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.     

GaAs窗口晶体位错分布及其对红外透射的影响

LEC法生长的轻掺铬高阻GaAs 晶体熔融KOH腐蚀坑SEM观察结果表明,轻掺Cr-GaAs晶体位错的径向分布呈“W”型。这是由于GaAs单晶生长过程中,晶体中的热弹应力的“W”分布和位错大量增殖时所需的临界应力共同作用的结果。红外透射测量结果表明:当位错密度不太高时,位错引入的电子态的体密度低,位错的径向分布对红外透过率的影响很小;当位错密度很高时,其对红外透过率的影响不可忽视。     

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。     

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.     

Strain evolution of SiGe-on-insulator fabricated by germanium condensation method with over-oxidation

Strain evolutions of SiGe film during Ge condensation processes of SiGe on silicon-on-insulator were studied in detail with assistance of X-ray diffraction. At the beginning of Ge condensation, SiGe on silicon-on-insulator with low Ge fraction was oxidized at higher temperature of 1150 °C, the strong plastic deformation of buried SiO₂ and Si-Ge intermixing relieved most of the strain in SiGe with increasing Ge fraction. When temperature was reduced to 900 °C for oxidation of SiGe layer with higher Ge fraction, Ge accumulation overmatched Si-Ge inter-diffusion, resulting in non-uniform profile of Ge in SiGe layer. During this period, plastic deformation of buried SiO₂ can be neglected and dislocation gliding plays a significant role in relieving strain in SiGe, which enlarges the surface roughness. The strain in SiGe increases gradually with condensation time for the thickness of SiGe layer reduces close to its critical thickness, even with higher Ge fraction. Intensive over-oxidation of germanium-on-insulator materials was suggested to be effective to fully relax the compressive strain but should be precisely controlled to avoid surface deterioration.     

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.     

Structural Analysis of Highly Relaxed GaSb Grown on GaAs Substrates with Periodic Interfacial Array of 90° Misfit Dislocations

We report structural analysis of completely relaxed GaSb epitaxial layers deposited monolithically on GaAs substrates using interfacial misfit (IMF) array growth mode. Unlike the traditional tetragonal distortion approach, strain due to the lattice mismatch is spontaneously relieved at the heterointerface in this growth. The complete and instantaneous strain relief at the GaSb/GaAs interface is achieved by the formation of a two-dimensional Lomer dislocation network comprising of pure-edge (90°) dislocations along both [110] and [1-10]. In the present analysis, structural properties of GaSb deposited using both IMF and non-IMF growths are compared. Moiré fringe patterns along with X-ray diffraction measure the long-range uniformity and strain relaxation of the IMF samples. The proof for the existence of the IMF array and low threading dislocation density is provided with the help of transmission electron micrographs for the GaSb epitaxial layer. Our results indicate that the IMF-grown GaSb is completely (98.5%) relaxed with very low density of threading dislocations (10⁵ cm⁻²), while GaSb deposited using non-IMF growth is compressively strained and has a higher average density of threading dislocations (>10⁹ cm⁻²).