Showing papers by "University of Science and Technology Beijing published in 1992"

Superconductivity and magnetic properties in Pr0.2Yb0.8−xLaxBa2Cu3O7−δ

Abstract: The crystal structure, superconductivity and magnetic properties of the title compounds have been investigated. The results revealed that with the concentration of Pr being constant, the transition temperatureTc of this system decreases with the increasing of La, or rather with the increasing of effective ionic radii of rare earth elements and the superconductivity disappears at aboutx=0.6. The coefficient of electron specific heat and the density statesN (0) which were infered from the magnetic measurements also decrease with the increasing of the effective radii. The relation between the radii of rare earth elements and hybridization is briefly discussed.

Carlsson-Gelatt-Ehrenreich technique and the Möbius inversion theorem.

Abstract: The M\"obius inversion theorem is first applied to different lattices so as to improve the Carlsson-Gelatt-Ehrenreich technique and give very fast convergence.

Oxidation and mechanical behavior of intermetallic alloys in the TiNbAl ternary system

Abstract: The isothermal oxidation behavior in a temperature range from 900 to 1100 °C and mechanical properties of intermetallic alloys with a wide range of compositions in the TiNbAl system were investigated A composition range of the alloys having excellent oxidation resistance is suggested The oxidation resistance maps summarizing the results of weight gain data and the constituents of oxide scale were established, as were the compressive strength maps as well The lowest weight gain was determined to be 1–2 mg cm −2 and 2–3 mg cm −2 for oxidation at 1100 °C and 1200 °C for 100 h respectively The compressive yield strength at 1100 °C and strain rates of 10 −2 s −1 and 10 −5 s −1 reached 450 MPa and 90 MPa respectively

Oxidation of Intermetallic Alloys in Ti-Al-Nb Ternary System

Abstract: The isothermal oxidation behavior of intermetallic alloys over a wide range of compositions in the Ti-Al-Nb system was investigated in a temperature range from 900 to 1,200°C. The results showed that the Al content and the ratio of Ti-to-Nb of the alloys affected the oxidation behavior in this ternary system. Al improved the oxidation resistance of alloys. If the ratio of Ti-to-Nb was either too high or too low, the oxidation resistance was reduced dramatically. The oxide constituents were determined. The formation of an integral protective Al2O3 scale layer was the basic condition for good oxidation resistance. A composition range of the alloys, having excellent oxidation resistance, was suggested. A map summarizing the results of the oxidation resistance and the oxide scale type was established. The oxidation mechanism was also discussed.

The structural model of Al-Mn decagonal quasicrystal based on a new AI-Mn approximant

Abstract: A new Al-Mn orthorhombic crystalline phase isostructural with Al60 Mn11 Ni4, Al20 Mn3 Cu2, and the T3 AlMnZn phase has been found in coexistence with the decagonal quasicrystal in a rapidly solidified Al-Mn alloy. From the structure of this Penrose-tiling approximant two sets of Penrose-type tiles were derived. A structural model of the Al-Mn decagonal quasicrystal based on these tiles was proposed. This model agrees well with the recent experimental X-ray structural determination of the Al-Mn decagonal quasicrystal.

Hydrogen effects on brittle fracture of the titanium aluminide alloy Ti-24Al-11Nb

Abstract: Varying amounts of hydrogen were dissolved in the titanium aluminide alloy Ti-24Al-llNb (atomic percent). Virtually all of this hydrogen probably precipitated as hydride on cooling because the terminal solubility in the dominant Ti3Al phase is very low at room temperature. Although the yield strength (YS) increased, the ultimate tensile strength (UTS), ductility, fracture stress in notched bend bars, and fracture toughness decreased with increasing amounts of hydride. The strength and fracture properties, for all hydride contents, did not change with testing speed below about 5 to 50 mm/min but decreased steeply for speeds greater than that. The presence of hydride decreased the critical value of testing speed by about an order of magnitude. Brittle cracks in bluntly notched bend bars, with or without hydride, nucleated at the notch root or at a distance below the root which was less than one fifth of the distance to the peak stress location. This result suggests that the cleavagelike cracking in this material is not controlled by normal stress alone but has some dependence on the applied strain. The fracture surfaces of notched or precracked specimens, with or without hydride, consisted entirely of cleavagelike fracture, but these cracks exhibited stable crack propagation. This permitted both the measurement of crack resistance or R curves and also observation of the initiation and propagation of the crack with increasing KI. The results showed that cracks initiated discontinuously at characteristic sites within the plastic zone and along the slip bands when the plastic deformation ahead of the precrack developed to a particular and reproducible extent. Literature cleavage models were compared to results for the present tests.

Orthorhombic crystalline approximants of the Al-Mn-Cu decagonal quasicrystal

Abstract: Two orthorhombic Al–Mn–Cu phases, π-Al20Mn3Cu2 and Y-AlMnCu, with large lattice parameters were found to coexist with the decagonal quasicrystal in Al10Mn3Cu0.1-0.2 alloys and they were shown to be crystalline approximants of the latter. These two phases have similar structural subunits and a close orientation relationship. They twin easily and the twin boundary of the π phase is a layer of the π phase and vice versa. Therefore, these two phases intergrow frequently and both translational and rotational twin domains are abundant.

Transient free convection and thermal stratification in uniformly-heated partially-filled horizontal cylindrical and spherical vessels

Abstract: An integral approach has been used to analyze the development of the free convection boundary layer on heated concave surfaces, such as those in horizontal cylinders or a sphere. Based on the non-dimensional laminar and turbulent velocity and temperature profiles closed form expressions for the boundary layer thickness, velocity scale as well as the boundary layer commencement after the point of instability are obtained. In addition, the mass flowrate to the thermal stratified region is given.

Structure of Al3Mn, an orthorhombic approximant of the decagonal quasicrystal

Abstract: A structural model of the orthorhombic approximant Y-A13Mn of the AlMn decagonal quasicrystal has been derived from another orthorhombic approximant π-AlMn. There are 124 Al atoms and 32 Mn atoms, with a composition of A13·88Mn, in a unit cell with a = 1·487 nm, b = 1·247 nm and c = 1·256nm, satisfying the symmetry of the space group of Pnma (or PnZ1a after refinement made on single-crystal X-ray analysis). The calculated X-ray powder diffraction pattern agrees well with the experimental results. The high-resolution electron microscopy image of Y-Al3Mn has been correlated with the [010] structural projection along the pseudo-tenfold axis.

Coercive Field in Sm2Fe17Nx Hydrogen Decrepitated Magnets

Abstract: The coercivity mechanism of Sm2Fe17Nx hydrogen decrepitated magnets is analysed on the basis of micromagnetism. The discrepancy between the experimentally realized coercivity and the ideal nucleation field as predicted by Brown is also found in Sm2Fe17Nx magnets. This discrepancy is due to the effect of the microstructure. Our analysis strongly indicates that the coercivity in Sm2Fe17Nx hydrogen decrepitated magnets is controlled by the nucleation mechanism. The anisotropy constants K1, K2, and the saturation polarization Js of Sm2Fe17Nx in the temperature range from 200 to 620 K are determined from the analysis of the approach to ferromagnetic saturation. Der Koerzitivfeldmechanismus von durch Wasserstoffversprodung hergestellten Sm2Fe17Nx-Magneten wird unter Zugrundelegung der Theorie des Mikromagnetismus analysiert. Der Unterschied zwischen den experimentell festgestellten Koerzitivfeldstarken und dem von Brown vorhergesagten idealen Keimbildungsfeld wird auch im Falle der Sm2Fe17Nx-Magnete beobachtet. Dieser Unterschied wird auf die Rolle der Mikrostruktur zuruckgefuhrt. Unsere Analyse gibt starke Hinweise darauf, das das Koerzitivfeld in durch Wasserstoffversprodung hergestellten Sm2Fe17Nx-Magneten durch einen Keimbildungsmechanismus bestimmt ist. Im Temperaturbereich von 200 bis 620 K werden die Anisotropiekonstanten K1, K2 und die Sattigungspolarisation von Sm2Fe17Nx mit Hilfe einer Analyse der Einmundung in die ferromagnetische Sattigung bestimmt.

Discommensurate microstructures in phason-strained octagonal quasicrystal phases of Mo-Cr-Ni.

Abstract: Phason-strained octagonal quasicrystal phases of Mo-Cr-Ni have been studied by electron diffraction and dark-field imaging. Diffraction patterns of the twofold axes show that some spots are shifted and split along the fourfold axis. Discommensurations (DCs) observed in dark-field images of the split reflections are aligned perpendicular to the fourfold axis. The density of the DCs in widely separated and adjacent regions may vary by more than 1 order of magnitude. Our observations are consistent with some of the results of the two-dimensional octagonal random tiling model of Li, Park, and Widom.

A dislocation model of shear fatigue damage and life prediction of SMT solder joints under thermal cycling

Abstract: A damage model of low cycle shear fatigue has been developed in dislocation theory. On the basis of this model, a formula including the factors of thermal cycle temperature, dwell time, and atmospheric oxidation has been established to predict the life of surface mount technology (SMT) solder joints under thermal cycling. This formula has been verified by experiments on the specimens of actual SMT assemblies. The results show that the life formula established coincides with the experimental results. >

Morphology of molybdena fractal clusters grown by vapour-phase deposition

Abstract: Molybdena fractal clusters of various shapes were obtained experimentally in an evaporation-deposition system The open ramified deposits grew after heating MoO3 powder at 750–900 °C for 05–2 h They demonstrate two structural forms: the stack-like formation and the net-like one Scanning electron microscopy observations revealed that the clusters consist of one of three different microstructures: needle-like whiskers, ribbon-like crystals, and fascicular crystals Each pattern displays the microscopical anisotropy although they are of macroscopical self-similar feature A nucleation-aggregation model is suggested to explain the non-equilibrium growth processes