He Yuehui

Phase composition, transition and structure stability of functionally graded cemented carbide with dual phase structure

The phase composition, phase transition and phase structure transformation of the wire-cut section of functionally graded WC-Co cemented carbide with dual phase structure were investigated by XRD phase analysis. It is shown that the composition of η phase in the core zone is Co3W3C (M6C type). The structure of cobalt based solid solution binder phase is fcc type. At the cooling stage of the sintering process, the phase transition of η phase, i.e. M6C→M12C and the martensitic phase transition of the cobalt based solid solution binder phase, i.e. fcc→hcp are suppressed, which facilitates the strengthening of the alloy. Because the instantaneous temperature of the discharge channel is as high as 10 000 °C during the wire cutting process, the processed surface is oxidized. Nevertheless, the oxide layer thickness is in micro grade. In the oxide film, η phase is decomposed into W2C and CoO, and cobalt based solid solution binder is selectively oxidized, while WC remains stable due to the existence of carbon containing liquid organic cutting medium.

Recycling of heavy metal alloy turnings to powder by oxidation-reduction process

Abstract The processes of direct recycling heavy metal turnings by oxidation–reduction technique have been investigated in details. The average particle size of recycled alloy powders was about 1.5 μm, and the shape of powder particle was regular when the final reduction temperature was 850 °C. The average size of the particle increased to 5 and 8 μm when increasing the reduction temperature to 900 and 950 °C, respectively. However, if the reduction temperature was above 900 °C, the surface of powder was complicated. Increasing reduction temperature from 900 to 950 °C, the particle of recycled powder grew slightly, and the content of oxygen decreased from 0.2314% to 0.1700%. It has been also found that the chemical composition of the recycled alloy powder is the same as the primary heavy metal turnings.

Physical simulation of hot deformation of TiAl based alloy

In order to establish a model between the grain size and the process parameters, the hot deformation behaviors of Ti-49.5Al alloy was investigated by isothermal compressive tests at temperatures ranging from 800 to 1 100 °C with strain rates of 10−3∓10−1 s−1. Within this range, the deformation behavior obeys the power law relationship, which can be described using the kinetic rate equation. The stress exponent, n, has a value of about 5.0, and the apparent activation energy is about 320 J/mol, which fits well with the value estimated in previous investigations. The results show that, the dependence of flow stress on the recrystallized grain size can be expressed by the equation: σ=K1drex−0.56. The relationship between the deformed microstructure and the process control parameter can be expressed by the formula: 1gdrex=−0.281 1gZ + 3.9081.

Determination of Li+ solid diffusion coefficient in LiMn2O4 by CITT

The capacity intermittent titration technique (CITT) was developed based on the ratio of potentio-charge capacity to galvano-charge capacity (RPG) method, to continuously determine the solid diffusion coefficient (D) of the intercalary species within insertion-host materials with a small voltage region. The linear equations of D vs the value of ratio of the potentio-charge capacity to the galvano-charge capacity (q) were given. By the CITT technique, the Li+ solid diffusion coefficients within LiMn2O4 at different voltages were determined. The results show that the values of D varied from 3.447 × 10−9 to 7.60 × 10−11 cm2/s in the voltage range of charge from 3.3 to 4.3 V as a function of voltage with “W” shape.

Grain refining of TiAl alloy by rapid deformation

Grain refining of TiAl intermetallics by rapid deformation was investigated. The as-cast TiAl alloy has very large grains and serious composition segregation. Most of the large grains of TiAl alloy can be refined after the first rapid deformation and heat treatment. However, a few of the coarse grains of TiAl still remained. When the second rapid deformation and heat treatment was conducted, all of the remaining coarse TiAl grains can be refined. As the grains were refined, the properties of TiAl alloy were significantly improved.

Optimization of Microstructure and Elimination of β(B2) Phase in As-Cast Ti-Al-Nb-W-B Alloy

The microstructures of as-cast Ti-Al-Nb-W-B alloy were investigated to assess the possibility of eliminating the triaxial β(B2) phase on the crystal boundary through heat treatment and multi-step forging thermal mechanical treatment. The microstructure evolution and growth mechanism were studied according to Ti-Al-Nb ternary phase diagram, and then the process of eliminating of the triaxial β(B2) phase on the crystal boundary was assessed. The results of the experiments indicate that the best way to eliminate β(B2) phase is holding at the 1260 °C for 8 h or 1220 °C for 36 h followed by furnace cooling, but the grains grow appreciably. On the other hand, after multi-step forging with a total deformation of more than 80%, the as-cast nearly lamellar structure can be refined obviously to the duplex microstructure, and the β(B2) phase is basically eliminated.

Heat treatment behavior of can-forged titanium aluminide alloy in (α+γ) region

The effect of initial microstructures on the heat treatment behavior of can-forged titanium aluminide alloy has been studied. Optical microscopy and EDX analyses results show that different initial microstructures, including difference in dislocation density, phase distribution and morphology, have inherent effects on the final heat treatment microstructure although at the same heat treatment temperature. An almost full lamellar (NFL) microstructure with an average α2/γ colony size of 20 µm was obtained through adjusting initial microstructure.

Effects of microstructures on the deformation and fracture behaviors of TiAl-based alloys

The fracture toughnessKIc, the tensile deformation and fracture behaviors of different microstructures ofγ-TiAl based alloy, Ti-33Al-3Cr-0.5Mo] were studied at room temperature. It is found that theKIc value is lower in the duplex microstructure, and increases with the increase of vol pct of the lamellar microstructure, and that the full lamellar has the highestKIc value. Cleavage was the dominant fracture mechanism in the duplex microstructure material. In contrast, for the full lamellar microstructure the high anisotropy of deformation and the large strain discontinuity at grain boundaries resulting in decohesion of grain boundaries are the main fracture processes.

Investigation on the morphology of martensite in carbon steels

The effect of carbon content on the morphology of martensite in carbon steels has been studied in depth. It is found that not all the packet martensites obtained in carbon steels quenched from elevated temperature are lath martensite. The packet martensite obtained thus should be divided into two categories: packet thin plate martensite (i. e. lath martensite) and packet plate martensite. The former is only found in low carbon steels, the latter mainly in medium and high carbon steels. The morphology of martensite in steels with different carbon contents has been researched in detail using optical microscopy, SEM and TEM. A new criterion is proposed for identifying the category of martensite. Based on this new criterion, it is found that as-quenched steels withCc⩽0.2% contains lath martensite; that with 0.2%<Cc<0.4% contains a mixture of lath and plate martensite. The structure of as-quenched steels withCc⩾0.4% is completely of plate martensite. The curve of the relative volume percentage of martensite without internal twins versus carbon content has been redetermined. Finally, the misinterpretation of experimental results by previous investigators is analysed.

Preparation of nanoporous BiVO4/TiO2/Ti film through electrodeposition for photoelectrochemical water splitting

A nanoporous BiVO4/TiO2/Ti film was successfully fabricated by electrodepositing a nanoporous BiOI film on nanoporous TiO2 arrays followed by annealing at 450°C for 2 h. The electrodeposition of BiOI film was carried out at different times (10, 30, 100, 500 and 1000 s) in Bi(NO3)3 and KI solution. The morphological, crystallographic and photoelectrochemical properties of the prepared BiVO4/TiO2/Ti heterojunction film were examined by using different characterization techniques. UV–vis spectrum absorption studies confirmed an increase in absorption intensities with increasing electrodeposition time, and the band gap of BiVO4/TiO2/Ti film is lower than that of TiO2/Ti. The photocatalytic efficiency of BiVO4/TiO2/Ti heterojunction film was higher compared to that of the TiO2/Ti film owing to the longer transient decay time for BiVO4/TiO2/Ti film (3.2 s) than that of TiO2/Ti film (0.95 s) in our experiment. The BiVO4/TiO2/Ti heterojunction film prepared by electrodeposition for 1000 s followed by annealing showed a high photocurrent density of 0.3363 mA cm−2 at 0.6 V versus saturated calomel electrode. Furthermore, the lowest charge transfer resistance from electrochemical impedance spectroscopy was recorded for the BiVO4/TiO2/Ti film (1000 s) under irradiation.