查看更多>>摘要：? 2021 Elsevier LtdThe aim of the current work was the experimental determination of limit stresses in the uniaxial cyclic compression test that lead to advancing ratcheting in different WC-Co hardmetal grades at elevated temperature. At stresses below the limit stress the plastic strain per cycle reduces and plastic strain accumulation stops after a characteristic number of load cycles. Special attention was paid to the microstructural influence on the onset of advancing ratcheting and the associated damage development at the microstructure level. WC-Co hardmetals are used in various areas such as forming and forging tools, where they are exposed to high temperatures and pressure loads. Their good high-temperature properties allow them to be used under these conditions, but these properties are influenced by the microstructure. Investigations of the mechanical properties of hardmetals had been carried out under monotonously increasing loads and cyclic tests at room temperature and elevated temperatures. In these tests, the effect of different stress ratios R = σmin/σmax on the fatigue behaviour of hardmetals were studied, However, no studies are known for WC-Co hardmetals and their limit stresses in relation to strain ratcheting under cyclic compressive loading at elevated temperature. Hence, in the current work the influence of increasing stresses as a function of microstructure and their effect on the evolution of the strain of six different WC-Co hardmetal grades are discussed. For this purpose, the materials were investigated by uniaxial cyclic compression tests at a stress ratio of R = ?∞ at 700 °C and 800 °C in vacuum. The investigated hardmetal grades differ on the one hand in their WC grain size, which varies between 0.4 μm and 2.0 μm, and on the other hand in their Co-content, which varies between 6 wt% and 12 wt%. The residual strain value εres at zero applied stress was observed to stabilize with increasing number of load cycles at low applied stress ranges. Strain ratcheting occurred above a critical stress range, referred to as the limit stress for strain ratcheting. Strain ratcheting is the accumulation of plastic strain with increasing number of load cycles in which no strain stabilization occurs. Further, for all investigated hardmetal grades, the limit stresses were observed to decrease with increasing temperature. In the following, the microstructure of one hardmetal grade was analysed after loading below and above the limit stress by scanning electron microscopy and electron backscatter diffraction (EBSD). The influence of strain stabilization and ratcheting was analysed with regard to damage development and deformation behaviour of the WC and Co-phases. Strain ratcheting was observed to result in the formation of cavities and nanopores at phase boundary triple points and WC/Co interfaces. Additionally, the EBSD data showed that the fcc Co phase was transformed into hcp Co. Therefore, it is assumed that on the one hand, a certain strain value needs to be exceeded for strain ratcheting to occur and, on the other hand, that besides dislocation movement, microdefect formation and phase transformation significantly contribute to the increase in strain.
查看更多>>摘要：? 2021 Elsevier LtdTiN coating was deposited on high-speed steel, cemented carbide and TiCN-based cermets using arc evaporation. The variation of substrates was to evaluate the effect of tool materials on the coating's microstructure, composition, hardness, adhesion and tribological properties. The TiN coating on all tool materials had a strong preferred TiN (111) orientation. The TiN coating on high-speed steel grew so fast in the vertical direction that the columnar crystal structure was noticeable. The columnar structure of TiN coating on TiCN-based cermets weakened for the reduced vertical growth rate. The Ti/N atomic content ratio of the TiN coating on high-speed steel was 0.84, and some N atoms formed an interstitial solid solution, which contributed to the increased hardness of the coating. However, the Ti/N atomic content ratio of TiN coating on TiCN-based cermets increased to 1.04. Due to the low residual stress, the adhesion of TiN coating on TiCN-based cermets and cemented carbide was noticeably higher than that of coating on high-speed steel. The wear mechanism of TiN coatings on all tool materials tested against with steel balls was mainly abrasive wear. An iron oxide film appeared on the coated TiCN-based sample, which directly caused the friction coefficient to decrease from 0.8 to 0.6. Therefore, the TiN coating on TiCN-based cermets had the best wear resistance.
查看更多>>摘要：? 2021As a highly controversial candidate superhard material, tungsten tetraboride (WB4) has attracted widespread attention due to its desirable mechanical properties. Here, single phase WB4 ceramics and WB4-based materials containing tantalum (Ta) have been successfully synthesized by a high-pressure reaction at 5.0 GPa and 1400 °C - 3000 °C. We systematically investigated the effect of processing temperature on the phase stability of WB4 and doping with different amounts of Ta on mechanical properties and thermal stability. The results confirm that increasing treatment temperature is beneficial to the synthesis of the WB4 phase, and high pressure can effectively inhibit the decomposition of WB4. Additionally, the hardness value of the WB4-based ceramic is found to decrease with the increase of the doped tantalum content. However, the hardness of WB4 is less than W0.9Ta0.1B4 (10 at. % Ta), indicating that a small amount of Ta doping is positive for the mechanical properties of WB4. The results show that the W0.9Ta0.1B4 ceramic synthesized at a temperature of 2800 °C and 5.0 GPa has excellent overall properties. Its Vickers hardness is 42 GPa (0.49 N), and its oxidation resistance temperature is 542 °C, which is much higher than the hardness and thermal stability of tungsten carbide (WC). This work can provide practical guide for the design of new transition metal boride superhard materials, and point out the visual direction for exploring the next generation of superhard materials.
查看更多>>摘要：? 2021 Elsevier LtdThe novel oscillating pressure sintering (OPS) technique was used to design and prepare 90 W-10Cu (mass fraction) refractory alloys under lower sintering temperature, and the effects of sintering temperature on microstructure, density, grain size and Vickers hardness were invested. When the sintering temperature was 1200°C, the density and Vickers hardness of the OPS sample are 99.38% and 168.7 HV1, respectively, which are higher than the hot-pressed sintered samples at the same temperature. The density of W-Cu refractory alloys could be effectively promoted under lower temperature by the oscillating pressure sintering. Moreover, the grain size could be reduced activity within 4 μm during the sintering process. This is mainly due to the liquid phase binder fully filling the W grains under oscillating pressure to form the closer network microstructure, which promotes improvement of the density and performance. The results show that the oscillating pressure sintering with low temperature is beneficial to the preparation of W-Cu refractory alloy with higher density, fine-grained and well property.
查看更多>>摘要：? 2021The densification mechanism of boron carbide is revealed by considering the effect of electromigration induced by electric current during spark plasma sintering (SPS). Different electric current values are obtained by choosing different heating rate, resulting in the different dislocation density as well as densification behavior. The apparent pressure-particle neck area relationship is applied to estimate the instantaneous electric current density, then a densification model was proposed which taking it into account. Higher electric current responds to lower dislocation density and lower apparent activation energy of the mechanism controlling densification. The densification mechanism exhibits a transformation from atom-diffusion-controlled creep deformation mechanism for low electric current to dislocation-glide-controlled mechanism for high electric current.
查看更多>>摘要：? 2021 Elsevier LtdTribological performance of polycrystalline diamond?tantalum (PCD-Ta) composites manufactured by powder metallurgy and HPHT sintering was studied. Different PCD/Ta concentrations were prepared and tested in a pin-on-disc configuration. The tribological interactions were investigated through depths of penetration, Coefficients of Friction (CoF), wear rates, and microstructural evaluation of pairs. The overall results indicated that tantalum percentage directly influenced the tribological behavior of composite pairs, and higher wear performance was observed in samples with lower binder content. Furthermore, microstructural analyses revealed that all pairs exhibited abrasive and adhesive wear mechanisms, and their intensities were closely dependent on the Ta concentration.
查看更多>>摘要：? 2021The deformation behavior and strengthening mechanism for Mo-3Nb single crystal with 〈111〉 orientation and polycrystal have been investigated and disclosed comprehensively in a wide temperature range by quasi-static compression with 7% plastic strain. The slip traces of single crystal and polycrystal at room temperature show different features, which long continuous slip traces appear on the surface of deformed Mo-3Nb single crystals while some branch-off slip lines can only be observed in large grains for polycrystals. The conjugate slip planes of single crystals are activated at the elevated temperature, while the slip lines of polycrystals are difficult to be observed because of the grain boundary sliding effect. The compressive stress of single crystals and polycrystals shows different results at different deformation temperatures, suggesting that grain boundaries in polycrystals play an important role in the strengthening mechanism. TEM analysis shows that dislocation entanglement is the main strengthening mechanism both in single crystal and polycrystal during the room temperature deformation. With the increase of temperature, the strengthening mechanism caused by dislocation entanglement becomes ineffective, while grain boundaries continue to hinder the dislocation movement and the strengthening effect greatly weakened due to the dynamic recovery.
查看更多>>摘要：? 2021 Elsevier LtdBy the powder metallurgy, as well as subsequent forging and annealing processes, a series of W-3Re-HfC (WRH) composites were fabricated successively. Microstructure and performance of WRH composites were investigated comparatively. It is found that the sintered, forged and annealed WRH composites are consistently made up of W-Re matrix, HfO2 and HfC second phase particles. The larger HfO2 particle is derived from the oxidation of HfH2 powder, playing an important role in purifying grain boundary and refining grain of matrix. The smaller HfC particle inside W-Re grains has a dispersion reinforcement effect. The forged WRH composites can obtain the highest relative density (99.9%), the micro-hardness (534.1 HV) and ultimate tensile strength (758 MPa), which is ascribed to the refinement of grains and dispersion strengthening. Although the performance of annealed WRH composites has a slight drop, an excellent interfacial matching between second phase particles and W-Re matrix can be produced, which facilitates to gain a better interfacial strength and homogenized microstructure.
查看更多>>摘要：? 2021 Elsevier LtdThe current study is focused on the preparation of Mo-10 vol%La2O3 and Mo-10 vol% La2Zr2O7 composite powders via low- and high-energy ball milling approaches as potential candidates for near-future high-temperature structural applications. The mechanical milling parameters play a critical role on the final powder's microstructure. When using the high-energy milling mode (using 800 rpm, ball-to-powder ratio (BPR) 100: 6), the homogeneous powder agglomerates are formed with refined laminated microstructure and more uniform ceramic phase distribution in both Mo-La2O3 and Mo-La2Zr2O7 systems compared to the powders produced by means of the low-energy milling mode (using 350 rpm, BPR 100: 6), where inhomogeneous powder mixture with less embedding of ceramic phases into Mo agglomerates was obtained. This study also focuses on the evaluation of high-temperature phase and microstructural stability of the produced composite powders treated at the temperature of 1300 °C under the different gaseous environments, including ambient, inert and reducing atmospheres. The Mo-10 vol% La2Zr2O7 composite powder exhibited better thermal stability during the high-temperature exposure in all tested atmospheres in comparison with the Mo-La2O3 composite powder, since it revealed less intensive formation of the intermediate phases, such as lanthanum oxymolybdates. Therefore, the Mo-10 vol% La2Zr2O7 composite powder was used further for consolidation by means of spark plasma sintering at 1600 °C. The successful production of Mo-La2Zr2O7 composite with homogeneous distribution of ceramic phase, the grain size about of 5 μm, and hardness of 3.4 GPa was not reported so far.
查看更多>>摘要：? 2021 Elsevier LtdThe CoCrFeNiMo0.4 high-entropy alloys (HEAs) were synthesized by a high-pressure solid-state reaction at 3–5 GPa and held for 3–10 min at 1200 °C. The microstructure and mechanical properties of the sample were influenced by the pressure and holding time, which was confirmed when the Co, Cr, Fe, Ni, and Mo atoms diffused to form HEAs. The increased pressure hindered the diffusion between atoms and thus inhibited the formation of HEAs. Under the influence of the enthalpy of mixing between elements, Fe had the strongest repulsive force on the system and the weakest diffusion effect, but increasing the holding time promoted the diffusion of Fe atoms. Compared with the microhardness reported in other studies, the maximum hardness in our work was 449.1 HV, which represents a maximum increase of approximately 80%. The synergistic effects of the solid-solution strengthening of the system and the serious lattice distortion effect caused by large Mo atoms and high pressure were the dominant mechanisms responsible for the enhanced microhardness of the sintered HEAs under extreme conditions.
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