[en] Highlights: • Reinforcement with low fractions and nano size can improve properties of DRTMCs. • Hierarchical structures design can improve strength-ductility trade-off of DRTMCs. • Oxidation and creep resistance are important for widening application of DRTMCs. • Numerical methods are beneficial for fundamental research of DRTMCs. Discontinuously reinforced titanium matrix composites (DRTMCs) are attractive materials for the application of aeronautics and astronautics, automotive and military fields, due to their low density, heat-resistance, and excellent room- and high-temperature properties. Recently, numerous literatures have been published about the DRTMCs, it is essential to summarize their research progress, in order to further comprehend their development and guide the future work. This work includes several significant aspects of DRTMCs: recent development in the reinforcements, microstructure characteristics, mechanical properties as well as modeling and calculation. The present review attempts to reveal the existing problems and propose further research directions for the aforementioned topics. In order to further improve the overall properties and extend the applications of DRTMCs, bringing the hierarchical structures of biological materials into the composites is an effective method. Furthermore, the numerical methods should be performed to study the composites fundamentally and obtain microscopic information of the composites.

[en] We apply transformation optics (TO) theory to investigate two-dimensional heat flux cloaks for arbitrarily shaped objects. The TO theory is applied to design a device through which heat flux travels around objects with arbitrary shapes, which greatly improves the flexibility of the cloak applications. The proposed theory is verified by numerical results, showing that the proposed method is capable of controlling the diffusive heat flow and cloaking a region with arbitrary geometries of interest. (paper)

[en] A generalized transformation is proposed to design an illusion device. The device can reshape an arbitrarily shaped perfect electrical conductor (PEC) into another dielectric object with arbitrary geometry. Such a device can evolve into an ideal invisibility cloak with non-conformal boundaries if the virtual space is filled with air. Furthermore, the validity of our proposed transformation is confirmed by two specific devices. One is to convert a regular polygonal PEC cylinder into a circular dielectric cylinder. Another one is to reshape a circular PEC cylinder into a regular polygonal dielectric cylinder.

[en] Microstructural evolution of as-rolled (TiB₂/Al)-Ti laminates in the process of multistep heat treatments and microstructural characteristics of the resulting TiB₂-TiAl composites were investigated in detail by scanning electron microscope (SEM) and transmission electron microscope. The desired TiB₂-TiAl composites exhibited a unique microlaminated structure consisting of alternating fully lamellar α₂-Ti₃Al/γ-TiAl layers and TiB₂-rich layers. Moreover, the formation mechanism of the microlaminated structure was elucidated. It is noteworthy that the TiB₂-TiAl composites showed improved high-temperature tensile properties and room-temperature fracture toughness due to the strengthening effect of TiB₂ particles and the structure effect of the unique microlaminated microstructure. In addition, the fracture behavior of the microlaminated TiB₂-TiAl composites during the dynamic tensile was characterized by combining use of the three-dimensional X-ray microscope (3D-XRM) and SEM. The results indicated that pores and cracks preferred to initiate in the brittle TiB₂-rich layer and showed a typical layered distribution, and then expanded into a nearby fully lamellar α₂/γ layer and kept propagating through the entire α₂/γ layer into another TiB₂-rich layer, repeatedly, and finally the main crack propagated through all the fully lamellar α₂/γ layers and TiB₂-rich layers, eventually resulting in failure. Thus, the fracture mechanism of the innovative microlaminated TiB₂-TiAl composites was proposed.

[en] Starting with graphite, TiB₂, and Ti-6Al-4V powders, the present work demonstrated that hybrid (TiC+TiB) network-strengthened Ti-6Al-4V—based composites can be fabricated via an integrated low-energy ball-milling and reaction hot-pressing-sintering technique. With the aid of phase equilibrium and powder densification kinetic calculations, the corresponding sintering parameters were optimized and tunable network microstructures were subsequently achieved. Tensile properties for these composites were examined at elevated temperatures of 500 °C, 550 °C, 600 °C, and 650 °C, the results of which indicated that the 50-μm network configuration with 5 vol pct reinforcer content led to the most enhanced tensile strength compared to both Ti-6Al-4V alloys and solely TiB-reinforced Ti-6Al-4V composites. The underlying strengthening mechanisms were mainly ascribed to carbon interstitial dissolution, reinforcer-assisted grain refinement, and extensive dispersoids. It was recognized from fractographic analyses that the matrix/reinforce interface contributed to the major crack propagation source at temperatures below 550 °C, leading to brittlelike fracture along the network boundary; however, once testing temperatures rose above 600 °C, matrix tearing and reinforcer cut-through mechanisms took place, giving rise to ductile fracture. Based on the experimental observations and theoretical calculations, future perspectives regarding the processing and microstructural manipulation for advanced high-temperature titanium matrix composites were also discussed.

[en] Based on complementary fractal geometry structures, we design a novel infrared quasi-three-dimensional (3D) nanocavity with a localized enhanced field with multiband resonant frequencies. The fractals offer the nanostructure two important characteristics, multiband functionality and a subwavelength effect. The electric field, power flow, and the field intensity distributions are given to indicate the internal mechanism of the localized enhanced field in the nanocavity. Additionally, the effective medium method is established to retrieve the permittivity and impedance of the structure. It is shown that a strongly enhanced localized field is achieved in the nanocavity at two different resonant frequencies by using the finite difference time domain method. The field intensity in the nanocavity is enhanced by a factor of up to 60 times over that of the incident light because of the important contribution of the loss factor in the permittivity. The surface plasmon hybridization is thought to play an important role in the strong localized field enhancement. The multiband property and high localized intensity offer the nanocavity great potential for applications in surface enhanced Raman scattering and other nanoscale novel devices. (paper)

[en] Highlights: • Two-scale laminate-network structure of composites is successfully fabricated by powder metallurgy. • Bending strain remarkably increases with similar strength, implying a strength-ductile balance is achieved. • Impact energy is enhanced due to two-scale structure, microstructure refinement and interface crack deflection. • Increasing reinforcement fraction in two-scale composites can enhance energy in propagation stage during impact process. TMCs are well known materials for their potential use in many fields. However, there are few research on the toughness especially impact toughness of TMCs. To improve impact property of TMCs, one novel kind of titanium matrix composites with two-scale laminate-network structure was fabricated via powder metallurgy and reaction hot pressing. From macro-scale, the novel composites were constituted by Ti6Al4V alloy layers and TiBw/Ti6Al4V composite layers. Moreover, the composite layer exhibited network structure and refined matrix microstructure from micro-scale. Layer thicknesses were well controlled by the masses of Ti6Al4V powder and TiB₂-Ti6Al4V mixture powders, and the volume fraction of TiBw reinforcement in the composite layer was adjusted by controlling TiB₂ raw material. Bending test results exhibited that the ultimate strain of two-scale composite doubled while bending strength was similar compared with TiBw/Ti6Al4V composites. The impact toughness of two-scale composites was nearly fivefold enhanced compared with monolithic TiBw/Ti6Al4V composites. This phenomenon can be attributed to the introduction of laminate structure, microstructure refinement and interface crack deflection. The analysis of impact curves and fractographs suggests that introducing Ti alloy layers can enhance plastic stage. Increasing TiBw reinforcement volume fractions can enhance the energy absorbed during crack propagation stage.

[en] We present an infrared perfect absorber model composed of gold nanobars and a photonic microcavity. The inevitable losses in metamaterials are taken as an advantage for high absorbance efficiency. By adjusting the structural geometry, the device can be used for refractive index sensing. In our calculation with a spacer thickness H = 90 nm it can yield more than 99% absorbance in the near-infrared frequency region. The full-width at half-maximum can be realized up to an extremely narrow value of 40.8 nm and the figure of merit can be obtained as high as 357. For sensing applications with a perfect absorber, our work can serve as a model of coupling between the localized surface plasmon within nanoparticles and the propagating surface plasmon along the planar metal layer. The novel concept has great potential to maintain its performance of localized surface plasmon in practical applications. (paper)

[en] The metallic periodic bowtie structure is systematically investigated using a three-dimensional finite difference time domain (FDTD) method. The oscillating dipoles picture is proposed to explain the novel enhancement of localized near-field. It is indicated that the shift and the intensity change of notable resonant dips in the transmission spectrum result from the variation of localized surface plasmon resonance (LSPR) conditions due to the different geometrical sizes of the structure and the array periodicity. The results are helpful for guiding the bowtie structures to the application of the near-field imaging and sensing in the subwavelength optics.

[en] The thermal cloak has been a long-standing scientific dream of researchers and engineers. Recently thermal metamaterials with man-made micro-structure have been presented based on the principle of transformation optics (TO). This new concept has received considerable attention, which is a powerful tool for manipulating heat flux in thermal imaging systems. However, the inherent material singularity has long been a captivation of experimental realization. As an alternative method, the scattering-cancellation-based cloak (or bi-layer thermal cloak) has been presented to remove the singularity for achieving the same cloaking performance. Nevertheless, such strategy needs prerequisite knowledge (geometry and conductivity) of the object to be cloaked. In this paper, a new thermal ground cloak is presented to overcome the limitations. The device is designed, fabricated and measured to verify the thermal cloaking performance. We experimentally show that the remarkably low complexity of the device can fully and effectively be manipulated using realizable transformation thermal devices. More importantly, this thermal ground cloak is designed to exclude heat flux without knowing the information of the cloaked object. - Highlights: • We present the first thermal carpet cloak. • The carpet can thermally cloak any shaped object without knowing the properties of the object to be cloaked. • Excellent agreements between simulation and experiment are observed.