In situ observations of crack arrest and bridging by nanoscale twins in copper thin films

In situ tensile experiments in a transmission electron microscope revealed that micro-cracks in ultrafine grained, free-standing, thin copper foils containing nanoscale twins initiated in matrix domains separated by the twins and then arrested at twin boundaries as twin boundary sliding proceeded. The adjacent microcracks eventually coalesced through shear failure of the bridging twins. To investigate the atomic mechanism of this rarely seen nanoscale crack bridging behavior, molecular dynamics simulations were performed to show that during crack propagation twin boundaries are impinged upon by numerous dislocations from the plastically deforming matrix. These dislocations react at the interface and evolve into substantially impenetrable dislocation walls that strongly confine crack nucleation and resist crack propagation, leading to the experimentally observed crack bridging behavior. The present results raise an approach to significantly toughening polycrystalline thin films by incorporating nanoscale twin structures into individual grains that serve as crack bridging ligaments.     

Effect of unwelded length on behaviour of brittle crack arrest in T-joint structure

In the large heat input weld joint of heavy gauge steel plate used for large container carriers, a brittle crack possibly propagates straight along a weld joint without diverging to the base metal. This phenomenon is discussed for its application to the large heat input weld of heavy gauge plate to ship structures. In this study, to arrest a brittle crack at the T-joint embedding unwelded face, the effect of the unwelded face on behaviour of brittle crack propagation/arrest in the T-joint structure was investigated and analysed. The ESSO test of the T-joint structure was carried out, supposing that a brittle crack which propagates along a weld joint of hatch coaming rush into T-joint of hatch coaming and strength deck. The test results showed that the brittle crack arrested at the T-joint embedding the unwelded face, and the brittle crack arrested easier, the longer the unwelded face. The results of static FEM analysis showed that the stress intensity factor of the brittle crack was increased by the unwelded face, until the brittle crack reached a flange. However, when the brittle crack propagated into the flange, the stress intensity factor of the brittle crack was decreased by the unwelded face. The crack-arrest effect of the unwelded face appears after the brittle crack propagates into the flange.     

Semiconducting properties of thin films with embedded nanoparticles

We demonstrate here the possibility of designing semiconducting thin films with controlled electrochemical properties. The thin films are composed of (i) an insulating binder and (ii) a semiconductor nanopowder which enables the fine tuning of the semiconducting properties of the layers. Thus, p- and n-type silicon particles (obtained from a top-down technique), or metal-oxide ZnO, SnO2 and NiO nanoparticles (synthesized using a bottom-up protocol) are successfully integrated into spin-coated or screen-printed thin films and used as semiconducting materials. The flat band potential (Vfb) of the films is then easily tuned from 0 V to ?1.138 V.     

Effect of distance of loading points on long brittle crack propagation/arrest behaviour

Abstract

The effect of the distance between loading points on a long brittle crack arrest behaviour was investigated in the ultrawide duplex ESSO tests. The effect of loading conditions was also analysed. Arrest of a long brittle crack was more facilitated in the tests with a distance between loading points of 5 m than in those with 10 m. A load drop was larger in the tests with a loading point distance of 5 m than in those with 10 m. If the specimen length is 6·8 m or more, or the distance between loading points are 10 m or more in the ultrawide duplex ESSO test, the effect of the unloading stress wave on the dynamic stress intensity factor of a crack penetrating into the test plate becomes negligible, within the range of brittle crack propagation rate between 500 and 800 m s^?1.     

A simple nanoindentation-based methodology to assess the strength of brittle thin films

In this work, we report a simple methodology to assess the mechanical strength of sub-micron brittle films. Nanoindentation of as-deposited tetrahedral amorphous carbon (ta-C) and Ti–Si–N nanocomposite films on silicon substrates followed by cross-sectional examination of the damage with a focused ion beam (FIB) miller allows the occurrence of cracking to be assessed in comparison with discontinuities (pop-ins) in the load–displacement curves. Strength is determined from the critical loads at which first cracking occurs using the theory of plates on a soft foundation. This methodology enables Weibull plots to be readily obtained, avoiding complex freestanding-film machining processes. This is of great relevance, since the mechanical strength of thin films ultimately controls their reliable use in a broad range of functional uses such as tribological coatings, magnetic drives, MEMS and biomedical applications.     

Strain gradient effect in nanoscale thin films

Compared to uniform deformation, gradient-dominant deformation experiments at the micro-scale have consistently shown remarkable strengthening effect. It is proposed in the literature that pronounced strain gradient, on the order of the inverse of a characteristic length scale, adds to the materials strength. The literature contains several formulations that account for the strain gradient in the constitutive equation. However, the role of microstructures in these formulations remains to be investigated. Due to the difficulties in micro/nano scale experimentation, attempts to investigate this mechanism so far have been confined to specimens with dimensions more than 10 microns, or to nano-indentation experiments. In this study, we use MEMS-based testing techniques to explore the effect of strain gradient in 100, 150, 200 and 485 nm thick freestanding Aluminum specimens with average grain size of 50, 65, 80 and 212 nm respectively. Strain gradient plasticity analysis show that the characteristic length scale for Aluminum is about 4.5 μm, which is similar to the values for copper and nickel reported in the literature. Experimental results suggest that the strain gradient effect is fundamentally related to dislocation-based mechanisms, and is absent at extremely small length scales (< 100 nm for Aluminum), where accommodation of dislocations in the crystal grains is not energetically favorable.     

Instability of crack propagation in brittle bulk metallic glass

The instability of the crack tip in brittle Mg-based bulk metallic glass (BMG) is studied. The formation of various fractographic surfaces of the BMG is associated with the instability of the fluid meniscus, which is due to viscous fluid matter being present on the fracture process zone. Depending on the values of the wavelength of the initial perturbation of the fluid meniscus and the local stress intensity factor, different fracture surface profiles, i.e. a dimple-like structure, a periodic corrugation pattern and a pure mirror zone are formed. The fractogaphic evolution is significantly affected by the applied stress. A decreased fracture surface roughness is observed under a low applied stress. An increased fracture surface roughness, which has frequently been reported by other researchers, is also observed in the present studies under a high applied stress. Unique fractographic features are attributed to the non-linear hyperelastic stiffening (or less softening) mechanism.     

Effect of toughness distribution in the thickness direction on long brittle crack propagation/arrest behaviour of heavy gauge shipbuilding steel

ABSTRACT

Brittle crack arrestability of the heavy gauge steel plates for shipbuilding is now an important issue for the recent mega container ships. In the present work, the brittle crack arrestability of the steel plate with different toughness distributions in thickness is examined in ultra-wide duplex ESSO tests. It is examined whether a running long brittle crack arrests or not in flat temperature condition in ultra-wide duplex ESSO test that are harder mechanical conditions similar to an actual ship hull condition. Test temperatures are selected at which arrest toughness, Kca, evaluated by temperature gradient type standard ESSO test is the same for two test plates. The steel plate with higher toughness in mid-thickness (t/2) than that in quarter thickness (t/4) could arrest a running long brittle crack although the plate with lower toughness in mid-thickness than that in quarter thickness could not arrest it. The typical split-nail shape appeared at the arrested crack front in the plate with higher toughness in mid-thickness than that in quarter thickness. The numerical analyses also demonstrate that the local stress intensity factor at the arrested crack tip is changing sensitively to the crack front shape. It suggests that the higher brittle crack arrestability appears due to the split-nail shape of the arrested crack front enhanced by the inhomogeneous toughness in thickness.     

The effect of multilayer structure on magnetic properties of FePt thin films

The Fe/Pt multilayer films with different structures were deposited by RF magnetron sputtering on glass substrates, and the L1₀-FePt films were obtained after theas-deposited samples were subjected to vacuum annealing at various temperatures. Results show that the Fe/Pt multilayer structure can effectively reduce the ordering temperature of FePt film, and the in-plane coercivity of [Fe (5.2 nm)/Pt (5.2 nm)]₇ multilayers can reach 161.2 kA/m after annealed at 350 ℃ for 30 min. When Fe and Pt layer thickness is equal, the coercivity of the film is the largest. On the other hand, the different Fe-Pt crystalline phases such as Fe₃Pt and FePt₃ phases are formed after annealing when the thickness ratio of Fe/Pt deviates from 1 after annealing. When Fe and Pt have the same thickness, the thinner single layer gets the lower ordering temperature and the larger coercivity.     

The effect of nitrogen pressure on cathodic arc deposited NbN thin films

NbN thin films were deposited on non-standard grade high speed steel (HSS) (79.90 wt.% Fe, 0.71 wt.% C, 6.09 wt.% W, 4.52 wt.% Mo, 3.95 wt.% Cr, 1.82 wt.% Co, 1.75 wt.% V and a hardness of 65 HRC) using cathodic arc deposition at 0.125, 0.5, 1.0 and 1.5 Pa nitrogen pressures (P_N2), with a bias voltage of − 150 V. X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Nanoindentation and Rockwell C analysis were used to characterize the thin films in order to identify the NbN phases and to investigate the influence of P_N2 on mechanical properties. Hexagonal β-Nb₂N, ε-NbN and δ′-NbN_0.95 are identified in XRD analysis. Hardness values derived by nanoindentation technique are 20 GPa for β-Nb₂N, ε-NbN and 40 GPa for δ'-NbN_0.95. Due to the complexity of phase system special attention was focused on identification of NbN phases by deconvolating the XRD peaks especially at 0.5 Pa in which both ε-NbN and β-Nb₂N were found. Rockwell C analysis revealed that the film adhesion is found to be poor at lower P_N2, due to the brittle nature of β-Nb₂N.     

单边裂纹电磁热止裂的动力学效应

对超强脉冲放电止裂瞬间产生的动力学效应及其对裂纹尖端的影响进行研究,通过位移势函数的求解给出准稳态各应力应变分量;通过拉氏变换得出具有动力学效应下的各应力应变分量。以单边裂纹的无限大薄板为例,具体计算出瞬时点热源下的准稳态和动力学效应两种情况下的裂纹尖端局部区域的应力及变化。研究结果表明,在点热源产生的很短时间内,动力学效应会在裂尖附近由于热击产生压应力的突变,带有惯性项的动力学效应能更有效的遏制裂纹的扩展。     

Switching effect on thin T1InSe films

This article examines the current-voltage characteristic of thin TlInSe₂ films in a static mode depending on the length and area of the contact. It has been found that, on thin TlInSe₂ films, the effect of switching with memory is observed, and, to get stable switching, the threshold current should not exceed the current of the stabilization of the state with a nonhomogenous current distributed over the cross section.     

连铸薄板坯表面纵裂纹的产生根源

通过580组数据测量及分析研究,认为产生薄板坯表面纵裂纹的原因除钢水质量、结晶器保护渣、连铸工艺参数等因素外,还与结晶器的结构,即与结晶器的背腔U形槽冷却水通道的几何参数和结晶器内腔曲面的形状有关.     

Grain level analysis of crack initiation and propagation in brittle materials

A study on the accuracy of cohesive models for capturing dynamic fragmentation of ceramic microstructures is presented. The investigation consists of a combined experimental/numerical approach in which microcracking and damage kinetics are examined by means of plate impact recovery experiments. The numerical analysis is based on a 2-D micromechanical stochastic finite element analysis. The model incorporates a cohesive law to capture microcrack initiation, propagation and coalescence, as well as crack interaction and branching, as a natural outcome of the calculated material response. The stochasticity of the microfracture process is modeled by introducing a Weibull distribution of interfacial strength at grain boundaries. This model accounts for randomness in grain orientation, and the existence of chemical impurities and glassy phase at grain boundaries. Representative volume elements (RVE) of ceramic microstructure with different grain size and shape distributions are considered to account for features observed in real microstructures. Normal plate impact velocity histories are used not only to identify model parameters, but also to determine under what conditions the model captures failure mechanisms experimentally observed. The analyses show that in order to capture damage kinetics a particular distribution of grain boundary strength and detailed modeling of grain morphology are required. Simulated microcrack patterns and velocity histories have been found to be in a good agreement with the experimental observations only when the right grain morphology and model parameters are chosen. It has been found that the addition of rate effects to the cohesive model results in microcrack diffusion not observed experimentally.     

Controlled biaxial deformation of nanostructured W/Cu thin films studied by X-ray diffraction

The deformation behaviour of 150 nm thick W/Cu nanocomposite deposited on polyimide substrates has been analysed under equi-biaxial tensile testing coupled to X-ray diffraction technique. The experiments were carried out using a biaxial device that has been developed for the DiffAbs beamline of SOLEIL synchrotron source. Finite element analysis has been performed to study the strain distribution into the cruciform shape substrate and define the homogeneous deformed volume. X-ray measured elastic strains in tungsten sub-layers could be carried out for both principal directions. The strain field was determined to be almost equi-biaxial as expected and compared to finite element calculations.     

应变影响下单晶Ge薄膜热导率分析

利用非平衡态分子动力学模拟方法研究了应变对Ge薄膜热导率的影响。结果表明系统应变对单晶Ge薄膜热导率产生明显影响,热导率随着拉伸应变的增大而减小,随着压缩应变的增大而增大,得出声子速率降低以及薄膜表面重构是产生该模拟结果的内在原因。同时,采用修正的Callaway模型对NEMD结果进行理论验证,两种方法得到的结果吻合得较好。理论结果表明应变弛豫时间对Ge单晶薄膜的热导率产生了重要影响。     

The pulsed-laser deposition of superconducting thin films

High-critical-temperature (high-T_c) superconductors have generated tremendous interest for the fabrication of superconducting thin films in advanced microelectronic applications. Superconducting thin films operating at liquid nitrogen temperatures offer great possibilities for faster, more sensitive and more precise electronic devices. The pulsed-laser deposition technique is an excellent method of fabricating high-T_c superconducting thin films with exceptional microstructural and property control, and its unique features have made it quite popular. This article discusses the nature of laser-solid-plasma interactions during the deposition process and processing aspects which result in films exhibiting excellent superconducting properties.     

Origin of the n-type field effect in polyaniline and oligoaniline thin films

We analyse the n-type field effect recently reported for protonated, layer-by-layer self-assembled polyaniline and oligoaniline thin films. An agreement is obtained between the experiments and a model, valid for three-dimensional variable-range hopping in a parabolic Coulomb gap. The results suggest that the wave-function decay length ξ and/or the dielectric constant ε vary with the position of the Fermi level (E_F), with a positive derivative dln[ξ(E_F)ε(E_F)]/dE_F in the order of a few eV⁻¹. The apparent discrepancy between the p-type doping and n-type field effect is explained by the electron-hole asymmetry characteristic for aniline derivatives.