Showing papers on "Laves phase published in 1998"

Microanalysis of two creep resistant 9–12% chromium steels

Abstract: The ferritic chromium steels P122 and P92 have been investigated using APFIM and TEM. The two steels are similar in composition with the exception of an addition of copper to P122, allowing a higher chromium content. The investigated materials were tempered at 770°C and isothermally aged at 600°C for times ranging from 0 to 10 000 h. The matrix and precipitates of type M 23 C 6 , MX and Laves phase have been analysed with APFIM. Steel P122 contains 0.9% copper and it was found that the matrix concentration of copper drops during ageing from 0.4% to an equilibrium level at 0.1%, which is in good agreement with previous thermodynamical calculations. No copper was found in M 23 C 6 , MX or Laves phase. Copper instead forms a separate phase. This phase was identified by TEM. During ageing the amount of tungsten in the matrix drops due to formation of Laves phase. This process is faster in steel P122 compared with steel P92, indicating an accelerating effect of copper on the nucleation of Laves phase. In both steels, enhanced concentrations of boron were found inside M 23 C 6 carbides. Phosphorous was found to segregate to a very narrow region at the carbide/matrix interface.

Point Defects in Binary Laves-Phase Alloys

Abstract: Point defect mechanisms in the binary C15 NbCr{sub 2} and NbCo{sub 2}, and C14 NbFe{sub 2} systems on both sides of stoichiometry was studied and clarified by both bulk density and X-ray lattice parameter measurements. It was found that the vacancy concentrations in these systems after quenching from 1000 C are essentially zero. The constitutional defects on both sides of stoichiometry for these systems were found to be of the anti-site type in comparison with the model predictions. However, thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in NbCr{sub 2} laves phase alloys after quenching from 1400 C. These could be completely eliminated by annealing at 1000 C. Anti-site hardening was found on both sides of stoichiometry for all three Laves phase systems studied. Furthermore, the thermal vacancies in NbCr{sub 2} alloys after quenching from 1400 C were found to soften the Laves phase. The anti-site hardening of the Laves phases is similar to that of the B2 compounds, while the thermal vacancy softening is unique to the Laves phase. Both the anti-site defects and thermal vacancies do not significantly affect the fracture toughness of the Laves phases.

Quasielastic neutron scattering study of hydrogen motion in C15-type

Abstract: In order to clarify the mechanism of hydrogen diffusion in cubic Laves phase , we have performed high-resolution quasielastic neutron scattering measurements on (x = 0.6 and 1.1) in the temperature range 10-320 K. It is found that the diffusive motion of hydrogen in this system can be described in terms of two jump processes: the fast localized H motion within hexagons formed by interstitial g sites and the slower hopping from one hexagon to the other. This model is also supported by neutron diffraction measurements showing that the sublattice of g sites in is split into hexagons well separated from each other. The behaviour of the elastic incoherent structure factor for suggests that only a fraction of the H atoms participate in the fast localized motion, and this fraction increases with temperature. Comparison of the properties of and other Laves phase hydrides with g-site occupation shows a clear correlation between the parameters of the two jump processes and the g-g distances (within the hexagons) and (between different hexagons).

Factors affecting the room-temperature mechanical properties of TiCr2-base Laves phase alloys

Abstract: Various effects on the room-temperature mechanical properties of TiCr 2 -base alloys have been assessed in efforts to improve the toughness of Laves phase intermetallics (AB 2 ). Systematic studies were performed on: (i) single-phase stoichiometric and nonstoichiometric TiCr 2 , (ii) TiCr 2 -base ternary Laves phases, and (iii) two-phase binary alloys containing TiCr 2 . In order to make quantitative comparisons among the different alloys, Vickers indentation was used to obtain hardness and fracture toughness values. Within the single-phase field, constitutional defects accounted for the compositional dependencies of properties, and may aid the synchroshear deformation process. Ternary Laves phases comprised of Fe, Nb, V or Mo additions to TiCr 2 indicated that alloying elements which partition to both A and B sublattices can improve the toughness. Stabilization of the cubic C 15 crystal structure also resulted in higher toughness values. Small amounts of the bcc β -phase effectively reduced crack lengths in the Laves phase, and two-phase alloys of (Ti,Cr)+TiCr 2 exhibited significant improvements in toughness.

The structures of hexagonal phases in Mg-Zn-RE (RE = Sm and Gd) alloys

Abstract: The structures of hexagonal μ-MgZnRE phases of Mg 28.3 Zn 65.2 Sm 6.5 and Mg 28.6 Zn 63.8 Gd 7.7 , were determined by single crystal X-ray structural analysis: μ-MgZnSm, P6 3 /mmc (No. 194), a = 14.619(1) A, c = 8.708(1) A, atoms/cell = 92, F(000) = 2484, μ = 27.44 mm -1 , D calc = 5.621 Mg m -3 , R = 0.0201 for the observed 1110 reflections with F obs > 4.0σ(F obs ) and μ-MgZnGd, P6 3 /mmc (No. 194), a = 14.633(2) A, c = 8.761(2) A, atoms/cell = 92, F(000) = 2527, μ = 28.51 mm -1 , D calc = 5.707 Mg m -3 , R = 0.0210 for the observed 1084 reflections with Fobs > 4.0σ(F obs ). The structure of μ-MgZnRE is closely related to that of the MgZn 2 Laves phase (P6 3 /mmc, a = 5.223 A, c = 8.566 A). The introduction of large RE elements increases the distortion of the icosahedral atomic arrangement around Zn and this produces a variety of icosahedral linkages in the μ-MgZnRE structure.

Electrochemical properties of over-stoichiometric ZrMn1-xVxNi1.4+y alloys with C15 Laves phase

Abstract: The hydrogen storage performance and electrochemical properties of ZrMn{sub 1{minus}x}V{sub x}Ni{sub 1.4+y} (x = 0.5, 0.7; y = 0.0, 0.2, 0.4, and 0.6) alloys, one type with a high amount of manganese, i.e., x = 0.5, and the other with a small amount of manganese, i.e., x = 0.7, were investigated. The relationship between discharge performance and alloy characteristics such as crystallographic parameters, surface morphology, reaction surface area, and exchange current density is also discussed. All of these alloys were found to have mainly a C15-type Laves phase structure by x-ray diffraction analysis. The equilibrium hydrogen pressure of the alloy increased as the amount of Ni increased in both types of alloys. In the case of ZrMn{sub 0.5}V{sub 0.5}Ni{sub 1.4+y} (y = 0.0, 0.2, 0.4, and 0.6) alloys, the discharge efficiency and the rate capability decreased as the amount of Ni increased, while these values increased for the ZrMn{sub 0.3}V{sub 0.7}Ni{sub 1.4+y} (y = 0.0, 0.2, 0.4, and 0.6) alloys. After measuring the specific reaction surface area and exchange current density to identify the reason why the electrode properties had a reverse tendency between the two systems, the authors confirmed that for the ZrMn{sub 0.5}V{sub 0.5}Ni{sub 1.4+y} (y = 0.0,more » 0.2, 0.4, and 0.6) alloys the major factor controlling the electrode properties was the specific reaction surface area, not the exchange current density, while, in the case of ZrMn{sub 0.3}V{sub 0.7}Ni{sub 1.4+y} (y = 0.0, 0.2, 0.4, and 0.6) alloys, mainly the exchange current density, not the specific reaction surface area, controlled the electrode properties.« less

Quantification of precipitates in advanced creep resistant 9-12% Cr steels

Abstract: In this paper a procedure is introduced for the quantification of precipitates appearing in 9-12%Cr-steels. Results gained from conventional transmission electron microscopic (TEM) investigations are compared with results from energy filtering transmission electron microscopic (EFTEM) investigations. The study was performed on a creep rupture specimen of the cast material G-X12CrMoWVNbN10-1-1 exposed at 600°C and ruptured after 33000 h. The size distribution of M23C6 carbides, MX phase and Laves phase were measured for both the un-stressed head and the stressed shank (gauge length) part of the investigated specimen. In particular, problems and sources of uncertainty concerning the quantitative determination of particle parameters in this type of steel are discussed. It is shown that quantification of the MX-particles by means of TEM bright field images is hardly possible. The size distributions of M23C6 carbides and MX phase overlap significantly which makes a separation based only on their size nearly impossible. Only Laves phase occupies a different size range. The difference between the size distributions of head and shank is pronounced only for M23C6 carbides. The measured particle distributions obey more closely a log-normal distribution rather than a normal distribution.

Microstructure and bulk reactivity of the nonevaporable getter zr57v36fe7

Abstract: We report on a structural investigation of the zirconium-based ternary alloy getter “St 707” (from SAES Getters, Milan, Italy) based on in situ x-ray diffraction (XRD) measurements at high temperature in inert gas atmosphere accompanied by scanning electron microscopy with energy dispersive x-ray analysis (SEM-EDX) and thermal analysis by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) investigations The XRD together with SEM-EDX confirms that the getter is biphasic before as well as after activation Hexagonal Zr and the cubic Laves phase Zr(V1−xFex)2 with a narrow compositional range (016⩽×⩽018) coexist in this material Upon heating, the in situ XRD shows a gradual decrease in the crystallinity of the Laves phase, which is attributed to its disintegration into small particles DSC measurements under moist Ar exhibit an exothermic irreversible transition at ≈ 675 K which is associated with chemical bulk reactions and indicative of the gettering process The operation of t

Evaluation of Zr ( Ni , Mn ) 2 Laves Phase Alloys as Negative Active Material for Ni‐MH Electric Vehicle Batteries

Abstract: Laves phase alloys of compositions (Zr, Ti)(Ni, Mn, M) X where M = Cr, V, Co, Al, and 1.9 < x < 2.1 with hexagonal C14 or cubic C15 structure have been studied in order to select the most suitable AB 2 alloys as an active material for nickel-metal hydride (Ni-MH) batteries. With the selected alloy, feasibility of MH negative electrodes using industrial technology and containing more than 97% of the alloy powder has been demonstrated. 22 Ah Ni-MH batteries for electric vehicle application have been assembled, and 600 cycles have been achieved at steady C/3 charge and discharge rates and 80% depth of discharge.

Structural stability of compounds (M = Al, Ni, Cu) studied by ab initio total-energy calculations and high-pressure x-ray diffraction

Abstract: A combination of ab initio total-energy calculations and high-pressure x-ray diffraction experiments has been used to study the phase stability of Y-based 1:2 compounds. The motivation for these investigations was to clarify the origin of the change of the structure among the compounds (T = 3d transition element) between the neighbouring elements Ni (: defect superstructure of cubic Laves phase C15) and Cu (: orthorhombic -type structure). As a test of the ab initio calculations the cubic Laves phase compound has been included in the investigations. The ab initio calculations confirm the reversal of the relative stabilities of the and C15 phases in and and give also the correct results for . Furthermore a pressure-induced structural transition is predicted for . The high-pressure x-ray experiments show that there is very good agreement between the calculated and the measured pressure dependence of the lattice parameters of up to about 10 GPa. Above 10 GPa the structure starts to become irreversibly amorphous. This instability may be a hint at the structural phase transition predicted by the calculations.

New ferromagnetic compound CaCo 2 (C15) synthesized at high pressure

Abstract: The compound CaCo2 with the C15 cubic Laves phase structure and an estimated density of 5.21 g/cm3 has been synthesized at 8.0 GPa pressure. Magnetization measurements showed that the compound CaCo2 is a ferromagnet with Curie temperature 528 K and magnetic moment per Co atom 1.75 μB at T=4.2 K. LMTO calculations of the electronic band structure showed that CaCo2 forms as a result of an s-d electronic transition of Ca and in the ground state it is a ferromagnet with a high magnetic moment per Co atom.

The anomalous mixed state of the C15-Laves phase superconductor : II. History dependence in field-cooled magnetization hysteresis

Abstract: =1 We report an unusual history dependence of the field-cooled magnetization in and around the peak-effect regime of the superconducting mixed state of . Such history effects in magnetization are shown to be absent in the peak-effect regime of the high- superconductor Bi-2212. Some qualitative similarities with and at the same time considerable quantitative differences from relatively old studies of the history dependence of the transport critical current density in single-crystal Nb and and amorphous films are examined. Our work indicates that the peak effect in is probably due to a first-order phase transition unlike the peak effect in Bi-2212, and that the latter could be due to a continuous or second-order phase transition. Differences in details of the history effects suggest that the origin of the peak effect in is likely to be different from those for these hard type-II superconductors.

Magnetism and microstructure in epitaxial TbFe 2 (111) thin films

Abstract: We report the epitaxial growth of the highly magnetostrictive Laves phase compound TbFe{sub 2}, oriented (111) on Mo (110) templates by means of molecular beam epitaxy. The film structure was investigated and related to the magnetic properties. Reflection high-energy electron diffraction showed the growth mode to be Stranski-Krastanov. Pronounced roughening was observed to follow strain relaxation, resulting in a film with deep grooves between islands. Differences of thermal expansion between the sapphire (11{bar 2}0) substrates and the magnetic epilayers place the films under tensile strain at room temperature and below. The magnetic properties of the films are analyzed as the magnetic response of single-domain particles subject to an anisotropic biaxial tensile strain. Dipolar domain coupling and in-plane strain anisotropy caused by the magnetostrictive distortion of domains are discussed for their relevance in explaining the anomalous magnetic hysteresis observed in these TbFe{sub 2} thin films. thinsp thinsp {copyright} {ital 1998} {ital The American Physical Society}