Disordered Fcc Structure Research Articles

Neutron-diffraction study of the rare-gas interstitial fullereneArC60

Neutron powder diffraction studies have been carried out between room temperature and 15 K on ${\mathrm{C}}_{60}$ powder, which has been hot isostatically pressed (HIPed) under 170 MPa of Ar and at 300 \ifmmode^\circ\else\textdegree\fi{}C to give ${\mathrm{ArC}}_{60}$. Rietveld analysis of the neutron diffraction patterns confirms the stoichiometry, in agreement with our earlier x-ray analysis. This analysis shows the Ar to be trapped in octahedral interstitial sites of a fcc lattice with a unit cell of \ensuremath{\sim}14.230 \AA{} at 285 K, which is unexpectedly close to that of pure ${\mathrm{C}}_{60}$\ensuremath{\sim}14.218 \AA{}. A phase transition from an orientationally disordered fcc structure to an ordered simple cubic structure occurs on cooling between 245 and 250 K, which is lower than found for pure unHIPed ${\mathrm{C}}_{60}$ (255--260 K). The Rietveld refinement of the diffraction pattern at 15 K indicates that the Ar does not influence the orientation of the ${\mathrm{C}}_{60}$ molecules.

Formation of nanocrystalline Cu 70Ti 20Ni 10 powders by mechanical alloying

The formation and thermal stability of nanocrystalline Cu 70Ti 20Ni 10 powders by mechanical alloying has been studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). After milling, the material has a disordered fcc structure, with nanocrystalline grain sizes around 4.7 nm determined from the fcc (111) diffraction peak. The DSC signal monotonically decreases with isothermal transformation owing to grain growth. The crystal grains do not grow by heating until the intermetallic TiCu 3 and NiTi phases nucleate at 450 °C.

Accurate Measurement of Lattice Misfit Between γ And γ' Phases in Nickel-Base Superalloys at High Temperatures

Accurate measurement of the lattice misfit between the γ and γ phases in Ni-base single crystal superalloys at high temperatures has successfully been achieved using a parallel beam mode X-ray diffractometer with a high temperature specimen stage. The superalloys have γ' precipitates, an ordered FCC structure based on Ni3Al, in a γ-matrix having a disordered FCC structure (LI2). The parallel beam mode, the optics of which is made from a Ge (111) channel cut crystal, generates a superior monochromatic and parallel CuKα1 beam, which makes it possible to resolve a cluster peak in Ni-base superalloys into individual γ and γ ‘ peaks. This peak separation was almost impossible using the CuKα1+2 doublet beam from the standard type para-focusing mode, due to the very small difference in lattice parameters between the two phases.

Thickness effects on the magnetic hysteresis of Co-Pt films

The magnetic and structural properties of CoxPt100−x films with x=40,45 and 50 at%, have been investigated. The magnetic studies were focused on the dependence of magnetic hysteresis on the film thickness. The sample thicknesses varied between 10 and 200 nm. The coercivities of the as-grown, disordered fcc structure films did not change very much with thickness for all compositions studied. Upon annealing at 650 and 700°C, a structural transformation takes place to the ordered face centered tetragonal (fct) phase which has a high anisotropy resulting in large coercivities that show a pronounced thickness dependence for all the compositions studied.

Determination of the Coherency Strain of γ and γ′ Phases in Nickel-Base Superalloys at High Temperatures

AbstractA method for using syncrotoron-radiation parallel-beam X-ray diffractometry for precise lattice parameters and strains of γ-γ′ type Nickel base superalloys at elevated temperature is described. The superalloys have γ′ precipitates which are an ordered L12 structure based on Ni3Al, in y-matrices having a disordered FCC structure. Lattice misfit between γ and γ′ phases was very small and peaks reflected from γ and γ′ phases made unresolved clusters of peaks.Profile fitting with a pseudo-Voigt function is used to resolve overlapping peaks. Instrumental broadening of the peak profile was removed using a deconvolution method. The standard errors of the calculated peak angle were less than 0.002°. The elastic strain of the γ′ precipitates in the alloys were smaller than those of γ-matrices.

Phase transformation behaviour of a low carat gold alloy for porcelain bonding during continuous heating

The phase transformation associated with hardening in a low carat gold alloy for porcelain bonding during continuous heating was studied by resistivity measurements, hardness tests, X-ray diffraction and scanning and transmission electron microscopies. A short-range order structure, based on Au-Pd, was produced at a lower temperature. At an elevated temperature, two types of precipitation were observed. One is a nodular reaction consisting of an L12 ordered structure based on Pd3Sn and a disordered FCC structure at grain boundary. The other is a formation of spherical L12 ordered precipitates based on Pd3Sn within the grain followed by Ostwald ripening. A fine dispersed precipitate of the latter contributed to the remarkable hardening.

Auger analysis of the fracture surfaces of ordered Ni 4Mo

The high temperature disordered FCC structure (alpha) in Ni-20 at. % Mo (Ni/sub 4/Mo) orders to a D1a structure (beta) upon aging at temperatures below the critical temperature of 1141 K. The ordering reaction occurs by the rearrangement of the atoms on the FCC lattice, so that the ordered beta retains a close-packed structure and a crystallographic relation to the alpha from which it is formed. The domains in beta form with six crystallographic variants relative to the alpha, leading to three types of domain boundaries. Upon isothermal aging, the structure of the fine domains developed inside the former alpha crystals is such that the beta crystals (domains) are those which were inherited from the former alpha crystals. In the range 973 - 1073 K, these high angle boundaries migrate, leaving a coarser beta domain structure in their wake. Upon aging to form beta, the strength and hardness increase, the magnitude depending upon the aging temperature. Concomitant with this strengthening is a decrease in ductility (e.g., from 80% to < 5% elongation at fracture). Fracture occurs along the high angle boundaries (the former alpha boundaries) even when these boundaries have migrated. In order to assess the role of grain boundarymore » segregants in the intergranular embrittlement in ordered Ni/sub 4/Mo, the fracture surfaces of samples ordered under two aging conditions have been examined using Auger Electron Spectroscopy. The results are discussed in terms of segregation-induced embrittlement.« less

Atomic Short Range Order and Pair Interaction of Disordered Au4Mn by Neutron Diffraction

Atomic diffuse scattering from a single crystal of Au 4 Mn with the disordered fcc structure was observed by neutron diffraction in the three dimensional reciprocal space. A cigar type diffuse pattern along the line of [1, K , 0] with three broad maxima at K =0.27, 0.50 and 0.73 was obtained. After some corrections and an arrangement of the pattern at the equivalent positions in the reciprocal space, the parameters of atomic short range order and pair interaction up to the 50th neighbor were determined by a method presented by Adachi-Nakashima: J. Phys. Soc. Japan 40 (1976) 25. The reproducibilities from the order parameters and the pair interactions thus determined were examined. The former gives a good reproducibility but the latter not. Origins of the pair interaction of this alloy were discussed.

Structural transformations in U 3Si

X-ray diffraction analysis has been used to examine the phase change introduced in U 3Si thermally, by irradiation at 60°C and by plastic deformation. Short irradiation exposures (burnup ⩽ 0.12 × 10 −3% of all atoms) and plastic deformation modified the room temperature structure of U 3Si, be tetragonal, to a disordered fcc structure, while heating above 756 ± 5 ° C caused an allotropic transformation to an ordered fcc structure of the Cu 3Au type. Lattice parameter determinations indicated that the irradiation-induced phase transformation was accompanied by a volume increase of 0.6%. Further irradiation resulted in a nearly complete destruction of the diffraction pattern as the U 3Si became amorphous. The damage introduced by irradiation or by plastic deformation annealed in the range 250–500 °C with a complete recovery to the bc tetragonal structure.

Effects of Mechanical and Thermal Treatment on the Structure and Magnetic Transitions in FeRh

Well-annealed bulk samples of the intermetallic compound FeRh exhibit a first-order transition from antiferromagnetic to ferromagnetic order on heating near 330°K. Plastic deformation has been found to convert the normal CsCl-type ordered structure of FeRh to a disordered fcc structure, which is only weakly magnetic and does not exhibit the first-order transition. On annealing the disordered material at Ta=510°K, three stages of effects have been observed. In the first, the fcc phase converts to a highly ordered CsCl-type structure, the magnetization at Ta increasing with a time constant of ∼10 min; the material is strongly ferromagnetic down to low temperatures. In the second stage, there is a much slower (τ≈400 min) increase in magnetization at Ta with a gradual return of the first-order transition at lower temperatures. In the third stage, there is no further increase in magnetization at Ta, but the first-order transition continues to increase in completeness and sharpness. The slow return of the first-order transition is attributed to the gradual attainment of perfect long-range order or, alternatively, to the annealing out of defects produced by plastic deformation.