Effects of the extrusion parameters on microstructure,texture and room temperature mechanical properties of extruded Mg-2.49Nd-1.82Gd-0.2Zn-0.2Zr alloy

Microstructure,texture,and mechanical properties of the extruded Mg-2.49Nd-1.82Gd-0.2Zn-0.2Zr alloy were investigated at different extrusion temperatures(260 and 320℃),extrusion ratios(10:1,15:1,and 30:1),and extrusion speeds(3 and 6 mm/s).The experimental results exhibited that the grain sizes after extrusion were much finer than that of the homogenized alloy,and the second phase showed streamline distribution along the extrusion direction(ED).With extrusion temperature increased from 260 to 320℃,the microstructure,texture,and mechanical properties of alloys changed slightly.The dynamic recrystallization(DRX)degree and grain sizes enhanced as the extrusion ratio increased from 10:1 to 30:1,and the strength gradually decreased but elongation(EL)increased.With the extrusion speed increased from 3 to 6 mm/s,the grain sizes and DRX degree increased significantly,and the samples presented the typical<2111>-<1123>rare-earth(RE)textures.The alloy extruded at 260℃ with extrusion ratio of 10:1 and extrusion speed of 3 mm/s showed the tensile yield strength(TYS)of 213 MPa and EL of 30.6%.After quantitatively analyzing the contribution of strengthening mechanisms,it was found that the grain boundary strengthening and dislocation strengthening played major roles among strengthening contributions.These results provide some guidelines for enlarging the industrial application of extruded Mg-RE alloy.

Understanding the creep behaviors and mechanisms of Mg-Gd-Zn alloys via machine learning

Mg-Gd-Zn based alloys have better creep resistance than other Mg alloys and attract more attention at elevated temperatures.However,the multiple alloying elements and various heat treatment conditions,combined with complex microstructural evolution during creep tests,bring great challenges in understanding and predicting creep behaviors.In this study,we proposed to predict the creep properties and reveal the creep mechanisms of Mg-Gd-Zn based alloys by machine learning.On the one hand,the minimum creep rates were effectively predicted by using a support vector regression model.The complex and nonmonotonic effects of test temperature,test stress,alloying elements,and heat treatment conditions on the creep properties were revealed.On the other hand,the creep stress exponents and creep activation energies were calculated by machine learning to analyze the variation of creep mechanisms,based on which the constitutive equations of Mg-Gd-Zn based alloys were obtained.This study introduces an efficient method to comprehend creep behaviors through machine learning,offering valuable insights for the future design and selection of Mg alloys.

Microstructure Evolution during Solidification of AZ91D Magnesium Alloy in Semisolid

The liquid quenching method was adopted to study the solidification morphology and microstructure of AZglD Mg alloy in semisolid. The results indicate that cooling rate has important effects upon the solidification structures. Under the cooling rate of liquid quenching, primary α-phase grows first by attaching on the original α grains, or independent nucleation and growth. The high cooling rate makes primary α-phase grow in ＂rags＂ or dendrite shape. Eutectic solidification is carried out in terms of both dissociated growth and symbiotic growth. The dissociated growth forms rough and large β-phase at grain boundaries, while symbiotic growth forms eutectic of laminar structure. The small liquid pool inside the original α-phase solidifies basically in the same way as that of intergranular liquid, but owing to less amount of liquid phase, the eutectic solidification is mainly carried out in the dissociated pattern.

Solidification Structure Formation of Undercooled Single Phase Alloys

Solidification structure variation of single phase alloy with undercooling prior to nucleation has been widely studied. The progress, especially during the last decade, is reviewed so as to give a comprehensive knowledge for it, in which the emphases are laid on the structure evolution mechanism and the potential application. Lastly, the future interesting subjects are presented.

Effect of the precipitation state on high temperature tensile and creep behaviors of Mg-15Gd alloy

Due to the effective precipitation strengthening effect of the β phase, Mg-Gd alloys exhibit excellent room temperature mechanical behaviors. However, when served at high temperatures, the metastable β phase will transform to other phases, resulting in severe performance degradation. In this study, we investigated the effect of precipitation state achieved by different heat treatments on high temperature tensile and creep behaviors of the Mg-15Gd alloy by comparing the properties of the as-cast, solid-solutioned(T4) and peak-aged(T6) alloys. The results showed that the tensile mechanical properties of the T6 alloy were always highest from room temperature to 300 ℃, in spite of an abnormal strength increase with temperature existed in the T4 alloy. For tensile creep properties, the T6 alloy exhibited the lowest steady creep rate below 235 ℃ while the T4 alloy possessed the best properties above 260 ℃. Microstructure characterization revealed that the transition was caused by the stress-promoted precipitation of β phase in the T4 alloy and rapid phase transformation in the T6 alloy at high temperatures. At 260 ℃, the calculated stress exponent n was 3.1 and 2.8 for the T4 and T6 alloys, respectively, suggesting the creep deformation mechanism was dislocation slip, which was further confirmed by the microstructure after creeping. Our findings can provide new insights into the heat treatment process of Mg-Gd alloys served at high temperatures.

Effect of dimensional expansion on carrier transport behaviors of the hexagonal Bi-based perovskite crystals

All-inorganic Cs_(3)Bi_(2)I_(9)(CBI)halide perovskites are sought to be candidate for photoelectrical materials because of their low toxicity and satisfactory stability.Unfortunately,the discrete molecular[Bi2I9]3−clusters limit the charge-transport behaviors.Herein,the defect halide perovskite based on trivalent Bi^(3+)is expanded to Cs_(3)Bi_(2)I_(6)Br_(3)(CBIB).Centimeter-size CBIB single crystal(Φ15×70 mm^(3))was grown by the vertical Bridgeman method.The powder X-ray diffraction analysis shows that CBIB has structure with lattice parameters of a=b=8.223Å,c=10.024Å,α=β=90°andγ=120°.The density functional theory(DFT)calculations demonstrate that the charge density distribution was enhanced after the dimensional expansion.The enhancement of carrier transport ability of(00l)in-plane is characterized before and after dimensional improvement.The obtained CBIB(001)exhibited an electron mobility up to 40.03 cm^(2)V^(−1)s^(−1)by time-of-flight(TOF)technique,higher than 26.46 cm^(2)V^(−1)s^(−1)of CBI(001).Furthermore,the X-ray sensitivity increases from 707.81μC Gy^(−1)cm^(−2)for CBI(001)to 3194.59μC Gy−1 cm^(−2)for CBIB(001).This research will deepen our understanding of Bi-based perovskite materials and afford more promising strategies for lead-free perovskite optoelectronic devices modification.

Twin boundary dominated electric field distribution in CdZnTe detectors

The performance of CdZnTe X/γ-ray detectors is strongly affected by the electric field distribution in terms of charge transport and charge collection.Factors which determine the electric field distribution are not only electric contact,but also intrinsic defects,especially grown-in twin boundaries.Here,the electric field distribution around twin boundaries is investigated in a CdZnTe bicrystal detector with a{111}–{111}twin plane using the Pockels electro-optic effect.The results of laser beam induced current pulses are also obtained by the transient current technique,and we discuss the influence of the twin boundary on the electric field evolution.These studies reveal a significant distortion of the electric field,which is attributed to the buildup of space charges at twin boundaries.Also,the position of these space charge regions depends on the polarity of the detector bias.An energy band model based on the formation of an n–n+–n junction across the twin boundary has been established to explain the observed results.

Metal–organic framework wafer enabled fast response radiation detection with ultra-low dark current

Semiconductive metal–organic frameworks(MOFs)have attracted great interest for the electronic applications.However,dark currents of present hybrid organic–inorganic materials are 1000–10,000 times higher than those of commercial inorganic detectors,leading to poor charge transportation.Here,we demonstrate a ZIF-8(Zn(mim)_(2),mim=2-methylimidazolate)wafer with ultra-low dark current of 1.27 pA·mm^(-2) under high electric fields of 322 V·mm^(-1).The isostatic pressing preparation process provides ZIF-8 wafers with good transmittance.Besides,the presence of redox-active metals and small spatial separation between components promotes the charge hopping.The ZIF-8-based semiconductor detector shows promising X-ray detection sensitivity of 70.82μC·Gy^(-1)·cm^(-2) with low doses exposures,contributing to superior X-ray imaging capability with a relatively high spatial resolution of 1.2 lp·mm^(-1).Simultaneously,good peak discrimination with the energy resolution of~43.78%is disclosed when the detector is illuminated by uncollimated 241Am@5.48 MeVα-particles.These results provide a broad prospect of MOFs for future radiation detection applications.

Microstructure evolution,mechanical properties and diffusion behaviour of Mg-6Zn-2Gd-0.5Zr alloy during homogenization

The microstructure evolution and mechanical properties of Mg-6 Zn-2 Gd-0.5 Zr alloy during homogenization treatment were investigated. The as-cast alloy was found to be composed of dendritic primaryα-Mg matrix, α-Mg +W（Mg;Zn;Gd;） eutectic along grain boundaries, and icosahedral quasicrystalline I（Mg;Zn;Gd） phase within α-Mg matrix. During homogenization process, α-Mg +W（Mg;Zn;Gd;） eutectic and I phase gradually dissolved into a-Mg matrix, while some rod-like rare earth hydrides（GdH2）formed within α-Mg matrix. Both the tensile yield strength and the elongation showed a similar tendency as a function of homogenization temperature and holding time. The optimized homogenization parameter was determined to be 505℃ for 16 h according to the microstructure evolution. Furthermore,the diffusion kinetics equation of the solute elements derived from the Gauss model was established to predict the segregation ratio of Gd element as a function of holding time, which was proved to be effective to evaluate the homogenization effect of the experimental alloy.

Structural and optical properties of Dy^(3+):YAlO3 phosphors for yellow light-emitting diode applications

The spectroscopic properties of a series of Dy^(3+)single-doped and Dy^(3+)/Nd^(3+),Dy^(3+)/Tb^(3+),and Dy^(3+)/Tm^(3+)co-doped YAlO_(3)(yttrium aluminum perovskite,YAP)phosphors were investigated and compared through the measurements of optical absorption,emission spectra,and fluorescence decay curves.For the Dy^(3+)ion single-doped samples,the intensity of each absorption band increases with an increment in Dy^(3+)ion doping concentration,and the identified strong absorption peak at 447 nm indicates that Dy^(3+):YAP phosphors are suitable to be pumped by a blue laser diode(LD).For all co-doped samples,absorption peaks of Dy^(3+)ion along with some of the absorption bands of Nd^(3+),Tb^(3+),and Tm^(3+)ions are observed.Under 351 and 447 nm excitation,a prominent emission peak at 572 nm was obtained in all the samples,corresponding to Dy^(3+):^(4)F_(9/2)→^(6)H_(13/2)transition.Here,2 at%Dy^(3+):YAP phosphor exhibits the highest yellow emission intensity under 447 nm pumping.Among the three kinds of Dy^(3+)co-doped phosphors,Dy^(3+)/Tb^(3+):YAP phosphor possesses the dominant yellow emission.The fluorescence decay curves show exponential behaviour and are fitted well.The Commission International de L’Eclairage(CIE)chromaticity coordinates were calculated following the respective emission spectra,and it is found that all the coordinates locate in the yellow region.The energy transfer(ET)processes were investigated and the concentration quenching mechanism was discussed.The obtained results suggest that Dy^(3+)-activated YAP phosphors are good candidates for yellow LED applications.

Microstructure, Microsegregation, and Mechanical Properties of Directional Solidified Mg–3.0Nd–1.5Gd Alloy

The microstructure, microsegregation, and mechanical properties of directional solidified Mg–3.0Nd–1.5Gd ternary alloys were experimentally studied. Experimental results showed that the solidification microstructure was composed of dendrite primary a（Mg） phase and interdendritic a（Mg） · Mg12（Nd, Gd） eutectic and Mg5 Gd phase. The primary dendrite arm spacing k1 and secondary dendrite arm spacing k2 were found to be depended on the cooling rate R in the form k1= 8.0415 9 10-6R-0.279 and k2= 6.8883 9 10-6R-0.205, respectively, under the constant temperature gradient of40 K/mm and in the region of cooling rates from 0.4 to 4 K/s. The concentration profiles of Nd and Gd elements calculated by Scheil model were found to be deviated from the ones measured by EPMA to varying degrees, due to ignorance of the back diffusion of the solutes Nd and Gd within a（Mg） matrix. And microsegregation of Gd depended more on the growth rate, compared with Nd microsegregation. The directionally solidified experimental alloy exhibited higher strength than the non-directionally solidified alloy, and the tensile strength of the directionally solidified experimental alloy was improved,while the corresponding elongation decreased with the increase of growth rate.

Terahertz emission from layered GaTe crystal due to surface lattice reorganization and in-plane noncubic mobility anisotropy

In this work, a model based on the optical rectification effect and the photocurrent surge effect is proposed to describe the terahertz emission mechanism of the layered GaTe crystal. As a centrosymmetric crystal, the optical rectification effect arises from the breaking of the inversion symmetry due to lattice reorganization of the crystal’s surface layer. In addition, the photocurrent surge originating from the unidirectional charge carrier diffusion-due to the noncubic mobility anisotropy within the layers-produces terahertz radiation. This is confirmed by both terahertz emission spectroscopy and electric property characterization. The current surge perpendicular to the layers also makes an important contribution to the terahertz radiation, which is consistent with its incident angle dependence. Based on our results, we infer that the contribution of optical rectification changes from 90%under normal incidence to 23% under a 40° incidence angle. The results not only demonstrate the terahertz radiation properties of layered GaTe bulk crystals, but also promise the potential application of terahertz emission spectroscopy for characterizing the surface properties of layered materials.

Structural Stability and Half Metallic Features of Zn_(0.5)Cr_(0.5)S under Pressure

Spin-polarized first-principle was performed to study the structural stability and the electronic states of Cr doped ZnS with the Cr component of 50% in zincblende （ZB）, wurtzite （W） and rocksalt （RS） structures under pressure. The results show that the zincblende and wurtzite structures become unstable under low pressures of about 4.68 and 9.61 GPa, respectively, but the rocksalt structure can be maintained up to an extremely high pressure of about 32.92 GPa. Both zincblende and wurtzite Zno.sCro.5S display half metallic features under pressure, while rocksalt Zno.sCro.sS exhibits metallic feature. The half metallic features can be ascribed to the stronger interactions between S-3p and Cr-3d states and the metallic feature is due to the higher crystal symmetry of rocksalt Zn0.5Cr0.5S. These results can provide helpful guidance for Cr doped ZnS to be used in spintronic devices.

Orientation and mobility control of 4HCB organic film for flexible X-ray detectors with high performance

In comparison to inorganic counterparts,organic semiconducting(OSC)crystalline films are promising for building large-area and flexible ionizing radiation detectors for X-ray imaging or dosimetry due to their tissue equivalence,simple processing and large-scale production accessibility.Fabrication processes,how-ever,hinder the ability to generate aligned and large-area films with high carrier mobility.In this work,the space-confined melt process is used to produce highly orientated 4HCB(4-hydroxycyanobenzene)OSC films with a large area of 15×18 mm^(2).The out-of-plane direction of the 4HCB film is<001>,and the benzene rings are found to be extensively overlapped inside the in-plane direction,according to the XRD patterns.The film exhibits a high resistivity up to 1012cm,and high hole mobility of 10.62 cm^(2)V^(−1)s^(−1).Furthermore,the 4HCB(80μm-thick film)based X-ray detectors can achieve a sensitivity of 93μC Gy air^(−1) cm^(−2)and on/offratio of 157.The device also shows steady flexibility,with no degradation in detecting function after 100 cycles of bending.Finally,the proposed 4HCB film detectors demonstrated a high-resolution X-ray imaging capability.The imaging of several materials with sharp edges(copper and polytetrafluoroethylene)has been obtained.This work has developed a fast but efficient approach for producing large-area,highly oriented OSC films for high-performance X-ray detectors.

ZnCl_2-assisted Synthesis of ZnSe Polycrystal

A chemical-assisted element direct-reaction method is developed to synthesize ZnSe compound semiconductor material at a relatively low temperature （-1000 ℃）. ZnSe polycrystal was obtained in the closed-tube systems with Zn-Se, Zn-Se-Zn（NHa）2CI2, ZnoSe-NH4CI and Zn-Se-ZnCI2. The as-synthesized samples were tested by X-ray diffraction （XRD）, thermogravimetric analysis （TGA） and analyzed by thermodynamic numerical method. The results demonstrate that the synthesis efficiency is higher than 99.96% for Zn-Se-ZnCl2 system at around 1000 ℃ for 3 weeks. It also exhibits that not only temperature, but also low apparent ratio of volume and surface area of the source materials and higher ZnCl2 content are required to achieve high synthesis efficiency. A SeCI transporting reaction synthesis process is proposed based on the thermodynamic analysis.