Search

The microstructure and tensile properties of twin and grain size strengthening Mg–6.85Gd–5.79Y–0.44Zr alloy have been investigated and characterized by optical microscopy (OM), scanning electron microscopy (SEM), and electron back-scattering diffraction (EBSD). Their room temperature tensile results exhibit that twin strengthening can enhance the strength of Mg–Gd–Y–Zr alloy but sacrifice its ductility or toughness. This should be attributed that lenticular {10−12} twins with high-stress localization directly lead to a brittle failure. In contrast, the strength, ductility, and toughness were synchronously improved for the grain size strengthening Mg–Gd–Y–Zr alloy, i.e., the ultimate tensile strength, uniform elongation, and static toughness increased to 254 (259–245) MPa, 1.02 (1.40–0.55)%, and 5.7 (6.7–4.2) MJ/m3, respectively. This ductility increment should have resulted from fine recrystallization that was free from stress concentration and accommodated the tensile strain by more non-basal slip and grain boundary sliding or grain rotation.

Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale1-10. So far these polaritons have been limited to two classes: bulk polaritons, which propagate inside a material, and surface polaritons, which decay exponentially away from an interface. Here we report a near-field observation of ghost phonon polaritons, which propagate with in-plane hyperbolic dispersion on the surface of a polar uniaxial crystal and, at the same time, exhibit oblique wavefronts in the bulk. Ghost polaritons are an atypical non-uniform surface wave solution of Maxwell's equations, arising at the surface of uniaxial materials in which the optic axis is slanted with respect to the interface. They exhibit an unusual bi-state nature, being both propagating (phase-progressing) and evanescent (decaying) within the crystal bulk, in contrast to conventional surface waves that are purely evanescent away from the interface. Our real-space near-field imaging experiments reveal long-distance (over 20 micrometres), ray-like propagation of deeply subwavelength ghost polaritons across the surface, verifying long-range, directional and diffraction-less polariton propagation. At the same time, we show that control of the out-of-plane angle of the optic axis enables hyperbolic-to-elliptic topological transitions at fixed frequency, providing a route to tailor the band diagram topology of surface polariton waves. Our results demonstrate a polaritonic wave phenomenon with unique opportunities to tailor nanoscale light in natural anisotropic crystals.

The static recrystallization process of a cold-rolled Mg-Zn-Gd alloy was tracked by a quasi-in-situ electron backscatter diffraction method to investigate the orientations of nuclei. The results show that orientation distribution of nuclei is associated with nucleation mechanism. The continuous static recrystallization nuclei display similar orientations to the parent grains with TD orientation. Differently, discontinuous static recrystallization nuclei formed within the parent grains (TD-45° orientation) show random orientations and a variety of misorientation angles but preferred axes 52 ¯ 73> or 52 ¯ 70>. Interestingly, a special oriented nucleation is found. Discontinuous static recrystallization nuclei originated from boundaries of the parent grain (TD-70° orientation) show concentrated TD orientations in another side due to the preferred misorientation relationship 70° 11 2 ¯ 0> (∑18b). It is speculated that these two special misorientation relationships are related to the dislocation type.

Phonon polaritons in polar van der Waals materials enable strong electromagnetic-field confinement and enhancement for deeply subwavelength scale light-matter interactions. Here we propose and theo...

Hexagonal boron nitride (hBN) supports two types of hyperbolic phonon polaritons (HPPs), whose properties of strong electromagnetic field confinement and low propagation loss have been proposed for various applications in nanophotonics. Conventionally, real-space imaging of HPPs by scattering-type scanning near-field optical microscopy (s-SNOM) with vertical polarization excitation contains both tip and edge launched polariton modes, which leads to hybrid interference fringes. In this work, we symmetrically study the tip and edge excited HPPs in both boron nitride with the natural distribution of boron isotopes (natural hBN) and 11B isotope-enriched boron nitride (99.2% 11B hBN). The intrinsic HPPs excited in 99.2% 11B hBN exhibit a lower damping rate and longer propagation length than that in natural hBN. We experimentally realize a tuning from tip-dominated to edge-dominated excited HPPs by rotating the polarization of incident light. The near-field electric field intensity (NEFI) of edge-excited HPPs Eedge and the angle β (between the hBN edge and the projective direction of the incident electric field on the hBN plane) present a sine function relationship as Eedge ∝ |sin β| under an s-polarized incident light. The NEFI of edge-excited HPPs in 99.2% 11B hBN shows a 10% enhancement compared to natural hBN under the same measurement conditions. Our findings demonstrate an effective approach to reducing phonon polariton damping and manipulating phonon polariton excitation in hBN, which are beneficial for developing HPPs-based nanophotonic applications.

Hot isostatic pressing (HIP) was applied to Mg-6Gd-3Y-0.5Zr (GW63) alloy to reduce shrinkage porosity, thus, to enhance the integrity and reliability of castings. During HIP process, shrinkage porosity was closed by grain compatible deformation and subsequent diffusion across the bonding interface. The amount of initial shrinkage porosity was the key factor for shrinkage porosity closure. HIP was testified to be effective on shrinkage porosity reduction in GW63 alloy due to its relatively narrow solidification range and resultant low content of initial shrinkage porosity in most sections, leading to higher tensile properties both in as-cast and cast-T6 condition. The improvement in tensile properties was mainly because of shrinkage porosity reduction and resultant effective rare-earth (RE) elements homogenization and precipitation strengthening.

We experimentally and theoretically investigated the propagation and subwavelength coupling of surface plasmons in carbon nanotube pairs (CNTPs). We found two plasmon modes in the Fourier transformed near-field images of CNTPs, while only one propagating plasmon mode in the single carbon nanotube (CNT). Using the finite element method (FEM), we calculated plasmon coupling between two carbon nanotubes. We attributed the observed two plasmon modes to the symmetric and asymmetric coupling of the plasmons between the two CNTs. Using numerical simulation, we further investigated the plasmon coupling strength and found that it is sensitive to the separating distance of CNTs. When the separation is larger than 5 nm, the plasmon coupling attenuates. We show that near-field optical imaging is a feasible way to characterize the plasmon coupling of CNTs, even if the separation of the CNTs is only a few nanometers.

Polaritons in anisotropic materials result in exotic optical features, which can provide opportunities to control light at the nanoscale

The rolled Mg-1.02Zn alloys with strong basal-textures and Mg-0.76Y alloys with weakened basal-textures were subjected to annealing in order to obtain different grain structures meanwhile keeping the textures nearly unchanged. Then tension testing and microstructure examination were applied to the Mg-1.02Zn and Mg-0.76Y alloys with different grain structures, aiming to validate the Hall-Petch relationship and related inner deformation modes. The tensile testing results show that the Mg-1.02Zn specimens exhibit two slope (k) of Hall-Petch relationships, that is, the coarse-grained (> 49 µm) specimens reveal an abnormally enhanced tendency of k (~ 472 MPa μm1/2) with increased strains while fine-grained (≤ 49 µm) specimens exhibit a slightly decreased tendency of k with increased strains. Contrastively, the Mg-0.76Y specimens follow a single slope of Hall-Petch relationship well in the grain regime with slightly declined k with increased strains. That could be attributed to the distinguished twinning responses and/or twinning-slipping interactions dependence of grain size in the studied alloys with different initial textures.

Hyperbolic phonon polaritons (HPhPs) in α-phase molybdenum trioxide (α-MoO3) have recently attracted significant attention. They propagate anisotropically along the flake of α-MoO3 and show anomalously concave wavefronts when being excited by a pointlike source. Such anisotropic propagation is governed by the anisotropic wavevectors of HPhPs, which have been studied in different works. However, extracting the complex-valued wavevectors of all HPhP modes directly from the observed anomalous wavefront remains elusive. Here, we theoretically and experimentally demonstrate that the complex-valued HPhP wavevectors can be accurately quantified by fitting the concave wavefront profiles with a modified damped sine-wave function in all allowed directions. To that end, HPhPs are launched by an infrared antenna on a thin flake of α-MoO3 and are imaged in real space by using scanning near-field optical microscope. From the recorded concave wavefronts of HPhPs, we have experimentally retraced both the real part and the imaginary part of polariton wavevectors along different propagation directions. Our results are of fundamental importance for analyzing the optical properties of HPhPs in α-MoO3, which can also be generic to other anisotropic optical/polaritonic systems.

The microstructure evolution and plastic deformation mechanism of Mg-6.58Gd-5.7Y-0.55Zr alloy were investigated by uniaxial compression at temperatures ranging from 300 °C to 450 °C and with an initial strain rate of 0.1 s−1, and characterized by optical microscope (OM), scanning electron microscope (SEM) and electron back-scattering diffraction (EBSD). The effect of temperature on twinning behavior was focused in detail. At relatively low temperature, such as 300 °C, multiple twin variant pairs can be activated in one grain, and the twining process was dominated by nucleation. However, at high temperature, most of the twins activated in one grain belong to the single twin variant pair with the highest Schmid Factor (SF), and twin growth seems to represent the twinning behavior at 450 °C. The twinning behavior of Mg-6.58Gd-5.7Y-0.55Zr alloy at temperatures ranging from 300 °C to 450 °C basically follows Schmid law, and can be well understood in terms of variation of stress state and activated deformation modes with compression temperature.

We theoretically propose and study in-plane anisotropic acoustic phonon polaritons (APhPs) based on a layered structure consisting of a monolayer (or few layers) {\alpha}-phase molybdenum trioxide ({\alpha}-MoO3) sandwiched between two metal layers. We find that the APhPs in the proposed sandwiched structures are a canalization (highly directional) electromagnetic mode propagating along with the layers and at the same time exhibit extreme electromagnetic-field confinement surpassing any other type of phonon-polariton modes. When a double layer of {\alpha}-MoO3 is sandwiched by two Au layers, twisting the two {\alpha}-MoO3 layers can adjust the interlayer polaritonic coupling and thus manipulate the in-plane propagation of the highly confined APhPs. Our results illustrate that the metal-MoO3-metal sandwiched structures are a promising platform for light guiding and manipulation at ultimate scale., Comment: 7 pages, 4 figures

The plasmonic response of gold clusters with atom number (N) = 100–70 000 was investigated using scanning transmission electron microscopy-electron energy loss spectroscopy. For decreasing N, the bulk plasmon remains unchanged above N = 887 but then disappears, while the surface plasmon firstly redshifts from 2.4 to 2.3 eV above N = 887 before blueshifting towards 2.6 eV down to N = 300, and finally splitting into three fine features. The surface plasmon's excitation ratio is found to follow N0.669, which is essentially R2. An atomically precise evolution picture of plasmon physics is thus demonstrated according to three regimes: classical plasmon (N = 887–70 000), quantum confinement corrected plasmon (N = 300–887) and molecule related plasmon (N

Quantitative morphological evolution is of great importance in nanochemistry. In this work, morphology of silver nanotriangles (AgNTs) is quantitatively evolved under the guidance of DNA. First, intact AgNTs are prepared relying on the protection of horseradish peroxidase. Then different regions of AgNTs are sequentially etched by C-rich DNA, leading to DNA-guided postshaping of AgNTs. In combination with atomically resolved images and theoretical simulation, a model is established to track the postshaping process. Since real-time morphological evolution of AgNTs is determined with spectra, a series of AgNTs with specific corners can be obtained by controlling incubation time. The DNA-guided postshaping is sequence and structure dual-dependent, and a mechanism is proposed based on metal-base interaction, surface energy of faces, and freedom of DNA structure. In addition, the postshaping is further used to design DNA-mediated biosensors. This study provides a precise and quantitative method of controlling morphology of anisotropic metallic nanomaterials.

The effect of initial tempers with different characteristics of precipitates and contents of supersaturated solute atoms, including the homogenized, peak–aged and over–aged conditions, on the tensile creep behavior of WE43 alloy has been investigated at 523 K. Results show that the peak–aged alloy at 523 K obtained superior creep resistance than the homogenized, peak–aged at 498 K and over–aged at 523 K alloy. A uniform dispersion of β precipitates was dynamically formed within steady–stage creep microstructure of the homogenized WE43 alloy after creep deformation of 200 h. It is found that the precipitate size and distribution is similar with the alloy aged equal time without the applied stress. In addition, the WE43 alloy in all tempers obtains similar precipitate size and distribution in their steady–stage creep microstructures. Therefore, it is inferred that the various initial tempers mainly affect the primary creep stage. Furthermore, numerous dislocations were detected between precipitates and the stress exponent n is 4.5, which is close to 5. Thereby, dislocation climb is suggested to be the creep mechanism. The reason for the peak–aged alloy at 523 K obtained superior creep resistance is that the initial uniform dispersion of β″ and β′ precipitates have smaller precipitate size and higher precipitate density than that of the homogenized and over–aged alloy, which are more effectively to hider dislocation climb. However, a deterioration of creep resistance was occurred in the peak−aged alloy at 498 K due to precipitate recovery when crept at 523 K. As a consequence, WE43 alloy in peak–aged temper at 523 K achieves the highest creep resistance.

The effect of cold plastic deformation by multi-directional forging prior to artificial aging treatment on the microstructure and mechanical properties of WE43 alloy was investigated. The results show that the pre-deformation produced high density of dislocations and deformation twins. After peak-aging treatment at 200 °C, the pre-deformation reduced the average size of fine β″ and β′ precipitates and promoted the formation of β 1 phase. Additionally, spheroid–shaped precipitates formed at twin boundaries and the deformation twins were thermally stable during the aging treatment. Mechanical properties reveal that the tensile yield and ultimate strength is significantly enhanced from 191±3 MPa to 270±15 MPa and 278±5 MPa to 320±18 MPa, respectively.

High temperature tensile–creep behavior of Mg–4Y–2.3Nd–1Gd–0.6Zr (wt%, WE43(T6)) alloy at 523–573 K was investigated. The creep stress exponent is equal to 4.6, suggesting the underlying dislocation creep mechanism. The activation energy is (199 ± 23) kJ/mol, which is higher than that for self-diffusion in Mg and is believed to be associated with precipitates coarsening or cross slip. The creep mechanism is further suggested to be dislocation climb at 523 K, while a cross slip at 573 K is possible. The metastable β′ and β1 phases in the WE43(T6) alloy were relatively thermal stable at 523 K and could be effective to hinder the dislocation climb, which contributed to its excellent creep resistance. However, at 573 K it readily transforms into equilibrium βe phase and coarsens within two hours, thereby causing a decrease of creep resistance. In addition, precipitate free zones approximately normal to applied stress direction (directional PFZs) developed during the creep deformation, especially at 573 K. Those zones became preferential sites to nucleate, extend and connect microcracks and cavities, which lead to the intergranular creep fracture. Improving the thermal stability of precipitates or introducing thermally stable fine plate-shaped precipitates on the basal planes of Mg matrix could enhance the high temperature creep resistance.

Mid-infrared is a spectral region of molecular vibration and rotation modes and thus, it has been widely used in chem/bio analysis. On-chip MIR waveguides combining attenuated total reflection spectroscopy provide an efficient way to minimize equipment size and benefit chemical trace analysis. But, inevitable surface roughness-induced scattering is harmful for waveguide mode propagation in traditional sensors. Two-dimensional materials are natural thin slabs with atomic-scale smooth surfaces and thus could be excellent for building weak surface scattering waveguides. Here, we used near-field microscopy to investigate a waveguide mode of 1T tin diselenide slabs at nanoscale resolution in 5.13-6.57 μm and manipulate the mode strengths and wavelengths by controlling the slab thickness. This work extends two-dimensional materials as building blocks for integrated MIR chips.

A variety of infrared applications rely on semiconductor superlattices, including, notably, the realization of high-power, compact quantum cascade lasers. Requirements for atomically smooth interface and limited lattice matching options set high technical standards for fabricating applicable heterostructure devices. The semiconductor twinning superlattice (TSL) forms in a single compound with periodically spaced twin boundaries and sharp interface junctions and can be grown with convenient synthesis methods. Therefore, employing semiconductor TSL may facilitate the development of optoelectronic applications related to superlattice structures. Here, it is shown that InAs TSL nanowires generate inter-sub-band transition channels due to the band projection and the Bragg-like electron reflection. The findings reveal the physical mechanisms of inter-sub-band transitions in TSL structure and suggest that TSL structures are promising candidates for mid-infrared optoelectronic applications.

A Mg-2.1Y-0.5Zn-0.6Cu-0.1Zr (WZC200, in at. %) alloy was developed and conducted to extrusion process to refine the grains as well as LPSO structure, aiming to investigate the mechanical behaviors at ambient and elevated temperatures and their relations with microstructures. A 14H-LPSO structure with stacking sequence of ABCBCACACACBCBA, consisting of twin-related building blocks with a novel CBCA-type, was revealed in the solution-treated WZC200 alloy. Fully recrystallized microstructure with grain size of ⁓14 μm, refined LPSO structure and weaken texture were obtained through extrusion process. The tension results tested at ambient and elevated temperatures revealed ultimate tensile strength (UTS) of 317 MPa along with excellent elongation-to-failure of 26.5% at RT, moderate UTS above 275 MPa with occurrence of serrated flow in the tensile curves in the range 150–200 °C, and obviously declined UTS (⁓150 MPa) along with superplastisity of 155% at 300 °C. The observed serrated flow in the range 150–200 °C was mainly attributed to the dynamic strain ageing. The dramatic drop in UTS at 300 °C was ascribed to fragmenting of LPSO phases and probably significant activation of non-basal slips, these two factors help to coordinate deformation between LPSO and the matrix and restrain the cavity nucleation, resultantly contributing to the observed superplasticity at this temperature.

We systematically studied surface plasmons reflection by graphene wrinkles with different heights on SiC substrate. Combined with numerical simulation, we found that the geometry corrugation of a few nanometer height wrinkle alone does not causes a reflection of graphene plasmons. Instead, the separated wrinkle from substrate exhibits a nonlinear spatial Fermi energy distribution along the wrinkle, which acts as a heterojunction. Therefor a higher graphene wrinkle induces a stronger damped region when propagating graphene surface plasmons encounter the wrinkle and get reflected.

Interference-free hyperbolic phonon polaritons (HPPs) excited by natural wrinkles in a hexagonal boron nitride (hBN) microcrystal are reported both experimentally and theoretically. Although their geometries are off-resonant with the excitation wavelength, the wrinkles compensate for the large momentum mismatch between photon and phonon polariton, and launch the HPPs without interference. The spatial feature of wrinkles is about 200 nm, which is an order of magnitude smaller than resonant metal antennas at the same excitation wavelength. Compared with phonon polaritons launched by an atomic force microscopy tip, the phonon polaritons launched by wrinkles are interference-free, independent of the launcher geometry, and exhibit a smaller damping rate (γ ≈ 0.028). On the same hBN microcrystal, in situ nanoinfrared imaging of HPPs launched by different mechanisms is performed. In addition, the dispersion of HPPs is modified by changing the dielectric environments of hBN crystals. The wavelength of HPPs is compressed twofold when the substrate is changed from SiO2 to gold. The findings provide insights into the intrinsic properties of hBN-HPPs and demonstrate a new way to launch and control polaritons in van der Waals materials.

Due to the ability to manipulate photons at nanoscale, plasmonics has become one of the most important branches in nanophotonics. The prerequisites for the technological application of plasmons include high confining ability (λ0 /λp ), low damping, and easy tunability. However, plasmons in typical plasmonic materials, i.e., noble metals, cannot satisfy these three requirements simultaneously and cause a disconnection to modern electronics. Here, the indium arsenide (InAs) nanowire is identified as a material that satisfies all the three prerequisites, providing a natural analogy with modern electronics. The dispersion relation of InAs plasmons is determined using the nanoinfrared imaging technique, and show that their associated wavelengths and damping ratio can be tuned by altering the nanowire diameter and dielectric environment. The InAs plasmons possess advantages such as high confining ability, low loss, and ease of fabrication. The observation of InAs plasmons could enable novel plasmonic circuits for future subwavelength applications.

The nontrivial topological origin and pseudospinorial character of electron wavefunctions make edge states possess unusual electronic properties. Twenty years ago, the tight-binding model calculation predicted that zigzag termination of 2D sheets of carbon atoms have peculiar edge states, which show potential application in spintronics and modern information technologies. Although scanning probe microscopy is employed to capture this phenomenon, the experimental demonstration of its optical response remains challenging. Here, the propagating graphene plasmon provides an edge-selective polaritonic probe to directly detect and control the electronic edge state at ambient condition. Compared with armchair, the edge-band structure in the bandgap gives rise to additional optical absorption and strongly absorbed rim at zigzag edge. Furthermore, the optical conductivity is reconstructed and the anisotropic plasmon damping in graphene systems is revealed. The reported approach paves the way for detecting edge-specific phenomena in other van der Waals materials and topological insulators.