the material.

The effects of heat treatments on the microstructure and electrochemical properties of the ZrCr 0.7Ni 1.3 multiphase alloy

Qualitative and quantitative X-ray analysis using the Rietveld method of the as cast ZrCr 0.7Ni 1.3 alloy, shows an average phase abundance of 62.7% C15-Laves phase, 36.6% of Zr 7Ni 10 and less than 1% of chromium. The annealed alloy at 1373 K reveals a complete disappearance of pure Cr with an important reduction of the amount of the Zr 7Ni 10 phase based on EDS/SEM and XRD results. Pressure-Composition-Temperature (PCT) measurements show that the alloy behaves as a single phase with faster kinetics. Electrochemical measurements shows a relatively high discharge capacity of the cast alloy, c=320 mAhg −1 after 20 cycles. Contrary to that, the discharge capacity of the annealed alloy decreases by about a factor of 2 with c=156 mAhg −1.

Phase structure, crystallography and electrochemical properties of Laves phase compounds Ti 0.8Zr 0.2V 1.6Mn 0.8− xM xNi 0.6 (M=Fe,Al,Cr,Co)

The effects of partial substitution of Mn by M (M=Fe,Al,Cr,Co) on the phase structures, crystallography and electrochemical properties of Laves phase compounds Ti 0.8 Zr 0.2 V 1.6 Mn 0.8−x M x Ni 0.6 ( M= Fe, Al, Cr, Co) have been studied. It is found by X-ray powder diffraction and energy dispersive X-ray spectrometer analysis that most alloys are composed of a AB 2-type hexagonal structure C14 Laves phase matrix and a BCC structure TiNi-based secondary phase alloy and some of them contain another BCC phase. The optical photomicrograph shows that the C14 Lave phase forms a continuous matrix and the TiNi-based secondary phase intersperses among it in the form of dendritic structures. With the substitution of Mn by Fe and Cr, the dendrites are larger, while Co and Al substitutions make the dendrites smaller. Electrochemical studies indicate that Cr substitution improves the cyclic durability of the alloys greatly despite the decrease of discharge capacity, which can be attributed to the suppression of surface segregation of Ti, Zr and V elements by the presence of Cr, which suppresses the oxidation and the dissolution of V and the oxidation of Ti and Zr greatly as evidenced by the XPS analysis. Other element substitutions such as the substitution with Fe, Al and Co do not have significant effect on the alloy electrode performance.

Spin-glass behavior of Zr(Fe xCr 1− x) 2 compounds

The magnetic properties of C14 Laves-phase Zr(Fe x Cr 1− x ) 2 (0.7⩽ x⩽0.85) alloys have been investigated. Arrott plots showed evidence of ferromagnetic ordering for alloys with x>0.7 but not for x=0.7. Zero field cooling/field cooling magnetization and Mössbauer spectroscopy data indicated spin-glass-like freezing for the sample with x=0.7 and a reentrant spin-glass behavior for samples with x>0.7. Dynamical behavior of the spin-glass sample ( x=0.7) was studied by means of AC susceptibility. Two critical lines described by T f (H)−T(0) ∼H 2/δ were obtained with δ=4.4 and 2.5, respectively.

Elastic moduli of the C15 Laves-phase materials TaV 2 , TaV 2 H(D) x and ZrCr 2

Elastic moduli of the C15 Laves-phase materials TaV 2 , TaV 2 H(D) x and ZrCr 2

Elastic moduli of the C15 Laves-phase materials TaV2, TaV2H(D)xand ZrCr2

Abstract The elastic moduli of the C15 Laves-phase materials TaV2, TaV2H(D),x and ZrCr2 have been measured using the technique of resonant ultrasound spectroscopy. The temperature dependences of the shear modulus and Young's modulus of TaV2 were found to be anomalous; the moduli increase with increasing temperature over the entire temperature range 4-345 K. In contrast, the shear modulus and Young's modulus of TaV2H0.34, TaV2H0.53 and TaV2D0 17 all decrease with increasing temperature. The bulk moduli of these materials are only weakly temperature dependent. The elastic moduli of the C15 Laves-phase compound ZrCr2 exhibit a normal temperature dependence over the range 20-300 K. The unusual elastic behaviour of these materials is accounted for by a model involving electronic contributions to the elastic constant c44. The symmetry of the C15 structure results in doubly degenerate electronic energy levels at the X point of the Brillouin zone. The strain dependence of these levels affects the elastic constant c44, which in turn contributes to the measured moduli of the polycrystalline samples. The predictions of the model depend critically on the distance of the Fermi level from the double-degeneracy point. The experimental results for TaV2, are qualitatively accounted for if the Fermi level passes very near this special point. It is suggested that the extra electrons contributed by H(D) raise the Fermi level in TaV2H(D)x and result in a more normal temperature dependence of the elastic moduli over the observed temperature range. Similarly, C15 ZrCr2 with its extra electron per formula unit may have a higher Fermi level and hence a more normal temperature dependence of the moduli.

The evolution of magnetic correlations and onset of magnetic order in Y(Co 1− xFe x) 2

Small-angle neutron scattering has been used to study the evolution of magnetic correlations in the C15 Laves-phase compound Y(Co 1− x Fe x ) 2 with 0.075⩽ x⩽0.175, close to the critical concentration for ferromagnetic order (0.1375< x c<0.15).

GdFe 2 alloy formation observed by STM

Ultra thin films of an alloy consisting of Gd and Fe were prepared under ultra high vacuum conditions on top of a W(110) surface. The films were characterized using scanning tunneling microscopy and low energy electron diffraction. We succeeded in preparing the initial stages of well ordered GdFe 2 films. Basing on atomically resolved STM images and LEED studies, a structure model for the thin GdFe 2 films was deduced. The crystallographic structure of the first and second ML GdFe 2 on W(110) was found to be different from the bulk structure (Laves phase C15).

Electrochemical properties of Zr (V xNi1− x) 3 as electrode material in nickel-metal hydridebatteries

Zr-V-Ni based alloys represent one group of alloys with promisingproperties for the use as hydrogen absorbing electrodes in an alkaline electrolyte. In particular,this group has been studied because these alloys are effective in overcoming the energy densitylimitations of present AB 5 type alloys. We have investigated the alloys Zr (V xNi 1−x) 3 in therange 0.29⩽x⩽0.5 by means of scanning electron microscopy (SEM) with electron probe X-Raymicroanalysis (EPMA), X-ray diffraction (XRD) and electrochemical measurements. The alloys are composed of different crystallographic phases. The main phase (a C15laves phase, space group Fd3m) consists of all three elements and was found to be responsible forthe high reversible capacity of a maximum of 375 mAh/g. The second phase is vanadium-nickelbased (b.c.c. structure, space group: Im3m). The discharge kinetics of this alloys was investigated in electrochemical experiments. Weobserved an increase of the high rate dischargeability with increasing nickel content. This is mainlydue to the catalytic effect of nickel for the hydrogen absorption-desorption process. The exchangecurrent density increases with increasing nickel content. Furthermore, we observed a fasteractivation for the alloys with higher nickel content.

Nonequilibrium behaviour in the field-cooled magnetization of the C15 Laves-phase superconductor CeRu2

We present results showing novel nonequilibrium effects in the field-cooled superconducting mixed state of CeRu2. This nonequilibrium response has uncanny similarity with that observed in the field-cooled state of the random-field Ising systems.

Magnetic Structure of C14-Laves Phase DyMn2

Magnetic Structure of C14-Laves Phase DyMn_<b>2</b>

Thermodynamic modelling of the Cu-Mg-Zn ternary system

The quasibinary MgCu 2-MgZn 2 system was optimized with the program BINGSS. All phase diagram data and thermodynamic values available in the literature were critically assessed before the optimization. Experimental investigations by EDX of ternary Cu-Mg-Zn alloys were specifically performed to provide missing data of the Cu solubilities of the Mg-Zn phases. The Laves-phases C15 (MgCu 2), C14 (MgZn 2) and C36 (τ,Mg 2CuZn 3) existing in the Cu-Mg, Mg-Zn and Cu-Mg-Zn systems were described by the “Compound-Energy-Formalism” with Cu-Zn exchange, Mg(Cu 1 − x Zn x ) and a weak tendency for antistructure atom formation (Cu and Zn on the Mg sublattices, Mg on the Cu-Zn sublattices). The binary intermetallic phases MgZn, Mg 2Zn 3 and Mg 2Zn 11 are modelled to have Cu-Zn exchange on one sublattice. The Bragg-Williams description of ordering was extended to describe the ternary range of the βCuZn phase. Estimates were made, where experimental data are not sufficient. Using the binary subsystems from the literature with small updates the Cu-Mg-Zn ternary system was calculated. In general, good agreement is obtained between calculations and experiments.