Residual strain evolution in steel samples: tension versus torsion

Torsion provides a unique opportunity to probe mechanical behavior of materials subjected to pure state of shear stress. In this research, identical steel alloy (12L14) hollow cylinder samples are subjected to predetermined amounts of plastic axial and shear strain such that their octahedral shear strain (an invariant) are identical for comparison. Measurements were made at the residual stress measuring facility at the High Flux Isotope Reactor in Oak Ridge (NRSF2), using a small gauge area in the direction of strain gradients (0.5 mm×0.5 mm) through the hollow cylinder wall thickness. These orthogonal strains are obtained for BCC Fe for three hkl’s. Three normal strains in the hoop, radial, and axial directions are obtained as a function of centroid position of the gauge volume through the 2 mm wall thickness. Significant differences in measured residual strains are noted between the torsion and the tension samples. The largest differences are found for the Fe (200) planes while the smallest differences are observed for the Fe (211) planes. This research demonstrates the need for a systematic study of residual strain as a function of applied stress path moving beyond tensile testing for solving real world problems.     

On the determination of the volume dependence of Grüneisen parameters in cubic and non-cubic solids

For a material of orthorhombic symmetry under hydrostatic stress, three generalized Grüneisen parameters relative respectively to the three principal directions of the deformation can be defined. Finite strain expressions for these parameters are derived in terms of the Lagrangian strain tensor and the frame-indifferent analogue of the Eulerian strain tensor. The expressions require for their evaluation the thermal expansion coefficients, the elastic moduli and their pressure and temperature derivatives, and the specific heat of the material, so that there are no arbitrary constants or “curve fitting.” In the case of cubic materials, it is possible to determine the unique Grüneisen parameter as a function of volume directly. For non-cubic materials, the volume dependence of the Grüneisen parameters is calculated using a quasi-harmonic finite strain model of equation of state. Numerical applications are given for some cubic and hexagonal metals and the results are discussed.     

Effect of elastic characteristics of the surface layer on the deformation properties of materials with an interface-controllable structure

Computer simulation is used for analyzing the possibility of changing the ultimate strain in samples of “interface” materials whose mechanical behavior is determined by strain localization at the interfaces of structural elements (blocks, grains, etc.) by controlled modification of surface layers. It is shown that a considerable improvement of the deformability of samples subjected to cyclic loading can be attained by reducing the Young modulus and the elastic limit of interfaces in the surface layer. This effect can be explained by the large volume of the material involved in irreversible strain accumulation, which suppresses strain localization in the vicinity of stress macroconcentrators and delays the crack formation.     

三维材料微结构设计与数值模拟

为了研究材料细观尺度的力学性能与失效行为,达到对材料微结构的“性能导向型”设计与性能预测的目的,通过程序设计结合有限元数值模拟的方法实现多元多相异质体材料微观组织结构的计算机仿真、材料微结构的细观力学计算与虚拟失效分析.以材料微观组织结构计算机仿真软件ProDesign构造的多晶体材料与多晶体基复合材料微结构的代表性体积单元为基础,基于对商业有限元软件ABAQUS的二次开发,实现对材料微结构细观力学的数值计算,并根据数值模拟结果预测微结构的材料性能,识别“材料结构弱点”,评估异质体材料微结构内微裂纹的启裂 关键词： 材料微结构 数值模拟 各向异性 虚拟失效     

Integrability of the equilibrium and compatibility equations for a viscoplastic medium with negative strain rate sensitivity

The equilibrium and compatibility equations for viscoplastic medium with an arbitrary material function relating the stress intensity to the strain rate intensity is considered. A general form of the function ensuring complete integrability of two-dimensional equations has been found. The obtained function has an N-shaped (spinodal) graph and in particular cases corresponds to a linearly viscous liquid and perfectly plastic solid. A change of the strain rate sensitivity sign corresponds to a change in the type of the system and passing over the discontinuity line in a solid. The obtained function provides decoupling of the operator in a pair of two-dimensional subspaces where the equations are exactly linearized. The results of this study allows us to extend the class of integrable problems to so-called “active materials” (or “materials with internal dynamics”), which have aroused considerable interest.     

Resonant electron tunneling in GaN/Ga1−x AlxN(0001) strained structures with spontaneous polarization and piezoeffect

Electron tunneling through the GaN/Ga_1?x Al_xN(0001) wurtzite strained structures is investigated by the pseudopotential and scattering matrix methods. It is shown that the results of multiband calculations at low aluminum concentrations (x<0.3) are adequately described within the single-valley model in the envelope wave function method accounting for the dependences of the effective mass on the energy and strain. Upon electron tunneling through two-barrier structures, sharp resonance peaks are observed at a barrier thickness of several monolayers and the characteristic collision time in the resonance region is equal to ～1 ps. The internal electric fields associated with spontaneous and piezoelectric polarizations lead to a “red” or “blue” shift in the resonance energy according to the thickness and location of barriers with respect to the polar axis. In the (GaN)_n(Ga_1?x Al_xN)_m superlattices, the internal fields can form the Stark ladder of electronic states at a small number of ultrathin layers even in the absence of external fields.     

Numerical simulation of ultrafast expansion as the driving mechanism for confined laser ablation with ultra-short laser pulses

Recently, a so-called “directly induced” laser ablation effect has been reported, where an ultra-short laser pulse (660 fs and 1053 nm) irradiates a thin Mo film through a glass substrate, resulting in a “lift-off” of the irradiated layer in form of a thin, solid, cylindrical fragment. This effect provides a new and very energy-efficient selective structuring process for the Mo back electrode in thin-film solar cell production. To understand the underlying physical mechanisms, a 3D axisymmetric finite element model was created and numerically solved. The model is verified by a direct comparison of experimental and numerical results. It includes volume absorption of the laser pulse, heat diffusion in the electron gas and the lattice, thermal expansion of the solid phase and further volume expansion from phase transition to fluid and gas, and finally the mechanical motion of the layer caused by the resulting stress wave and the interaction with the substrate. The simulation revealed that irradiation of the molybdenum layer with an ultra-short pulse causes a rapid acceleration in the direction of the surface normal within a time frame of a hundred picoseconds to a peak velocity of about 100 m/s. The molybdenum layer continues to move as an oscillating membrane, and finally forms a dome after about 100 ns. The calculated strain at the edges of the dome exceeds the tensile stress limit at fluences that initiate the “lift-off” in experimental investigations. In addition, the simulation reveals that the driving mechanism of the “lift-off” is the ultrafast expansion of the interface layer and not the generated gas pressure.     

Carbon‐Coated Supraballs of Randomly Packed LiFePO4 Nanoplates for High Rate and Stable Cycling of Li‐Ion Batteries

One of the key strategies used to obtain high‐rate Li‐ion battery is the reduction of the Li‐ion path length inside the active materials and the enhancement of the ionic diffusion outside the active materials. It is demonstrated that electrochemical performance can be improved significantly at high C‐rates using carbon‐coated spherical aggregates or “supraballs” of randomly packed olivine LiFePO₄ (LFP) nanoplates as cathode active materials. 258 nm LFP nanoplates with 30 nm thickness are synthesized through a high‐temperature solvothermal method, in which short lithium‐ion channels are formed perpendicular to the top or bottom planes. These thin nanoplates are formed into carbon‐coated “supraballs” through a spray‐drying and thermal annealing process, in which nanoplates are not stacked but randomly packed due to relatively fast drying. Internal and external nanoplate ion diffusion is therefore enhanced simultaneously due to the optimal molecular crystalline structure and interparticle pore structures of the nanoplates. Indeed, the initial capacity of the carbon‐coated supraballs is 162 mAh g^?1 (173.34 mAh cm^?3) at 0.1 C and more than 80% is retained (≈130.91 mAh g^?1) at 50 C. Furthermore, they offer durable cycling stability (>500 cycles) at 1 C without compromising their capacity.     

Dynamics of composite nonlinear systems and materials for engineering applications and energy harvesting – The role of nonlinear dynamics and complexity in new developments

This engineering research focus issue was inspired by the workshop organised by us on March 19th 2012 at the Lublin University of Technology (in the framework of CEMCAST) titled “Dynamics of functionally graded materials and systems with hysteresis” The collection of the present research papers followed our fruitful discussions focussing on “Dynamics of composite nonlinear systems and materials for engineering applications and energy harvesting” which is now the title of the present volume. In the next two sections we briefly discuss the topics included in this focus issue.     

New field equations in the 5-dimensional projective unified field theory

New field equations of the Projective Unified Field Theory are presented which avoid potential difficulties of former versions with respect to the equivalence principle. The physical interpretation of this new version remains unchanged: constancy of the “gravitational constant”, electromagnetic polarization of the vacuum, definiteness of the energy of the stationary scalaric field, etc. Furthermore, the Klein-Gordon field and the Dirac field are treated.     

Geometrical coupling between inhomogeneous strain fields: Application to fluctuations in ferroelastics

The compatibility conditions of elasticity theory are applied to the calculation of strain fluctuations in ferroelastic materials. A ferroelastic transition with an acoustic phonon mode instability and an order-disorder transition with striction are considered in the framework of the Landau-Ginzburg functional. Anisotropy of strain correlation functions and their critical behaviour in the symmetric phase near the transition point are analysed and compared with the results of the Ornstein-Zernike theory of fluctuations. Characteristic shape of the correlation functions (“butterflies”) are predicted.     

Band structure of collective modes and effective properties of binary magnonic crystals

In this paper a theoretical study of the band structure of collective modes of binary ferromagnetic systems formed by a submicrometric periodic array of cylindrical cobalt nanodots partially or completely embedded into a permalloy ferromagnetic film is performed. The binary ferromagnetic systems studied are two-dimensional periodic, but they can be regarded as three-dimensional, since the magnetization is non uniform also along the z direction due to the contrast between the saturation magnetizations of the two ferromagnetic materials along the thickness. The dynamical matrix method, a finite-difference micromagnetic approach, formulated for studying the dynamics in one-component periodic ferromagnetic systems is generalized to ferromagnetic systems composed by F ferromagnetic materials. It is then applied to investigate the spin dynamics in four periodic binary ferromagnetic systems differing each other for the volume of cobalt dots and for the relative position of cobalt dots within the primitive cell. The dispersion curves of the most representative frequency modes are calculated for each system for an in-plane applied magnetic field perpendicular to the Bloch wave vector. The dependence of the dispersion curves on the cobalt quantity and position is discussed in terms of distribution of effective “surface magnetic charges” at the interface between the two ferromagnetic materials. The metamaterial properties in the propagative regime are also studied (1) by introducing an effective magnetization and effective “surface magnetic charges” (2) by describing the metamaterial wave dispersion of the most representative mode in each system within an effective medium approximation and in the dipole-exchange regime. It is also shown that the interchange between cobalt and permalloy does not necessarily lead to an interchange of the corresponding mode dispersion. Analogously to the case of electromagnetic waves in two-dimensional photonic crystals, the degree of localization of the localized collective modes is expressed in terms of an energy concentration factor.     

Pressure coefficients of the photoluminescence of the II–VI semiconducting quantum dots grown by molecular beam epitaxy

Luminescence properties of CdTe and CdSe quantum dots have been studied under high hydrostatic pressure. The luminescence pressure coefficients of the II–VI quantum dots appear to be very similar to the pressure coefficients of the band-gap of bulk CdTe and CdSe, respectively. In contrary to that, the luminescence pressure coefficients of the III–V quantum dots are significantly lower than pressure coefficients of energy gaps of the appropriate dot materials. The discrepancy can be explained by the theoretical model, which takes into account effects of strain on pressure coefficients in thin strained layers. The experimentally observed pressure-induced quenching of the QDs luminescence is attributed to the “zinc-blende–cinnabar” phase transition in CdTe QDs and to the “zinc-blende–rock-salt” phase transition in CdSe QDs.     

Molecular dynamics simulation of compression of single-layer graphene

The compression of a single-layer graphene sheet in the “zigzag” and “armchair” directions has been investigated using the molecular dynamics method. The distributions of the xy and yx stress components are calculated for atomic chains forming the graphene sheet. A graphene sheet stands significant compressive stresses in the “zigzag” direction and retains its integrity even at a strain of ～0.35. At the same time, the stresses which accompany the compressive deformation of single-layer graphene in the “armchair” direction are more than an order in magnitude lower than corresponding characteristics for the “zigzag” direction. A compressive strain of ～0.35 in the “armchair” direction fractures the graphene sheet into two parts.     

Structure-dependent optical anisotropy of porphyrin Langmuir–Schaefer films

We have studied the optical anisotropy of Langmuir–Schaefer layers of PdC₁₀OAP porphyrin, deposited onto gold substrates with thickness in the range 0–16 monolayers (ML). Deposition has been carried out at two values of the surface pressure Π, corresponding to different layer structures. In one case (Π=30 mN/m), molecules are well ordered in stacks oriented edge-on with respect to the substrate. In the other (Π=10 mN/m), a complex reorganization of the system happens several days after deposition, to form a mesoscopic two-dimensional lattice. The spectra measured by reflectance anisotropy spectroscopy (RAS) in two cases are clearly characterized. In the former, the line shape is dominated by a characteristic, large structure appearing in coincidence with the Soret band of the molecule, the development of which from a “peak-like” to a “derivative-like” appearance occurs at a well-defined critical thickness Θ_c (8 ML). In the latter, the line shape is always “peak-like”. We explain both line shapes in terms of morphological characteristics of the layer, occurring at different thickness values. The present results clearly show the potential of RAS to characterize efficiently the deposition of organic materials, and suggest that in short time it will be used as an in situ and real time spectroscopy, as already done in inorganic growth.     

Small-Angle Neutron Scattering Wave Interference on Ferromagnetic Alloys

The detection conditions and features of direct and scattered neutron wave interference are studied on magnetized Co₆₇Fe₃₁V₂ alloy slabs. The angular intensity distributions of neutrons passed through a sample are measured for the opposite polarization directions of the initial neutron beam. The sought-for effect that is induced by the magnetic scattering on crystal structure irregularities in specimens manifest itself by different areas of peaks “without neutron spin flip.” The ratio of these areas depends on the thermal treatment mode, sample thickness and strength of the magnetic field applied to the sample. The peaks “with neutron spin flip” are due to the mechanism of neutron wave passage through magnetononcollinear boundaries. The methods for experimental data acquisition and processing are reported as well.     

边修饰Net-Y纳米带的电子结构及机械开关特性的应变调控效应

利用密度泛函理论和非平衡态格林函数相结合的方法,系统地研究了边修饰Net-Y纳米带的电子结构和器件特性的应变调控效应..计算表明:本征纳米带为金属,但边缘的氢或氧原子端接能使其转变为半导体.应变能有效地调控纳米带带隙的大小,适当的应变使能带结构从间接带隙转变为较小的直接带隙,这有利于光的吸收.应变也能改变纳米带的功函数,压缩应变能明显减小功函数,这有利于纳米带实现场发射功能.特别是应变能有效地调控纳米带相关器件的I-V特性,能使其开关比(I_on/I_off)达到106,据此,可设计一个机械开关,通过拉伸及压缩纳米带使其可逆地工作在“开”和“关”态之间.这种高开关比器件也许对于制备柔性可穿戴电子设备具有重要意义.     

绿色植被可见-近红外反射光谱模拟材料研究进展

成像光谱技术能够同时获取目标的图像特征和光谱特征,很容易识别与背景环境光谱特征区别较大的传统伪装材料。近年来,成像光谱得到了迅速发展,经历了多光谱技术到高光谱技术的跨越,传感器的探测波段数、光谱分辨率、空间分辨率的显著提高。得益于各国ISR无人机技术的应用,高光谱传感器由星载拓展到机载,可以在更近距离对军事伪装目标进行识别,对具有重要价值的军事目标的生存能力构成巨大挑战。目前,应对高光谱的伪装材料主要设计思路是,选择材料或材料体系具有与环境背景相似的颜色和光谱反射特征（传感器探测范围内）进行复合,目的是与环境背景达到“同色同谱”来躲避高光谱侦察。绿色植被是最常见的伪装背景,也是本领域绝大部分研究的光谱模拟对象,其反射光谱曲线在可见近红外波段具有：“绿峰”、“红边”、“近红外高原”和“水吸收带”四个主要特征,分别由叶片的组织结构以及叶绿素和水分产生。离体叶绿素光热稳定性较差,不能直接用作伪装材料,所以寻找和合成稳定性好、具有类叶绿素结构及光谱特征的分子是当前的研究热点之一。此外,铬绿和钴绿是常用的伪装颜料,具有类似绿色植被“绿峰”、“红边”和“近红外高原”光谱反射特性,研究者将其与高吸水填料复合来引入“水吸收峰”,大致模拟出绿色植被反射光谱,但是想要实现精确模拟,仍存在一些难以解决的问题。从绿色植被光谱特征出发,分别阐述了模拟绿色植被可见光区和近红外光区光谱特征的材料选择依据及体系;同时介绍了它们在精确模拟植被光谱时存在的问题,以及通过改性和复合来提升光谱相似度和耐候性的相关研究工作,总结并展望了绿色植被光谱模拟材料要解决的重难点问题和发展方向。     

Influence of external factors on the self-organization of lead and tin telluride nanostructures on the BaF2(111) surface under conditions close to the thermodynamic equilibrium

The morphology of PbTe and SnTe nanostructures grown on BaF₂(111) substrates from the vapor phase in a vacuum under conditions close to the thermodynamic equilibrium has been investigated using atomic force microscopy. The equilibrium shape of PbTe and SnTe quantum dots and the statistical parameters of arrays of these quantum dots have been studied as a function of the thermodynamic conditions of growth, the crystal lattice mismatch between the materials of the quantum dots and substrate, and elastic properties of these materials. It has been established that, when the BaF₂(111) substrate is deformed under external mechanical loading, the self-organization of dislocations on the BaF₂(111) surface can result in the formation of a nanoscale ordered strain relief, which can be used for the fabrication of nanostructures. The morphology of this relief depends on the external load and on the temperature at which the substrate is deformed. It has been shown that the deformation effect on the surface of the substrate and light irradiation of the growth zone of nanostructures affect the nucleation of islands and kinetic processes occurring on the surface of the substrate during their growth. Using the influence of external factors on the BaF₂(111) surface under certain thermodynamic conditions, it is possible to grow SnTe and PbTe nanostructures with different morphologies: continuous epitaxial layers with a thickness of less than 10 nm, homogeneous arrays of quantum dots with a high lateral density (more than 10¹¹ cm²), quasi-periodic lateral nanostructures (nanowires), “single” and “coupled” quantum dots, and “molecules” of quantum dots.