Papers by Author: Fu Chi Wang

Abstract: Chemical vapor transport deposition (CVTD) is an effective method for preparing large tungsten coatings for space thermionic reactors. In this study, a high-density, high-work-function polycrystalline tungsten coating was prepared using a WCl₆ transport agent in a concentric tube-type closed transport system. The relationship between the kinetics and the microstructures of the CVTD polycrystalline tungsten coating at the substrate temperature of 1593 K-1793 K and system pressure of 15.93 Pa-106.8 Pa was studied, which provided a basis for the preparation of high-quality tungsten coatings. At a low temperature or a low pressure, the activation energy was approximately 2 kJ/mol, the deposition rate was almost independent of the temperature changes, and the control mechanism was mass transport limited. The tungsten coating had nodules on the surface with pores in the grain boundaries and grew preferentially along <111>. At a high temperature and a high pressure, the apparent activation energy was approximately 90 kJ/mol, the value of order was approximately 1, and the control mechanism in this process range was surface limited. The tungsten coating exhibited a hexagonal pyramidal structure, and the growth direction was preferred along <110>. The average work function of the tungsten coating prepared at a temperature of 1673 K and a system pressure of 106.80 Pa was as high as 5.20 eV.

Abstract: La_1-xSrxTiO_3+δ (LST) has been studied in many fields due to its excellent physical and chemical properties. However, there are rare reports on its optical properties, especially reflectivity. Our previous work has prepared LST coatings by atmospheric plasma spraying technique, but the reflectivity is much lower than that of LST powder itself. In this paper, LST coating was prepared by knife coating which is a traditional convenient method. The phase structure and surface morphology of the coating were examined by X-ray diffraction and scanning electron microscopy, respectively. The optical property was characterized by UV-visible-near infrared spectroscopy and the laser ablation tests were carried out by YSL-2000 Fiber Laser (IPG Co) with the wavelength of 1070 nm and the spot size of 10 mm×10 mm. Results show that the reflectivity of the LST coatings depends on the weight ratio of LST and binder. A high-reflectivity coating can be prepared by knife coating with appropriate content of LST.

Abstract: In this paper, in situ TiB reinforced Ti-3Al, Ti-6Al and Ti-6Al-4V matrix composites were prepared by arc-melting technique utilizing the reaction between Ti and TiB₂, and then forged in the α+β phase field. Phase identification was carried out via X-ray diffraction. Microstructure of the composites was studied by optical microscopy (OM) and scanning electron microscopy (SEM). Mechanical properties of the composites after forging were measured at various temperatures by tensile experiment. The results showed that Ti-6Al-4V-2TiB composite exhibits fine equiaxed matrix microstructure with a grain size of 5-10μm. The tensile strength and elongation of the composite at room temperature reached 1069MPa and 10.0%, respectively.

Abstract: The in-situ synthesized TiB reinforced titanium matrix composites have been prepared by spark plasma sintering technique at 950–1250°C, using mixtures of 10wt% TiB2 and 90wt% Ti powders. The effects of the sintering temperature on the mechanical properties (Vickers microhardness, yield strength and Young`s modulus) of the composites were investigated. SEM was used to analyze the reaction process and the microstructure of the compacts synthesized at different sintering temperatures. The results indicated that the in situ synthesized TiB grow rapidly with increasing sintering temperature. The composite sintered at 1250°C have the highest relative density of 99.2%. However, the composite sintered at 950°C exhibits the best Vickers microhardness of 4.64GPa and yield strength of 989MPa, respectively.

Abstract: Si/Al composites with different Si particle sizes were fabricated using spark plasma sintering process for electronic packaging. The density, thermal conductivity, coefficient of thermal expansion and flexural strength of the composites were investigated. Effect of Si particle size on structure and properties of the Si/Al composites were studied. The results showed that the Si/Al composites synthesized by spark plasma sintering were composed of Si and Al. Al was uniformly distributed among the Si phase, leading to a high thermal conductivity (>120 W/m·k). The relative density of the Si/Al composites decreased with increasing Si particle size. Small Si particle size produced small grains, leading to a low coefficient of thermal expansion and a high strength. There is an optimal matching among the thermal conductivity, coefficient of thermal expansion and flexural strength when the Si particle size was 44 um.

Abstract: In order to research the formation and spread of adiabatic shear bands in Ti-6Al-4V targets, LS-DYNA code is used to simulate the ballistic impact process. The projectile used in the impact test is a flat-headed steel cylinder with diameter of 7.62mm and length of 39mm. The results of simulated and impact test are in good agreement. Multiple adiabatic shear bands form in Ti-6Al-4V targets under high-speed ballistic impact. Adiabatic shear bands were found to extend parallel with a certain distance. The formation and distribution of adiabatic shear bands was related to the breaking-off of projectiles, which was caused by the distribution of maximum shear stress in Ti-6Al-4V targets and projectiles.

Abstract: Reticulated ceramic/metal composites (RCMC) are characterized by the ceramic skeleton and metallic matrix both continuous in three-dimensional space, and this special structure makes full use of the ceramic phase’s high strength and the metal phase’s toughness. Correspondingly, the deformation and damages mechanisms of RCMC under dynamic load conditions are very complicated and the related studies are quite important. In the current study, the dynamic properties of RCMC are investigated by using finite element methods based on the factitious microstructures of SiC/Al composites. The ceramic/metal interface is induced by employing a “constrained-tie-break” method, so as to study the effects of interface when suffering from a dynamic impact loads. Effects of the phase distribution, shape, ratio, as well as the size are systematically analyzed. The methods proposed in this paper would be helpful for predicting the dynamic behaviors of RCMC, and developing new ceramic/metal composites.

Abstract: Some results of an experimental study on high strain rate deformation of TC21 alloy are discussed in this paper. Cylindrical specimens of the TC21 alloys both in binary morphology and solution and aging morphology were subjected to high strain rate deformation by direct impact using a Split Hopkinson Pressure Bar. The deformation process is dominated by both thermal softening effect and strain hardening effect under high strain rate loading. Thus the flow stress doesn’t increase with strain rate at the strain hardening stage, while the increase is obvious under qusi-static compression. Under high strain rate, the dynamic flow stress is higher than that under quasi-static and dynamic flow stress increase with the increase of the strain rate, which indicates the strain rate hardening effect is great in TC21 alloy. The microstructure affects the dynamic mechanical properties of TC21 titanium alloy obviously. Under high strain rate, the solution and aging morphology has higher dynamic flow stress while the binary morphology has better plasticity and less prone to be instability under high strain rate condition. Shear bands were found both in the solution and aging morphology and the binary morphology.

Abstract: An electrical – thermal coupled finite element model (FEM) is developed to investigate the temperature distribution during spark plasma sintering (SPS) with a pre-designed graded graphite die. The sample used in this investigation consists of five layers with different contents of Ti and TiB (45 wt % Ti, 55 wt %Ti, 65 wt % Ti, 75 wt % Ti and 85 wt % Ti). The temperature distribution in gradient Ti-TiB composites was obtained. Owing to the use of the die with changing cross section, a temperature difference of 142K in the axial direction inside the sample is achieved, while the maximum radial temperature difference is 6.2 times less than the axial one.

Abstract: Dynamic mechanical properties of AZ31 magnesium alloy plate were carried out using split Hopkinson pressure bar (SHPB) with compression direction 0° and 90° from normal direction respectively. Optical microscopy (OM) and scanning electronic microscopy (SEM) were used for the observation of microstructure and fractograph. OM observations showed that cracks initiated and propagated along localized deformed bands consisted of twin intersection for 0° specimen, and that cracks initiated and propagated along localized transformed bands consisted of fine equiaxed dynamic recrystallized grains for 90° specimen. SEM observations showed that the fractograph exhibited typical ductile dimple fracture pattern for 0° and 90° specimens due to the thermal accumulation in localized bands caused by severe plastic deformation. The fracture mechanisms are anisotropic for AZ31 magnesium alloy plate, which contributed to the dynamic mechanical anisotropy of AZ31 magnesium alloy plate.