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TiC–Mo–Ni cermets were prepared by vacuum sintering using Ni powder as well as the self-synthesized ultrafine TiC and Mo powders as raw materials. Ni used as sintering additive was added to activate the sintering process. The effects of TiC grain size and sintering temperature on mechanical properties and microstructure of TiC–Mo–Ni cermets were investigated. The results showed that Mo was dissolved into TiC phase to generate (Ti, Mo)C solid solution and form a typical core–rim structure. The grain size of cermets was greatly affected by the particle size of raw material TiC powders. In addition, the increases of oxygen content and grain size in TiC powder were detrimental to the densification of TiC–Mo–Ni cermets. By using the TiC powder obtained under the calcium treatment temperature of 1523 K, the TiC–20Mo–10Ni cermet sintered at 1773 K in vacuum had a high relative density (98.8 pct), uniform microstructure, fine grain size (1.49 μm) and excellent mechanical properties (with the Vickers hardness, fracture toughness, transverse rupture strength of 2081 HV30, 11.43 MPa·m1/2 and 813 MPa, respectively). Moreover, the increase of TiC grain size led to the transition of fracture mechanism from intergranular fracture to transgranular fracture, which was favorable for the improvement of toughness. [ABSTRACT FROM AUTHOR]

In this work, high-performance 93W–4.9Ni–2.1Fe–1La2O3 (wt pct) alloy was fabricated by a two-stage sintering method and subsequent vacuum heat-treatment, which had a high densification degree (99.4 pct) and fine grain sizes of W (only 9.76 μm). The utilization of ultrafine W–Ni–Fe composite powder, addition of La2O3 particles and optimized sintering technique effectively resulted in structure refinement. Moreover, the second-phase particles were mainly dispersed at grain boundaries (GBs), besides a small amount of them in the interior of W grains. The as-prepared alloys after sintering first at 1400 °C for 2 hour and then at 1500 °C 30 minutes exhibited enhanced mechanical properties, with a maximum tensile strength of ~ 1024 MPa, elongation of ~ 22.7 pct and hardness of ~ 457 HV1. The excellent strength could be attributed to various mechanisms, namely fine-grained strengthening and oxide dispersion strengthening. Meanwhile, the ductilization is predominantly benefitted from a synergistic effect of low angle grain boundaries, intragranular oxide particles and vacuum heat-treatment process. This work not only promotes the development of oxide-reinforced WHAs for challenging engineering applications but also provides a new perspective for the design of refractory composite materials. [ABSTRACT FROM AUTHOR]

0.7BiFeO3–0.3BaTiO3 ceramics were synthesized through the solid-state reaction method under different calcination temperatures (Tcal) and calcination times (td) to investigate their effects on the microstructure and electrical properties of the ceramics. The grains and phase structure change upon increasing Tcal or td. However, too high values of Tcal or td result in the volatilization of Bi2O3, which deteriorates the electrical properties of the ceramics. For Tcal = 800 °C and td = 2 h, a morphotropic phase boundary with mixed rhombohedral and pseudo-cubic phases appears, and the lowest leakage density and the maximum grain size are obtained, which leads to an improvement in the electrical properties (Pr = 26.7 µC/cm2, EC = 28.5 kV/cm, d 33 ∗ = 237 pm/V, and TC = 419 °C). This study indicates that Tcal and td influence considerably the electrical properties of the 0.7BiFeO3–0.3BaTiO3 ceramics. [ABSTRACT FROM AUTHOR]

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The present study utilized W nanopowders (prepared by a two-stage reduction process consisting of steps of carbothermic pre-reduction and hydrogen deep reduction) to produce fine-grained 90W-7Ni-3Fe and 93W-4.9Ni-2.1Fe alloys. The effects of sintering temperature [in a range of 1573 K to 1723 K (1300 °C to 1450 °C)] and time on the microstructure and mechanical properties of W-Ni-Fe alloys were studied in detail. The microstructures of the prepared alloys at 1573 K and 1673 K (1300 °C and 1400 °C) indicated that the grain sizes of W grains are < 5 μm. However, the grain size dramatically increases to > 5 μm after sintering for > 60 minutes at 1723 K (1450 °C). The observation shows the as-sintered alloys with different holding times exhibit different fracture modes. With increasing holding time, the hardness of two alloys first increases and then decreases at the same sintering temperature. It is also concluded that the bending strength of alloys increases with the increase of sintering temperature. The maximum hardness of 381 and 393.6 HV was obtained after sintering at 1573 K (1300 °C) for 60 minutes for 90W-7Ni-3Fe and 93W-4.9Ni-2.1Fe alloys, respectively, while the highest bending strength of 1381.4 and 1278.8 MPa could be achieved at 1673 K (1400 °C) for 120 minutes. The excellent mechanical properties of W-Ni-Fe alloys could be attributed to the grain refinement strengthening. [ABSTRACT FROM AUTHOR]

To explore the influence of cyclic pre-deformation on the mechanical behavior of ultrafine-grained (UFG) materials with a high stacking fault energy (SFE), UFG Al processed by equal-channel angular pressing (ECAP) was selected as a target material and its tensile behavior at different pre-cyclic levels D (D = Ni / Nf, where Ni and Nf are the applied cycles and fatigue life at a constant stress amplitude of 50 MPa, respectively) along with the corresponding microstructures and deformation features were systematically studied. The cyclic pre-deformation treatment on the ECAPed UFG Al led to a decrease in flow stress, and a stress quasi-plateau stage was observed after yielding for all of the different-state UFG Al samples. The yield strength σYS, ultimate tensile strength σUTS, and uniform strain ɛ exhibited a strong dependence on D when D ≤ 20%; however, when D was in the range from 20% to 50%, no obvious change in mechanical properties was observed. The micro-mechanism for the effect of cyclic pre-deformation on the tensile properties of the ECAPed UFG Al was revealed and compared with that of ECAPed UFG Cu through the observations of deformation features and microstructures. [ABSTRACT FROM AUTHOR]

A method of preparing Mo2C via vacuum carbothermic reduction of MoS2 in the temperature range of 1350-1550°C was proposed. The effects of MoS2-to-C molar ratio (α, α = 1:1, 1:1.5, and 1:2.5) and reaction temperature (1350 to 1550°C) on the reaction were studied in detail. The phase transition, morphological evolution, and residual sulfur content of the products were analyzed by X-ray diffraction, field-emission scanning electron microscopy, and carbon-sulfur analysis, respectively. The results showed that the complete decomposition of MoS2 under vacuum is difficult, whereas activated carbon can react with MoS2 under vacuum to generate Mo2C. Meanwhile, higher temperatures and the addition of more carbon accelerated the rate of carbothermic reduction reaction and further decreased the residual sulfur content. From the experimental results, the optimum molar ratio α was concluded to be 1:1.5. [ABSTRACT FROM AUTHOR]

In this paper, micro-electromechanical systems (MEMS) technology and the bionic principle are used to develop a low-frequency high-sensitivity three-dimensional omni-vector hydrophone that can obtain vector information of an underwater sound field by imitating the auditory principle of a fish's lateral line organ. The key features are smaller size, better consistency, better low-frequency characteristic, higher sensitivity, and rigid mounting, which thus allow a spatial acoustic source to be detected directionally by a single hydrophone. The bionic MEMS microstructure was designed and fabricated and consists of two components: the vertical detection unit including a four-beam-cilium structure and a level detection unit including a double T-shaped beam structure. On the basis of theoretical analysis, the structure size and layout location of the piezoresistors are determined by simulation analysis and the double cilia type microstructure is fabricated integrally by MEMS manufacturing technology; after which the acoustic package of the microstructure is complete and the prototype is produced. Finally, this paper presents the experimental characterization of the microdevice, validating the concept and the analytical models used. The test results show that the three-dimensional vector hydrophone has a flat frequency response curve, exhibits a sensitivity of −185 dB (X, Y) and −181 dB (Z) (1 kHz, 0 dB reference 1 V/uPa) and shows a good directivity pattern in the form of an '8' shaped. More importantly, the depth of the concave point reaches 47.7 dB, and the asymmetry is only 0.5 dB, indicating that the three-dimensional vector hydrophone has great advantages in spatial orientation, which is suitable for applications in sonar systems. [ABSTRACT FROM AUTHOR]