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In single crystals, plastic deformations are predominantly governed by dislocation movement and interactions. The group of dislocations that creates strain gradients, known as geometrically necessary dislocations (GNDs), also deterministically contributes to strain hardening, micron-scale size effects, fatigue, and Bauschinger effect. During bending large strain gradients naturally emerge which makes this deformation mode exceptionally suitable to study the evolution of GNDs. Here we present bi-directional bending experiment of a Cu single crystalline microcantilever with in situ characterisation of the dislocation microstructure in terms of high-resolution electron backscatter diffraction (HR-EBSD). The experiments are complemented with dislocation density modelling to provide physical understanding of the collective dislocation phenomena. We find that dislocation pile-ups form around the neutral zone during initial bending, however, these do not dissolve upon reversed loading, rather they contribute to the development of a much more complex GND dominated microstructure. This irreversible process is analysed in detail in terms of the involved Burgers vectors and slip systems. We conclude that at this scale the most dominant role in the Bauschinger effect and corresponding strain hardening is played by short-range dislocation interactions. The in-depth understanding of these phenomena will aid the design of microscopic metallic components with increased performance and reliability.

BACKGROUND: Vascular transplantation is a hot research topic in vascular surgery at present. However, the detection of mechanical properties of preserved vascular wall needs to be solved urgently. OBJECTIVE: To summarize the techniques used by researchers to study arterial wall mechanics and the problems faced in this research direction. METHODS: The first author searched CNKI, WanFang, PubMed, and Google Scholar databases for relevant literature published from 2016 to 2022. The Chinese search terms were “aortic blood vessel, arterial wall, mechanics, elasticity, detection method” and the English search terms were “aortic vessels, arterial walls, mechanical model, elastic structure, mechanical testing.” The relevant literature about the distribution of mechanical structure of aortic blood vessel walls and relevant detection methods were retrieved. Finally, 54 articles regarding mechanical and micromechanical testing were induced for review. RESULTS AND CONCLUSION: The arterial wall is characterized by complex microstructure, which affects the mechanical properties of vascular tissue. The main components include collagen and elastin fibers, proteoglycans, vascular smooth muscle cells, and matrix. These components are often damaged during cryopreservation. Among them, vascular smooth muscle cells play a key role in the active mechanical response of the arteries, and collagen and elastin determine the passive mechanical response. After the treatment of vascular grafts, the technologies of detecting vascular wall skeleton damage still need to be further explored or summarized to obtain a set of more accurate methods, so as to evaluate vascular wall skeleton damage in multiple dimensions and help in [ABSTRACT FROM AUTHOR]

This article reviews the microstructural evolution in ultrafine-grained and nanotwinned austenitic stainless steels that have been subjected to hydrogen embrittlement (HE) and fatigue cracking. It provides guidelines for the development of high-strength austenitic steels without sacrificing HE and fatigue performance. The author focuses on the hydrogen-induced ductility loss and short fatigue crack growth associated with deformation-induced martensitic transformation, using micro-tension and micro-fatigue testing technologies. In type 304 metastable austenitic stainless steel, the microstructure produced by high-pressure torsion depends strongly on the processing temperature. Nanocrystalline austenite with enhanced strength and moderate ductility can be obtained at a processing temperature of ~423-573K, whereas dual-phase microstructures comprising austenite and martensite are formed by processing at room temperature. Introducing ultrafine grains and nanotwin bundles mitigates the hydrogen-induced ductility loss in metastable austenitic steel by controlling the dynamic martensitic transformation. The microstructure refinement also contributes to enhanced resistance to short fatigue crack growth by changing the route of the damage accumulation process via phase transformation and detwinning. [ABSTRACT FROM AUTHOR]

Digital image correlation (DIC) in combination with Scanning Electron Microscopy (SEM) and in-situ mechanical testing has great potential to study the micro-mechanical behavior of materials. Yet, the application of a high-quality speckle pattern for DIC remains challenging at this scale. Here, we present a versatile polymer particle-based speckle pattern with adjustable speckle size (0.17–1.04 μm) to cover a wide range of magnifications (500x to 5000x) in SEM. This speckle does not completely cover the surface, ensuring the visibility of deformations taking place in the studied object. The quality of the pattern is analyzed to determine the precision, noise, and correctness of the results obtained by SEM-DIC. Out-of-plane displacements are also considered. Depending on the choice of speckle size and imaging parameters, high quality SEM-DIC patterns were obtained. The measurable strain with this method ranges between 0.05% up to 100%. To illustrate the versatility of the speckling method, three different micromechanical test cases on polymer materials are presented. These showed that global and local deformation can clearly be visualized at different magnifications.

This study focuses on the influence of additive manufacturing process strategies on the specimen geometry, porosity, microstructure and mechanical properties as well as their impacts on the design of metamaterials. Filigree additively manufactured NiTi specimens with diameters between 180 and 350 µm and a nominal composition of Ni50.9Ti49.1 (at %) were processed by laser powder bed fusion in a first step. Secondly, they structures were characterized by optical and electron microscopy as well as micro tomography to investigate the interrelations between the process parameters, specimen diameters and microstructure. Each specimen was finally tested in a micro tensile machine to acquire the mechanical performance. The process strategy had, besides the resulting specimen diameter, an impact on the microstructure (grain size) without negatively influencing its quality (porosity). All specimens revealed a superelastic response while the critical martensitic phase transition stress decreased with the applied vector length. As a conclusion, and since the design of programmable metamaterials relies on the accuracy of FEM simulations, precise and resource-efficient testing of filigree and complex structures remains an important part of creating a new type of metamaterials with locally adjusted material behavior. [ABSTRACT FROM AUTHOR]

Scientific processes produce huge amounts of data that are usually acquired, transformed and analyzed on a regular basis. Translating these processes into automatable and reproducible workflows is considered to be an efficient way to support scientists in performing repeated processes that would otherwise be time-consuming and error-prone tasks. Consequently, the quality of scientific research can be accelerated and enhanced. In this article, we present for the first time a use-case of KadiStudio as a tool to automate analysis procedures of scientific data that are repeatedly acquired from in situ scanning electron microscope (SEM) micromechanical testing. KadiStudio provides a desktop-based workflow editor as part of the ecosystem of Kadi4Mat: Karlsruhe Data Infrastructure for material Science. The presented workflow includes nodes for processing and analysis of different types of data, namely mechanical response in text format and a series of SEM images in video file format acquired during in situ SEM deformation tests. In addition, the raw and analyzed data are automatically uploaded to the KadiWeb repository via nodes based on the kadi-apy library.

SiC f /BN/SiC Ceramic-Matrix Composites are candidate materials for aero-engines, but their interphase stability after potential low-temperature water-vapour exposures during flight cycles is not well known. The examination of these composites exposed for 50, 250 and 500 h at low temperature (65 °C) and 95% relative humidity was therefore performed, in order to understand if resulting oxidation products affected the mechanical properties at the micro-scale. The composites were subject to fibre push-out tests to compare pristine from degraded composites. It was found that whilst the sample exposed for 50 h had no significant change from pristine, the samples exposed for 250 and 500 h had a clear decrease in interfacial shear strengths measured. Parallel studies also revealed that whilst damage was strongly localised, the diffusion of water within the composite was not fully complete at exposure times between 50 and 250 h. The permeability of the CMC was shown to be affected at longer exposure times where differences in mechanical performances even between tows and within tows were noted. [ABSTRACT FROM AUTHOR]

Micromechanical characterization of the oxygen-rich layer (ORL) of a Ti-6Al-4V alloy due to high-temperature oxidation was investigated at room temperature. The tensile strength of the pre-oxidized specimens linearly decreased as a function of the surface fraction of ORL in relation to the gage section, demonstrating a competition between oxygen strengthening and embrittlement. Electron-probe microanalyses and nanoindentation testing aimed at locally assessing the elastic and hardness response of the material as a function of the oxygen content. These properties were used in finite element simulations to quantify stress profiles within the oxygen-graded material for different ORL thickness/specimen thickness couples. • Oxygen embrittlement was investigated in Ti alloys using micromechanical testing. • High oxygen-rich layer/specimen thickness ratio was possible using ultrathin specimen. • Competition between oxygen strengthening and embrittlement was observed. • Linear relationship was established between elastic modulus and oxygen concentration. • Numerical simulations on oxygen-graded Ti alloys demonstrated size effects on the "core/shell" response. [ABSTRACT FROM AUTHOR]

Small disks are often the specimen of choice for exposure in nuclear reactor environments, and this geometry invariably limits the types of mechanical testing that can be performed on the specimen. Recently, shear punch testing has been utilized to evaluate changes arising from neutron irradiation in test reactor environments on these small disk specimens. As part of a broader effort to link accelerated testing using ion irradiation and conventional neutron irradiation techniques, a novel microshear specimen geometry was developed for use with heavy-ion irradiated specimens. The technique was demonstrated in pure Cu irradiated to 11 and 110 peak dpa with 10 MeV Cu ions. At 11 peak dpa, the Cu specimen had a high density of small voids in the irradiated region, while at 110 peak dpa, larger voids with an average void swelling of ~20% were observed. Micropillar and microshear specimens both exhibited hardening at 11 dpa, followed by softening at 110 dpa. The close alignment of the new microshear technique and more conventional micropillar testing, and the fact that both follow intuition, is a good first step towards applying microshear testing to a wider range of irradiated materials. [ABSTRACT FROM AUTHOR]

SiC-bonded diamond materials produced by pressureless reactive infiltration of diamond preforms with silicon show high hardness and wear resistance. These properties are due to the relatively high diamond volume content of approximately 50 vol% and the mechanically strong interface between diamond and SiC. To determine the bending strength of individual interfaces between diamond and SiC, micro-cantilevers were prepared by focused ion beam milling at 13 grain boundaries and in-situ bending tests were carried out in a scanning electron microscope. The determined strength of cantilevers showing interface fracture was 10.4 ± 4.0 GPa. Fracture surfaces were analyzed to verify the fracture behavior and initiation. In addition to fracture at the interface diamond/SiC, fracture occurred inside the SiC grains and at the SiC/silicon interface at comparable strength values. The results prove the high diamond/SiC-interface bonding strength.

Nickel-based alloys are extensively used in a wide range of extreme environments because of their exceptional mechanical properties. The excellent strength of these alloys is derived from the addition of long-range ordered precipitates, introduced by thermal aging. The interactions between the dislocations and LRO precipitates dictate the deformation modes and plastic response in these alloys. While the majority of studies have focused on L12-structured precipitate-strengthened Ni-based alloys, less work has considered the Ni-based alloys containing Pt2Mo-structured, Ni2(Cr,Mo)-typed precipitates. In these alloys, Pt2Mo-structured precipitates enable room-temperature deformation twinning in addition to slip, which increases strain hardenability measured from bulk mechanical testing. Although previous geometric-based model suggested that deformation twinning is favored over slip, the factors that influence the activation between twinning versus slip have not been thoroughly explored in this class of Ni-based alloys. In this work, molecular dynamics examined the possible types of dislocation and Pt2Mo-structured precipitate interactions at low temperature. Combined with in situ micromechanical testing, the role of resolved shear stresses on dislocation partials were shown to directly influence the activation of slip versus twinning. Additionally, using an energy-based approach, molecular dynamics results demonstrated a novel twin formation process, caused by the dislocation interaction with the Pt2Mo-structured precipitates.

Feedstock powder characteristics such as composition (specific alloying elements and concentrations and impurity levels), microstructure, thickness/composition of surface oxide layers, and particle size distribution play a crucial role in determining the overall mechanical properties of cold sprayed deposits. Herein, we report on two deposits consolidated via cold spray processing from differently-sourced batches of nominally identical elemental refractory powders under identical spraying conditions, which exhibit bending strength and ductility values that differ by more than a factor of two – and with the weakest sample displaying negligible effective ductility. Through chemical, microstructural and micromechanical characterization of both the feedstock powders and cold sprayed deposits, we consider the possible influences of feedstock characteristics on the mechanical performance of cold spray consolidated deposits. It is shown that while differences in interstitial oxygen and hydrogen content may result in differences in the intrinsic yield characteristics of the feedstock material, both feedstocks maintain the ductile behavior required to induce good metallurgical bonding upon impact of optimally sized powder particles. We conclude that the deposits formed from the two feedstock powders are indistinguishable and exhibit high ductility when characterized locally within the relatively undeformed bulk of a single particle or splat. However, the two sprayed deposits show low ductility or brittle behavior when loaded in tension across intersplat boundary domains comprising material that has undergone extensive deformation. In addition, one feedstock incorporates a broader particle size distribution, with a long tail of larger-than-optimal particles. These larger particles are accelerated below the critical impact velocity, resulting in observed imperfect metallurgical bonding at splat interfaces and higher porosity at such intersplat boundaries. This result, combined with the overall differences in the morphology of the intersplat boundary network between the two deposits, explains the difference in macroscopic mechanical behavior. We conclude that accurate control of the powder feedstock particle size distribution is essential for optimizing the mechanical integrity of cold sprayed refractory elements and alloys. [ABSTRACT FROM AUTHOR]

For the last decades, new reparation or fabrication processes have been studied to replace traditional rebar by roving of different mineral or organic fibres to avoid corrosion issues. Such materials refer to the family of cementitious composite. Their tensile strength would directly depend on the proportion of reinforcement and strongly on the interfacial mechanical properties between fibres and cementitious matrix. From now, evaluation of interfacial properties was mostly limited to the use of force–displacement curves obtained from mechanical experiments. This work presents a new methodology using micromechanical tension stiffening tests combined with X‐ray computed tomography (XRCT) observations, performed at the Anatomix beamline at Synchrotron SOLEIL, and specific image processing procedures. Multi‐XRCT acquisitions with suitable scanning strategy are used to image the whole fibre‐matrix interface along centimetric samples at four to five different levels of loading magnitude. Intensive image processing is then performed on tomographic images including digital volume correlation (DVC), image subtraction and Hessian‐based filtering. This experiment allows to study damage mechanisms at small scale. The proposed methodology shows great potential to provide both qualitative and quantitative elements on interfacial mechanical behaviour such as crack growth and crack orientation. The interface between mortar and sufficiently small multi‐fibre yarn used in this paper is shown to behave in certain condition as traditional rebar interface producing conical cracks in the surrounding matrix rather than debonding in mode 2, permitting a much higher energy dissipation during debonding. According to this study, conical cracks repartition and geometry are mostly influenced by the cementitious matrix. The spacing between cracks goes from 50 to 100 μm, and the angle between crack normal vector and yarn orientation goes from 35° to 50°. [ABSTRACT FROM AUTHOR]

Summary: A site‐specific xenon plasma focused ion beam preparation technique for microcantilever samples (1–20 µm width and 1:10 aspect ratio) is presented. The novelty of the methodology is the use of a chunk lift‐out onto a clean silicon wafer to facilitate easy access of a low‐cost probe type indenter which provides bending force measurement. The lift‐out method allows sufficient room for the indenter and a line of sight for the electron beam to enable displacement measurement. An electroplated nanotwinned copper (NTC) was cut to a 3 × 3 × 25 µm microbeam and in situ mechanically tested using the developed technique. It demonstrated measured values of Youngs modulus of 78.7 ± 11 GPa and flow stress of 0.80 ± 0.05 GPa, which is within the ranges reported in the literature. Lay Description: In this paper a site specific method is present for making particularly small mechanical tests samples, of the order of 100th the size of a human hair. These small samples can then be used to determine the mechanical properties of the bulk material. Copper with a nano twinned grain structure is used as a test medium. Ion milling was used to cut the sample to shape and a micro probe was used for mechanical testing. Ion milling can cut away very small volumes of material as it accelerates ions at the surface of the sample, atomically machining the sample. Micro probes are a cost‐effective small‐scale load measurement devices, however, they require a large area for accessing the sample. The indenter requirements are a problem when making you samples with ion milling as ion millers are best at making small cuts. Our aim was to design a cutting strategy which reduces the amount of cutting required while allowing samples to be fabricated anywhere on the sample. We used a chunk lift out technique to remove a piece of material which is then welded to a wafer of silicon this gives sufficient space around the sample for ion milling and testing. The additional space allowed easy access for the probe. A 3 × 3 × 10 µm micro cantilever beam was cut out from copper, this beam was then bent. The force from bending and distance bent was measured and converted into Youngs modulus which is a measure of flexibility. The modulus value measured was comparable to the values reported in other papers. [ABSTRACT FROM AUTHOR]

• Fracture toughness of an AlTiN coating was characterized by nanomechanical measurements. • Microcantilever bending resulted in lower toughness compared to nanoindentation. • The test method and the coating microstructure influence the measurement results. • Fracture toughness of AlTiN is higher when compared to the literature data on TiN. Metal nitride coatings are increasingly used in MEMS applications. The brittle nature of these coatings makes fracture one of the main failure mechanisms during operation. Therefore, quantifying the fracture toughness of nitride coatings in a reliable fashion is important to understand the failure behavior and to optimize device performance and reliability. This study investigated the fracture toughness of AlTiN coatings produced by cathodic arc evaporation, a widely used technique in the industry. Nanoindentation and microcantilever bending-based measurements indicated a fracture toughness of 6.6 and 4.8 MPa·m1/2, respectively. The lower toughness results of microcantilever bending were attributed to the columnar growth of the coating, which promoted crack propagation perpendicular to the film surface. The results provide useful data toward a better understanding of the fracture of hard coatings and give insight into the advantages and disadvantages of different measurement methods. [ABSTRACT FROM AUTHOR]

In single crystals, plastic deformations are predominantly governed by dislocation movement and interactions. The group of dislocations that creates strain gradients, known as geometrically necessary dislocations (GNDs), also deterministically contributes to strain hardening, micron-scale size effects, fatigue, and Bauschinger effect. During bending large strain gradients naturally emerge which makes this deformation mode exceptionally suitable to study the evolution of GNDs. Here we present bi-directional bending experiment of a Cu single crystalline microcantilever with in situ characterisation of the dislocation microstructure in terms of high-resolution electron backscatter diffraction (HR-EBSD). The experiments are complemented with dislocation density modelling to provide physical understanding of the collective dislocation phenomena. We find that dislocation pile-ups form around the neutral zone during initial bending, however, these do not dissolve upon reversed loading, rather they contribute to the development of a much more complex GND dominated microstructure. This irreversible process is analysed in detail in terms of the involved Burgers vectors and slip systems. We conclude that at this scale the most dominant role in the Bauschinger effect and corresponding strain hardening is played by short-range dislocation interactions. The in-depth understanding of these phenomena will aid the design of microscopic metallic components with increased performance and reliability. • Cyclic bending of a copper single crystalline microcantilever revealed a non-reversible evolution of dislocation structures. • Microstructures obtained by high-resolution EBSD measurements are recovered by continuum dislocation density simulations. • TEM confirmed that the dislocation pile-ups formed during the initial bending do not dissolve upon reserved loading. • Upon cyclic loading a rather complex dislocation microstructure develops that changes its polarisation in every cycle. • The strong strain hardening and the Bauschinger-effect was mainly caused by the short-range dislocation interactions. [ABSTRACT FROM AUTHOR]

We propose a minimum input mechanical model of pulp fibres. A pulp fibre can be idealised as a complex compound bar, microfibrils render its behaviour anisotropic. The objective of this work is twofold: (i) develop an anisotropic viscoelastic material model of a pulp fibre that shows the essential characteristics with a minimum number of material parameters and (ii) establish suitable experiments with pulp fibres for obtaining these parameters. The model consists of an additive combination of elastic potentials for an isotropic and an anisotropic part. Viscoelasticity is implemented by a generalised Maxwell model. We suggest an incompressible treatment of the material to relate experiments and material parameters in a well-defined manner. By this means, the calibration of the parameters is performed with only two micromechanical experiments, atomic force microscopy nanoindentation on the fibre's surface and uniaxial tensile tests. We advocate this strategy for certain applications, such as in paper network simulations or fibres in a dry state, when the fibre's compressibility is of minor importance. The results of this analysis show a good performance of the proposed model applied to pulp fibres. [ABSTRACT FROM AUTHOR]

Application of a multilayered coating architecture based on two alternating hard materials provides a strategy to enhance the mechanical properties of hard coatings for the metal cutting industry. Within this work, the mechanical behavior of CVD TiN/TiBN multilayer coatings was correlated with their microstructure and compared to the respective single-layers. Multilayers with different bilayer periods (1400, 800, 300 and 200 nm) were prepared in an industrial-scale thermal CVD plant by alternate variation of the feed gas composition. Complementary X-ray diffraction, Raman spectroscopy and transmission electron microscopy investigations confirmed a dominating face-centered cubic TiN structure, accompanied by hexagonal TiB 2 and amorphous TiB in the B containing coatings. The addition of B and a decreasing bilayer period resulted in a decreasing grain size and increasing hardness. Micromechanical bending tests revealed an increase of strength and fracture toughness with addition of B and increasing layer number. While the highest strength was observed in a TiBN single-layered coating, the multilayered TiN/TiBN with a bilayer period of 200 nm was the hardest and at the same time toughest of the investigated coatings. Unlabelled Image • Bilayer period λ in CVD TiN/TiBN multilayer coatings was systematically varied. • Coatings are dominated by crystalline TiN with fractions of TiB 2 and amorphous TiB. • Amorphous TiB detected in multilayer stack by cross-sectional Raman spectroscopy • TiBN single layer yielded the highest fracture stress. • TiN/TiBN multilayer with lowest λ shows highest hardness and fracture toughness. [ABSTRACT FROM AUTHOR]

Abstract Environmental barrier coatings (EBCs) play a vital role in protecting advanced turbine components subjected to extreme service environments, where mechanical, thermal and chemical behaviors typically dominate the design criteria. This study focuses on investigating the mechanical properties of individual phases within a HfO 2 -Si ceramic matrix composite (CMC) structure through implementation of notched microcantilevers and micropillar splitting experiments. The microcantilever experiments resulted in fracture toughnesses ranging between 1.38–1.52 MPa-m1/2 for the Si-rich phase and 2.26–2.38 MPa-m1/2 for the HfO 2 -rich phase depending on the method of analysis. From our micropillar splitting experiments, we found fracture toughnesses of 1.13 ± 0.39 MPa-m1/2 for the Si-rich phase and 1.18 ± 0.26 MPa-m1/2 for the HfO 2 -rich phase. Comparisons with bulk single edge V-notched beams (SEVNBs) suggest the micropillar results are accurate whereas the microcantilever experiments may overestimate K IC of the Si-rich and HfO 2 -rich phases by ˜0.2–1.2 MPa-m1/2, most likely, due to dimensional errors that affect specimen and model compliances. [ABSTRACT FROM AUTHOR]

Small-scale metallic glasses have many applications in microelectromechanical systems (MEMS) and sensors which require good mechanical properties. Bending, tensile and compression properties of metallic glasses at micro/nano-scale have been well investigated previously. In this work, by developing a micro robotic system, we investigated the torsional behavior of Fe-Co based metallic glass microwires inside a scanning electron microscope (SEM). Benefiting from the in situ SEM imaging capability, the fracture behavior of metallic glass microwire has been uncovered clearly. Through the postmortem fractographic analysis, it can be revealed that both spiral stripes and shear bands contributed to the fracture mechanism of the microscale metallic glass. Plastic deformation of the microwires include both homogenous and inhomogeneous plastic strain, which began with the liquid-like region, then a crack formed because of shear bands and propagated along the spiral direction. Although the metallic glass microwire broke in brittle mode, the shear strain was not lower than that of conventional metal wires. Moreover, we found an inverse relationship between the plastic strain and the loading rate. [ABSTRACT FROM AUTHOR]

Abstract The fracture toughness of nanostructured materials depends on anisotropic physical properties of individual microstructural features, their texture and/or topology. In this work, intentionally sculptured grain boundaries of low cohesive energy were used to form "weak" and "tough" crack propagation directions within a nanocrystalline TiN film, allowing to correlate the directional arrangement of grains and anisotropy of fracture toughness. By using a selective micromechanical testing approach, two different cracking directions were probed in a scanning electron microscope by loading microcantilever beam specimens prepared parallel and perpendicular to the stacked direction of the alternately tilted columnar grains. The fracture toughness along the sculptured grain boundaries was ~30% higher due to effective multiple crack deflection at the kink planes, which was not observed along weak cleavage planes in the stacked direction. The results indicate the fundamental importance of microstructural design in the synthesis of tough nanostructured ceramics, whose anisotropic mechanical properties can be controlled effectively by incorporating dedicated microstructural features of well-defined topology, orientation and density. Graphical abstract Unlabelled Image Highlights • Weakly bonded grain boundaries designed in a zig-zag pattern were used to enhance fracture toughness of a brittle TiN ceramic. • The toughening effect originates from a multiple crack deflection at the tilted grain boundaries. • The observed fracture anisotropy was identified in directional arrangement of columnar grains within the material volume. • It resulted in a difference of 30% between weak and tough planes inherently formed in two in-plane directions. [ABSTRACT FROM AUTHOR]

Fe-Cr alloy has great potential as an interconnect material for SOFCs at temperatures up to 800 °C, with Cr providing excellent oxidation resistance through the formation of a protective chromium oxide layer. However, internal brittle oxides formed at grain boundaries (GBs) can lead to severe cracking and cause material failure. Here, we report the formation of internal GBs oxide in an Fe-15 wt%Cr alloy. We carried out microstructural and chemical characterization of the intergranular oxide and performed micromechanical testing and FEM simulation to study the fracture behavior at the GBs. Our findings provide insights for improving the oxidation resistance in the material design. [Display omitted] • GB oxidation in Fe-15Cr alloy studied by combining microstructural and micro-mechanics analysis. • Possible oxidation mechanisms leading to intergranular internal oxidation are discussed. • Kirkendall effect contributes to formation of GB oxide pores. • GB oxides are prone to cracking as evidenced by nanoindentation near the GB. [ABSTRACT FROM AUTHOR]

• Modified procedure for the J -integral testing on micro-scale cantilever beam specimens. • Old procedure delivers wrong J -values for smaller crack lengths. • Numerical case study to determine the plastic eta-factor, η pl. • Fitting procedure for the determination of correct η pl -values. The current paper presents a modified procedure for the J -integral testing on micro-scale cantilever beam specimens. For such tests, a non-dimensional factor is required that relates the plastic part of the area below the load vs. load-point displacement curve to J. This parameter is called the "plastic eta-factor", η pl. Up to now, a constant value of η pl ≈ 2 has been used, which is also valid for deeply notched single-edge bend specimens. A comprehensive numerical case study is performed to determine η pl for cantilever beam specimens as a function of the relative crack length a / W , the relative cantilever length L / W , the relative base distance d / W , and the load, i.e. the degree of plasticity in the specimen. It is shown that η pl ≈ 2 is applicable only, if the crack length is large, a / W ⩾ 0.5. For smaller a / W -ratios, which are commonly used in the fracture mechanics testing of nano- and micrometer sized materials, η pl can become much smaller and also dependent on the d / W -ratio. A fitting procedure is presented that enables the determination of the correct value of η pl , and modifications of the J -integral testing procedure for cantilever beam specimens are proposed. Finally, a numerical validation of the procedure is presented. [ABSTRACT FROM AUTHOR]

Compression tests at low strains were performed to theoretically analyze the effects of anisotropic elasticity, misorientation, grain boundary (GB) stiffness, interfacial dislocations, free surfaces, and critical force on dislocation pile-ups in micro-sized Face-Centered Cubic (FCC) Nickel (Ni) and α -Brass bi-crystals. The spatial variations of slip heights due to localized slip bands terminating at GB were measured by Atomic Force Microscopy (AFM) to determine the Burgers vector distributions in the dislocation pile-ups. These distributions were then simulated by discrete pile-up micromechanical calculations in anisotropic bi-crystals consistent with the experimentally measured material parameters. The computations were based on the image decomposition method considering the effects of interphase GB and free surfaces in multilayered materials. For Ni and α -Brass, it was found that the best predicted step height spatial profiles were obtained considering anisotropic elasticity, free surface effects, a homogeneous external stress and a certain critical force in the material to equilibrate the dislocation pile-ups.

The enhanced properties of nanomaterials make them attractive for advanced high-performance materials, but their role in promoting toughness has been unclear. Fabrication challenges often prevent the proper organization of nanomaterial constituents, and inadequate testing methods have led to a poor knowledge of toughness at small scales. In this work, the individual roles of nanomaterials and nanoarchitecture on toughness are quantified by creating lightweight materials made from helicoidal polymeric nanofibers (nano-Bouligand). Unidirectional ( θ $\theta $  = 0°) and nano-Bouligand beams ( θ $\theta $  = 2°-90°) are fabricated using two-photon lithography and are designed in a micro-single edge notch bend (µ-SENB) configuration with relative densities ρ ¯ $\overline \rho $ between 48% and 81%. Experiments demonstrate two unique toughening mechanisms. First, size-enhanced ductility of nanoconfined polymer fibers increases specific fracture energy by 70% in the 0° unidirectional beams. Second, nanoscale stiffness heterogeneity created via inter-layer fiber twisting impedes crack growth and improves absolute fracture energy dissipation by 48% in high-density nano-Bouligand materials. This demonstration of size-enhanced ductility and nanoscale heterogeneity as coexisting toughening mechanisms reveals the capacity for nanoengineered materials to greatly improve mechanical resilience in a new generation of advanced materials., (© 2023 Wiley-VCH GmbH.)

Gas-atomized Al 6061 powder was studied in as-atomized, heat-treated, and homogenized conditions, and low-coverage cold spray deposits were produced from these powders to allow individual splats to be investigated. The microstructures were investigated by electron microscopy, and the mechanical behavior of the particle/splat interiors was evaluated by the compression of micropillars milled from the samples. The as-atomized and heat-treated powders exhibit cell-like solidification microstructures with Mg-Si and Fe-Al-Si secondary phases around the cell boundaries. These powders have flow stresses of ≈ 185 MPa with pronounced strain bursts due to rapid, stochastic dislocation motion confined within the cells. In the homogenized powders, the Mg-Si phase has dissolved, the Fe-Al-Si has coarsened, and the grain boundaries have unpinned. The stress-strain curves are smoother due to the dissolution of cell boundary obstacles, but the flow stress is about 30 MPa higher due to the increased solute content giving more sluggish dislocation motion. The microstructures of the splat interiors resemble those of the powders closely, but the flow stresses measured are all much higher (280-290 MPa) with little effect of powder microstructure or deposition pressure. This is consistent with the high dislocation density reaching a saturation value in all of the splats studied. [ABSTRACT FROM AUTHOR]

Ti-Si-N nanocomposite thin films usually consist of a few nm sized crystals embedded in an amorphous matrix. Here we study the influence of the Si content in Ti-Si-N films – prepared by reactive magnetron sputtering using Ti targets alloyed with intermetallic TiSi 2 (to obtain Si contents of 0, 10, 15, 20, 25 at.% in the targets) – on fracture toughness ( K IC ) and hardness. Single cantilever bending experiments show that Ti-Si-N exhibits K IC values of up to 3.0 ± 0.2 MPa√m, which are significantly higher than those for TiN with 1.9 ± 0.2 MPa√m. Complementary nanoindentation experiments reveal also a higher hardness for Ti-Si-N with 34 ± 1 GPa as compared to 26 ± 1 GPa for TiN. The film structure is carefully studied by X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. [ABSTRACT FROM AUTHOR]

Superlattice films are generally known for their exceptional high hardness compared to their monolithic constituents. Recently, we have shown that CrN/TiN superlattice films exhibit a peak in fracture toughness for a bilayer period of 6.0 nm, similar to the former reported peak in hardness. We propose that a dominating factor for obtaining such favourable material properties is the interface constitution between the individual layers. To proof this notion, we have intentionally modified the interface sharpness by post-deposition vacuum annealing of the samples at different temperatures. This promotes interdiffusion of Ti or Cr into its adjacent layers and gradually changes the interfaces to interphases (because TiN and CrN form a solid solution). In order to obtain reliable K IC fracture toughness values as a function of the annealing temperature, in-situ micromechanical cantilever bending tests on ex-situ vacuum annealed freestanding films were performed. High temperature loads take also place during machining processes like dry cutting or high-speed cutting, and are thus of high practical relevance. [ABSTRACT FROM AUTHOR]

This study presents a novel approach of micromechanical interfacial testing in loading mode III. The interfaces beneath single copper interconnect structures are investigated. The method is based on lateral nanoindentation experiments, utilized to probe customized copper torsion structures manufactured by the Dual Damascene process. Thus, the test structures resemble product-like length-scales and properties. For the investigation of interface properties, the experimental load-displacement data were reviewed. Two different interfaces occurring in Back End of Line (BEoL) structures are probed: Copper to Ta/Ta-N barrier as well as Copper to Si-N. SEM images and FIB cross-sections are performed, validating the delamination of the targeted interface. While the first interface shows a very high strength and cannot be delaminated by the here presented experiments, the latter one was brought to delamination in mode III several times. The tests yielded reproducible load-displacement data, the load at delamination for the interface Copper to Si-N was measured to be 130 ± 40 μN. A simple analytical approximation for the maximum shear stresses applied in mode III at the interface of interest is provided. The proposed testing approach is able to reproducibly manufacture and test large quantities of the reality like test structures, while avoiding time-consuming FIB based preparation routines. This can help to evaluate the interface properties in modern BEoL stacks and could lead to simulation based design improvements. • Novel experimental setup for torsional loading of micron sized interfaces • Micromechanical investigations of the interface strength of interconnects • For the first time individual interconnect structures are tested in Mode III. • Two different interfaces are tested, both occurring in BEoL structures. • Estimates for their maximum interface strength are presented. [ABSTRACT FROM AUTHOR]

Digital image correlation (DIC) in combination with Scanning Electron Microscopy (SEM) and in-situ mechanical testing has great potential to study the micro-mechanical behavior of materials. Yet, the application of a high-quality speckle pattern for DIC remains challenging at this scale. Here, we present a versatile polymer particle-based speckle pattern with adjustable speckle size (0.17–1.04 μm) to cover a wide range of magnifications (500x to 5000x) in SEM. This speckle does not completely cover the surface, ensuring the visibility of deformations taking place in the studied object. The quality of the pattern is analyzed to determine the precision, noise, and correctness of the results obtained by SEM-DIC. Out-of-plane displacements are also considered. Depending on the choice of speckle size and imaging parameters, high quality SEM-DIC patterns were obtained. The measurable strain with this method ranges between 0.05% up to 100%. To illustrate the versatility of the speckling method, three different micromechanical test cases on polymer materials are presented. These showed that global and local deformation can clearly be visualized at different magnifications. [Display omitted] • A versatile and easily applicable particle based speckle pattern is developed for SEM-DIC measurements. • The presented SEM-DIC method shows low parasitic strains (±0.2%) without the use of any correction scheme for magnifications up to 5000x. • Microscale strain measurement feasibility is showcased via three case studies, spanning small to large strains. [ABSTRACT FROM AUTHOR]

This study aims at determining the strength of a fully lamellar Ti-45Al-2Nb-2Mn (at.%) + 0.8 (vol.%)TiB 2 alloy, with nanoscale lamellar thicknesses. With this aim, the average lamellar thickness of a centrifugally cast alloy was refined one order of magnitude, down to just a few nanometers, by water quenching, followed by a low temperature ageing treatment. The strengthening induced by the lamellar thickness reduction down to just a few nanometers, determined as a function of lamellar orientation by micropillar compression, was found to be very significant, even higher than what can be achieved by high Nb alloying. [ABSTRACT FROM AUTHOR]

The mechanical response of oxidized grain boundaries in a nickel alloy used for nuclear applications has been investigated by performing microcantilever bend tests. It was found that whilst failure can proceed along the oxide-metal interface not all oxidized grain boundaries exhibit intergranular failure. The presence of an external surface oxide has been identified as playing a crucial role in influencing the mechanical response. By removing the surface oxide, using a focused ion beam, tests were performed on the same grain boundaries with and without a surface oxide layer, and showed that surface oxides can suppress or delay fracture. Taking into account the effect of the surface oxide on microcantilever tests, it was possible to present the most accurate parameterization of the local stress at failure of oxidized grain boundaries to date and to predict the experimentally observed behavior via realistic cohesive damage finite element simulations, which further underline the experimental observations. [ABSTRACT FROM AUTHOR]

Single crystalline microcantilevers of Fe–3wt% Si were bent while electrochemically hydrogen (H) charged in situ inside a miniaturized electrochemical cell and are compared with cantilevers bent in air. Yield point decreases, crack initiation and propagation were observed for the cantilevers bent in the presence of H, while notch blunting occurred for the cantilevers bent in air. The results show H enhanced dislocation nucleation and H pinning of dislocation at the crack tip are responsible for embrittlement. [ABSTRACT FROM AUTHOR]

This paper reveals potential micro mechanisms underlying time-dependent anelasticity observed in Al-(1 wt%)Cu thin films. The analyzed deformation mechanisms involve dislocation motion and interaction with solute diffusion, grain boundaries and precipitates. In order to investigate the role of these mechanisms, Al-(1 wt%)Cu alloy thin films are heat treated to systematically change the precipitation state, while characterizing the grain boundary distribution with electron backscatter diffraction. Micromechanical characterization is performed by microbeam bending, nano-tensile creep testing and nano-indentation. Results in microbeam bending reveal, for all precipitate and grain boundary states considered, a similar time-dependent evolution of the anelastic strain after load release. The magnitude of the recovered strain is also observed to be independent of the precipitate or grain boundary configuration. The nano-tensile creep test also indicates the same time-dependent anelastic evolution, indicating that the loading state does not affect the underlying mechanisms. Analysis of strain bursts in nano-indentation shows that pinning of dislocations by Cu solutes is unaffected by the precipitation state. Based on uniaxial creep and time-dependent anelasticity measurements in pure Al specimens, it is made plausible that the time-dependent anelasticity originates from diffusion-limited glide or climb of dislocation segments that are pinned at Cu solutes or in dislocation structures, which provide an internal driving force. [ABSTRACT FROM AUTHOR]

Though hard and superhard ceramic nanocrystalline thin films exhibit extraordinary strength, they typically suffer from brittleness. The lack of plasticity and associated toughness in these materials is due to brittle fracture along grain boundaries of a low cohesive energy, which is favored over dislocation emission. The catastrophic intergranular fracture of ceramic films is undesirable in many applications and limits their exploitation potential despite of other exceptional physical properties. We demonstrate here that it is possible to increase fracture toughness of ceramic nanostructured materials of more than 150% by a dedicated grain boundary orientation design with respect to the direction of the expected crack path without loss of hardness. The concept was applied to nanocrystalline monolithic TiN thin films which were mechanically tested as notched and unnotched microcantilever specimens inside a scanning electron microscope. The films with a columnar chevron-like grain morphology exhibit multiple crack deflections at the kink planes of the repeatedly tilted grains resulting in energy dissipation at the crack tip and in an increase of fracture surface area. The results document that the number of predefined cross-sectional crack turns as well as the film density play decisive roles in the measured overall material fracture resistance. We suggest that by a dedicated design of grain boundary orientations, it is possible to synthetize novel types of hard and tough nanomaterials. [ABSTRACT FROM AUTHOR]

• High entropy nitride coatings were deposited by reactive DC magnetron sputtering. • Silicon addition leads to decreased indentation modulus. • 30% higher elastic deformability with Si, leading to increased damage tolerance. We present micromechanical experiments of DC magnetron sputtered high-entropy nitride thin films, single-phase (Al,Ta,Ti,V,Zr)N with nearly equimolar metal-fractions, and of their Si-alloyed variant. The (Al,Ta,Ti,V,Zr)N coating exhibits a hardness of ∼30 GPa and a fracture toughness of 2.4 MPa√m. Alloying this high-entropy nitride with ∼5 at% Si does not influence the hardness and fracture toughness but lowers significantly the elastic response. Consequently, the elastic strain to failure increases by more than 30% leading to a significantly improved damage tolerance for the Si alloyed (Al,Ta,Ti,V,Zr)N. [ABSTRACT FROM AUTHOR]

[Display omitted] • A validated PBO/EP single fiber pull-out test method is proposed. • The IFSS was found to have an inverse relationship with the test temperature. • The accuracy of single fiber pull-out was verified by COMSOL simulation results. In the micromechanical test of fiber-reinforced composites, single fiber pull-out test is the most widely used test method. However, for high performance fibers/matrix, different test equipment and sensor accuracy can affect the single fiber pull-out test results very much. In this study, an innovative single fiber pull-out test method is proposed for testing the interfacial shear strength (IFSS) of high performance fibers/resins at different temperatures. Single fiber pull-out test was carried out on PBO fiber/epoxy resin (EP) composites at room temperature and elevated temperature. The IFSS was found to have an inverse relationship with the test temperature. The IFSS value of PBO/EP decreased from 25.25 MPa at room temperature to 6.25 MPa as the temperature approached the glass transition temperature. COMSOL simulations showed that fiber pull-out with angular tilt exhibited better interfacial strength, demonstrating the validity of the method. The results of this paper provide ideas to study the effect of temperature on the mechanics of fiber-reinforced resin matrix composites and expand the way to design stiffer, stronger and more robust composites. [ABSTRACT FROM AUTHOR]

A new method for the preparation of freestanding thin film samples for mechanical testing in transmission electron microscopes is presented. It is based on a combination of focused ion beam (FIB) milling and electron-beam-assisted etching with xenon difluoride (XeF 2 ) precursor gas. The use of the FIB allows for the target preparation of microstructural defects and enables well-defined sample geometries which can be easily adapted in order to meet the requirements of various testing setups. In contrast to existing FIB-based preparation approaches, the area of interest is never exposed to ion beam irradiation which preserves a pristine microstructure. The method can be applied to a wide range of thin film material systems compatible with XeF 2 etching. Its feasibility is demonstrated for gold and alloyed copper thin films and its practical application is discussed. [ABSTRACT FROM AUTHOR]