Effect of Cu-Rich Phase Precipitation on the Microstructure and Mechanical Properties of CoCrNiCux Medium-Entropy Alloys Prepared via Laser Directed Energy Deposition

CoCrNiCux (x=0.16,0.33,0.75,and 1) without macro-segregation medium-entropy alloys (MEAs) was prepared using laser directed energy deposition (LDED).The microstructure and mechanical properties of CoCrNiCux alloys with increas-ing Cu content were investigated.The results indicate that a single matrix phase changes into a dual-phase structure and the tensile fracture behaviors convert from brittle to plastic pattern with increasing Cu content in CoCrNiCux alloys.In addi-tion,the tensile strength of CoCrNiCux alloys increased from 148 to 820 MPa,and the ductility increased from 1 to 11％with increasing Cu content.The nano-precipitated particles had a mean size of approximately 20 nm in the Cu-rich phase area,and a large number of neatly arranged misfit dislocations were observed at the interface between the two phases due to Cu-rich phase precipitation in the CoCrNiCu alloy.These misfit dislocations hinder the movement of dislocations during tensile deformation,as observed through transmission electron microscopy.This allows the CoCrNiCu alloy to reach the largest tensile strength and plasticity,and a new strengthening mechanism was achieved for the CoCrNiCu alloy.Moreover,twins were observed in the matrix phase after tensile fracture.Simultaneously,the dual-phase structure with different elastic moduli coordinated with each other during the deformation process,significantly improving the plasticity and strength of the CoCrNiCu alloy.     

NbCr2/ Nb合金高温力学行为

在800～1200℃的变形温度,0.001～0.1 s~(-1)的应变速率条件下对通过机械合金化+热压工艺制备的成分为Nb-22.5Cr的NbCr_2/Nb合金进行了高温压缩试验,研究了合金的高温力学行为,并通过透射电子显微镜观察分析了合金的变形机制。结果表明:NbCr_2/Nb合金的峰值强度随着变形温度的升高,应变速率的降低而下降。基体Nb的变形机制主要为位错的滑移;而NbCr_2的变形机制是通过层错、孪晶、不全位错等方式进行。     

外延生长的Gd掺杂CeO2薄膜内位错的透射电子显微学表征

利用脉冲激光沉积（ pulsed laser depositon, PLD）方法在YSZ（ Y2 O3 stabilised zirconia）单晶衬底上外延生长了Gd掺杂的CeO2薄膜（gadolinium doped CeO2,GDC）。利用透射电子显微镜（TEM）对GDC／YSZ界面以及GDC薄膜内部的位错结构进行了表征。实验发现,界面处存在周期性分布的失配位错,界面失配主要通过失配位错释放。 GDC薄膜内部存在两种不同的位错,其中一种为纯刃型位错,另外一种为混合型位错。     

Large‐Scale Growth of Two‐Dimensional SnS2 Crystals Driven by Screw Dislocations and Application to Photodetectors

2D SnS₂ crystals are attracting increasing attention owning to the huge potential for electronic and optoelectronic applications. However, batch production of 2D SnS₂ crystals via a simple vapor process remains challenging by far. Moreover, the growth mechanism for vapor growth of 2D SnS₂ is not well documented as well. Herein, a simple approach is presented for preparation of large‐scale 2D SnS₂ crystals on mica sheets and it is demonstrated that these 2D crystals follow a screw‐dislocation‐driven (SDD) spiral growth process. The synthesized 2D crystals show hexagonal and truncated triangular shapes with the lateral size ranging from a few micrometers to dozens of micrometers. Observations of key features for screw dislocations, such as helical fringes, dislocation hillocks, and herringbone contours, solidly confirm the SDD spiral growth behavior of the SnS₂. Possible mechanism is proposed in this work to show the generation and propagation of screw dislocations. Furthermore, in order to explore the optoelectronic property of the SnS₂, photodetectors based on 2D SnS₂ crystals are fabricated. The resulting device shows excellent operating characteristics, including good photo‐stability and reproducibility as well as a fast photoresponse time (≈42 ms), which enable the SnS₂ a promising candidate for photodetectors.     

Effect of obstacle strength and spacing on the slope of Haasen plot

The rate sensitivity of multiple obstacle aluminium alloy system was measured using the stress relaxation method. A discrepancy was observed between the slope of the Haasen plot (rate sensitivity of dislocation–dislocation interaction) in pure metals and in alloys strengthened by multiple obstacles. Considering a simplifying assumption of the constant obstacle force–distance profile, it is suggested that slope of the Haasen plot is governed by glide dislocations’ length taking part in a thermally activated event. The relative strength and spacing of obstacles (forest dislocations, solutes and precipitates) is proposed to affect the thermally activated dislocation length, which in turn manifests as the difference in slope of the Haasen plot.     

Simulation Charpy impact energy of functionally graded steels by modified stress-strain curve through mechanism-based strain gradient plasticity theory

In the present work, Charpy impact energy of functionally graded steels produced by electroslag remelting composed of graded ferritic or austenitic layers in both crack divider and crack arrester configurations has been modeled by finite element method. The yield stress of each layer was related to the density of the statistically stored dislocations of that layer and assuming by Holloman relation for the corresponding stress-strain curves, tensile strengths of the constituent layers were determined via numerical method. By using load-displacement curves acquired from instrumented Charpy impact tests on primary specimens, the obtained stress-strain curves from uniaxial tensile tests were modified. The data used for each layer in finite element modeling were predicted modified stress-strain curves obtained from strain gradient plasticity theory. A relatively good agreement between experimental results and those obtained from simulation was observed.     

High Hole Mobility of GaSb Relaxed Epilayer Grown on GaAs Substrate by MOCVD through Interfacial Misfit Dislocations Array

The structural property of GaSb epilayers grown on semi-insulator GaAs (001) substrate by metalorganic chemical vapor deposition (MOCVD) using Triethylgallium (TEGa) and trimethylantimony (TMSb),was investigated by variation of the Sb:Ga (V/III) ratio.An optimum V/III ratio of 1.4 was determined in our growth conditions.Using transmission electron microscopy (TEM),we found that there was an interfacial misfit dislocations (IMF) growth mode in our experiment,in which the large misfit strain between epilayer and substrate is relaxed by periodic 90 deg.IMF array at the hetero-epitaxial interface.The rms roughness of a 300 nm-thick GaSb layer is only 2.7 nm in a 10 μm×10 μm scan from atomic force microscopy (AFM) result.The best hole density and mobility of 300 nm GaSb epilayer are 5.27×10 6 cm 3 (1.20×10 6) and 553 cm 2 ·V 1 ·s 1 (2340) at RT (77 K) from Hall measurement,respectively.These results indicate that the IMF growth mode can be used in MOCVD epitaxial technology similar to molecular beam epitaxy (MBE) technology to produce the thinner GaSb layer with low density of dislocations and other defects on GaAs substrate for the application of devices.     

Collinear unequal crack series in magnetoelectroelastic materials: Mode I case solved via new real fundamental solutions

For the first time, real fundamental solutions are proposed for in-plane magnetoelectroelastic governing equations. Dislocation simulation method is used to derive singular integral equations for collinear unequal crack series under mode I loadings. Special cases of a single crack and two collinear equal cracks are used to validate the proposed solution. The effects of crack space, crack length and magnetic/electric loading are discussed. Real fundamental solutions have an advantage over complex fundamental solutions, because real analyses are more straightforward than complex analyses. Moreover, the model of collinear unequal cracks is more practical than that of collinear equal cracks.