Showing papers on "Laves phase published in 1995"

Formation of Ti–Zr–Cu–Ni bulk metallic glasses

Abstract: Formation of bulk metallic glass in quaternary Ti–Zr–Cu–Ni alloys by relatively slow cooling from the melt is reported. Thick strips of metallic glass were obtained by the method of metal mold casting. The glass forming ability of the quaternary alloys exceeds that of binary or ternary alloys containing the same elements due to the complexity of the system. The best glass forming alloys such as Ti34Zr11Cu47Ni8 can be cast to at least 4-mm-thick amorphous strips. The critical cooling rate for glass formation is of the order of 250 K/s or less, at least two orders of magnitude lower than that of the best ternary alloys. The glass transition, crystallization, and melting behavior of the alloys were studied by differential scanning calorimetry. The amorphous alloys exhibit a significant undercooled liquid region between the glass transition and first crystallization event. The glass forming ability of these alloys, as determined by the critical cooling rate, exceeds what is expected based on the reduced glass transition temperature. It is also found that the glass forming ability for alloys of similar reduced glass transition temperature can differ by two orders of magnitude as defined by critical cooling rates. The origins of the difference in glass forming ability of the alloys are discussed. It is found that when large composition redistribution accompanies crystallization, glass formation is enhanced. The excellent glass forming ability of alloys such as Ti34Zr11Cu47Ni8 is a result of simultaneously minimizing the nucleation rate of the competing crystalline phases. The ternary/quaternary Laves phase (MgZn2 type) shows the greatest ease of nucleation and plays a key role in determining the optimum compositions for glass formation.

Quasicrystalline and crystalline precipitation during isothermal tempering in a 12Cr-9Ni-4Mo maraging stainless steel

Abstract: A thorough microstructural investigation has been performed on a high strength maraging steel of the type 12%Cr-9%Ni-4%Mo-2%Cu-1%Ti. The major precipitate formed during isothermal aging at 475 C is a quasicrystalline phase possessing icosahedral symmetry termed R{prime}-phase with a typical chemical composition of 48%Mo-33%Fe-13%Cr-2%Ni-4%Si. At 550 C the major precipitate is trigonal R-phase with a typical composition of 45%Mo-31%Fe-18%Cr-4%Ni-2%Si. At 550 C also Laves phase with a composition of 48%Mo-35%Fe-13%Cr-2%Ni-2%Si could be observed. At both 475 and 550 C an ordered phase termed L-phase precipitated. This minority phase has an ordered face centered cubic (f.c.c.) structure of type L1{sub 0}. Its composition is typically 9%Fe-4%Cr-52%Ni-15%Mo.-16%Ti-4%Al. R{prime}-phase formed at 475 C transformed to R-phase and Laves phase during aging at 550 C. In an analogous manner, R-phase and Laves phase formed at 550 C transformed to R{prime}-phase during subsequent aging at 475 C. This transformation was rationalized by a strong similarity in crystal structure between quasicrystalline R{prime}-phase of icosahedral symmetry and Frank-Kasper phases such as R-phase and Laves phase.

Elastic constants of the C15 laves phase compound NbCr2

Abstract: Elastic properties of a solid are important because they relate to various fundamental solid-state phenomena such as interatomic potentials, equations of state, and phonon spectra. Elastic properties are also linked thermodynamically with specific heat, thermal expansion, Debye temperature, and Gruneisen parameter. Most important, knowledge of elastic constants is essential for many practical applications related to the mechanical properties of a solid as well: load-deflection, thermoelastic stress, internal strain (residual stress), sound velocities, dislocation core structure, and fracture toughness. In order to understand better the physical properties and deformation behavior of the C15 compound NbCr{sub 2}, the authors have studied its elastic properties in this paper. In Section 2, the experimental methods are described, including the preparation of the sample and the measurement of the elastic constants. In Section 3, the experimental results are presented and the implications of these experimental results are discussed. Conclusions are drawn in Section 4.

Laves phase in superalloy 718 weld metals

Abstract: The important consequence of the solidification in cast or welded superalloy 718 is the segregation of Nb and the formation of laves phase. Laves phase is a brittle intermetallic topologically close-packed phase with hexagonal structure, known for its detrimental effect on mechanical properties at room temperature [1]. Although data available with regard to wrought materials is somewhat elaborate it is not true for welds in general and for electron beam welds in particular. In this letter the formation of laves phase in high heat input gas tungsten arc (GTA) and low heat input electron beam (EB) welds of 2 mm thick superalloy 718 in as-welded and post-weld heat treated (PWHT) conditions is evaluated. The results have a bearing on the tensile ductility and other properties of welds. Sheets of superalloy 718 of thickness 2 mm in solution treated condition (chemical composition in Table I) were autogenously welded by automatic GTA and EB welding processes, resulting in full penetration using the weld process parameters listed in Table II. The as-welded samples were subjected to two PWHT schedules: direct duplex ageing and solution treatment followed by ageing. Solution treatment was carried out at 980 °C for 20 rain with air cooling and the duplex ageing was carried out at 720 °C for 8 h with furnace cooling to 620 °C for 8 h with air cooling. The as-welded and heat treated samples were then subjected to scanning electron microscopic (SEM) examination and quantitative electron probe micro-analysis (EPMA) for the analysis of microsegregation of elements and determination of the formation of laves phase. Figs 1 and 2 show SEM micrographs of the aswelded microstructures of EB and GTA weld metals, respectively. Essentially, the solidified structure is of dendritic type. EB weld metal showed relatively finer

High temperature mechanical properties of C15 Laves phase Cr2Nb intermetallics

Abstract: High temperature deformation behavior of C15 Cr2Nb intermetallic compounds was observed by compression tests. These intermetallic compounds are deformable at temperatures above 1473 K. The flow stress decreases with increasing temperature and is little sensitive to small changes in alloy composition. Stress-strain curves are sensitive to temperature and also to the strain rate. At intermediate temperatures (also at intermediate strain rates) an apparent stress peak and a subsequent decrease in flow stress were observed. At high temperatures (also at low strain rates) a steady state flow was observed immediately after yielding. The stress exponent in the constitutive equation is about 3.5 while the apparent activation energy is 470 kJ mol−1. The activation of a considerable amount of dislocations was observed at temperatures where compressive plastic deformation is accomplished.

Improvement of Creep Rupture Strength of High Cr Ferritic Steel by Addition of W

Abstract: The effect of W addition on creep rupture strength (CRS) of high Cr ferritic steel was evaluated with three kinds of steels; base steel of 9Cr-1Mo-V-Nb-N, 0.7%W addition to base steel, and 1.7%W addition with decreased No addition to 0.5%. The latter two steels have the same values of Mo+0.5W viz. Mo equivalent. Further, the reasons for the beneficial effect of W was analyzed. The CRS at 600°C for 1000 h increases with W addition at a rate of 35 MPa/%W. As the hardness remains almost constant during the creep test, the enhancement of CRS could not be attributed to the formation of precipitate or cluster during the creep test. The W addition was found to increase the partition of Nb to VN, resulting in the lattice expansion of VN and enhancing the CRS through coherency strain around VN. This effect accounts for about half of the increase of CRS by W addition.The W addition also causes the precipitation of a film-like Laves phase along the subgrain boundary. Inconsequently, Cr2C precipitates simultaneously. The Laves phase contributes to the other half of the CRS increase, possibly through suppressing the growth of subgrain during the creep test.

Mechanically induced structural and magnetic changes in the GdAl2 Laves phase.

Abstract: Changes of the structure and magnetic behavior of the ${\mathrm{GdAl}}_{2}$ Laves phase by high-energy ball milling were followed by x-ray diffraction, ac magnetic susceptibility, and dc magnetization measurements. The starting compound crystallizes in the cubic ${\mathrm{MgCu}}_{2}$ structure and is a ferromagnet at lower temperatures with a Curie temperature of about 170 K. Upon mechanical milling, the ordered ${\mathrm{GdAl}}_{2}$ compound is found to become atomically disordered nanocrystalline material. Atomic disorder in ${\mathrm{GdAl}}_{2}$ is evidenced by the disappearance of a number of x-ray diffraction peaks, the decrease of lattice parameter, the increase of lattice strain, and the drastic changes of high-field magnetization curve at 4.2 K and of the nature of magnetic ordering upon milling. On the basis of experimental observations, a special type of atomic disorder namely quadruple-defect disorder is suggested. A constant average-crystallite size of about 21 nm is found after relatively short periods of milling. The material remains in the same crystalline structure as the starting compound even after prolonged periods of milling. While the Curie temperature of ${\mathrm{GdAl}}_{2}$ is slightly decreased by milling, another magnetic phase is detected with a magnetic ordering temperature of about 45 K during the intermediate stage of milling. This magnetic phase has been proved to be a spin-glass phase [Zhou and Bakker, Phys. Rev. Lett. 73, 344 (1994)]. With increasing milling time, both the fraction of the spin-glass phase and its freezing temperature ${\mathit{T}}_{\mathit{f}}$ increase, while the amount of ferromagnetic phase decreases. After long-period milling the original ferromagnetic phase disappears and the material completely becomes the spin-glass phase with a final ${\mathit{T}}_{\mathit{f}}$ of 65 K. Therefore, by mechanical milling well-defined nanocrystalline spin glass is created. The appearance of this spin-glass phase is due to accumulation of quadruple-defect disordering in the lattice of the ${\mathrm{GdAl}}_{2}$ Laves phase. The importance of using magnetic measurements as a structural probe is emphasized.

Gd substitutions in the TmCo2 Laves phase: the onset of long-range magnetic order in the itinerant subsystem

Abstract: We found that in TmCo2 the molecular field does not exceed the critical value necessary to induce long-range magnetic order in the Co d-electron subsystem, which is in contrast to the other heavy RCo2 compounds. Below Tc=3.8 K a first-order magnetic phase transition at 3.4 K exists, which is due to a rearrangement of the Tm 4f magnetic moments. When substituting Tm by Gd long-range magnetic order appears in the Co d subsystem. The onset of the magnetic order in the d-electron subsystem can most clearly be seen from thermal expansion measurements. There is an increasing positive volume anomaly with increasing Gd content below Tc. The estimated magnetostriction constant lambda 111 of TmCo2 is -4.1*10-3, in agreement with the single-ion model. Pronounced discontinuities at Tc are characteristic for the transport properties (resistivity and thermopower) for TmCo2 as well as for the pseudobinary Tm1-xGdxCo2 compounds. At elevated temperatures the physical properties of all these compounds are dominated by spin fluctuations.

Stacking fault energy of the NbCr2 Laves phase

Abstract: A total energy study has been performed on the NbCr2 Laves phase using first-principles electronic structure calculations based on the full-potential linear muffin-tin orbital method. For the two Laves phase structures, C15 and C14, cohesive energies and heats of formation were obtained. A method was developed to calculate the stacking fault energy, γ, in C15 Laves phases, using only two quantities: the cohesive energy difference between C15 and C14 Laves phases, and the C15 lattice constant. For C15 NbCr2, the calculated stacking fault energy is 90mJ m−2. The calculated result is in good agreement with an experimental result deduced from an extended dislocation node.

Icosahedral and related phase formation in rapidly quenched Ti-Zr-Fe alloys

Abstract: We report the formation of the icosahderal phase (i-phase) in some rapidly quenched Tix, Zry, Fez, (45 ≤ × ≤ 70; 5 ≤ y ≤ 25; 20 ≤ z ≤ 30) alloys. Transmission electron microscopy (TEM) studies show that unlike the Ti-Zr-Ni i-phase, i(TiZrFe) is strongly disordered, showing strong spot anisotropy and intense localized diffuse scattering. These features, and a smaller quasilattice constant for i(TiZrFe) (a 0 = 4.85A) than for i(TiZrNi) (a 0 = 5.12), indicate that i(TiZrFe) is more closely related to the icosahedral phases found in Ti-3d transition metal-Si alloys. TEM studies also reveal many related crystalline phases dominated by local polytetrahedral order, including α(TiZrFe), a bcc phase that is a 1/1 approximant to the i-phase, the λ-phase, an orthorhombic phase that appears to be an approximant to a decagonal phase, a Laves phase of the hexagonal MgZn2-type, and a Ti2Ni-type fcc phase. Evidence for ordering in the α(TiZrFe) and bcc solid solution phases is presented.

Aging phenomena before the precipitation of the bulky Laves phase in Fe-10%Cr ferritic alloys

Abstract: The detailed study through microstructural observation on the initial stage of precipitation behavior of the Laves phase in the 9--12%Cr ferritic steels is slightly difficult, because the matrix phase is martensite containing a high number density of dislocations and, secondly, the similar size and shape of carbides are formed with the Laves phase during aging treatments. In the present research, the precipitation behavior of the Laves phase, particularly, focusing on an initial stage of it, was investigated using sample Fe-10%Cr ferritic alloys.

Magnetic properties of C14-laves phase RMn2

Abstract: The temperature dependence of magnetic susceptibility and magnetization of metastable C14-RMn 2 (R=Y, Gd, Tb, Dy and Yb), synthesized at high pressures, has been measured YMn 2 is an itinerant paramagnet over the whole range of temperatures GdMn 2 , TbMn 2 and DyMn 2 are ferromagnetic at low temperatures, while their susceptibilities beyond T C obey the Curie-Weiss law YbMn 2 is a paramagnet down to 42 K whose susceptibility suggests antiferromagnetic interactions These magnetic properties are discussed together with those of other stable C14-RMn 2 Similary to stable C14- and C15-RMn 2 , the appearance of local magnetic moments of itinerant 3d Mn electrons of C14-RMn 2 at low temperatures are determined by the Mn–Mn distance The Mn atom has a magnetic moment for light rare earths, whereas no moment for heavy rare earths The critical substance for the onset of Mn moments is TbMn 2

Hydrogen-occluding alloy and hydrogen-occluding alloy electrode

Abstract: Disclosed are a hydrogen-occluding alloy electrode comprising an alloy powder wherein the alloy comprises a Ti--V solid solution mother phase and a secondary phase predominantly containing a Ti--Ni phase or an AB 2 Laves phase in which the secondary phase forms a three-dimensional reticulate skeleton in the alloy; said hydrogen-occluding alloy powder surface-treated with hydrofluoric acid to provide a titanium hydride surface and surface-coated with at least one metal of Ni, Cu and Co.