Chen Zhenhua

Solid-liquid mixed casting of Al-Si alloy

The paper presents a novel material preparation technology—Solid-liquid mixed casting technology. In the technology, large amounts of homogeneous alloy powder or heterogenous powder with perfect wettability are added into the superheated melt. After strong agitation, the mixed melt can be cast or hot processed. Applying solid-liquid mixed casting, three kinds of Al-Si alloys were investigated. The results show that, when the mass of powder accession to alloy melt is about 1, the mean size of primary Si in hyper-eutectic alloy can be controlled at less than 5 µm; and the mean grain size of α phase in hypo-eutectic alloy is less than 10 µm. This technology has the advantage of preparing material with very fine microstructure by fairly simple casting process, and may be a new practicable and valuable metal preparation technology.

Effect of Substrate Temperature on Carbon Nitride Thin Films Prepared by Radio Frequency Sputtering

Carbon nitride thin films have been deposited by radio frequency sputtering at different substrate temperatures. The electronic structure and optical properties of the films have been systematically studied for the different substrate temperatures. The maximum N concentration in the films arrived at 41 at.%. The binding energy of core levels C 1s and N 1s produces a large shift in the range of 3.5-0.3 eV depending on substrate temperature Ts. The band gap is at about 0.61-1.22eV. N atom concentration and shift of core level as well as electron band gap decrease with increasing Ts, which illustrates that raising Ts is not a good way to form carbon nitride films. Ultraviolet-visible near infrared spectra show that the films have a good transparency in near infrared region, but there is a sharp absorption peak around 2720 nm. The peak disappears when Ts is higher than 400° C.

The effect of thermal exposure on the microstructure and hardness of as-cast Mg–Zn–Al alloys with Sn addition

Abstract Microstructural evolution and hardness changes of as-cast Mg-6Zn-2Al alloys with Sn addition during prolonged thermal exposure to 150 °C and 200 °C were studied using optical microscopy, X-ray diffraction, scanning electron microscopy and hardness tests. Neither obvious grain growth nor new phases were detected in these alloys during thermal exposure. There were initial precipitation hardening peaks followed by long-term hardness stability during thermal exposure to 150 °C, indicating that all the alloys exhibited excellent microstructural stability. There were two peaks in the hardness vs. annealing time curves of these alloys during thermal exposure to 200 °C, which corresponded to the precipitation of MgZn and Mg2Sn. The overall hardness of these alloys decreased with prolonged time, resulting from the aggregation and coarsening of Mg2Sn.

Bifurcation theory model for the glass transition

Abstract Based on the basic fact that the glass transition is a physical phenomenon which occurs in nonequilibrium solidification conditions, the dynamical process of rapid solidification is discussed by using nonlinear theory and a bifurcation theory model for the glass transition is established. It is shown that in rapid solidification a multiple steady states phenomenon can take place because of the serious nonequilibrium of the solidification condition and the nonlinear coupling between the solidification rate and the solidification temperature, and this multiple steady states phenomenon can be understood as the glass transition. Then, the thermodynamic behavior of the rapid solidification process is analyzed by using nonequilibrium thermodynamics, and this thermodynamic analysis shows the possibility of the solidification system becoming unstable when the solidification system is far from the equilibrium solidification point to some extent because of the rapid solidification condition. This model can explain some experimental features of the glass transition such as the changes of specific heat Cp, expansion coefficient α, diffusion constant D, and viscosity η and the cooling rate dependence of Tg. And also, the critical cooling rates of forming glass and the reduced glass transition temperatures of some alloys estimated by this model are consistent with the experimental results.

A novel multi-layer spray deposition

A novel spray deposition technology with the characteristics of multi-layer deposition, the combination of forced external cooling and internal water-cooling of the substrate was developed. Compared with the conventional spray deposition technology, the novel technology exhibits the specific advantages, including larger solidification rate, higher dimensional precision, and ease to manufacture large preforms. The law of multi-layer spray deposition was studied, and Al-Fe-V-Si alloy with good high-temperature mechanical properties and 6066Al/SiCp composites with excellent room-temperature mechanical properties are prepared by this novel technology.

The constitution of multicomponent quasi-crystalline alloys

The structural similarity plays an important role in the formation of the quasi-crystalline phase. Acoording to this, the authors suggest that a principle of composition additivity in the constitution of quasi-crystalline alloys probably exists, which means that some new quasi-crystalline alloys can be constituted by adding several quasi-crystalline alloy compositions. By means of a rapid solidification technique, new alloys were obtained and characterized.

Preparation and study of Al-Cr, Al-Cr-Mn quasicrystalline powders

The production technique of Al-Cr, Al-Cr-Mn alloy quasicrystalline powders and their morphology, composition, diffraction pattern and thermal stability were studied. Computer processing of elevated-temperature X-ray diffraction data, enabled kinetic phase transformation diagrams to be plotted. It was shown that in Al-Cr, Al-Cr-Mn alloys, the composition and cooling rate dramatically affect the formation of the icosahedral phase; at a cooling rate of 105−106 Ks−1, nearly full icosahedral phase Al82Cr18, Al82 (Mn, Cr)18 powders could be produced. For Al-Cr alloy, the thermal stability of quasicrystalline phase is improved with the increasing chromium content. The addition of the third constituent, manganese, can also improve the thermal stability of quasicrystalline phase.

Effects of water phase concentration on the emulsion polymerization of polyaniline

The emulsion polymerization of aniline in three-phase system of xylene-functionalized protonic acid-water was carried out using (NH4)2S2O4 as oxidant. The influences of water phase concentration on the viscosity, conductivity, transmittance of polyaniline(PAN) latex and its powders were studied. The results show that the properties of PAN prepared through the emulsion polymerization are influenced by the amount of water used in the polymerization. The morphology of PAN varies with the water phase concentration used in the polymerization, which may result in the change of properties of PAN latex and its powders. When the volume fraction of water (φ) is about 20%–30%, the prepared PAN powder has higher conductivity, and the PAN latex has appropriate viscosity and particle size. The consumption of xylene was reduced at high φ value.

Application of a Novel Multilayer Spray Forming Technology in the Preparation of Large Dimension Aluminium Alloy Blanks

Principle of multilayer spray forming and preparation technique of tubes and billets by it were presented and described in the article. The marked characteristic of multilayer spray forming technology is the to-and-fro movement of spray system, so the preform is deposited layer by layer with the surface of the deposit retaining relatively low temperature during the spray forming process. The two outstanding advantages of multilayer spray forming are as follows: suitable for manufacturing large dimension blanks, higher solidification cooling rate. Now Al-Fe-V-Si alloy (8009) and SiC particle reinforced aluminium alloy metal matrix composite billets of 300–600 mm in diameter and tubular blanks of size up to 650 mm in outer diameter, 360 mm in inner diameter and 1 200 mm in length have been made with the technology. After extrusion, the material has good properties.

THE CRYSTALLINE PHASES IN RAPIDLY SOLIDIFIED AL65CU20FE15 ALLOY POWDERS

An Al{sub 65}Cu{sub 20}Fe{sub 15} alloy with the composition of Al-65at%, Cu-20at% and Fe-15at% was prepared from 99.5% pure starting materials. Alloy powders were then produced with a rapidly solidified (RS) device developed by Chen et al. The powders obtained have an average particle size of about 15{mu}m and their cooling rates are estimated to be 10{sup 5}--10{sup 7} K/S. The powders were uniformly sprinkled on to microgrids for observation by transmission electron microscope (H-800 transmission electron microscope equipped with an EDAX SW9100 composition analysis system).