High‐Entropy Alloy (HEA)‐Coated Nanolattice Structures and Their Mechanical Properties

Nanolattice structure fabricated by two‐photon lithography (TPL) is a coupling of size‐dependent mechanical properties at micro/nano‐scale with structural geometry responses in wide applications of scalable micro/nano‐manufacturing. In this work, three‐dimensional (3D) polymeric nanolattices are initially fabricated using TPL, then conformably coated with an 80 nm thick high‐entropy alloy (HEA) thin film (CoCrFeNiAl_0.3) via physical vapor deposition (PVD). 3D atomic‐probe tomography (APT) reveals the homogeneous element distribution in the synthesized HEA film deposited on the substrate. Mechanical properties of the obtained composite architectures are investigated via in situ scanning electron microscope (SEM) compression test, as well as finite element method (FEM) at the relevant length scales. The presented HEA‐coated nanolattice encouragingly not only exhibits superior compressive specific strength of ≈0.032 MPa kg^?1 m³ with density well below 1000 kg m^?3, but also shows good compression ductility due to its composite nature. This concept of combining HEA with polymer lattice structures demonstrates the potential of fabricating novel architected metamaterials with tunable mechanical properties.

     

On the mixed Mode II/III fatigue threshold behaviour for aluminium alloys 2014‐T6 and 7075‐T6

This paper describes an investigation into the fatigue threshold behaviour of two structural aluminium aerospace alloys, Al 2014‐T6 and Al 7075‐T6, when subjected to Mode II, Mode III and mixed Mode II/III loading. A unique four‐point shear loading test rig was employed to cyclically load sharply edge‐notched square bar specimens using an increasing load technique. The main aim of the work has been to generate Mode II–Mode III interaction diagrams for the fatigue threshold in each case, in order to facilitate improved design procedures for components fabricated from these alloys, which are susceptible to fatigue cracking under predominantly shear type loading. Aircraft are subjected to structural loads consisting of: pressurization, tension/compression, bending, shear and torsion, both on the ground and in flight. Representative fatigue fracture surfaces have been examined using scanning electron microscopy.     

针刺C/SiC复合材料拉-压疲劳特性与失效机理EI北大核心CSCD

研究了室温下针刺C/SiC复合材料的拉-压疲劳特性,并与其拉-拉疲劳特性进行了对比。结果表明:针刺C/SiC复合材料的拉-压疲劳强度略低于拉-拉疲劳强度;两种循环载荷下都存在迟滞现象,随着循环数的增大迟滞环不断右移,且偏斜程度和包围面积不断增大。采用扫描电子显微镜对失效试件的断口形貌和微观结构的观察表明:除了垂直于加载方向的基体开裂以及界面脱粘,拉-压循环加载下的细观失效机制还包括平行于加载方向的基体开裂以及层间的开裂。这些平行于加载方向的损伤使得纤维受力状态恶化,最终削弱了针刺C/SiC复合材料拉-压疲劳强度。     

A study of the microscopic deformation behavior of Nb3Sn composite superconducting tape using the acoustic emission technique

Nb₃Sn-based composite superconducting tapes have been widely used because of their excellent properties such as high critical current density, low AC loss and high critical temperature. However, one of the disadvantages of Nb₃Sn-based composite superconducting tape is that the Nb₃Sn compound often exhibits multiple cracking owing to its intrinsic brittleness when subjected to mechanical loading such as bending, winding, and operation. Such cracking eventually causes severe degradation of the critical current density. Therefore, it is very important to understand the microscopic deformation behavior of Nb₃Sn-based composite superconducting tape under mechanical loading.In this study, the microscopic deformation behavior and the fracture mechanism of Nb₃Sn-based composite superconducting tape were investigated using acoustic emission (AE) technique at room temperature. The tensile behavior of Nb₃Sn-based composite superconducting tape was analyzed using the AE parameters including amplitude, duration time, and event count, which are representative of various deformation parameters such as elasticity, yielding, and cracking. The results show that the AE technique is very effective for evaluating the deformation behavior of Nb₃Sn-based composite superconducting tape.     

Microstructure and mechanical behavior of ECAP processed AZ31B over a wide range of loading rates under compression and tension

In this work, a commercial magnesium alloy, AZ31B in hot-rolled condition, has been subjected to severe plastic deformation via four passes of equal channel angular pressing (ECAP) to modify its microstructure. Electron backscatter diffraction (EBSD) was used to characterize the microstructure of the as-received, ECAPed and mechanically loaded specimens. Mechanical properties of the specimens were evaluated under both compression and tension along the rolling/extrusion direction over a wide range of strain rates. The yield strength, ultimate strength and failure strain/elongation under compression and tension were compared in detail to sort out the effects of factors in terms of microstructure and loading conditions. The results show that both the as-received alloy and ECAPed alloy are nearly insensitive to strain rate under compression, and the stress–strain curves exhibit clear sigmoidal shape, pointing to dominance of mechanical twinning responsible for the plastic deformation under compression. All compressive samples fail prematurely via adiabatic shear banding followed by cracking. Significant grain size refinement is identified in the vicinity of the shear crack. Under tension, the yield strength is much higher, with strong rate dependence and much improved tensile ductility in the ECAPed specimens. Tensile ductility is even much larger than the malleability under compression. This supports the operation of 〈c + a〉 dislocations. However, ECAP lowers the yield and flow strengths of the alloy under tension. We attempted to employ a mechanistic model to provide an explanation for the experimental results of plastic deformation and failure, which is in accordance with the physical processes under tension and compression.     

Ermüdungsverhalten und Dauerfestigkeit von Graphit und Aluminium – infiltriertem Graphit

Fatigue behaviour and endurance limit of graphite and of aluminium‐infiltrated graphite Fatigue properties of polycrystalline, isotropic graphite FU2590 and of FU2590 infiltrated with AlSi7Mg (FU2590/AlSi7Mg) were investigated in reversed bending tests at 25 Hz at numbers of cycles below 10⁷ and in tension‐compression tests at 20 kHz below 10⁹ cycles. The open porosity of Graphite (10‐11 Vol.‐%) was infiltrated with the aluminium alloy using the squeeze casting infiltration method, which led to an increase of the bending strength by 50 %, increase of tensile strength by 30 % and increase of stiffness by 15 %. Fully reversed tension‐compression loading of FU2590 delivers a mean endurance limit at 10⁹ cycles at the normalized maximum stresses (i.e. maximum tension stress of a cycle divided by the static strength) of 0,65±0,03. Mean numbers of cycles to failure of 10⁴ were found in fully reversed bending tests at the normalized maximum stress of 0,78. The infiltrated material shows approximately 30 % higher cyclic strength in reversed bending tests, and the mean endurance limit under tension compression loading increases by 15 %. The increased endurance limit of the infiltrated material is caused by the increased stiffness. The increased toughness of graphite due to the infiltration with aluminium is of additional beneficial influence at the higher cyclic stresses investigated in reversed bending tests and in static tests.     

Mechanical behaviour and microstructural characterization of 3D four‐directional braided SiO2f/SiO2 composites

In this paper, SiO_2f/SiO₂ composites reinforced by 3D four‐directional braided quartz preform were prepared by the silica sol‐infiltration‐sintering method in a relatively low sintering temperature (450 °C). To characterize the mechanical properties of the composites, mechanical testing was carried out under various loading conditions, including tensile, flexural and shear loading. The microstructure and the fracture behaviour of the 3D four‐directional braided SiO_2f/SiO₂ composites were studied. The tensile strength, flexural strength and the in‐plane shear strength were 30.8 MPa, 64.0 MPa and 22.0 MPa, respectively. The as‐fabricated composite exhibited highly nonlinear stress–strain behaviour under all the three types of loading. The tensile and flexural fracture mechanisms were fully discussed. The fracture mode of the 3D four‐directional braided SiO_2f/SiO₂ composite in the Iosipescu shear testing was based on a mixed mechanism because of the multi‐directivity of the composite. Owing to low sintered temperature, the fibre/matrix interfacial strength was weak. The SiO_2f/SiO₂ composites showed non‐catastrophic behaviour resulting from extensive fibre pull‐out during the failure process.     

Improved Room Temperature Plasticity of Zr41.2Ti13.8Cu12.5Ni10Be22.5 Bulk Metallic Glass by Channel‐Die Compression

In this work, the effect of channel‐die compression (CDC) on the mechanical behavior of the Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 bulk metallic glass is analyzed. The results indicate that CDC can be successfully used as a pre‐deformation process to effectively enhance the room temperature plastic strain ability of metallic glasses. The origin of the improved mechanical properties is most likely due to the creation during CDC of a heterogenous microstructure consisting of hard and soft regions able to hinder the rapid propagation of shear bands.     

A comparison of results obtained using different methods to assess the elastic properties of ceramic materials exemplified for Ba<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>Co<Subscript>0.8</Subscript>Fe<Subscript>0.2</Subscript>O<Subscript>3−δ</Subscript>

Different mechanical testing methods have been used to assess the elastic behavior of the mixed ion electron conducting perovskite Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF). Although the main aim is the comparison of the testing techniques for ceramic material in general, experiments have been performed at RT and up to 900 °C to illustrate the capabilities of the utilized methods for a perovskite with a stiffness anomaly. BSCF specimens in disc shape and tubular geometry provided by different suppliers were tested in biaxial bending by ring-on-ring testing and under uniaxial stress in O-ring and under compression loading. The elastic modulus was determined as a function of temperature up to 900 °C. In addition, the room temperature elastic modulus was measured using depth-sensitive indentation.     

Three‐dimensional mixed‐mode (I and II) crack‐front fields in ductile thin plates — effects of T‐stress

It has been well‐established that the non‐singular T‐stress provides a first‐order estimate of geometry and loading mode (e.g. tension versus bending) effects on elastic–plastic crack‐front field under mode I loading conditions. The objective of this paper is to exam the T‐stress effect on three‐dimensional (3D) crack‐front fields under mixed‐mode (modes I and II) loading. To this end, detailed 3D small strain, elastic–plastic simulations are carried out using a 3D boundary layer (small‐scale yielding) formulation. Characteristics of near crack‐front fields are investigated for a wide range of T‐stresses (T/σ₀ = ?0.8, ?0.4, 0.0, 0.4, 0.8). The plastic zones and thickness and angular and radial variations of the stresses are studied, corresponding to two values of the remote elastic mixity parameters M_e = 0.3 and 0.7, under both low and high levels of applied loads. It is found that different T‐stresses have a significant effect on the plastic zones size and shapes, regardless of the mode mixity and load level. The thickness, angular and radial distributions of stresses are also affected markedly by T‐stress. It is important to include these effects when investigating the mixed‐mode ductile fracture failure process in thin‐walled structural components.