Externally applied stress sign and film elastic properties effects on brittle film fracture

Rectangular stainless steel samples with TiN film deposited on the front lateral surface were loaded in three-point bending to the maximum normal strain of 6%. Scanning electron microscopy showed that vertical cracks appeared in the tension zone when the tensile strain exceeded 1.5%, while horizontal cracks appeared in the compression zone when the compressive strain exceeded –2.9%. Film cracks in the compressive zone originate from the tensile stress imposed by the plastically deformed substrate due to the Poisson’s expansion. Taking plastic deformation and Poisson’s expansion of the substrate in compression into account, theoretical analysis of normal stress distribution along the cracked film segment in compression is presented. Substrate strain and film elastic properties affect film cracking in the compressive zone. At larger compressive strain, some transverse cracks along with buckling cause the film spallation. The presented method is useful for studying brittle film fracture with variable strain levels in a single sample.     

In-situ strain localization analysis in low density transformation-twinning induced plasticity steel using digital image correlation

The effect of deformation temperature on the strain localization has been evaluated by an adapted digital image correlation (DIC) technique during tensile deformation. The progress of strain localization was traced by the corresponding strain maps. The electron backscatter diffraction analysis and tint etching technique were utilized to determine the impact of martensitic transformation and deformation twinning on the strain localization in both elastic and plastic regimes. In elastic regime the narrow strain bands which are aligned perpendicular to the tension direction were observed in temperature range of 25 to 180 °C due to the stress-assisted epsilon martensite. The strain bands were disappeared by increasing the temperature to 300 °C and reappeared at 400 °C due to the stress-assisted deformation twinning. In plastic regime strain localization continued at 25 °C and 180 °C due to the strain-induced alfa-martensite and deformation twinning, respectively. The intensity of plastic strain localization was increased by increasing the strain due to the enhancement of martensite and twin volume fraction. The plastic strain showed more homogeneity at 300 °C due to the lack of both strain-induced martensite and deformation twinning.     

ESSENTIAL WORK OF FRACTURE AND THE PHASE TRANSFORMATION IN β-iPP

The possibility of phase transformation toughening is demonstrated by the example of the β-modification isotactic polypropylene (β-iPP), which undergoes the β–α transformation (i.e., from hexagonal to monoclinic) during mechanical loading. The β–α recrystallization was examined on essential work of fracture (EWF) specimens with two processing conditions. Differential scanning calorimetry and wide-angle x-ray diffraction demonstrate the occurrence of this β–α transformation. Some structural changes can be identified in EWF specimens by using nano-indentation machine. Toughness of the α- and β-iPP is studied and compared with the EWF concept by using static-loaded deeply double-edge-notched tensile specimens. The main effect of the β–α transformation is a large increase in the specific plastic work consumed in the necked zone.     

Changes in surface morphology and work function caused by corrosion in aluminum alloys

Corrosion behavior of a material is complicated and can affect strongly its surface physical and chemical properties. In this study, the effect of corrosion on surface morphology and electron work function (EWF) of various Al alloys was comparatively investigated using a scanning electron microscopy and a scanning Kelvin probe. The experimental results showed that the change in EWF with corrosion time was not linear, but firstly increased and then decreased. The theoretical treatments demonstrated that such a change in EWF did not only depend on the surface roughness, but also correlate with oxide layers formed at the surfaces of Al alloys. The present results can also be used to reasonably explain the correlation between surface roughness and EWF reported previously.     

Deformation behaviour of body centered cubic iron nanopillars containing coherent twin boundaries

Molecular dynamics simulations were performed to understand the role of twin boundaries on deformation behaviour of body-centred cubic (BCC) iron (Fe) nanopillars. The twin boundaries varying from 1 to 5 providing twin boundary spacing in the range 8.5–2.8 nm were introduced perpendicular to the loading direction. The simulation results indicated that the twin boundaries in BCC Fe play a contrasting role during deformation under tensile and compressive loadings. During tensile deformation, a large reduction in yield stress was observed in twinned nanopillars compared to perfect nanopillar. However, the yield stress exhibited only marginal variation with respect to twin boundary spacing. On the contrary, a decrease in yield stress with increase in twin boundary spacing was obtained during compressive deformation. This contrasting behaviour originates from difference in operating mechanisms during yielding and subsequent plastic deformation. It has been observed that the deformation under tensile loading was dominated mainly by twin growth mechanism. On the other hand, the deformation was dominated by nucleation and slip of full dislocations under compressive loading. The twin boundaries offer a strong repulsive force on full dislocations resulting in the yield stress dependence on twin boundary spacing. The occurrence of twin–twin interaction during tensile deformation and dislocation–twin interaction during compressive deformation has been discussed.     

Engineering the work function of armchair graphene nanoribbons using strain and functional species: a first principles study

First principles density functional theory calculations were performed to study the effects of strain, edge passivation, and surface functional species on the structural and electronic properties of armchair graphene nanoribbons (AGNRs), with a particular focus on the work function. The work function was found to increase with uniaxial tensile strain and decrease with compression. The variation of the work function under strain is primarily due to the shift of the Fermi energy with strain. In addition, the relationship between the work function variation and the core level shift with strain is discussed. Distinct trends of the core level shift under tensile and compressive strain were discovered. For AGNRs with the edge carbon atoms passivated by oxygen, the work function is higher than for nanoribbons with the edge passivated by hydrogen under a moderate strain. The difference between the work functions in these two edge passivations is enlarged (reduced) under a sufficient tensile (compressive) strain. This has been correlated to a direct-indirect bandgap transition for tensile strains of about 4% and to a structural transformation for large compressive strains at about - 12%. Furthermore, the effect of the surface species decoration, such as H, F, or OH with different covering density, was investigated. It was found that the work function varies with the type and coverage of surface functional species. Decoration with F and OH increases the work function while H decreases it. The surface functional species were decorated on either one side or both sides of AGNRs. The difference in the work functions between one-sided and two-sided decorations was found to be relatively small, which may suggest an introduced surface dipole plays a minor role.     

单轴拉伸下氧化锆纳米柱相变机理的分子动力学研究

通过分子动力学模拟,观察到[001]取向的四方氧化锆纳米柱在拉伸载荷下具有两个线弹性变形的应力-应变关系.这一现象是四方结构向单斜结构相变的结果 .为了进一步阐明应力-应变曲线,进行了包括晶体结构分析和原子应变计算在内的详细研究.晶格取向强烈影响塑性变形机制,即[001]和[111]取向的纳米柱在拉伸载荷下经历相变,而沿[110]取向的纳米柱导致脆性断裂.观察到显著的温度效应,随着温度从300K升高到1500K,弹性模量从573.45GPa线性降低到482.65GPa.此外,还用轻推弹性带(NEB)理论估算了相变能垒,观察到相变能垒随温度的升高而降低.这一工作将有助于加深对氧化锆的四方相到单斜相转变和纳米尺度力学行为的理解.     

Deformation behavior of Fe-based bulk metallic glass during nanoindentation

Fe-based bulk metallic glasses (BMGs) normally exhibit super high strength but significant brittleness at ambient temperature. Therefore, it is difficult to investigate the plastic deformation behavior and mechanism in these alloys through conventional tensile and compressive tests due to lack of distinct macroscopic plastic strain. In this work, the deformation behavior of Fe₅₂Cr₁₅Mo₉Er₃C₁₅B₆ BMG was investigated through instrumented nanoindentation and uniaxial compressive tests. The results show that serrated flow, the typical plastic deformation feature of BMGs, could not be found in as-cast and partially crystallized samples during nanoindentation. In addition, the deformation behavior and mechanical properties of the alloy are insensitive to the applied loading rate. The mechanism for the appearance of the peculiar deformation behavior in the Fe-based BMG is discussed in terms of the temporal and spatial characteristics of shear banding during nanoindentation. Supported by the National Natural Science Foundation of China (Grant Nos 50571109, 10572142 and 56771102) and the National Basic Research Program of China (973 Program)(Grant No 2007CB613900)     

Zr-4合金压缩形变行为的研究

密排六方结构的Zr呈现弹塑性各向异性,轧制工艺会使材料内部产生晶间应力.准确地评估Zr合金内部的晶间应力分布并明确其微观形变机制,对其服役能力和使用寿命的准确评判具有重要的科学意义和应用价值.利用中子原位衍射技术结合弹塑性自洽(EPSC)模拟分析了Zr-4合金的压缩形变行为,加载方式为沿轧板厚度方向压缩.研究中辅以非原位的背散射电子衍射测试进行织构演化分析及透射电镜(TEM)测试分析缺陷形态.EPSC模拟可以定量地给出不同形变量下的形变机制,并且计算结果可由TEM实验佐证.研究表明:当形变量较小(0.55%)时,柱面{10ˉ10}?11ˉ20?(?a?型)滑移起主导作用;随着塑性形变量的增加,锥面滑移的作用增强,且锥面{10ˉ11}?11ˉ23?(?c+a?型)滑移的作用大于柱面{10ˉ10}?11ˉ20?(?a?型)滑移,少量的锥面{10ˉ11}?11ˉ20?(?a?型)和{10ˉ12}?11ˉ20?(?a?型)滑移也存在.     

Strain Effect on the Absorption Threshold Energy of Silicon Circular Nanowires

In this work,the influence of strain on threshold energy of absorption in Silicon circular nanowires is investigated.For this purpose,we first have used the density functional theory(DFT) to calculate the electron and hole effective masses.Then,we have obtained absorption threshold energy with two different procedures,DFT and effective mass approximation(EMA).We have also obtained the band structures of Si nanowires both DFT and EMA.The results show that:i) the expansive strain increases the hole effective mass while compressive strain increases the electron effective mass,ii) the electron and hole effective masses reduce with decreasing the wire size,iii) the absorption threshold energy decreases by increasing strain for compressive and tensile strain and its behavior as a function of strain is approximately parabolic,iv) the absorption threshold energy(for all sizes) obtained using EMA is greater than the DFT results.     

Some effects of inhomogeneous strain on surface properties of metals: I. Flat surface

Jelium model calculations using the many electron formalism and a parameterized charge density distribution have been used to evaluate the change in surface energy, barrier height and work function due to the presence of a tensile and compressive strain gradient at the surface of Cs, Rb, K and Na. The method is useful for short range strain fields (? 4–5 atom spacings), but not for larger fields. The surface energy change due to inhomogeneous strain is almost twice as large as that due to homogeneous strain. The work function changes due to inhomogeneous strain are ~10–10² times larger than those due to homogeneous strains. The results are of considerable importance to general fracture and stress corrosion cracking studies.     

温度和应变速率耦合作用下纳米晶Ni压缩行为研究

本文利用低温力学测试系统研究了电化学沉积纳米晶Ni在不同温度和宽应变速率条件下的压缩行为. 借助应变速率敏感指数、激活体积、扫描电子显微镜及高分辨透射电子显微镜方法, 对纳米晶Ni的压缩塑性变形机理进行了表征. 研究表明, 在较低温度条件下, 纳米晶Ni的塑性变形主要是由晶界位错协调变形主导, 晶界本征位错引出后无阻碍的在晶粒内无位错区运动, 直至在相对晶界发生类似切割林位错行为. 并且, 在协调塑性变形时引出位错的残留位错能够增加应变相容性和减小应力集中; 在室温条件下, 纳米晶Ni的塑性变形机理主要是晶界-位错协调变形与晶粒滑移/旋转共同主导. 利用晶界位错协调变形机理和残留位错运动与温度及缺陷的相关性揭示了纳米晶Ni在不同温度、不同应变速率条件下力学压缩性能差异的内在原因.     

Stretchable electromagnetic interference shielding and antenna for wireless strain sensing by anisotropic micron-steel-wire based conductive elastomers

We prepare stretchable elastic electromagnetic interference(EMI) shielding and stretchable antenna for wireless strain sensing using an elastic composite comprising commercial steel wool as a conducting element. The prepared elastic conductor shows anisotropic electrical properties in response to the external force. In the stretchable range, the electrical resistance abnormally decreases with the increase of tensile deformation. The EMI shielding effectiveness of the elastic conductor can reach above-30 d B under 80% tensile strain. The resonance frequency of the dipole antenna prepared by the elastic conductor is linearly correlated with the tensile strain, which can be used as a wireless strain sensor. The transmission efficiency is stable at about-15 d B when stretched to 50% strain, with attenuation less than 5%. The current research provides an effective solution for stretchable EMI shielding and wireless strain sensing integrated with signal transmission by an antenna.     

2024铝合金激光多次冲击诱导残余应力状态分布规律

采用高功率激光器多次冲击2024铝合金,用X射线衍射技术分析了冲击区域的残余应力,研究了冲击残余应力状态分布规律,并用其评价激光冲击强化效果。研究表明,随着冲击次数增加,塑变量及塑性应变梯度逐渐减小,测点是双向压应力状态,而4次冲击时,塑性应变梯度增大,光斑中心是单向压应力状态,其他点是双向压应力状态。当激光功率密度为2.8 GW/cm2时,3次冲击强化效果最佳,材料是二向压应力状态,残余最大主应力及应力强度的均值最大,方差最小,分布基本均匀,塑性应变梯度较小。     

Molecular dynamics study of coupled layer thickness and strain rate effect on tensile behaviors of Ti/Ni multilayered nanowires

Novel properties and applications of multilayered nanowires(MNWs) urge researchers to understand their mechanical behaviors comprehensively.Using the molecular dynamic simulation,tensile behaviors of Ti/Ni MNWs are investigated under a series of layer thickness values(1.31,2.34,and 7.17 nm) and strain rates(1.0 × 10~8 s~(-1) ≤ε≤5.0 × 10~(10) s~(-1)).The results demonstrate that deformation mechanisms of isopachous Ti/Ni MNWs are determined by the layer thickness and strain rate.Four distinct strain rate regions in the tensile process can be discovered,which are small,intermediate,critical,and large strain rate regions.As the strain rate increases,the initial plastic behaviors transform from interface shear(the shortest sample) and grain reorientation(the longest sample) in small strain rate region to amorphization of crystalline structures(all samples) in large strain rate region.Microstructure evolutions reveal that the disparate tensile behaviors are ascribed to the atomic fractions of different structures in small strain rate region,and only related to collapse of crystalline atoms in high strain rate region.A layer thickness-strain rate-dependent mechanism diagram is given to illustrate the couple effect on the plastic deformation mechanisms of the isopachous nanowires.The results also indicate that the modulation ratio significantly affects the tensile properties of unequal Ti/Ni MNWs,but barely affect the plastic deformation mechanisms of the materials.The observations from this work will promote theoretical researches and practical applications of Ti/Ni MNWs.     

Investigation of local work function distribution of field emission cathode from carbon materials doped by Ba with the help of AFM/SEM/X-ray

The joint application of atomic force microscopy (AFM) and SEM/X-ray micro-analyzer for investigation of electron work function (EWF) distribution over the surface of field emission cathode (FEC) is described in the paper. This method was used to investigate the graphite cathodes surface after barium doping. The possibility of defining the average value of EWF over the surface of FEC is shown. The results of AFM measurements are compared with results of field emission tests. The reduction of work function down to 3–3.2 eV was found after barium doping.     

On mechanical stability of molybdenum

Investigations of the strength properties of materials under different loading conditions are of practical importance in many engineering applications. The knowledge of elastic moduli as a function of strain is required for determination of strength properties. In the present work, we have determined the elastic moduli of molybdenum through first principles study of the energy changes under three different loading conditions namely ‘uni-axial tensile deformation’ along [0 0 1] direction, ‘uni-axial tensile loading’ along [0 0 1] direction and ‘hydrostatic tensile loading’. The stability conditions for the system are expressed in terms of the elastic moduli and analyzed along the deformation paths corresponding to these three loading modes. The theoretical spall strength (σ_S), tensile strength (σ_T) along [0 0 1] direction and hydrostatic tensile strength (σ_H), are evaluated as a stress at the first onset of the instability for three loading conditions, respectively. The calculated equilibrium volume and elastic moduli are compared with that reported from experimental and other theoretical works.     

Strain-tunable electronic,elastic, and optical properties of CaI2 monolayer: first-principles study

ABSTRACT

The effects of biaxial strain on the electronic structure and the elastic and optical properties of monolayer CaI₂ were studied using first-principles calculations. The two-dimensional (2D) equation of state for monolayer CaI₂ as fit in a relative area of 80–120% is more accurate. The band gap can be tuned under strain and reached a maximum at a tensile strain of 4%. Under compressive strains, the absorption spectrum showed a significant red shift at higher strains. The static reflectance and static refractive index decreased in the strain range of ?10% to 10%.     

Electron work function of Rb-Cs alloys

The temperature and concentration dependences of the electron work function (EWF) of rubidium, cesium, and nine Rb-Cs alloys are studied by the Fowler photoelectric method under the conditions of an ultra-high vacuum. It is shown that the polytherms of the electron work function for the pure components and alloys decrease by a linear law with increasing temperature. Based on the experimental data obtained, it is concluded that the concentration dependence of the EWF for the binary Rb-Cs system has a linear (additive) character within the limits of experimental error (about 1%). The presence of the azeotropic point on the state diagram of the system is not visualized in the concentration dependence of the work function.     

Deformation textures produced in diamond anvil experiments, analysed in radial diffraction geometry

Diamond anvil cells may not only impose pressure upon a sample but also a compressive stress that produces elastic and plastic deformation of polycrystalline samples. The plastic deformation may result in texture development if the material deforms by slip or mechanical twinning, or if grains have a non-equiaxed shape. In radial diffraction geometry, texture is revealed by variation of intensity along Debye rings relative to the compression direction. Diffraction images (obtained by CCD or image plate) can be used to extract quantitative texture information. Currently the most elegant and powerful method is a modified Rietveld technique as implemented in the software package MAUD. From texture data one can evaluate the homogeneity of strain in a diamond anvil cell, the strain magnitude and deformation mechanisms, the latter by comparing observed texture patterns with results from polycrystal plasticity simulations. Some examples such as olivine, magnesiowuestite, MgSiO(3) perovskite and ε-iron are discussed.