Interaction of intermetallic compounds of rare-earth metals, nickel, and aluminum with hydrogen

AtT=293 K andp _H2≤10 MPa. we synthesized hydridesR _3Ni8Al-H_7.2–15.2 (R=Nd. Sm. Gd. Tb. Dy, Ho, Er, Tm, and Lu). The X-ray analysis revealed an isotropic deformation of the original crystal lattice due to hydrogen penetration. For the hydrides obtained, the lattice constanta exceeds that of the original intermetallide by 3.9–6.0% and the constantc and the cell volumeV are increased by 6.0–12.6% and 15.6–26.4%, respectively. The volume expansion per absorbed hydrogen atom is (2.4–3.6)·10⁻³ nm⁻³. By construction of the isotherms of hydrogen desorption for the systemsR _3Ni>8Al-H, we established the existence of the α-solid hydrogen solution and the region of its coexistence with β-hydride, the region of coexistence of β- and γ-hydrides, and the β- and γ-hydride phases. Heats of the phase transitions were determined as follows: ΔH=31.4±0.8 kJ/(mole H₂) in the system Gd₃Ni₈Al-H for the transition γ→β, ΔH=39.8±1.1 kJ/(mole H₂) in the system Tb₃Ni₈Al−H for the transition β→α. and ΔH=37.1±5.1 kJ/(mole H₂) in the system Tm₃Ni₈Al−H for the transition γ→β. Among the products of hydrogenation, intermetallic compounds Lu₃Ni8Al and Tm₃Ni₈Al. we revealed the β′- and γ-phases. We also found that, atp _H2≤10 MPa andT=400–430 K, the phases of Sm₃Ni₈Al and Gd₃Ni₈Al decay into two hydridesRH_2–3 andR(Ni.Al)₅H _x . Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 1, pp. 76–82, January–February, 2000.     

Strength and toughness of beryllium-niobium intermetallic compounds

Bend and compression strengths, fracture toughness, and high-temperature microhardness of Be---Nb intermetallic compounds were measured at temperatures up to 1200 °C. Be₁₂Nb and Be₁₇Nb₂ materials exhibited brittle behavior at temperatures below 1100 °C in bending and below 800 °C in compression. Hot isostatically pressed (HIP) Be₁₂Nb had the highest low-temperature strengths (250 MPa in bending and 2750 MPa in compression) resulting from its greater fracture toughness (K_IC = 4 MPa m^1/2) compared with the other Be---Nb materials, vacuum hot pressed (BHP) Be₁₂Nb, and HIP Be₁₇Nb₂, which had . Measured strengths for the HIP Be₁₂Nb were more than twice that measured for the VHP Be₁₂Nb or for HIP Be₁₇Nb₂. The HIP Be₁₂Nb also exhibited good high-temperature mechanical properties, having a bend strength of 250 MPa at 1200 °C, compared with less than 100 MPa for the VHP Be₁₂Nb. However, intergranular embrittlement was observed at intermediate temperatures, reducing the HIP Be₁₂Nb bend strength and fracture toughness below those measured for the other materials. HIP Be₁₇Nb₂ exhibited poor low-temperature properties, but high-temperature bend strengths of 740 MPa at 1100 °C and 400 MPa at 1200 °C were measured. Strength in compression was similar for all materials above 800 °C, decreasing sharply to about 600 MPa at 1000 °C and to 200 MPa at 1200 °C. Microhardness and indentation creep tests also revealed similar high-temperature behavior among the materials. Power-law creep exponents ranging from 4.1 to 6.6 and activation energies of 220–290 kJ mol⁻¹ were measured for the beryllides, with the HIP Be₁₂Nb having the highest activation energy for creep.     

Selective synthesis and superconductivity of In-Sn intermetallic nanowires sheathed in carbon nanotubes

We demonstrate a simple and reproducible technique to synthesize crystalline and superconducting In-Sn intermetallic nanowires sheathed in carbon nanotubes (CNTs). The method is based on the catalytic reaction of C(2)H(2) over a mixture of both SnO(2) and In(2)O(3) particles. Importantly, tetragonal β-In(3)Sn and hexagonal γ-InSn(4) nanowires with diameters of less than 100?nm are selectively synthesized at different SnO(2) to In(2)O(3) weight ratios. CNTs may serve as cylindrical nanocontainers for continuous growth of liquid-phased In(1-x)Sn(x) nanowires during growth process as well as for their solidification into In-Sn intermetallic nanowires during the cooling process. Microscopic and spectroscopic analyses clearly reveal evidence of a core-shell structure of the CNT-sheathed In-Sn intermetallic nanowires. Magnetization measurements show that the superconducting In-Sn nanowires have a critical magnetic field higher than the value of their bulk intermetallic compounds. Our method can be adopted to the nanofabrication of analogous binary and ternary alloys.     

High-strength,high-temperature intermetallic compounds

Materials that are solid at high temperatures are in demand for high-temperature structural applications, and materials that have high values of strength-to-weight and stiffness-to-weight are desired for aircraft and space applications. Basic properties that are insensitive to processing history can be used to provide a preliminary ranking of single-phase substances. A compilation is presented of 293 intermetallic compounds (or metal-metalloid compounds) that melt at T 1500° C. By displaying the data by crystal structure on plots ofT _m against the specific gravity candidates for optimum specific strength and specific stiffness can be recognized for materials that are likely to have similar plastic properties.     

Combustion synthesis of TiNi intermetallic compounds

The microstructural characteristics of NiTi intermetallic shape memory compounds produced by combustion synthesis have been investigated. It was found that the microstructure consists mainly of NiTi parent phase and NiTi₂ second phase. The faceted or coarse dendritic NiTi₂ phase, which is produced using a low cooling rate of the synthesized liquid product, can be modified to provide an evenly distributed, fine dendritic structure by increasing the cooling rate. This fine dendritic product can be readily hot-rolled into plate exhibiting the shape memory effect (SME). The initial nickel particle size has an important influence on the microstructure and also on the SME transition temperature. The morphology of the microstructure can also be modified to that of conventionally produced alloy by solution treatment. It is proposed that the synthesis reaction mechanism occurs by two combustion stages, i.e. precombustion and combustion. The particle size plays a key role in the precombustion stage.     

Reactive processing of nickel-aluminide intermetallic compounds

NiAl have been fabricated by reactive sintering compacts of ball-milled powder mixtures containing Ni and Al. The reaction mechanism, as well as phase and microstructural development, were investigated by analyzing compacts quenched from different temperatures during reactive hot compaction. It was found that the reaction process was strongly affected by pressure, heating rates, heat loss from the sample to the environment. The application of 50 MPa prior to the reaction resulted in the intermetallic-formation reaction initiating at a temperature (480°C) much lower than that (550°C) when no pressure was applied. At high heating rate (50°C/min), when the heat loss is small, the formation of NiAl occurs rapidly via combustion reaction. On the other hand, if the heat loss is significant as in slow heating rate (10°C/min), the reaction process is controlled by solid-state diffusion. The phase formation sequence for the slow solid-state reaction was determined to be: NiAl₃ Ni₂Al₃ NiAl NiAl (Al-rich) + Ni₃Al NiAl.     

Combustion synthesis of TiNi intermetallic compounds

The synthesis of the TiNi intermetallic compound using the thermal explosion mode of the combustion synthesis technique has been used to determine the heat of fusion, ΔH _m (7.77 kcal mol⁻¹), of the TiNi intermetallic and the heat capacity,C _p1 (17.96 cal mol⁻¹ K⁻¹), of the liquid-phase TiNi. The effect of heating rate and degree of dilution of the Ti + Ni powder compact reactants with previously synthesized TiNi on the ignition,T _ig, and combustion,T _c, temperatures in an argon atmosphere have been determined. It was found thatT _c was dependent on heating rate and dilution with TiNi, whereasT _ig remained unchanged with respect to these two process variables.     

Laser-controlled synthesis of nickel-aluminum intermetallic compounds

Laser-initiated self-propagating high-temperature synthesis (SHS) in a disperse Ni-Al powder composition was studied. The X-ray diffraction measurements reveal the presence of nickel-aluminum intermetallic phases, the relative content of which depends on the regime of laser action. The regimes making possible a laser-controlled SHS of Ni-Al intermetallic compounds are considered.     

Theory of superconductivity in heavy-fermion compounds

A single-order-parameter and two-gap model (a modified Suhl two-gap model) is proposed for heavy-fermion superconductors. A simple relation between two gaps is found. The thermodynamic quantities in the superconducting state are calculated in terms of this model. It is shown that the critical values of the physical quantities are quantitatively in agreement with experiment. The behavior of the specific heat, of the ultrasonic attenuation, and of the NMR rate are also in good agreement with those of CeCu₂Si₂ and UBe₁₃ at all temperatures.     

A family of ductile intermetallic compounds

Stoichiometric intermetallic compounds have always been touted for their attractive chemical, physical, electrical, magnetic and mechanical properties, but few practical uses have materialized because they are brittle at room temperature. Here we report on a large family of fully ordered, stoichiometric binary rare-earth intermetallic compounds with high ductility at room temperature. Although conventional wisdom calls for special conditions, such as non-stoichiometry, metastable disorder or doping to achieve some ductility in intermetallic compounds at room temperature, none of these is required in these unique B2 rare-earth compounds. Ab initio calculations of YAg, YCu and NiAl crystal defect energies support the observed deformation modes of these intermetallics.     

Superconductivity in ternary Heusler intermetallic compounds

We report the discovery of superconductivity in a number of stoichiometric A₂BC ordered ternary intermetallic alloys isostructural to the prototype ferromagnetic Heusler alloy, Cu₂MnAl. Ni in the superconducting Ni₂BC phases appears to be behaving as a full d-band constituent. The highest superconducting critical temperature T_c, for the Ni phases surveyed is exhibited by Ni₂NbSn (T_c = 3.4 K).     

Structure and superconductivity in ternary systems of compounds

A survey of the known highT _c ternary systems of compounds is presented. Special emphasis has been placed on the part played by the crystal structure and bonding in the exhibition of highT _c superconductivity. The superconducting and other physical properties of the ternary borides have been discussed in relatively more detail. Finally, the available guidelines from crystal and solid state chemistry points of view, have been reiterated for a search for new highT _c materials.     

Optical properties of coloured platinum intermetallic compounds

The optical properties of the intermetallic compound PtAl₂ were altered by the addition of 5% –25% copper by mass. It was found that these additions cause the colour of the compound to change from the brass-yellow of PtAl₂ through orange to copper-pink. The colour of the intermetallic compounds has been described using the CIELab and chromaticity colour-measuring systems. The effect of the copper additions on the lattice parameter and band structure is discussed, related to the associated change in colour of the intermetallic compound. The effect of the copper additions on the physical and mechanical properties of the material has been studied.     

Structural instabilities and superconductivity in La-214 compounds

In La_2–x Ba _x CuO₄ (LBCO) the transition to a low-temperature tetragonal phase and the suppression of superconductivity occur at the carrier concentration p 1/8 per copper. We will discuss the roles of various material parameters that control this instability. An unusual lattice softening has been found by ultrasonic measurement on La_2–x Sr _x CuO₄ (LSCO). This softening is present only in an in-plane shearing mode and is ascribed to the growth of structural fluctuations in the normal state.These phenomena are closely related because both the structural change in LBCO and the applied strain in LSCO lift the degeneracy of in-plane oxygen sites. They clarify the importance of strong coupling between the normal-state electronic system and the lattice by a Peierls-type mechanism.     

Atomic ordering and superconductivity in A-15 compounds

Twenty-six different binary A-15 compounds were investigated to determine if superconductivity could be related to the degree of long-range atomic ordering on the crystallographic lattice sites. Significant changes in the superconducting transition temperatures and critical fields were produced by quenching from high temperatures followed by low-temperature annealing. These changes inT _cwere accompanied by changes in the degree of long-range ordering as determined using x-ray diffraction methods. The results can be interpreted within the framework of Weger's linear-chain model only when the B-element is a nontransition element. When both the A-and B-components are transition elements, however, the superconducting behavior will apparently depend on the nature of d-electron interactions between the component atoms. Complete ordering is not always an essential requirement for optimizing the superconducting properties.     

The oxidation resistance and hardness of some intermetallic compounds

Thirty-nine intermetallic compounds have been oxidation-tested in air. These include all the known compounds in the Al-V, Al-Ni and Al-La systems, three compounds each from the Al-Mn and Cu-Zn systems, four zinc-rich transition-metal phases, six intertransition metal Laves phases, and the equiatomic compounds of antimony with aluminium, indium and nickel respectively. From the oxidation results the temperature corresponding to a weight change of 1 mg cm⁻² (4h)⁻¹ was determined by interpolation. An attempt is made to classify the compounds in terms of the ratio of this temperature to the melting point of the appropriate compound or its oxidation product. The hardness of the compounds and of additional Al-3d compounds has been estimated, and in conjunction with the oxidation data forms the basis for some observations on the nature and incidence of binary intermetallic compounds of high atomic ratio.     

Possible coexistence of superconductivity and magnetism in layered compounds

The possibility of a coexistent superconducting and magnetic phase in layered transition metal dichalcogenides of the type MX ₂ A _x is investigated (M =transition metal; X = S, Se; A = magnetic ion; x 0.25). Describing such systems with a model in which a highly anisotropic electron gas interacts with a quasi-two-dimensional Heisenberg magnet, we find the following conditions for coexistence: (1) The coupling between adjacent layers of ferromagnetically ordered magnetic ions needs to be antiferromagnetic (S ^z = 0) in order to have no pair-breaking internal fields. (2) The exchange interaction between conduction electrons and magnetic ions must be very small ( 0.003 eV), since otherwise, due to the high concentration of localized magnetic moments, spin-flip scattering processes and spin fluctuations would destroy superconductivity. The theoretical prediction for the persistence of superconductivity up to concentrations of x 0.25 of magnetic ions is compared with recent experiments on Eu-intercalated TaS ₂ and NbS ₂ and related compounds showing a tendency for ferromagnetically ordered layers of Eu impurities and antiferromagnetic coupling between neighboring layers but no superconductivity for x exceeding a few percent. Reasons for the quick disappearance of superconductivity in these systems and criteria for possible observation of high-magnetic-impurity-concentration superconductivity in other layered compounds are given.     

Abnormal weight changes of intermetallic compounds when heated

Monotonic weight gains are most usual when intermetallic compounds are heated in air to successively higher temperatures, but about 15% of compositions exhibit other behaviour in which mainly losses were partial or dominant. From a specimen population of binary phases with components drawn from many parts of the periodic table, it was found that abnormality was markedly found when components came from the early B Groups. It is suggested that the high volatility of phases such as beta brass and related structures may be responsible.     

Studies on the oxidation behaviour of intermetallic compounds

The oxidation behaviour of the AB₅ type intermetallic compounds CaNi₅ and LaNi₅ has been studied at different temperatures ranging from 100 to 800° C. The kinetic results indicate that the element A (Ca, La) is first oxidized rapidly followed by slower oxidation of nickel. X-ray diffractograms of the oxidized LaNi₅ show the formation of ternary phases like LaNiO₃ and La₂NiO₄ at temperature as low as 400° C. A comparison between the oxidation behaviour of the two compounds reveals that CaNi₅ is more resistant to oxidation and decomposition than the LaNi₅ system.