Effect Of Structural Relaxation Research Articles

Relaxation and crystallization of amorphous FeCrAlTiMo coatings: Effects on mechanical properties and lead-bismuth eutectic corrosion resistance

The microstructure evolution of amorphous FeCrAlTiMo coatings during annealing process, and the effects of structural relaxation and crystallization on its mechanical properties and LBE corrosion resistance are systematically investigated. With increasing annealing temperatures, three coatings with different microstructure, including relaxation and crystallization, could be obtained. The structural relaxation is beneficial to improve the LBE corrosion resistance of the coating, but it is accompanied by the loss of mechanical properties (interface bonding strength and wear resistance). The crystalline coating with multi-phase microstructure of BCC, FCC and HCP (<2％) exhibits the best LBE corrosion resistance and mechanical properties.

Temperature dependence of spatial nanoheterogeneities of shear modulus in supercooled glycerol.

The boson peak in the terahertz vibrational spectrum carries information about nano-heterogeneities in the shear modulus in glass formers. Its evolution upon heating or cooling in a supercooled liquid state may shed light on the temperature dependence of heterogeneities. For this purpose, an analysis of the light scattering spectra of supercooled glycerol in the spectral range of the boson peak and fast relaxation was carried out and the parameters of the boson peak in the temperature range 180-330K were determined. The temperature dependent frequency of the boson peak was then expressed in terms of the mean-square amplitude of the shear modulus fluctuations. This was done using the heterogeneous elasticity theory in combination with the perturbation theory on small fluctuations and Ioffe-Regel criterion for transverse vibrations in glass formers. The contribution of structural relaxation effects to phonon damping becomes significant with increasing temperature. It is shown here that structural relaxation largely determines the temperature dependence of the mean-square fluctuations of the shear modulus at high temperatures. By solving the inverse problem, the temperature dependence of shear modulus fluctuations was obtained. It shows a rapid decrease above ∼250K with a linear extrapolation going to zero at the so-called Arrhenius temperature TA = 350K. Comparison with literature data on the Landau-Placzek ratio shows that they have a similar temperature dependence at T < TA, which is explained by the appearance of nanometer scale spatial heterogeneities below TA. This is confirmed by the temperature dependence of the amplitude of the boson peak.

A Numerical Model to Predict the Relaxation Phenomena in Thermoset Polymers and Their Effects on Residual Stress during Curing, Part II: Numerical Evaluation of Residual Stress.

This article proposes a numerical routine to predict the residual stresses developing in an epoxy component during its curing. The scaling of viscoelastic properties with the temperature and the degree of conversion is modeled, adopting a mathematical formulation that considers the concurrent effects of curing and structural relaxation on the epoxy's viscoelastic relaxation time. The procedure comprises two moduli: at first, the thermal-kinetical problem is solved using the thermal module of Ansys and a homemade routine written in APDL, then the results in terms of temperature and the degree of conversion profiles are used to evaluate the viscoelastic functions, and the structural problem is solved in the mechanical module of Ansys, allowing the residual stresses calculation. The results show that the residual stresses mainly arise during cooling and scale with the logarithm of the Biot number.

Anelasticity of Mg-Mn and Mg-Gd alloys

Anelastic relaxation at sub-resonance frequency (from 0.01 to 100 Hz), temperature (from 0 to 450°C) and amplitude (ε0 from 5×10−5 to 10−3) range in binary Mg-0.8 wt%Mn (0.36 at%) and Mg-2 wt%Gd (0.31 at%) in as-extruded and annealed states was experimentally studied and analyzed, to characterize acting thermally activated and structural relaxation effects. Activation parameters for thermally activated anelastic effect and structural studies suggest that it is the grain boundary relaxation with dominating large angle boundaries, while the high-temperature background is mainly controlled by dislocations sliding. Pseudo peak at ∼290 °C (Mg-Mn) and ∼410 °C (Mg-Gd) is the result of the recrystallization process in the as-extruded samples, which indicates that the addition of Gd restricts the recrystallization of Mg compared with the addition of Mn. Analysis of amplitude dependencies of internal friction in the frames of Granato and Lücke approach suggests higher density of movable dislocations in as-extruded Mg-Mn alloy.

Origin of the Nonmonotonic Pressure Dependence of the Band Gap in the Orthorhombic Perovskite CsPbBr3.

The perovskite CsPbBr3 exhibits an unusual nonmonotonic dependence of the band gap on increasing pressure to about 2.0 GPa as compared to conventional semiconductors. Using the first-principles calculation method, we show that under pressure, isotropic volume deformation induces considerable compression of the Pb-Br bond length and thus an enhanced interaction between atomic orbitals of the antibonding valence band maximum states and the mostly nonbonding conduction band minimum states, resulting in a monotonic decrease in the band gap. On the other hand, structural relaxation tends to reduce the strain energy by decompressing the Pb-Br bond length and simultaneously compressing the Pb-Br-Pb bond angle, which increases the band gap energy. We find that the competition between the volume deformation effect and structural relaxation effect is the origin of the nonmonotonic behavior of the dependence of the band gap on pressure.

Bicrystallography-informed Frenkel–Kontorova model for interlayer dislocations in strained 2D heterostructures

In recent years, van der Waals (vdW) heterostructures and homostructures, which consist of stacks of two-dimensional (2D) materials, have risen to prominence due to their association with exotic quantum phenomena originating from correlated electronic states harbored by them. Atomistic scale relaxation effects play an extremely important role in the electronic scale quantum physics of these systems, providing means of manipulation of these materials and allowing them to be tailored for emergent technologies. We investigate such structural relaxation effects in this work using atomistic and mesoscale models, within the context of twisted bilayer graphene — a well-known heterostructure system that features moiré patterns arising from the lattices of the two graphene layers. For small twist angles, atomic relaxation effects in this system are associated with the natural emergence of interface dislocations or strain solitons, which result from the cyclic nature of the generalized stacking fault energy (GSFE), that measures the interface energy based on the relative movement of the two layers. In this work, we first demonstrate using atomistic simulations that atomic reconstruction in bilayer graphene under a large twist also results from interface dislocations, although the Burgers vectors of such dislocations are considerably smaller than those observed in small-twist systems. To reveal the translational invariance of the heterointerface responsible for the formation of such dislocations, we derive the translational symmetry of the GSFE of a 2D heterostructure using the notions of coincident site lattices (CSLs) and displacement shift complete lattices (DSCLs). The workhorse for this exercise is a recently developed Smith normal form bicrystallography framework. Next, we construct a bicrystallography-informed and frame-invariant Frenkel–Kontorova model, which can predict the formation of strain solitons in arbitrary 2D heterostructures, and apply it to study a heterostrained, large-twist bilayer graphene system. Our mesoscale model is found to produce results consistent with atomistic simulations. We anticipate that the model will be invaluable in predicting structural relaxation and for providing insights into various heterostructure systems, especially in cases where the fundamental unit cell is large and therefore, atomistic simulations are computationally expensive.

Structural Relaxation, Rejuvenation and Plasticity of Metallic Glasses: Microscopic Details from Anelastic Relaxation Spectra

The lack of periodicity and long-range order poses significant challenges in explaining and modeling the properties of metallic glasses. Conventional modeling of nonexponential relaxation with stretched exponents leads to inconsistencies and rarely offers information on microscopic properties. Instead, using quasi-static anelastic relaxation, we have obtained relaxation-time spectra over &gt;10 orders of magnitude of time for several metallic glasses. The spectra enable us to examine in microscopic detail the distribution of shear transformation zones and their properties. They reveal an atomically-quantized hierarchy of shear transformation zones, providing insights into the effect of structural relaxation and rejuvenation, the origin of plasticity and the mechanisms of the alpha and beta relaxation.

Effects of Structural Relaxation of Glass-Forming Melts on the Overall Crystallization Kinetics in Cooling and Heating.

In the theoretical treatment of crystallization, it is commonly assumed that the relaxation processes of a liquid proceed quickly as compared to crystal nucleation and growth processes. Actually, it is supposed that a liquid is always located in the metastable state corresponding to the current values of pressure and temperature. However, near and below the glass transition temperature, Tg, this condition is commonly not fulfilled. In such cases, in the treatment of crystallization, deviations in the state of the liquid from the respective metastable equilibrium state have to be accounted for when determining the kinetic coefficients governing the crystallization kinetics, the thermodynamic driving force of crystallization, and the surface tension of the aggregates of the newly evolving crystal phase including the surface tension of critical clusters considerably affecting the crystal nucleation rate. These factors may greatly influence the course of the overall crystallization process. A theoretical analysis of the resulting effects is given in the present paper by numerical solutions of the J(ohnson)-M(ehl)-A(vrami)-K(olmogorov) equation employed as the tool to model the overall crystallization kinetics and by analytical estimates of the crystallization peak temperatures in terms of the dependence on cooling and heating rates. The results are shown to be in good agreement with the experimental data. Possible extensions of the theory are anticipated and will be explored in future analysis.

Convoluted Magnetoresistance and Magnetic Reversal Processes in Ni–Fe Segmented Cylindrical Nanodots with Tunable Size and Composition for Technological Applications

AbstractDue to their unique properties, bi‐segmented systems are currently used in several technological applications such as sensing devices, high density magnetic data storage systems, spintronics and microelectromechanical components, among others. In this study, the magnetoresistance and magnetic properties of Ni–Fe bi‐segmented cylindrical nanodots in a broad range of diameters and heights are discussed. The power of the First Principles approach, as considered in the density functional theory formulation, is used to study the structural and magnetic relaxation effects, and atomistic simulations, through the Fast Monte Carlo methodology, are employed to explore the magnetoresistance and magnetic behaviors of these systems. By means of the magnetic hysteresis and magnetoresistance signals, convoluted magnetization reversal schemes are discussed. These effects take place depending on the size of the Ni and Fe components as a result of the interplay among exchange interactions and size and shape effects induced by dipolar interactions. Since size has become an experimental controllable parameter, due to the enriched phenomena, the effects discussed in these bi‐component systems are useful for the design and production of devices for technological applications with relevance beyond the observed in the more restricted single component systems.

Electronic properties of InBixP1-x alloy in the P-rich range calculated using first-principles calculations

First-principles calculations are carried out to explore the structural and electronic properties of the P-rich InBixP1-x versus bismuth fraction. It is found that the bowing coefficient for the lattice constant is minor (b=0.042 Å) due to the structural relaxation effect of the bond lengths in InBixP1-x. In the P-rich range, the InBixP1-x alloy has a direct bandgap. The result for the bond length shows that the average bond length in the InBixP1-x alloy becomes long with increasing bismuth fraction, resulting in weakening the total p−p coupling interaction and the total s−s coupling interaction. The descending of the conduction band minimum (CBM) is due to the weakened total p−p coupling interaction while the ascending of the valence band maximum (VBM) is owing to the weakened total s−s coupling interaction. Because of the descending of the CBM and the ascending of the VBM, the bandgap energy is shrunk. It is also found that the initial bismuth level is introduced below the VBM of InP due to the large strain of the In-Bi bond. In order to represent the bandgap energy in the P-rich range, the combination of a linear equation and the fusion-repelling model is proposed. It can offer an accurate description by setting V=1.39 eV2 and γ=1.61 eV. Besides, the positive formation energy shows that the Bi incorporation in InP is energetically unfavorable. This work is helpful for understanding the structural and electronic properties of the P-rich InBixP1-x alloy.

Unraveling structural relaxation induced ductile-to-brittle transition from perspective of shear band nucleation kinetics in metallic glass

Structural relaxation induced ductile to brittle transition seriously hampers the practical applications in metallic glasses. Although there have been many researches focusing on the hidden physical mechanism of ductile to brittle transition, the correlation between ductile to brittle transition and shear band nucleation kinetics is still unclear. In this work, by applying for one new studying strategy based on the nanoindentation, the evolution of the thermodynamic property, the macroscopic mechanical behavior and the corresponding shear band nucleation kinetics including the nucleation site type, the nucleation site density and the nucleation rate with annealing time at one Sub-glass transition temperature, were systematically investigated. We found that there appears one critical annealing state corresponding to the ductile to brittle transition and then the ductile-to-brittle transition induced by structural relaxation is the comprehensive result of the transition of the shear band nucleation mode, the nucleation site density and the nucleation rate. These results provide new enlightenment for the structural relaxation effect on the shear band nucleation and the macroscopic deformation behaviors, and for developing new metallic glasses with improved mechanical properties.

Magnetic Transition State Searching: Beyond the Static Ion Approximation

The effect of structural relaxations on the magnetocrystalline anisotropy energy (MAE) was investigated by using density functional theory (DFT). The theory of the impact of magnetostructural coupling on the MAE was discussed, including the effects on attempt frequency. The MAE for ferromagnetic FePt (3.45 meV/formula unit) and antiferromagnetic PtMn (0.41 meV/formula unit) were calculated within the local density approximation (LDA). The effects of the structural relaxation were calculated and found to give a &lt;0.5% reduction to the MAE for the ferromagnet and ∼20% for the antiferromagnet.

Dynamical relaxation and stress relaxation of Zr-based metallic glass

The relationship among dynamic relaxation, deformation and microstructural heterogeneity of amorphous alloy is one of the important research contents in the field of amorphous physics. In this paper, we utilize Zr&lt;sub&gt;48&lt;/sub&gt;(Cu&lt;sub&gt;5/6&lt;/sub&gt;Ag&lt;sub&gt;1/6&lt;/sub&gt;)&lt;sub&gt;44&lt;/sub&gt;Al&lt;sub&gt;8&lt;/sub&gt; bulk amorphous alloy with excellent glass forming capability and good thermal stability as a research carrier, and investigate the effects of temperature and structural relaxation on dynamic relaxation and deformation behavior through dynamic mechanical analysis and stress relaxation experiments. The results show that the dynamic relaxation spectrum of the model alloy is sensitive to temperature, showing four stages as the temperature increases, namely, iso configuration, aging, glass transition and crystallization. Based on the isothermal measurement of dynamic mechanical parameters under the glass transition temperature, the structural relaxation causes the amorphous alloy to migrate from the non-equilibrium high energy state to the low energy state, and the evolution of internal friction with aging time can be described by Kohlrausch-Williams-Watts (KWW) stretched exponential function. In addition, based on the KWW equation and activation energy spectrum, the activation of heterogeneous structure in the stress relaxation process of model alloy is analyzed, which involves the transformation from elastic deformation to nonelastic deformation. Owing to the microstructural heterogeneity and energy fluctuations in amorphous alloys, the activation of deformation units is not a single characteristic time, but follows a certain distribution. By considering that the characteristic time distribution of activation of deformation units is symmetric Gauss distribution or asymmetric Gumbel distribution on a logarithmic time scale, the heterogeneous activation process in stress relaxation response can be reconstructed.

Importance of Structural Relaxation on the Electrical Characteristics and Bias Stability of Solution-Processed ZnSnO Thin-Film Transistors.

Effect of structural relaxation (SR) on the electrical characteristics and bias stability of solution-processed zinc–tin oxide (ZTO) thin-film transistors (TFTs) were systematically investigated by controlling the annealing time of the ZTO semiconductor films. Note that SR was found to increase with increased annealing time. Due to the increased SR, the ratio of oxygen vacancies (VO) increased from 21.5% to 38.2%. According to increased VO, the mobility in the saturation region was exhibited by a sixfold increase from 0.38 to 2.41 cm2 V−1 s−1. In addition, we found that the threshold voltage negatively shifted from 3.08 to −0.95 V. Regarding the issue of bias stability, according to increased SR, positive-bias stress of the ZTO TFTs was enhanced, compared with reverse features of negative-bias stress. Our understanding is expected to provide a basic way to improve the electrical characteristics and bias stability of rare-metal-free oxide semiconductor TFTs, which have not been sufficiently studied.

Graphene Quantum Dot with Divacancy and Topological Defects: A Novel Material for Promoting Prompt and Delayed Fluorescence of Tunable Wavelengths

This work demonstrates the unique approach of introducing divacancy imperfections in topological Stone-Wales type defected graphene quantum dots for harvesting both singlet and triplet excitons, essential for fabricating fluorescent organic light-emitting diodes. Here, we first reveal that structural relaxation of these systems establishes the high-spin triplet state as the stable ground state at room temperature, thereby significantly increasing their potential in designing spintronic devices. Our extensive electron-correlated computations then demonstrate that the energetic ordering of the singlet and triplet states in these relaxed structures can trigger both prompt and delayed fluorescence of different wavelengths through various decay channels. Particularly, the position of divacancy determines the tunability range of the emission wavelengths. In addition, our results obtained from both multi-reference singles-doubles configurationinteraction (MRSDCI) and first-principles time-dependent density functional theory (TDDFT) methodologies highlight that the synergetic effects of divacancy-position,structural relaxation and spin multiplicity critically govern the nature and magnitude of shift exhibited by the most intense peak of the absorption profile, crucial for designing optoelectronic devices.

Carbon and vacancy centers in hexagonal boron nitride

Creation of defect with predetermined optical, chemical and other characteristics is a powerful tool to enhance the functionalities of materials. Herewith, we utilize density functional theory to understand the microscopic mechanisms of formation of defects in hexagonal boron nitride based on vacancies and substitutional atoms. Through in-depth analysis of the defect-induced band structure and formation energy in varying growth conditions, we uncovered a dominant role of interdefect electron paring in stabilization of defect complexes. The electron reorganization modifies the exchange component of the electronic interactions which dominates over direct Coulomb repulsion or structural relaxation effects making the combination of acceptor- and donor-type defect centers energetically favorable. Based on an analysis of a large number of defect complexes we develop a simple picture of the inheritance of electronic properties when individual defects are combined together to form more complex centers.

Machine learning based prediction of the electronic structure of quasi-one-dimensional materials under strain

We present a machine learning based model that can predict the electronic structure of quasi-one-dimensional materials while they are subjected to deformation modes such as torsion and extension/compression. The technique described here applies to important classes of materials such as nanotubes, nanoribbons, nanowires, miscellaneous chiral structures and nano-assemblies, for all of which, tuning the interplay of mechanical deformations and electronic fields is an active area of investigation in the literature. Our model incorporates global structural symmetries and atomic relaxation effects, benefits from the use of helical coordinates to specify the electronic fields, and makes use of a specialized data generation process that solves the symmetry-adapted equations of Kohn-Sham Density Functional Theory in these coordinates. Using armchair single wall carbon nanotubes as a prototypical example, we demonstrate the use of the model to predict the fields associated with the ground state electron density and the nuclear pseudocharges, when three parameters - namely, the radius of the nanotube, its axial stretch, and the twist per unit length - are specified as inputs. Other electronic properties of interest, including the ground state electronic free energy, can then be evaluated with low-overhead post-processing, typically to chemical accuracy. We also show how the nuclear coordinates can be reliably determined from the pseudocharge field using a clustering based technique. Remarkably, only about 120 data points are found to be enough to predict the three dimensional electronic fields accurately, which we ascribe to the symmetry in the problem setup, the use of low-discrepancy sequences for sampling, and presence of intrinsic low-dimensional features in the electronic fields. We comment on the interpretability of our machine learning model and discuss its possible future applications.

Effect of structural relaxation at bellow crystallization temperature on internal stress of Ni–Nb–Zr thin film amorphous alloys diaphragm for micro electromechanical systems sensors

In this paper, the effect of structural relaxation at temperatures below crystallization on internal stress of Ni–Nb–Zr thin film amorphous alloy (including thin film metallic glass: TFMG) diaphragms was investigated. We fabiricated the Ni–Nb–Zr diaphragm samples with four compositions. Before fabrication of diaphragm structure by etching, Ni–Nb–Zr thin films on Si substrate were annealed at a temperature below crystallization (473, 523, and 573 K) for structural relaxation. By performing bulge tests on annealed Ni–Nb–Zr diaphragms, we were able to determine their mechanical properties. As the result, Young’s modulus of all samples increased slightly with increasing annealing temperature because of the decrease of the free volumes during structural relaxation. Based on the results of internal stress, structural relaxation of all samples occurred below 473 K. Moreover, the effects of annealing temperature on internal stress differed by compositions. They were thought to be caused by the rate of structural relaxation.

Unraveling the microstructural heterogeneity and plasticity of Zr50Cu40Al10 bulk metallic glass by nanoindentation

Unraveling the microstructural heterogeneity is an important issue to understand the physical and mechanical properties of metallic glasses. Structural relaxation below the glass transition temperature Tg and cold rolling at ambient temperature are effective ways to tune the state within the potential energy landscape of metallic glasses, modifying the microstructural heterogeneity. With the help of nanoindentation technique, by scrutinizing the local reduced modulus and hardness of a typical Zr50Cu40Al10 metallic glasses in different energy states (from structural relaxed state to rejuvenated state), we demonstrate that the enhancement of microstructural heterogeneity plays an important role in tailoring the mechanical behavior. The internal friction coefficient is obtained by fitting the experimental load-displacement curves with finite element simulation. It is found that internal friction coefficient decreases with the increase of concentration of flow defects. The stress exponent obtained from the creep stage shows a strong dependence on the structural state of metallic glasses. Effects of structural relaxation and rejuvenation on the structural state are rationalized in terms of the relative flow defects concentration. Finally, the statistics of discrete displacement bursts during the deformation process were probed. We find that the displacement bursts behavior is sensitive to the structural energy state (microstructural heterogeneity). The displacement bursts phenomenon changes from obvious (relaxed state) to unapparent behaviors (rejuvenated state). By given a physical schematic, the correlation between the displacement bursts size and the shear band size in the local plastic zones is well established, which will lead to a deeper understanding of the rejuvenation-induced plasticity of metallic glasses.

Relaxation effect on crystal nucleation in a glass unveiled by experimental, numerical, and analytical approaches

In this work, we significantly extend our new approach for analyzing nucleation kinetics below the glass transition, Tg, which considers the structural relaxation effect. Using a 5BaO·8SiO2 (B5S8) glass, we showed a significant stepwise change in the nucleation relaxation time, τsr, with temperature in the Tg region, in contrast with the expected monotonous variation with temperature: τsr is four times shorter for the supercooled liquid than for the glass. Then, using a combination of new experimental data and theoretical (analytical and numerical) analyses, we advance an interpretation within the Classical Nucleation Theory (CNT) framework that brings together the nucleation rates in the T > Tg and T < Tg ranges. We show once more that the so-called “breakdown” of the CNT is merely the result of underestimated nucleation rates due to incomplete glass relaxation during limited experimental nucleation times, confirming our recent findings for Li2O·2SiO2 and 2Na2O·CaO·3SiO2. Finally, for the first time, we also show that due to the almost complete crystallization of the B5S8 glass, and likely of other glasses that exhibit sufficiently high nucleation and growth rates, the relaxation process cannot be finished in the low-temperature range and the ultimate steady-state nucleation regime cannot be reached. These combined results shed light on the complex nucleation behavior below Tg.