Dislocation-induced damping in metal matrix composites

The damping response of crystalline metals and alloys is generally associated with the presence of defects in the crystal lattice. The disturbance of these defects, usually in response to an applied cyclic load, dissipates energy, a mechanism known as internal friction. The various defects commonly found in crystalline materials include point defects (e.g. vacancies), line defects (e.g. dislocations), surface defects (e.g. grain boundaries) and volume defects (e.g. inclusions). Among these, dislocations are noteworthy because they play a critical role, not only in the damping response of crystalline materials, but also in the overall mechanical behaviour of the materials. Among the various structural materials actively being developed, metal matrix composites (MMCs) have received considerable attention as a result of their potential to combine reinforcement properties of strength and environmental resistance, with matrix properties of ductility and toughness. Of interest is the generally observed phenomenon that MMCs exhibit unusually high concentrations of dislocations, an observation typically attributed to the difference in coefficient of thermal expansion between matrix and reinforcement. The objectives of the present paper are to provide an overview of the sources of dislocation generation in MMCs, and to provide insight into the effects that dislocations have on the damping response of MMCs. The presence of dislocations in MMCs is highlighted on the basis of transmission electron microscopy studies, and the dislocation damping mechanisms are discussed in light of the Granato-Lücke theory.     

A review of computational phononics: the bulk,interfaces, and surfaces

Broad-based interest in microscale heat transport in novel materials, engineered phononic materials, metamaterials, and their relevant systems has created significant demand for computational approaches to aid in investigation and design of materials that support phonons. This review describes the significant improvements that have been made and new needs that have emerged for capabilities associated with the computability of phonons. The technical scope encompasses issues, especially relevant to bulk, interface, and surface effects. Traditional approaches such as molecular dynamics, lattice dynamics, and Boltzmann transport equation continue to advance the field but are frequently extended to the limits of their physical or numerical validity. New materials beyond traditional group-IV, III–V, and II–VI semiconductors, phenomena that critically depend on scattering, such as in low-dimensional nanostructures, materials with interior surfaces and defects, and in high-temperature environments, continue to push these limits. The basis for the traditional calculation methods shares their origins with the earliest theories for thermal transport, acoustic waves in solids, spectroscopy and dynamical crystal lattices. These will remain in wide use in the future. But computing methods and the accompanying advances in microprocessor technologies have enabled growth of phonon computing models and methods in sophistication, accuracy, fidelity and complexity that will lead to fundamental impacts beyond the classic types of problems for which they were developed. With their increasingly integrated use for design and research, the myriad developments that presently exist must be understood for their suitability for certain applications and their ability to aid in the pursuit of new technologies.     

Deformation mechanism and in-situ TEM compression behavior of TB8 β titanium alloy with gradient structure

Severe plastic deformation is known to induce grain refinement and gradient structure on metals'sur-faces and improve their mechanical properties.However,the fundamental mechanisms behind the grain refinement and micromechanical properties of materials subjected to severe plastic deformation are not still well studied.Here,ultrasonic surface rolling process(USRP)was used to create a gradient microstructure,consisting of amorphous,equiaxed nano-grained,nano-laminated,ultrafine laminated and ultrafine grained structure on the surface of TB8 β titanium alloy.High energy and strain drove element co-segregation on sample surface leading to an amorphous structure during USRP processing.In situ transmission electron microscope compression tests were performed in the submicron sized pillar extracted from gradient structure and coarse grain,in order to reveal the micromechanics behavior of different grain morphologies.The ultrafine grained layer exhibited the lowest yield stress in comparison with single crystal and amorphous-nanocrystalline layers;the ultrafine grained layer and single crystal had an excellent strain hardening rate.The discrepancy among the grain sizes and activated dislocation sources led to the above mentioned different properties.Dislocation activities were observed in both compression test and microstructure evolution of USRP-treated TB8 alloy.An evolution of dislocation tangles and dislocation walls into low angle grain boundaries and subsequent high angle grain bound-aries caused the grain refinement,where twinning could not be found and no phase transformation occurred.     

Modeling of irradiation hardening of polycrystalline materials

High energy particle irradiation of structural polycrystalline materials usually produces irradiation hardening and embrittlement. The development of predictive capability for the influence of irradiation on mechanical behavior is very important in materials design for next-generation reactors. A multiscale approach was implemented in this work to predict irradiation hardening of iron based structural materials. In the microscale, dislocation dynamics models were used to predict the critical resolved shear stress from the evolution of local dislocation and defects. In the macroscale, a viscoplastic self-consistent model was applied to predict the irradiation hardening in samples with changes in texture. The effects of defect density and texture were investigated. Simulated evolution of yield strength with irradiation agrees well with the experimental data of irradiation strengthening of stainless steel 304L, 316L and T91. This multiscale modeling can provide a guidance tool in performance evaluation of structural materials for next-generation nuclear reactors.     

不同因素下镍铜双层膜表面摩擦行为的研究

材料在摩擦接触过程中的弹塑性变形对基体的力学性能具有重要影响。为研究镍铜双层膜在接触过程中的变形行为和力学特性,本文从原子轨迹、原子晶格结构变化、接触力和基体内部位错等方面,详细研究了表面纹理密度、纹理方向、晶体学方向、磨粒半径和接触深度等因素对摩擦接触过程的影响。结果表明：纳观纹理表面以及镀层的引入对接触力产生影响。镍膜晶体学方向对滑动接触过程影响显著,存在接触力最小的晶体学方向;凸体的分布角度对摩擦过程的影响较小;在界面作用下,特定纹理密度表现出一定的减摩作用;基体材料的接触力随着磨粒半径和接触深度的增大而增大;在不同因素及水平下,基体表现出不同的位错缺陷程度和原子堆积现象。     

Investigations of structural defects,crystalline perfection,metallic impurity concentration and optical quality of flat-top KDP crystal

KDP crystal grown using flat-top technique has been characterized using X-ray and optical techniques with the aim of correlating the defects structure and impurity concentration in the crystal with its optical properties. Crystallographic defects were investigated using X-ray topography revealing linear and arc like chains of dislocations and to conclude that defects do not originate from the flat-top part of the crystal. Etching was performed to quantify dislocation defects density. The crystalline perfection of the crystal was found to be high as the FWHM of the rocking curves measured at several locations was consistently low 6–9 arc s. The concentration of Fe metallic impurity quantified using X-ray fluorescence technique was approximately 5 times lower in the flat-top part which falls in pyramidal growth sector as compared to the region near to the seed which lies in prismatic sector. The spectrophotometric characterization for plates cut normal to different crystallographic directions in the flat-top potassium dihydrogen phosphate (FT-KDP) crystal was performed to understand the influence of metallic impurity distribution and growth sectors on the optical transmittance. The transmittance of the FT-KDP crystal at 1064 nm and its higher harmonics (2nd, 3rd, 4th and 5th) was determined from the measured spectra and the lower transmission in the UV region was attributed to increased absorption by Fe metallic impurity at these wavelengths. The results are in agreement with the results obtained using X-ray fluorescence and X-ray topography. Birefringence and Mach–Zehnder interferometry show that except for the region near to the seed crystal the optical homogeneity of the entire crystal was good. The laser-induced damage threshold (LDT) values are in the range 2.4–3.9 GW/cm². The LDT of the plate taken from the flat-top region is higher than that from the bottom of the crystal, indicating that the flat-top technique has good optical quality and is comparable to those reported using rapid growth technique. The results indicate that the structural defects, crystalline quality and impurity concentration have a correlation with the optical properties of the FT-KDP crystal.     

Kinematics, electromechanics, and kinetics of dielectric and piezoelectric crystals with lattice defects

A mathematical framework is formulated to address the electromechanical behavior of dielectric and piezoelectric solids containing lattice imperfections. The macroscopic displacement gradient encompasses recoverable elasticity, deviatoric plasticity arising from dislocation glide, and volumetric deformation attributed to point vacancies in the crystal. A linear connection on the spatial manifold of deformed lattice vectors describes gradients of stretch and rotation at the microscale caused by continuous distributions of various classes of crystal defects. It is shown that parallel transport of a lattice director vector with respect to this connection about a closed loop yields a discontinuity with contributions from the torsion of the connection (physically, from dislocations) and its curvature (physically, from rotational defects such as domain walls, and from gradients in vacancy concentration). Classical balance laws of electrostatics and mass and momentum conservation are invoked. A free energy function dependent upon lattice distortion, polarization, temperature, and defect densities is suggested. Thermodynamically consistent kinetic relations for dislocation glide and vacancy diffusion are then derived, with the chemical potential for the latter depending upon defect density, electric potential, hydrostatic pressure, and vacancy energy. The theory also explicitly considers mass rearrangement at the free surface of the substance. Two forms of the contribution of vacancies to the free energy are investigated in detail: a logarithmic function common in chemical mixing theory, and a quadratic function analogous to the convex strain energy used in continuum elasticity theory. For the latter case, the analytical solution of the diffusion equation in one spatial dimension, at steady state, illustrates the effects of defect charge, defect energy, and mechanical stress on the distribution of vacancies in a dielectric thin film. A specific example is given of how compressive residual stresses observed in experiments may be correlated with the equilibrium concentration of vacancies within grains of a polycrystalline dielectric thin film, influencing its electrical performance.     

Revealing nano-chemistry at lattice defects in thermoelectric materials using atom probe tomography

The population of all non-equilibrium lattice defects in materials is referred to as microstructure. Examples are point defects such as substitutional and interstitial atoms, and vacancies; line defects such as dislocations; planar defects such as interfaces and stacking faults; or mesoscopic defects such as second-phase precipitates. These types of lattice imperfections are usually described in terms of their structural features, breaking the periodicity of the otherwise regular crystalline structure. Recent analytical probing at the nanoscale has revealed that their chemical features are likewise important and characteristic. The structure of the defects as well as their individual chemical composition, that is their chemical decoration state, which results from elemental partitioning with the adjacent matrix, can significantly influence the electrical and thermal transport properties of thermoelectric materials. The emergence of atom probe tomography (APT) has now made routinely accessible the mapping of three-dimensional chemical composition with sub-nanometer spatial accuracy and elemental sensitivity in the range of tens of ppm. Here, we review APT-based investigations and results related to the local chemical decoration states of various types of lattice defects in thermoelectric materials. APT allows to better understand the interplay between thermoelectric properties and microstructural features, extending the concept of defect engineering to the field of segregation engineering so as to guide the rational design of high-performance thermoelectric materials.     

Microscale study of poly(vinyl alcohol) as an effective additive for inhibiting recrystallization in ice slurries

Scanning tunneling microscopy (STM) was used to investigate the surface morphology of ice crystals containing adsorbed poly(vinyl alcohol) (PVOH) molecules inside a cold room at −7.0°C. PVOH was used as a substitute for antifreeze protein (AFP) type I, which is an effective additive for making ice slurries resistant to recrystallization. The STM images revealed microscale grooves on ice crystals made from PVOH solutions, indicating that PVOH molecules significantly influence the surface structure of the ice crystal. The length of each groove was similar to that of a PVOH molecule, indicating that PVOH molecules were adsorbed on the ice crystal surfaces. The interaction force between PVOH molecules and the ice surface was discussed by assuming a molecular structure of PVOH on the ice crystal surface, and the depression of the local freezing point was analyzed based on the surface curvature of ice revealed by STM.     

Titanium enrichment and strontium depletion near edge dislocation in strontium titanate [001]/(110) low-angle tilt grain boundary

Dislocations are linear lattice defects in a crystalline solid. Since the unusual atomistic environment of the dislocation may greatly influence various material properties, control of the composition would offer more opportunities to obtain unique one-dimensional structures. In the present study, we have characterized the structure of dislocations in a low-angle tilt grain boundary of strontium titanate (SrTiO₃). High-spatial resolution elemental mapping by electron energy loss spectroscopy combined with scanning transmission electron microscopy has enabled visualization of the enrichment of titanium (Ti) and the depletion of strontium (Sr) near the dislocation cores. The Ti enrichment and the Sr depletion have been observed at all of the dislocations, and the grain boundary is considered to be Ti excess. The extra Ti ions are located on the positions different from the normal perovskite lattice, suggesting that the local structure is largely reconstructed. It has been proposed that tensile strain at the dislocations may be a cause of the Ti enrichment.     

The weak-beam technique applied to the analysis of materials properties

The so-called weak-beam (WB) technique has been widely employed to elucidate dislocation near-core properties. To a very large extent WB images reflect the intimate structure of lattice defects through signals that may, however, be significantly convoluted. This contribution reviews selected situations where various factors affecting images must be taken into account. A less-common method to investigate crystal order under WB is also reported.     

KTiOAsO4晶体的生长缺陷研究

研究ＫＴｉＯＡｓＯ４的生长缺陷,对于改善它的性质和应用前景有很大的意义,本文利用化学腐蚀光学显微术和同步辐射Ｘ射线形貌术研究了ＫＴｉＯＡｓ析缺陷,实验结果表明,两种腐蚀剂对ＫＴＡ晶体的表面缺陷效果显著,ＫＴＡ晶体中主要的缺陷有铁电畴生产层,形界,位错和包裹物,了这些缺陷形成的原因。     

ADP晶体的生长丘、台阶微观形貌及台阶棱边能

ADP晶体{100}面族生长的实时与非实时AFM(atomic force microscopy,AFM)研究表明,过饱和度σ处于0.005～0.04,生长温度介于293～313K之间时,晶面上观察到位错生长丘和其它晶体缺陷所形成的生长丘,晶面主要为台阶推进方式生长;位错生长丘上空洞的出现与位错弹性理论相符;随过饱和度σ降低,台阶形貌会发生相应变化;生长温度为298K时,台阶棱边能不小于6.2×10-7J/cm2.     

Mechanochemical reactions between structural defects in magnetic fields

An influence of steady, pulse and microwave magnetic fields on mechanochemical reactions between structural defects in doped ionic crystals has been studied. The effects of dislocation depinning, dislocation mobility increase, macroplastic unhardening and magnetoresonant softening in magnetic fields have been revealed. It is shown that the magnetosensitive spin-dependent reactions between structural defects is the reason of magnetoplastic effects in nonmagnetic crystals. It means that the electron and nuclear spin states of the paramagnetic defects play an important role in the formation of mechanical properties of crystals in magnetic fields and without them.     

Defect-induced microstructures: an X-ray diffraction analysis

Single crystal X-ray diffraction was applied in order to investigate defect-induced microstructures in radiation damaged zircon. The formation of domains with different degrees of order was observed and in particular, it was possible to distinguish two types of defects: isolated lattice defects and dislocations. These lattice deformations have a great influence on the structural and physical properties of the materials.     

天然金刚石晶体缺陷与性能的研究

利用红外光谱和X射线形貌术等方法对辽南天然金刚石晶体进行了晶体类型、缺陷及性能的研究。结果发现,有的晶体存在有各种缺陷,如位错、亚结构或孪晶界等。对部分位错做了详细地讨论,并确定其特征量,同时还发现,晶体中位错的存在对其透光性能有明显的影响。     

Influence of particle size on electrical transport properties of La0.67Sr0.33MnO3 manganite system

A systematic investigation of lanthanum-based manganite, La0.67Sr0.33MnO3, has been undertaken with a view to understand the influence of varying particle sizes on electrical transport properties. With a view to obtain materials with varying particle size, they were prepared by sol-gel route, sintering at four different temperatures. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD data has been analyzed by Rietveld refinement technique and it has been confirmed that the materials have rhombohedral crystal structure with R3c space group. Metal-insulator transition temperatures (Tp) were found to decrease continuously with decreasing particle size where as ferro to paramagnetic transition temperatures (Tc) are found to remain constant. The magnetoresistance (MR) values are found to increase with decreasing particle size. With a view to understand the conduction mechanism, the electrical resistivity data have been analyzed both in the ferromagnetic metallic (T < Tp) as well as high temperature paramagnetic insulating (T > Tp) regions.     

Intrinsic Nanoscale Disorder and the Physical Properties of HTSC

Recent STM studies revealed nanoscale electronic disorder on the crystal surface in many cuprates. In BSCCO, strong correlations between oxygen defect distributions on its surface and both the gap map and the coherence peak amplitude showed that the off-center distortions in the positions of oxygen atoms are responsible for most of the electronic disorder. How do these nanoscale inhomogeneities affect the bulk macroscopic physical properties (such as transport properties) of these compounds? What is the effect of a local oxygen disorder on these properties? Persistent circulating supercurrents, which are known to bypass regions of a reduced order parameter (macroscopic crystal defects), have been used to investigate superconducting properties. Our investigations identified universal (sample independent) features in these properties (such as Josephson effects, filamentary and percolative flow of the transport current, etc.) which can be attributed to the presence of a nanoscale inhomogeneity. Local oxygen redistribution, induced either by careful low temperature annealing or by room temperature aging, was found to modify substantially both the superconducting and the normal state properties.     

激光冲击强化对300M钢微观组织和力学性能的影响

采用激光冲击强化(LSP)技术在300M钢表层制备梯度纳米结构,并借助三维表面形貌仪、扫描电镜(SEM)、透射电镜(TEM)、X射线衍射仪(XRD)、纳米压痕仪及拉伸试验机对300M钢不同脉冲能量LSP处理后的微观组织演变和力学性能变化进行表征。结果表明,300M钢经LSP处理后表层形成梯度纳米结构,随着脉冲能量的增加,表层晶粒尺寸从15 nm(3J)细化至10 nm(7J)左右,晶粒出现非晶化;同时,次表层组织中形成了大量位错缠结及形变孪晶等亚结构缺陷,且随着脉冲能量的增加位错密度急剧增高,同时形变孪晶数量也随之增多。LSP后300M钢表层纳米压痕硬度得到显著提高,且随着脉冲能量的增加而增加;强度和塑性得到一定程度的改善,断口形貌由典型的韧性断裂转变为韧-脆混合型断裂。