Microstructural characteristics of tungsten-base nanocomposites produced from micropowders by high-pressure torsion (original) (raw)
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Ultrafine-grained Tungsten by High-Pressure Torsion – Bulk precursor versus powder processing route
IOP Conference Series: Materials Science and Engineering, 2019
The continuous enhancements and developments in the field of power engineering, as well as the uprising of nuclear fusion technology, demand novel high performance materials featuring exceptional strength and damage tolerance as well as durability in harsh environments. Ultra-fine grained bulk materials fabricated by high-pressure torsion, exhibiting a grain size less than 500 nm are promising candidates for these applications. Tungsten, the material of choice for plasma-facing materials in fusion reactors, is expected to exhibit even more enhanced properties by precise doping with impurity atoms, strengthening grain boundary cohesion. In order to allow this meticulous control of chemical composition, in-house mixing of the raw material powders is preferable to use of commercially available alloys. Several challenges arise in powder processing of tungsten via high-pressure torsion, originating in the intrinsic strength and high melting point of the material, and in the affinity of t...
Microstructure effects on tensile properties of tungsten-Nickel-Iron composites
Metallurgical Transactions A, 1988
Controlled processing of heavy alloys containing 88 to 97 pct W resulted in high sintered densities and excellent bonding between the tungsten grains and matrix. For these alloys, deformation and fracture behavior were studied via slow strain rate tensile testing at room temperature. The flow stress increased and the fracture strain decreased with increasing tungsten content. The tradeoff between strength and ductility resulted in a maximum in the ultimate tensile strength at 93 pct W. Microstructure variations, notably grain size, explain sintering temperature and time effects on the properties. During tensile testing, cracks formed on the surface of the specimens at tungsten-tungsten grain boundaries. The crack density increased with plastic strain and tungsten content. The surface cracks, though initially blunted by the matrix, eventually increased in density until catastrophic failure occurred. An empirical failure criterion was developed relating fracture to a critical value of the surface crack tip separation distance. Application of the model explains the effects of microstructural variables on tensile properties.
Cumhuriyet Science Journal, 2019
Tungsten matrix composites reinforced with TiB2 and Y2O3 particles were fabricated by milling under ambient/cryogenic conditions and Ni activated sintering. Powder blends constituting the W-1 wt. % Ni-2 wt. % TiB2-1 wt. % Y2O3 composition were mechanically milled for 12 h under ambient condition or cryomilled for 10 min or sequentially milled under ambient and cryogenic conditions. Milling was carried out in a high-energy ball mill under ambient condition whereas cryogenic milling was conducted in externally circulated liquid nitrogen. Milled powders were compacted using a hydraulic press and the pellets were sintered at 1400°C for 1 h under Ar / H2 gas flowing conditions. The effects of different milling types on the microstructural and mechanical properties of the sintered composites were investigated. After sintering, in addition to dominant W phase, small amounts of WB and NiW phases were detected in all sintered samples. The application of cryomilling after milling at ambient condition provided the disappearance of the clustered TiB2 and Y2O3 particles in the sintered sample: They were located at the grain boundaries of W1Ni matrix and homogeneously distributed through the microstructure. Sequentially milled and sintered composite had the highest relative density (95.77 %) and the highest microhardness (7.23 GPa) values among the samples. Nanoindentation tests showed that there was an improvement in the hardness and elastic modulus of W matrix phase, which yielded the values of 8.9 and 373.7 GPa, respectively.
Development of Nanostructured Tungsten Based Composites for Energy Applications
2012
Tungsten (W) based materials can be used in fusion reactors due to several advantages. Different fabrication routes can be applied to develop tungsten materials with intended microstructure and properties for specific application including nanostructured grades. Therein, innovative chemical routes are unique in their approach owing numerous benefits. This thesis summarizes the development of W-based composites dispersed-strengthened by rare earth (RE) oxides and their evaluation for potential application as plasma facing armour material to be used in fusion reactor. Final material development was carried out in two steps; a) fabrication of nanostructured metallic tungsten powder dispersed with RE-oxides and b) powder sintering into bulk oxide-dispersed strengthened (ODS) composite by spark plasma process. With the help of advanced characterization tools applied at intermediate and final stages of the material development, powder fabrication and sintering conditions were optimized. The aim was to achieve a final material with a homogenous fine microstructure and improved properties, which can withstand under extreme conditions of high temperature plasma. Two groups of starting materials, synthesized via novel chemical methods, having different compositions were investigated. In the first group, APT-based powders doped with La or Y elements in similar ways, had identical particles' morphology (up to 70 µm). The powders were processed into nanostructured composite powders under different reducing conditions and were characterized to investigate the effects on powder morphology and composition. The properties of sintered tungsten materials were improved with dispersion of La 2 O 3 and Y 2 O 3 in the respective order. The oxide dispersion was less homogeneous due to the fact that La or Y was not doped into APT particles. The second group, Ydoped tungstic acid-based powders synthesized through entirely different chemistry, contained nanocrystalline particles and highly uniform morphology. Hydrogen reduction of doped-tungstic acid compounds is complex, affecting the morphology and composition of the final powder. Hence, processing conditions are presented here which enable the separation of Y 2 O 3 phase from Y-doped tungstic acid. Nevertheless, the oxide dispersion reduces the sinterability of tungsten powders, the fabricated nanostructured W-Y 2 O 3 powders were sinterable into ultrafine ODS composites at temperatures as low as 1100 °C with highly homogeneous nano-oxide dispersion at W grain boundaries as well as inside the grain. The SPS parameters were investigated to achieve higher density with optimum finer microstructure and higher hardness. The elastic and fracture properties of the developed ODS-W have been investigated by micro-mechanical testing to estimate the materials' mechanical response with respect to varying density and grain size. In contrast from some literature results, coarse grained ODS-W material demonstrated better properties. The developed ODS material with 1.2 Y 2 O 3 dispersion were finally subjected to high heat flux tests in the electron beam facility "JUDITH-1". The samples were loaded under ELM-like thermal-shocks at varying base temperatures up to an absorbed power density of 1.13 GW/m 2 , for armour material evaluation. Post mortem characterizations and comparison with other reference W grades, suggest lowering the oxide contents below 0.3 wt. % Y 2 O 3. As an overview of the study conducted, it can be concluded that innovative chemical routes can be potential replacement to produce tungsten based materials of various composition and microstructure, for fusion reactor applications. The methods being cheap and reproducible, are also easy to handle for large production at industrial scale.
Comparison of mechanical and microstructure properties of tungsten alloys for special purposes
Sustainable Engineering and Innovation
Tungsten belongs to group of refractory metal that possess extraordinary resistance to heat and wear and it is the heaviest engineering material. Because of its properties tungsten is used for special purposes. This paper presents the results of mechanical and microstructure research on the example of the characteristic heavy tungsten alloys 91W-6Ni-1.8Fe-1Co and 93W-5Ni-1.6Fe-0.3Co with different Ni/Co ratios. The proper Ni/Co ratio is important to obtain a favorable microstructure and mechanical properties of these materials. The distribution of the W, Ni, Co and Fe elements in tungsten phase and binder phase, which can influence on mechanical properties of tungsten alloys. The SEM analysis and mechanical results show that the alloy, which has Ni/Co within the given limits, posses a finer microstructure and better mechanical properties that is very important for the maintenance of the quality of tungsten alloys for special purposes.
Materials Science Forum, 2008
A tungsten heavy alloy (92%W, Ni-Co matrix) is subjected to severe plastic deformation (SPD) by high pressure torsion (HPT) at room temperature up to equivalent strains of 0.7, 5.3, 10.7 and 14.3. The microstructure and the mechanical properties are investigated by cylindrical compression samples at quasi-static and dynamic loading. The harder spherical W particles are homogeneously deformed within the softer matrix, becoming ellipsoidal at medium strains and banded at high strains without shear localization or fracture. Results of quasi-static loading show that the strength is approaching a limiting value at strains of ~10. At this strain for the matrix a grain size of ~80 nm and for W a cell size of ~250 nm was observed, suggesting strain concentration on the matrix. The initial yield stress of 945 MPa for the coarse-grained condition is increased thereby to an ultimate value of 3500 MPa, while a peak stress of ~3600 MPa is reached. Such remarkably strength has never been reported...
Mechanical Properties of Forged Tungsten Heavy Alloys
Acta Polytechnica CTU Proceedings
Tungsten heavy alloys are composite materials containing spherical tungsten particles embedded in binder matrix. Their excellent mechanical properties can be further improved by rotary forging. This paper aims to gain deeper understanding of the forging process by investigating the local elastic modulus, hardness, and residual stress of individual phases in W6Ni3Co pseudo-alloy. The resulting global properties of the composite material such as stress-strain behavior, fracture toughness and fatigue crack growth rate behavior are also studied. The results show that sintered and quenched material consists of highly textured matrix containing nearly perfect single crystal spheres of pure W. The rotary forging leads to significant lattice deformations destroying the texture and significantly increasing the hardness of both WNiCo matrix and W particles and making residual stresses in W particles anisotropic with increased compression along the longitudinal axis of the forged part.
Effect of matrix alloy and cold swaging on micro-tensile properties of tungsten heavy alloys
Materials Letters, 2006
The tensile properties of two types of tungsten heavy alloys, W -5.6% Ni -1.4% Fe and W -4.7% Ni -2.2% Co, were investigated as a function of matrix alloy (Ni -Fe vs. Ni -Co) and specimen orientation with respect to the cold swaging axis. Orientation dependent rod properties were measured using a micro-tensile testing unit developed at LLNL. Tungsten particles were slightly oblong in the swage direction in both alloys and particle -particle bonding was also apparent in both alloys. In the W -Ni -Fe alloy separation between W particles and the Ni -Fe matrix was observed, as well as Ni -Fe matrix cracking. The W -Ni -Co alloy showed no evidence of separation between W and the matrix alloy or Ni -Co matrix cracking. Differences in the matrix material condition appear to manifest themselves in the observed tensile properties and fracture surfaces. The ultimate tensile strength and elongation at failure values of the W -Ni -Co alloy were larger than those of the W -Ni -Fe alloy. D
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Tungsten based alloys are extensively used in the defence application due to their high density and ability to withstand very high temperature. Solid state processing of pure and W based alloys is, however, a difficult task due to the high sintering temperature (2700°C) required to sinter them. Liquid phase sintered W heavy alloys has been used as penetrators, but they can’t be used for applications above 1200°C. Recently has shown that the sintering temperature of pure W could be brought down to ~1800°C from the conventional 2700°C by making the W-powder nanostructured prior to sintering. Present work aims at developing Oxidation resistant W-based refractory alloys through mechanical alloying is followed by solid state sintering at modest temperature. Proper sintered alloys has to be studied. Sintered density of W50Mo50 after sintering for 300 min at 1500°C was 92.5%, and it could be considered as a very significant extent of densification at this relatively low temperature, when c...
Journal of Materials Engineering and Performance, 2016
WC-W 2 C/ZrO 2 nanocomposites were synthesized by pressure-less sintering (PS) and spark plasma sintering (SPS) of tungsten carbide/yttria-stabilized tetragonal zirconia, WC/TZ-3Y. Prior to sintering, WC/ TZ-3Y powders were totally ball-milled for 20 and 120 h to obtain targeted nano (N) and nano-nano (N-N) structures, indicated by transmission electron microscopy and powder x-ray diffraction (PXRD). The milled powders were processed via PS at temperatures of 1773 and 1973 K for 70 min and SPS at 1773 K for 10 min. PXRD as well as SEM-EDS indicated the formation of WC-W 2 C/ZrO 2 composites after sintering. The mechanical properties were characterized via Vicker microhardness and nanoindentation techniques indicating enhancements for sufficiently consolidated composites with high W 2 C content. The effects of reducing particle sizes on phase transformation, microstructure and mechanical properties are reported. In general, the composites based on the N structure showed higher microhardness than those for N-N structure, except for the samples PS-sintered at 1773 K. For instance, after SPS at 1773 K, the N structure showed a microhardness of 18.24 GPa. Nanoindentation measurements revealed that nanoscale hardness up to 22.33 and 25.34 GPa and modulus of elasticity up to 340 and 560 GPa can be obtained for WC-W 2 C/ZrO 2 nanocomposites synthesized by the low-cost PS at 1973 K and by SPS at 1773 K, respectively.