Papers by Keyword: Tungsten

Abstract: The main aim of studies on dynamic behaviour of construction materials at high strain rates is to determine the variation of mechanical properties (strength, plasticity) as a function of the strain rate and temperature. On the basis of results of dynamic tests on the properties of constructional materials the constitutive models are formulated to create numerical codes applied to solve constructional problems with computer simulation methods. In the case of military applications connected with the phenomena of gunshot and terminal ballistics it's particularly important to develop a model of strength and armour penetration with KE projectile founded on reliable results of dynamic experiments and constituting the base for further analyses and optimization of projectile designs in order to achieve required penetration depth. Static and dynamic results of strength investigations of the EN AW-7012 aluminium alloy (sabot) and tungsten alloy (penetrator) are discussed in this paper. Static testing was carried out with the INSTRON testing machine. Dynamic tests have been conducted using the split Hopkinson pressure bars technique at strain rates up to 1,2·10⁴ s^-1 (for aluminium alloy) and 6·10³ s^-1 (for tungsten alloy).

Abstract: Tungsten is a promising armour material for plasma facing components of nuclear fusion reactors (NFR) because of its low sputtering rate and favourable thermo-mechanical properties (high melting point and good thermal conductivity). This paper reports some results of an experimental campaign carried out for investigating the microstructural characteristics and the mechanical properties of tungsten (99.97% purity; 5% porosity) for fusion applications. Tungsten has been heat treated at 500 °C and 800 °C with increasing soaking time. The samples in as-supplied condition and after each step of the heat treatments have been examined by optical microscopy and TEM observations, X-ray diffraction (XRD) and micro-hardness tests. The original material has a dislocation density of 1.5 x 10¹⁰ cm^-2 and a mean grain size of 65 μm. Grain size is not affected by the heat treatment at 500 °C which induces only a weak decrease of dislocation density leading to a little smaller hardness. The microstructure can be considered substantially stable even if a weak recovery of dislocations takes place. On the contrary, grain growth is observed after heating at 800 °C: 10 hours of treatment nearly doubles the average grain size.

Abstract: The diffusion process of multiple He atoms in W is simulated by a molecular dynamics (MD) method with the W-H-He analytic bond-order potential. The diffusivities of different number of helium (He) atoms in W are determined by the mean squared displacement (MSD) method at different temperatures. The diffusivity-temperature (D-T) relationship is fitted to the Arrhenius equation to obtain the pre-factor and the diffusion barrier. Under the temperature of 1200K He atoms diffuse together, and above 1200K they separate from each other. When the number of He atoms is greater than three, all He atoms oscillate at the tetrahedral interstitial site (TIS) instead of diffusing under 400K. In the temperature range of 400-1200K, the diffusion barriers of He atoms, the number of which is from two to five, are 0.098, 0.170, 0.125 and 0.112eV, respectively. Contrasting with one He atom (0.058eV), the higher diffusion barriers reflect a greater difficulty in diffusion of multiple He atoms in W. In addition, when the number of He atoms is over five, vacancies are formed in W, and He atoms occupy the vacancies.

Abstract: Due to its excellent thermophysical properties, tungsten has been used as structural materials for divertor components of fusion reactors. With the development of technology, the helium cooled high performance divertor requires the high reliable joining between tungsten and ferritic martensitic high chromium steel. However, the difference of thermal expansion coefficients between tungsten and steel causes high thermally residual stresses, which will yield failure of the joint. Therefore, the preparation of the joint between tungsten and steel is a key issue for divertor application. A brazing process, using rapidly solidified Ni-based foil-type filler and a vanadium slice as intermediate materials, was developed to investigate the joining of tungsten and steel for divertor components, and the microstructure and mechanical properties of the joint were also studied. The elements and phases compositions in the boning regions were analyzed by electron probe microanalysis and X-ray diffraction. Micro-hardness distribution and tensile strength of joint were measured by nanoindenter and mechanical testing machine, respectively. The results indicated that the integral bonding was achieved at the bonding regions of the brazed joint. The typical microstructure of the joint was consisted of W/V and V/steel brazed seams separated by a V slice. The hard and brittle vanadium borides with 25 GPa hardness were produced at the W/V and V/steel brazed seams. The as-bonded W/steel joint with tensile strength of 143MPa was obtained, and specimens appeared a brittle fracture mode and fractured in the brittle vanadium boride layers during tensile testing.

Abstract: Pulsed electric-current sintering was applied to the bonding of tungsten to titanium. The influence of bonding condition on the bond strength of joint was investigated by observing the microstructure. The bonding process was carried out at bonding temperature from 773 to 1273 K for 1.8 ks at a bonding pressure of 40 MPa. The bond strength of the joint bonded at the temperature higher than 1173 K was around 200 MPa. This joint fractured in the tungsten during tensile test. SEM-EDX observation revealed that W diffused into Ti at the joint interface of the joint bonded at the temperature higher than 973 K.

Abstract: This article deals with the analysis and evaluation of coatings applications made by Physical vapour deposition on ceramics substrates. Main focus is on the application of tungsten and tungsten nitride coatings for functionally graded materials (FGM). The article focuses on analysis and evaluation of tungsten and tungsten nitride coatings with ceramics substrates. Firstly suitable deposition parameters of W and WN coatings, prepared by magnetron sputtering, were tested. Test coatings on tool steel substrate exhibited high adhesion and good mechanical and technological properties. Both tungsten and tungsten nitride coatings on selected ceramics substrates exhibited good mechanical a technological properties. Properties such as hardness, reduced elastic modulus and wear are discussed for each coating/substrate combination. Both coatings seem to be good candidates for application in field of FGM. The results of the presented experiment shows that the adhesion and the final quality of the coating depends on the type of chemical bond in the coating and in the substrate, and on the ability of the coating to deform plastically.

Abstract: In this paper, the penetration characteristic of tungsten penetrator was studied. Tungsten and aluminum was used as the material of the penetrator and target, respectively. Dimensions of the aluminum target were 100 X 100 X 5 (mm³). The penetrator with cylinder conical tip and flat tip was used in the impact test and simulation. The range of the initial velocity was from 400 to 1600 m/s. The angle of the incidence on the penetrator with cylinder cone tip was 0 ̊. The angle of the incidence on the penetrator with flat tip was 0 ̊, 30 ̊ , 60 ̊ . To obtain results of the simulation, AUTODYN 3D code was used. The residual mass and velocity was obtained to compare with results between the impact test and simulation.

Abstract: Addition of nanopowders of tungsten and aluminum oxyhydroxide to the weld pool has shown positive effect on the structure of metal during arc gas welding with a consumable electrode. It has been shown that introducing the powders decreases dendrite size and leads to the formation of a more equilibrium micrоstructure.

Abstract: This study relates to a micromechanics based finite element model of the effect of oxidation on heat transfer and mechanical behavior of a (Zr,W)B₂ ceramic at high temperature. An adaptive remeshing technique is employed in both heat transfer and thermal stress analysis models. A global-local modeling technique is used to combine finite elements with infinite elements for thermal stress analysis. Temperature and thermal stress distributions in the ceramic and the oxides are presented.

Abstract: The tungsten modified Pd-based nanocatalyst for ethanol oxidation in alkaline media was prepared by an ethylene glycol reduction method, and the catalytic performances of the prepared catalysts were evaluated. The results show that the addition of tungsten improved the dispersion of metal nanoparticles on the support surface and thus increased the current density of Pd-based catalyst for ethanol electrooxidation. In addition, it was found that in the tungsten modified Pd-based catalyst, tungsten interacted with palladium, leading to a low onset potential and a decreased active energy for ethanol oxidation. On the other hand, it was also observed that the addition of tungsten improved the poison resistance of Pd-based catalysts for ethanol oxidation.