Search

The structure or short-range order of clusters in undercooled metallic melts is influenced, to some extent, by the interfacial free energy between the cluster and the melt. Analyses of the effects of interfacial energy on the cluster structure based on the Gibbs equation show a possibility that atoms in the clusters tend to be packed more loosely with the increasing cluster size (or the undercooling). Nucleation may occur, following these analyses, when clusters reach a definite size and atoms in the clusters relax to some extent to form the crystal structure. Indirect support to this viewpoint is provided by the present results of cluster-induced nucleation experiments on undercooled Ge73.7Ni26.3 alloy melts.

Droplets of Zr41Ti14Cu12.5Ni10Be22.5 glass-forming alloy with different sizes are solidified in a drop tube containerless process. The glass transition temperature Tg of Zr41Ti14Cu12.5Ni10Be22.5 glassy spheres solidified with different cooling rates is investigated by using a differential scanning calorimeter. It was found that all the amorphous spheres show an increase of Tg with the heating rate. The glassy spheres have a unique value for the glass transition activation energy Eg = 435.50 kJ/mol, which is independent of cooling rate q. The insensitivity of Tg to q is interpreted by an extension of the free volume model for flow.

Ti-based bulk metallic glasses (Ti4oZr25Cu9Ni8Be18)100-xNbx with x = 0 to 5at.% are prepared by copper-mold casting. The glass formation ability is almost unchanged by addition of Nb. The compression plasticity is, however, apparently changed, from 3% at x = 0 to 13% at x = 3, about 330% increases at the strain rate of 1 × 10−4s−1. The increment of the plasticity can be attributed to the segregation of Nb in the area of shear bands during the compression processing. An effective way to increase the plasticity of Ti-based bulk metallic glasses is thus proposed.

Cylinder-shaped Cu80 Ni20 alloy melt is undercooled and solidified by the combination of the electromagnetic levitation technique and the flux treatment method. Nearly constant temperature gradient of 8-10K/cm is realized for the cylindrical melts with different undercooling levels at the bottom ends. The experimental results reveal that with the increase of the undercooling of the melts from 35 to 220K, the microstructures undergo transition from coarse dendrites to granular grains, unidirectional dendrites, and finally to equiaxed grains.

By employing the first-principles pseudopotential plane-wave method, the physical properties of zincblende ZnO are investigated in comparison with those of the common wurtzite structure. Zincblende ZnO is predicted to be a direct gap semiconductor. Compared to the wurtzite structure, the zincblende ZnO is characterized by smaller bandgap and pressure coefficient, larger electron effective mass, increasing static dielectric constants and more covalent bonding. Furthermore, the optical properties including dielectric function and energy loss function of zincblende ZnO were obtained and analysed with some features. These aspects reveal promising applications of zincblende ZnO in optoelectronic devices.

The compression of Zr44.4Nb7Cu13.5Ni10.8Be24.3 bulk metallic glass (BMG) is investigated at room temperature up to 39 GPa using {in situ} high-pressure energy dispersive x-ray diffraction with a synchrotron radiation source. The equation of state of the BMG is obtained by the calculation of the radial distribution function. Pressure-induced structural relaxation is exhibited. It is found that below about 6 GPa, the existence of excess free volume contributes to the rapid structural relaxation, which gives rise to rapid volumetric change, and the structural relaxation results in structural stiffness under higher pressure.

The high-pressure behaviour of the superconductor MgB2with a hexagonal structure has been investigated by thein situsynchrotron radiation x-ray diffraction method under pressures up to 42.2 GPa in a diamond anvil cell. An abrupt decrease of about 7% in the unit cell volume of this material occurs in the pressure range of 26.3-30.2 GPa. A split of the Raman spectrum was also observed. The jump of the compression curve and Raman spectrum are ascribed to an isostructural transition in MgB2at a pressure of 30.2 GPa.

The Pd39Ni10Cu30P21 bulk metallic glass is isothermally relaxed under various pressures. The degree of the structural relaxation is evaluated in terms of the enthalpy recovery behaviours involved in the irreversible glass transition processes by using a temperature-modulated differential scanning calorimetry technique. A roughly Linear increase of the recovery enthalpy is observed within the experimental pressure range fi-om 2.67 to 4.45 GPa, which reflects the release of the frozen-in enthalpy in the as-quenched glass with increasing relaxation pressure. The pressure dependence of the timescale of the enthalpy recovery processes is also exhibited.

Elastic properties of platinum nitride (PtN) are studied by first-principles calculations with the fully relativistic full potential linearized augmented plane-wave (LAPW) method, the plane-wave ultrasoft pseudopotential (PW-PP) and the projector-augmented wave (PAW) methods. The results reveal that: (1) the scalar relativistic scheme is sufficient to treat the valence electronic structure, i.e. the spin-orbit effect has little effect on the bulk modulus value of platinum nitride; (2) the all-electron full potential method is no more accurate than the pseudopotential and PAW-based methods when calculating the lattice constant and bulk modulus properties of the platinum nitride; (3) platinum nitride in zinc-blende structure is unstable and its crystal structure is still an open problem.

Effects of high pressure (6 GPa) on the solid state phase transformation kinetic parameters of aluminum bronze during the cooling process are investigated, based on the measurement and calculation of its solid state phase transformation temperature, duration and activation energy and the observation of its microstructures. The results show that high pressure treatment can reduce the solid phase transformation temperature and activation energy in the cooling process and can shorten the phase transformation duration, which is favorable when forming fine-grained aluminum bronze.

Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glasses (BMG) are annealed at a temperature of 603 K under ambient and high pressures in the range of 3−6 GPa. The effect of high pressure annealing on the nanocrystallization process of compressed specimens is investigated by x-ray diffraction, differential scanning calorimetry and transmission electron microscopy. Experimental results show that the grain size of the crystalline phase decreases with the increasing pressure. For the Zr41Ti14Cu12.5Ni10Be22.5 BMG annealing at 603 K in the pressure range of 0−6 GPa, the activation energy 159.68 kJ/mol and the activation volume ΔV* = 0.94 cm3/mol are determined. The mechanism for the effects of the high pressure on the nanocrystallization process of the BMG is discussed.

The effect of yttrium addition on glass formation of a ZrCuAlSi alloy is investigated. The maximum diameter 8mm of the glassy rods for (Zr46.3Cu43.3Al8.9Si1.5)100−xYx alloy with x = 2.5 is obtained by copper mould casting. Apparent enhancement of the glass formation ability is found with addition of yttrium, mainly due to the purification of the alloy melt and the suppression of formation of the primary phases by yttrium.

The solid state reaction of Ni50Ti50 multilayer under high pressure was carried out to investigate the effect of pressure on the amorphization process. Under the pressure of 20kbars amorphization reaction could not occur at 250° C while amorphous phase formed by isothermal annealing at same temperature in vacuum as well as lower pressure. It is suggested that the effect of pressure on diffusion played a predominant role, and Ni atom should diffuse into Ti layers with vacancy mechanism rather than interstitial one.

The structural transition process of the superconducting phase in Bi1-xPbxSrCaCu2Oy (x=0, 0.2) samples has been investigated at temperatures ranging from -190°C to 800°C by means of electron diffraction. Two structural phase transitions have been found at the temperatures about 310°C and 490°C respectively for the BiSr CaCu2Oy sample, and about 310°C and 470°C for the Bi0.8Pb0.2 SrCaCu2Oy sample. After these transitions a new structural phase forms.

The A15 Nb3Si with a smaller lattice parameter (α=0.5093nm) has been synthesized from the amorphous Nb77Si23 alloy under high pressure by using Bridgman anvils apparatus. Superconductivity measurements showed that the A15 phase produced from Nb77Si23 has the highest TC (19.1K) among all that can be obtained from Nb-Si alloys.

The solid-state reaction technique has been used to form the amorphous Ni50Ti50 film. The alternative layers were prepared by an electron beam evaporating apparatus. The amorphization was determined by means of x-ray diffraction and confirmed by DSC experiments.