A multi-wavelength mid-IR laser based on BaGa4Se7 optical parametric oscillators

A multi-wavelength mid-IR laser consisting of 3.05 μm, 4.25 μm, and 5.47 μm BaGa4Se7(BGSe)optical parametric oscillators (OPOs) switched by DKDP electro-optic switches with one 10 Hz/7.6 ns pumping wave is demonstrated. Maximum energies at 3.05 μm, 4.25 μm, and 5.47 μm are 1.35 mJ, 1.03 mJ, and 0.56 mJ, respectively, corresponding to optical-to-optical conversion efficiencies of 9.4%, 7.6%, and 4.2%. To the best of our knowledge, this study is the first of generation of three mid-IR wavelength lasers using electro-optic switches. Furthermore, this study provides a viable solution for a high-energy or high-power, compact, or even portable multi-wavelength mid-IR laser device that employs a single pumping wave

Strain hardenability of a gradient metallic alloy under high-strain-rate compressive loading

Subjected to an impact loading, metallic alloys usually display strain softening due to the highly localized shear deformation accompanied by temperature rise, which causes the decrease of resistance to an applied loading and even catastrophic failure. In this work, strain hardening under high-rate compressive loading is firstly achieved in a developed metallic alloy with the gradient microstructure of both phases and grain sizes. Mechanical responses at different compressive loading rates all show a unique strain hardening. Rate dependencies of yielding and strain hardening are revealed and illustrated by a modified Johnson-Cook constitutive model. Dynamic deformation and fracture are characterized as macroscopic shear with the shielded cracks. The inherent toughening mechanisms are indicated as gradient straining with phase transformation, twinning and dislocation tangling. This work demonstrates that the highly localized shear deformation accompanied by temperature rise can be prevented by the gradient microstructure with martensitic transformations, twinnings and dislocations. It is full of interest to create a feasible route of developing the strain hardenable metallic alloys under impact loadings via architecturing the gradient microstructure. (C) 2019 The Authors. Published by Elsevier Ltd

Structural Rejuvenation of Metallic Glasses and Its Effect on Mechanical Behaviors

Metallic glasses (MGs) are formed by the deep undercooling of high-temperature melt up to the glass transition temperature, and this process avoids the crystallization of the melt into ordered configurations of atoms. The atomic packing of MGs lacks a long-range periodicity. MGs reside at metastable energy states far away from the equilibrium of thermodynamics, but they are jammed in dynamics. These features provide MGs with remarkable mechanical, physical, and chemical properties, such as very high strength that is close to the ideal limit. However, the plastic deformation of MGs at room temperature is easily localized to form nanoscale shear bands, thereby resulting in limited macroscopic plasticity. More-over, physical ageing spontaneously reduces their energies toward an equilibrium state, thereby further weakening the plastic deformation ability of MGs, which is known as ageing-induced brittleness. Recent studies have shown that MGs can be rejuvenated with external energy injection into more disordered high-energy states in structure. This process, which is the inverse of physical ageing, can effectively improve the global plasticity of MGs and is expected to solve the problems of shear banding and physical ageing that restrict the applications of such materials. Therefore, the relevant aspects of the rejuvenation of MGs have attracted increasing interest. This article first introduces methods for the rejuvenation of MGs starting from the concepts of ageing and rejuvenation of glasses, and then summarizes the influencing factors of rejuvenation and the effects of rejuvenation on plasticity and other mechanical behaviors of MGs. Furthemore, the physical mechanism of rejuvenation is discussed briefly. Finally, several conclusions are drawn in this field, and some important problems that deserve further investigation are proposed

Effective Energy Density of Glass Rejuvenation

Glasses with rejuvenated structures usually exhibit improved room-temperature plasticity, which facilitates their applications. However, glass rejuvenation requires external energy injection to "shake up" the frozen-in disordered structure. In this work, we give the answer to how much the required energy is. According to the constitutive model of amorphous plasticity, we find that the applied stress higher than the steady-state flow value can effectively induce the structural disordering in terms of the generation of free volume. Therefore, the effective energy density (EED) of structural rejuvenation is defined as the integral of this effective stress on the corresponding strain. By tailoring the applied strain, strain rate, temperature and initial free volume, different degrees of structural rejuvenation are achieved, which show a generally linear correlation with the defined EED. This work deepens the understanding of glass rejuvenation from an energy perspective

Unraveling the threshold stress of structural rejuvenation of metallic glasses via thermo-mechanical creep

The competition between physical aging and structural rejuvenation determines the physical and mechanical properties of glassy materials. Thus, the rejuvenation-aging boundary must be identified quantitatively. In this work, we unravel a stress boundary to distinguish rejuvenation from aging via the thermo-mechanical creep of a typical Zr-based metallic glass. The crept glasses were rejuvenated into high-enthalpy disordered states when the applied stress exceeded a threshold that was numerically close to the steady-state flow stress; otherwise, the glasses were aged. A theoretical model for glass creep was adopted to demystify the observed stress threshold of rejuvenation. The model revealed that the thermo-mechanical creep beyond the threshold stress could activate sufficient shear transformations to create a net free volume, thus leading to structural rejuvenation. Furthermore, we derived the analytical expressions for the threshold and flow stresses. Both stresses can act as the rejuvenation-aging boundary, which is well supported by experimental creep data. The present work procures a deeper understanding of the rejuvenation mechanism of glasses and provides useful implications for abstaining from glass aging

Comparative study of amorphous and crystalline Zr-based alloys in response to nanosecond pulse laser ablation

In this work, we comprehensively investigate the response of amorphous and crystalline Zr-based alloys under nanosecond pulse laser ablation. The in situ multiphysics processes and ablation morphologies of the two alloy targets are explored and compared. The results indicate that the dynamics of laser-induced plasma and shock waves obey the idea blast wave theory and are insensitive to the topological structures of targets. Both targets experience significant superheating and culminate in explosive boiling. This ablation process leads to the formation of a hierarchical structure in the resultant ablation crater: microdents covered by widespread nanovoids. The amorphous target shows shallower microdents and smaller nanovoids than their crystalline counterparts because the former has a smaller heat-affected zone and experiences a higher degree of superheating. The hierarchical structure can adjust the surface wettability of targets from initial hydrophilic to hydrophobic, showing an increase of the contact angle approximately 119% for amorphous alloy compared with the crystal approximately 64%. This work demonstrates that amorphous alloys have a better performance against nanosecond pulse laser ablation and provides a feasible and one-step method of wettability modification for either amorphous or crystalline alloys

High-frequency elastic moduli and internal frictions of Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass during glass transition and crystallization

Temperature dependent Young's modulus, shear modulus and the related internal frictions of Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit-1) bulk metallic glass (BMG) were measured by using the modified piezoelectric ultrasonic composite oscillator technique (M-PUCOT) at longitudinal and torsional resonance under several tens of kilohertz, respectively. An irreversible internal friction peak accompanied by a very small modulus relaxation (about 10%) are observed near the onset crystallization temperatures T-x in each resonance mode, and the peak values in these two modes are almost the same, indicating that the supercooled liquid region (SLR) is far from liquid state under high frequency vibration with micro strain (similar to 10(-7)) amplitude. What is more, the internal friction peak value decreases obviously after a preheating process to SLR (similar to 410 degrees C) and the final values of moduli are proportional to the crystallinity. All these interesting phenomena during the glass transition and crystallization can well be interpreted through a three-parameter anelastic solid phenomenological model

Melatonin Mitigates Water Deficit Stress in <i>Cenchrus alopecuroides</i> (L.) Thunb through Up-Regulating Gene Expression Related to the Photosynthetic Rate, Flavonoid Synthesis, and the Assimilatory Sulfate Reduction Pathway

Melatonin can improve plant adaptability to water deficit stress by regulating the biosynthesis of flavonoids and improving the reactive oxygen species-scavenging enzyme system. However, it remains unclear whether melatonin mitigates the effects and causes of water deficit stress in Cenchrus alopecuroides. We conducted a PEG-simulated water stress pot experiment to determine whether and how exogenous melatonin alleviates water deficit in C. alopecuroides. The experiment was divided into four treatments: (1) normal watering (Control), (2) 40% PEG-6000 treatment (D), (3) 100 μmol·L−1 melatonin treatment (MT), and (4) both melatonin and PEG-6000 treatment (DMT). The results showed that melatonin can alleviate water deficit in C. alopecuroides by effectively inhibiting plant chlorophyll degradation and MDA accumulation while increasing antioxidant enzyme activities and photosynthetic rates under water deficit stress. The transcriptome results indicated that melatonin regulates the expression of genes with the biosynthesis pathway of flavonoids (by increasing the expression of PAL, 4CL, HCT, and CHS), photosynthesis-antenna proteins (by increasing the expression of LHC), and sulfur metabolism (the expression of PAPSS and CysC is up-regulated in the assimilatory sulfate reduction pathway), while up-regulating the transcription factors (AP2/ERF-ERF-, C2H2-, WRKY-, Tify-, bHLH-, NAC-, and MYB-related). These findings revealed the possible causes by which melatonin mitigates water deficit stress in C. alopecuroides, which provided novel insights into the role of melatonin in water deficit stress

Nanosecond pulsed laser-induced formation of nanopattern on Fe-based metallic glass surface

Fe-based metallic glasses (MGs) have attracted much attention because of their cheap raw materials, outstanding soft magnetic properties and superior catalytic activity. Meanwhile, the fabrication of micro/nano-structures on its surface could further improve its functional properties. In this study, it was attempted to fabricate micro/ nano-structures on a Fe-based MG (Fe52Cr13Mo12C15B6Er2, in at. %) surface by nanosecond pulsed laser irradiation technology. The surface characteristics and microstructural evolution of Fe-based MG were investigated. The experimental results showed that under different laser fluences, the laser-irradiated areas exhibited distinguished microstructures, i.e., nanoparticles, the network nanostructures or a combination of these two microstructures. Furthermore, oxygen and erbium were enriched inside the network nanostructures. By analyzing the microstructural evolution, formation mechanisms of the nanoparticles and the network nanostructures were discussed. The nanoparticles were actually caused by laser-induced element enrichment (i.e. amorphous erbium oxide) and the mismatch of its wettability with the substrate; the formation of the network nanostructures was attributed to the diffusion and connection of nanoparticles under the combined influence of recoil pressure and surface topography