W. K. Wang

Influence of pressures on the crystallization process of an amorphous Fe73.5Cu1Nb3Si13.5B9 alloy

Amorphous Fe73.5Cu1Nb3Si13.5B9 alloys, prepared by a melt-spinning technique, were annealed at a temperature of 823 K under pressures in the range of 1–5 GPa and ambient pressure. The high pressure experiments were carried out in a belt-type pressure apparatus. The microstructure of the annealed alloys has been investigated by x-ray diffraction, electron diffraction, and transmission electron microscopy. Experimental results show that the initial crystalline phase in these annealed alloys is α-Fe solid solution (named as α-Fe phase below), and high pressure has a great influence on the crystallization process of the α-Fe phase. The grain size of the α-Fe phase decreases with the increase of pressure (P). The volume fraction of the α-Fe phase increases with increasing the pressure as the pressure is below 2 GPa, and then decreases (P>2 GPa). The mechanism for the effects of the high pressure on the crystallization process of amorphous Fe73.5Cu1Nb3Si13.5B9 alloy and latent applications of high-pressure anne...

Nucleation and growth of feather-like boron nanowire nanojunctions

Nanojunctions made from one-dimensional nanostructures could provide fascinating opportunities to reveal new mesoscopic physics and could offer huge technological potential as well. Here we report highly controlled fabrication of boron nanowire nanojunctions with unilateral feather-like morphology by radio-frequency magnetron sputtering of a boron and boron oxide mixture in argon atmosphere. The branched boron nanowires nucleated on the same sidewall of the stems, grew airwards, and aligned in parallel to form multiple T- and/or Y-type nanojunctions. The boron nanofeathers self-assembled into large-scale, highly ordered arrays on various substrates without the use of patterned catalysts. This process might enable the creation of nanometre-size heterojunctions of a wide variety of one-dimensional nanostructures.

Boron nitride nanotube branched nanojunctions

One-dimensional boron nitride (BN) nanotube nanojunctions are of great interest for both fundamental and applied research because of their stable and excellent mechanical and physical properties. Large quantities of highly pure BN nanotube multiple Y- and/or T-junctions were synthesized by annealing pure boron nanowire precursors in N-2 atmosphere at 1500 degrees C. Transmission electron microscopy and electron energy-loss spectroscopy studies reveal that the products possess a concentric tubular structure and stoichiometric BN composition. The boron nanostructured precursors react with nitrogen at high temperature to form the corresponding BN components, retaining their original morphologies. A non-equilibrium interdiffusion of nitrogen and boron might be involved in the transformation process of the solid boron nanowires to hollow BN nanostructures. Our results illustrate the technological potential of BN nanoscale multiple junctions being incorporated into future nanoscale mechanical and electronic devices.

Evaluation of effective mass transport coefficients through comparison of solidification on the ground and on board a satellite

Mass transport caused by buoyancy convection in front of the solid–liquid interface was evaluated in terms of measurements of primary dendritic spacing combining with separation of the effective (or integral) mass transport coefficient DL. It was shown that DL in normal gravity (1g) condition was 1.64 times as high as that in microgravity (μg) condition at the cooling rate (v) of 0.056 K/s for Pd40Ni40P20 alloy. The higher DL value is due to the contribution of buoyancy convection on the ground.

Collective interdiffusion in compositionally modulated multilayers

Interdiffusion in Fe–Ti, Ag–Bi, Fe–Mo, Ni–Si, Mo–Si, Nb–Si, and Ag–Si multilayers (MLs) was investigated by an in situ low-angle x-ray diffraction technique. Temperature-dependent interdiffusivities were obtained which can be described as Arrhenius relations. The interdiffusion characteristics of the MLs were summarized. The extremely small values of the prefactor D0 and the marked correlation between the D0 and activation energy He for interdiffusion suggest that a collective atomic jumping mechanism involving 8–15 atoms govern the interdiffusion in the MLs.

Interactions between the interface of titanium and fullerene

X‐ray diffraction, photoemission, and Auger electron spectroscopy studies are reported of the interactions between the interface of titanium and C60 solid film during low‐temperature annealing. The structure of C60 at the Ti/C60 interface is disrupted by the Ti atoms when Ti is deposited onto the surface of C60 film. Titanium atoms react with carbon atoms to form amorphous Ti carbide during low‐temperature annealing. This interaction is related to the solid state amorphization reaction of the deposited Ti layer with C60 film, which occurs with a driving force of a negative heat of formation in the Ti–C system and a dominant diffusion of carbon into the Ti overlayer.

Synthesis of porosity-free nanocrystalline materials with ultrafine grain size by annealing amorphous alloy under high pressure

A porosity-free nanocrystalline Fe735CulNb3Si135B9 alloy with an ultrafine grain size of 4-6 nm has been synthesized. Experiments were carried out by annealing amorphous Fe735CulNb3Si135B9 alloy at a temperature of 823 K under a pressure of 5 GPa in a belt-type pressure apparatus. The microstructure of the samples annealed respectively under high pressure and normal pressure was investigated using X-ray diffraction (XRD), electron diffraction (ED) and transmission electron microscopy (TEM). Analysis results show that the high pressure makes the size of crystalline phase become much smaller,from 10-12 nm for the normal pressure to 4-6 nm for 5 GPa, without changing the lattice type of the crystalline phase, which is attributed to the effects of pressure on the crystallization process of amorphous alloy

Effects of pressure on the solidification of Al–Mn alloy

The solidification of A1-20 wt% Mn alloy was investigated under pressures up to 6 GPa, It was found that the solidification products under pressures below 4 GPa were about the same, composed mainly of Al and Al6Mn. A new Al-Mn phase with needle like morphology and Al nanocrystallites in size less than 20 nm were obtained in the quenched alloy at 6 GPa. Structure analysis by transmission electron microscopy and x-ray diffraction indicated that the new phase had a C-center othorhombic unit cell with lattice constants of a = 0.7565(4) nm, b = 1.2965(6) nm, and c = 0.7801(6) nm, The composition was determined to be Al77.5Mn22.5 by election probe microanalysis. The phase evolution during solidification under different pressures was discussed. Our experimental results show that the pressure, as a basic thermodynamic variable like temperature, may play an important role on the solidification of the alloy.

Influence of buoyancy convection on solute distribution in Pd40Ni40P20 alloy

The differences in the solute distribution in microstructure of Pd40Ni40P20 alloy solidified on board a Chinese retrievable satellite and on the ground were studied. In comparison with those crystallized under normal gravity conditions (1 g), it was found that the P content was lower, but the Pd content was higher in the primary phase in microgravity conditions (μ g). In the eutectic region the P content, however, was increased but the Pd content was decreased. The differences in solute distribution crystallized under 1 g and μ g conditions show the influence of buoyancy convection on the mass transport coefficient in liquids under normal gravity conditions.

NUCLEATION OF THE AL4MN ALLOY DURING CONTAINERLESS SOLIDIFICATION IN A DROP TUBE

The undercooling and nucleation of Al4Mn alloy have been investigated by containerless solidification in a 1.2 m long drop tube, where molten droplets of the alloy solidified in the vacuum. Droplets in various sizes were collected at the bottom of the tube. It is found by x‐ray and electron diffractions that besides the orthorhombic Al6Mn and β‐Mn phases, two approximants of the decagonal quasicrystal, one approximant of the icosahedral quasicrystal and decagonal domains can be observed in the as‐solidified droplets. However, due to the different cooling rate or undercooling in the samples of different sizes, the phases mentioned above will appear in different amounts. The relationship between the phase composition and the sample size is discussed on the basis of the classical nucleation and growth theories.