Anisotropic Stress Relaxation Research Articles

Significant reduction of anisotropy in stress relaxation aging and mechanical properties improvement for 2195 Al-Cu-Li alloy subjected to plastic loading

Although the plastic loading can enhance creep deformation and yield strength, the anisotropic Stress Relaxation Aging (SRA) behavior and mechanism under plastic loading remain unclear, which presents a significant challenge in accurately shaping aluminum alloy panels. In this study, the SRA behavior of 2195-T4 Al-Cu-Li alloys were thoroughly studied under initial loading stresses within the elastic (210/250 MPa) and plastic (380/420 MPa) ranges at 180 ℃ by stress relaxation and tensile tests as well as microstructure characterization. The findings reveal that compared with those under elastic loadings, in-plane anisotropy (IPA) values of the stress relaxation amount, yield strength and fracture elongation under plastic loadings are reduced by 60%-80%, 70%-90% and 72%-89%, respectively. Similarly, IPA values of precipitate size in grains and Precipitation-Free Zones (PFZ) width at grain boundaries under plastic loading decrease by 31.4% and 94.4% respectively. These results indicate plastic loading significantly weakens the anisotropic SRA behavior, owing to numerous uniformly distributed fine T1 phases and small IPA values of both T1 precipitates size and PFZ width in various loading directions. Compared with those of elastic loading-aged alloys, yield strength of plastic loading-aged alloys shows high strength-ductility because of the combined effect of closely dispersed fine T1 precipitates, narrowed PFZ and numerous sheared and rotated T1 phases at different locations during tensile process. The uniformly distributed larger Kernel Average Misorientation (KAM) and Schmidt factor values of the plastic loading-aged alloy, as well as the cross-slip generated, also help to enhance the strength and ductility of the alloy.

Vitrimer composites based on aligned graphite flakes: Accelerated and anisotropic stress relaxation and malleability

Vitrimers have attracted great interest in the past a dozen years and several ways have been developed to improve their appealing dynamic properties caused by the networks rearrangement. However, it is still a great challenge to improve their dynamic properties like stress relaxation and malleability by introducing fillers that commonly restricted chain mobility in vitrimer composites. Herein, vitrimer composites including single-layer epoxy vitrimer (SEV) composites and multi-layer epoxy vitrimer (MEV) composites were prepared by introducing aligned graphite flakes (GFs) into epoxy vitrimer matrix. Different from the suppressing effect of particle fillers in vitrimer networks, the aligned GFs improved the dynamic properties of vitrimer composites, which accelerated the stress relaxation behaviors and optimized the malleable properties. Meanwhile, the vitrimer composites exhibited unique anisotropic stress relaxation and malleability interestingly. For instance, the characteristic time (τ) of stress relaxation decreased from 1190s (reference epoxy vitrimer) to 863s (MEV composites-horizontal) and 773s (MEV composites-vertical) at 160 °C, while the critical temperature (Tv) of malleability decreased from 160 °C (reference epoxy vitrimer) to 155 °C (MEV composites-horizontal) and 143 °C (MEV composites-vertical). Moreover, the activation energy (Ea) of vitrimer composites increased and their prefactor τ0 decreased, resulting in their improved stability at service condition and promoted dynamic properties at malleable state. The accelerated and anisotropic dynamic properties of vitrimer composites was achieved by introducing aligned GFs for the first time.

Time-dependent shape of bistable unsymmetric carbon fibers-epoxy thin laminates

Carbon fiber-epoxy thin laminates, manufactured adopting a [0/90] non-symmetric lay-up, are characterized by two stable curved shapes, so being bistable. These composite thin laminates are attracting great interest for their potential application as novel morphing structures and power harvesting devices in many engineering fields. The curvature changes of rectangular bistable plates were monitored at room temperature by a laser scanner system over a time span of 10 d. These changes were ascribed to the relaxation of thermal stresses over time. Anisotropic stress relaxation behavior was then studied looking at the matrix dominated properties, such as the transverse modulus in accelerated high temperature experiments and the moisture absorption. Then, a simple viscoelastic model was proposed together with a finite element analysis of the distortion arising from laminate cooling at the end of high temperature cure process. The experimental and numerical results showed that the transverse elastic modulus and the hygroscopic effect are the main factors affecting the shape relaxation of bistable laminates.

Tensor Evaluation of Anisotropic Stress Relaxation in Mesa-Shaped SiGe Layer on Si Substrate by Electron Back-Scattering Pattern Measurement: Comparison between Raman Measurement and Finite Element Method Simulation

A strained SiGe layer will be used in next-generation transistors to improve device performance along with device scaling. However, the stress relaxation of the SiGe layer may be inevitable in nanodevices, because the SiGe layer is processed into a nanostructure. In this study, we evaluated the anisotropic stress relaxation in mesa-shaped strained SiGe layers on a Si substrate by electron backscattering pattern (EBSP) measurement. Moreover, we compared the results of EBSP measurement with those of anisotropic Raman measurement and finite element method (FEM) simulation. As a result, the anisotropic stress relaxation obtained by Raman spectroscopy was confirmed by EBSP measurement. Additionally, we obtained a good correlation between the results of EBSP measurement and FEM simulation. The σxx and σyy stresses were markedly relaxed and the σzz and σxz stresses were concentrated at the SiGe layer edges. These stresses were mostly relaxed in the distance range from the SiGe layer edges to 200 nm. Therefore, in a SiGe nanostructure with a scale of less than 200 nm, stress relaxation is inevitable. The results of EBSP and Raman measurements, and FEM simulation show a common tendency. We believe that EBSP measurement is useful for the evaluation of stress tensors and is complementary to Raman measurement.

Anisotropic viscoelasticity of polymers

A director theory of solutions and melts of flexible-chain polymers that is based on a simple law of anisotropic stress relaxation is developed. A spectral approach is used to study its structure. The simplest equation for the director that describes the dependence of the orientation of anisotropic viscoelastic fluids on shear rate is presented. The most appropriate flows for determining the material properties of polymer fluids with a single preferred direction are discussed. In addition, a new classification of anisotropic fluids is proposed.

Polarized Raman microscopy of anisotropic stress relaxation in strained-Si-on-insulator stripes

Using a high-numerical-aperture lens, we studied strain relaxation in the [110]-oriented strained-silicon-on-insulator (SSOI) stripes. A complete set of Si optical phonon Raman bands was observed. For a 50 nm thick SSOI, the symmetrical biaxial tensile stress in a relatively wide stripe is found to transform to the uniaxial [110] stress at its edge. The same uniaxial stress is observed in the narrow stripes with widths <200 nm, the value, probably, being a critical width for a complete SSOI transverse stress relaxation.

Role of Substrate Misorientation in Relaxation of 3C-SiC Layers on Silicon

We analyze in detail the evolution of curvature of 3C-SiC layers grown on vicinal silicon substrates. A common feature of (100) and (111) oriented layers is a strongly asymmetric wafer bending that may suggest an anisotropic stress relaxation within the layer. A comparative study of the curvature, lattice parameter, surface morphology and structural defects for the off-angles ranging from 0.5° to 6° is performed in order to confirm or disprove this hypothesis. We find a homogeneous, tensile in-plane lattice deformation. We also show the correlation of the orientation of the high and low curvature axes with the morphology and defect pattern of the layer.

Nucleation, growth, and deformation of one-dimensional Ge nanostructures on the Si(5 5 12)-2 × 1 surface

Evolution of the strained Ge-nanofacet, self-assembled on the Si(5 5 12)-2 × 1 template held at 530 °C, has been observed in the atomic scale using scanning tunneling microscopy. Initially, a Ge π-bonded chain is formed on the preferential adsorption site, a dimer–adatom row at the (2 2 5) subunit in a (5 5 12) unit, which results in subunit-switching between (3 3 7) and (2 2 5). By repeated subunit-switchings, the seed of sawtooth-like facet, that is, a single (1 1 3) subunit, is sorted out at the center of double (2 2 5) subunits. Then, the stable (1 1 3) facet expands until the (1 1 2) [or (3 3 5)] facet on the other side of sawtooth-like facet borders on the edge of the next (1 1 3) facet, and, in the end, the one-dimensional (1D) nanofacet with high aspect-ratio and well-defined cross-section (average width: 14 nm, and height: 0.58 nm) is completed at 1.5 monolayer (ML) of Ge coverage. At higher substrate temperature, 600 °C, the cross-section of nanofacet becomes smoother with additional faces (1 1 1) and (2 2 5), while its height becomes less uniform. When the anisotropic stress relaxation arrives at the limit with additional Ge, the (1 1 3) facet starts to have 2 × 2 structure and lose 1D symmetry. Finally, at 4 MLs of Ge coverage, it converts into a Ge hut with faces, (1 1 3), (1 1 2), (15 3 23), and (3 15 23).

Determination of In concentration in InGaAs/GaAs 0 0 1 epilayers in the early stage of anisotropic stress relaxation

A method has been developed for determination of In concentration in strained layers of InGaAs/GaAs heterostructures. Chemical composition and lattice strain were evaluated from the reciprocal space maps obtained for asymmetric reflections. It was observed that beginning of relaxation of the In 0.13Ga 0.87As/GaAs (0 0 1) system with lattice misfit Δ a/ a = 9.3 × 10 −3 and the critical thickness t cMB = 15 nm can be detected for layers of thickness exceeding t ≅ 70 nm ≅ 4.5 t cMB. The principal relaxation mechanism is due to the slipping of 60° misfit dislocations on the tilted (1 1 1) glide planes. The accuracy of indium concentration measurements was estimated to Δ x = ±0.01.

Ultraviolet-induced birefringence in hydrogen-loaded optical fiber

A precision phase-shifting approach to fabricate various phase-shifted gratings using different combinations of polarized ultraviolet (UV) light is demonstrated. In doing so, the difference between s- and p-polarized light reported by others is confirmed. However, we reveal added complexity for the role of hydrogen and deuterium in the UV-induced process. Previous arguments for the origins are systematically ruled out by reviewing existing literature. We note that the birefringence is made up of at least two components with different thermal stabilities, one consistent simply with molecular hydrogen being present in the system. Overall the birefringence, by deduction, is associated with anisotropy in hydrogen reactions within the fiber. As a result they lead, through known mechanisms of dilation in glass, to anisotropic stress relaxation that can be annealed out, with or without hydrogen remaining, at low temperatures close to 125 °C.

InAs/InP(0 0 1) quantum wire formation due to anisotropic stress relaxation: in situ stress measurements

We report in situ and in real time quantitative measurements of stress along [1 1 0] and [1 1 ̄ 0] directions during the formation of InAs/InP(0 0 1) quantum wires (QWr) and consequent stress relaxation. Results show a strong stress anisotropy due to the distortion of As–In bonds along [1 1 0] and As–As dimerization along [1 1 ̄ 0] . This anisotropy is claimed to be the origin of QWr formation instead of self-assembled quantum dots. Anisotropic stress relaxation associated to QWr formation is shown to be characteristic of heteroepitaxial systems involving different group V elements grown by MBE under group V stabilized surface (2×4 reconstruction).

Neutron diffraction measurements of residual stresses in alumina/SiC nanocomposites

The addition of 200 nm SiC particles to alumina improves its room temperature mechanical and tribological properties. A number of explanations have been proposed, all of which stem from the residual stresses caused by the expansion mismatch between alumina and SiC during cooling from processing temperatures. This paper describes neutron diffraction measurements of these stresses. The hydrostatic stresses were in accord with a simple elastic model and showed that stress relaxation ceased at ∼ 1300°C. Pressures of almost 2 GPa were present in the SiC particles. The average strains in the alumina grains also had a deviatoric component, which was tentatively interpreted in terms of anisotropic stress relaxation at high temperature. This mechanism reduces the thermal expansion anisotropy of the matrix grains. Possible explanations for nanocomposite properties are discussed in the light of the results. Although each mechanism is weak considered in isolation, the possibility for cooperative action exists.

A limiting effect in narrow permalloy lines

Large stresses can occur in the Permalloy films employed for bubble propagation and detection. In narrow lines and features anisotropic stress relaxation can lead to strong magnetic anisotropies. It is shown that these anisotropies can have a dominant influence on the magnetic behaviour of narrow lines and on the chevron sensor.