Local Atomic Rearrangement Research Articles

Fracture behavior and deformation-induced structure changes of a Ti-based metallic glass using micro-sized cantilevers

Micro-cantilever bending tests are performed to quantify the fracture toughness and reveal the deformation mechanism of a Ti-based metallic glass. Plastic deformation is localized in shear bands initiating from the roots of the notches of the cantilevers. Although the load-displacement curves show considerable ductile failure characteristics, the notch fracture toughness values that obtained from the bending tests are extremely low (∼4.7 MPa m). Size dependent fracture is thought to be observed, where the fracture toughness increases with decreasing the uncracked ligament size of the cantilevers. TEM bright field images show that the shear bands with the width of 4–7 nm present brighter contrast. The regions in vicinity of the shear bands show dark zones (ranging from 3 to 5 nm in size) surrounded by continues bright zones. Cu nanocrystals are found not only inside the shear bands, but also around them. The free volume theory together with the simulation results is applied to explain the formation of shear bands in this Ti-based metallic glass during the bending deformation. The observed Cu nanocrystals are thought to be formed as a consequence of plastic-deformation-assisted crystallization. It is suggested that local atomic rearrangements lead to shifting of Cu nanocrystals towards more stable positions and cause the formation of Cu nanocrystals in regions subjected to high shear stress field.

Infrasonic Nanocrystal Formation in Amorphous NiTi Film: Physical Mechanism, Reasons and Conditions

The physical mechanism, reasons and conditions of nanocrystal formation in an amorphous NiTi metal film, stimulated by infrasonic action, are formulated. Nanostructural elements of an amorphous medium (relaxation centers) containing disordered nanoregions with two-level systems are considered to be responsible for this process. When exposed to infrasound, a large number of two-level systems are excited, significantly contributing to inelastic deformation and the formation of nanocrystals. The physical mechanism of the nanocrystallization of metallic glass under mechanical action includes both local thermal fluctuations and the additional quantum tunneling of atoms stimulated by shear deformation. A crystalline nanocluster appears as a result of local atomic rearrangement growing increasingly exposed to infrasound. It is possibly unstable in the absence of infrasound. When the radius of the nanocluster reaches a critical value, a potential well appears, in which a conducting electron is localized to form a phason (stable nanocrystal). Estimated values of the phason’s radius and the depth of the nanometer potential well is about 0.5 nm and 1 eV, respectively. It forms a condition of stable phason formation. The occurrence of the instability of the amorphous state and following transformation to the nanostructured state is based on the accumulation of the potential energy of inelastic deformation to a critical value equal to the latent heat of the transformation of the amorphous state into the nanostructured state.

Hydration-driven stabilization and volume collapse of grain boundaries in Mg2SiO4 forsterite predicted by first-principles simulations

Abstract Grain boundaries in mantle minerals are of critical importance to geophysical and geochemical processes of the Earth’s interior. One of the fundamental issues is to understand how the water (H2O) component influences the properties of grain boundaries in silicate materials. Here, we report the results of the structure and stability of several tilt grain boundaries in Mg2SiO4 forsterite over the pressure range 0 to 15 GPa using density functional theory-based first-principles simulations. The results suggest greater energetic stability and hydration-driven volume collapse (negative excess volume) at zero pressure for the majority of hydrous grain boundaries relative to the anhydrous (dry) ones. All the hydrous grain boundaries become increasingly favorable at elevated pressures as the calculated hydration enthalpy systematically decreases with increasing pressure. The hydrous components at the interfacial regions are predominantly in the hydroxyl form and, to a lesser extent, in the molecular H2O form. Their calculated ratio ranges from 1.6 to 8.7 among the different grain boundary configurations. Our structural analysis also reveals that the hydroxyls are bound to either both Mg and Si or to Mg only. In comparison, the molecular species are bound only to Mg sites. Besides direct oxygen-hydrogen bonding, intermolecular hydrogen bonding becomes important with compression. On the basis of our results, we suggest that local atomic rearrangements caused by dissociative adsorption of water facilitate efficient compaction of the boundary interfaces, which, in turn, results in greater relative stability of hydrous grain boundaries. This means that water prefers to be incorporated within the grain boundaries over the bulk of silicate materials.

Atomistic modelling of thermal-cycling rejuvenation in metallic glasses

Cycling of a metallic glass between ambient and cryogenic temperatures can induce higher-energy states characteristic of glass formation on faster cooling. This rejuvenation, unexpected because it occurs at small macroscopic strains and well below the temperatures of thermally induced structural change, is important, for example, in improving plasticity. Molecular-dynamics simulations elucidate the mechanisms by which thermal cycling can induce relaxation (reaching lower energy) as well as rejuvenation. Thermal cycling, over tens of cycles, drives local atomic rearrangements progressively erasing the initial glass structure. This arises mainly from the heating stage in each thermal cycle, linked to the intrinsic structural heterogeneity in metallic glasses. Although, in particular, the timescales in MD simulations are shorter than in physical experiments, the present simulations reproduce many physically observed effects, suggesting that they may be useful in optimizing thermal cycling for tuning the properties of metallic glasses and glasses in general.

Insights into the nucleation, grain growth and phase transformation behaviours of sputtered metastable β-W films

Metastable phase in tungsten film is of great interests in recent years due to its giant spin Hall effects, however, little information has been known on its nucleation, growth and phase transformation. In this paper, a 900 nm-thick tungsten film with double-layer structure (α-W underlayer and β-W above it) was produced on SiO2/Si substrate by high vacuum magnetron sputtering at room temperature. The structural properties of β-W were systemically investigated by X-ray diffraction, transmission electron microscopy, thermodynamic calculation, first-principle and phase-field simulations. It is found that the β-W nucleation is energetically favoured on the SiO2 surface compared to the α-W one. As the film thickening proceeds, β-W[211] turns to be preferred direction of growth owing to the elastic strain energy minimization, which is verified by phase-field simulations. Moreover, the β→α phase transformation takes place near the film-substrate interface while the rest of the film keeps the β-W phase, leading to a double-layer structure. This localized phase transition is induced by lower Gibbs free energy of α-W phase at larger grain sizes, which can be confirmed by thermodynamic calculation. Further in-situ heating TEM analysis of the as-deposited film reveals that the β→α phase transformation is fulfilled by α/β interface propagation rather than local atomic rearrangements. Our findings offer valuable insights into the intrinsic properties of metastable phase in tungsten.

Comparison of short-range order in irradiated dysprosium titanates

The structural response of Dy2TiO5 oxide under swift heavy ion irradiation (2.2 GeV Au ions) was studied over a range of structural length scales utilizing neutron total scattering experiments. Refinement of diffraction data confirms that the long-range orthorhombic structure is susceptible to ion beam-induced amorphization with limited crystalline fraction remaining after irradiation to 8 × 1012 ions/cm2. In contrast, the local atomic arrangement, examined through pair distribution function analysis, shows only subtle changes after irradiation and is still described best by the original orthorhombic structural model. A comparison to Dy2Ti2O7 pyrochlore oxide under the same irradiation conditions reveals a different behavior: while the dysprosium titanate pyrochlore is more radiation resistant over the long-range with smaller degree of amorphization as compared to Dy2TiO5, the former involves more local atomic rearrangements, best described by a pyrochlore-to-weberite-type transformation. These results highlight the importance of short-range and medium-range order analysis for a comprehensive description of radiation behavior.

A classical molecular dynamics simulation method for the formation of “dry” gels from boro-aluminosilicate glass structures

In contact with water, glass transforms into amorphous and porous structures called gels. A simulation method based on classical molecular dynamics is proposed here to mimic “dry” gels forming from initial oxide glass structures. Six glass compositions were investigated. Two behaviours were evidenced depending on the initial glass composition, and in particular on the quantity of elements removed. If a large quantity of soluble elements (B, Na) was removed, it induced an increase in the average pore size within the gels, and the time needed to stabilise the gel structure increased because more local atomic rearrangements occurred. The gel network displayed a higher proportion of Si-Q4 at the expense of Si-Q3 and a lower average ring size compared to the glass network, irrespective of the glass composition. Surface effects were also highlighted in the dry gels, such as the presence of 3-coordinated Al and a decrease in the average angle Si-O-Si and Al-O-Al. These findings will be compared to both wet gels and experimental data in further studies, to help find the best procedure to simulate such structures.

Thermoelectric and Thermal Properties of a (GeTe)2/Sb2Te3 Interfacial Phase‐Change Memory Device

The thermoelectric and thermal properties of an interfacial phase‐change memory (iPCM) device, in which the resistive switching layer (RSL) is the (GeTe)2/Sb2Te3 superlattice, are investigated. To clarify the effects of intralayer exchange of the GeTe layer and the van der Waals (vdW) gap in the superlattice, a device model consisting of a 10 nm‐scale RSL connected to W(111) electrodes is focused. The two (GeTe)2/Sb2Te3 superlattice structures and one intercalation structure of GeTe in Sb2Te3, which are called inverted Petrov (IP), Ferro (FR), and Kooi–De Hosson (KH) structures, are examined. The calculated thermoelectric properties suggest that doping or charge trapping can enhance the difference in the thermoelectric properties and electric resistance contrast due to a phase change. Phonon thermal transport is almost ballistic for all three structural phases, whereas the KH structure shows the lowest thermal conductance and a large effect of phonon–phonon scattering. The theoretical calculations show that thermal and thermoelectric properties could be useful for identifying local structural change and atomic rearrangement.

Atomic configurations of planar defects in μ phase in Ni-based superalloys

μ topologically close-packed phase commonly forms in superalloys and its intrinsic deformation is critical for their performance. Atomic configurations of planar defects in μ phase have been investigated by aberration-corrected scanning transmission electron microscopy and geometrical structure analysis. It is found that the basal slip in μ phase is accomplished by synchroshear inside Laves triple-layers, whereas shear deformations on non-basal planes are associated with long-range diffusions or local atomic rearrangements. Distinct from that on (11¯02) pyramidal planes, displacement vectors of planar faults on (1¯101) pyramidal planes deviate from the slip plane, leading to the contraction or dilation faulted regions.

Intrinsic and extrinsic effects on the brittle-to-ductile transition in metallic glasses

The effects of cooling rate, temperature, and applied strain rate on the tensile deformation behavior of a Cu64Zr36 metallic glass (MG) are investigated using large-scale molecular dynamics simulations. An increase in the quenching rate during sample preparation, as well as an increase of the temperature or the applied strain rate, affects the activation of shear transformation zones (STZs) and, consequently, the shear-banding processes, which ultimately causes a brittle-to-ductile transition in the deformation behavior of MGs. A quantitative interpretation for the observed enhanced ductility in MGs with an increasing quenching rate is obtained by sampling the saddle points on the potential energy surface. High quenching rates lead to lower energy barriers for activation of a local atomic rearrangement (STZ) as compared to those MGs obtained at low quenching rates. Although the glassy structure does not show significant variations with increasing temperature, the kinetic energy of the atoms increases dramatically, which allows the atoms to rearrange easily; therefore, the probability of homogeneous thermal activation of STZs increases. Finally, a large number of STZs can also be activated by deformation at high strain rates when a large amount of elastic energy is stored in the glassy matrix. Consequently, a high density of STZ events and, therefore, a more complex percolation process results in a low probability for strain localization and formation of critical shear bands. Our results provide an atomistic understanding for the strain localization mechanisms in metallic glasses and shed more light on the brittle-to-ductile transition.

Atomistic Modelling of Thermal-Cycling Rejuvenation in Metallic Glasses

Cycling of a metallic glass between ambient and cryogenic temperatures can induce higher-energy states characteristic of glass formation on faster cooling. This rejuvenation, unexpected because it occurs at small macroscopic strains and well below the temperatures of thermally induced structural change, is important, for example, in improving plasticity. Molecular-dynamics simulations elucidate the mechanisms by which thermal cycling can induce relaxation (reaching lower energy) as well as rejuvenation. Thermal cycling, over tens of cycles, drives local atomic rearrangements progressively erasing the initial glass structure. This arises mainly from the heating stage in each thermal cycle, linked to the intrinsic structural heterogeneity in metallic glasses. Although, in particular, the timescales in MD simulations are shorter than in physical experiments, the present simulations reproduce many physically observed effects, suggesting that they may be useful in optimizing thermal cycling for tuning the properties of metallic glasses and glasses in general.

Heating-induced phase transitions in confined amorphous tetra-silicene

Heating-induced phase transitions of confined amorphous tetra-silicene (t-silicene) are studied via molecular dynamics (MD) simulations. Glassy models containing 6066 atoms interacted via Stillinger-Weber potential are heated up from 300 K to 5000 K at the heating rate of 1011 K s−1. We find glass-crystal-liquid phase transition indicated that amorphous t-silicene is a less stable glass. Crystallization of amorphous t-silicene occurs at around K while subsequent melting of crystallized t-silicene appears at around K. The latter is close to that of hexa-silicene. Thermodynamics and evolution of structure of models upon heating are analyzed in details. We find that crystallization of amorphous t-silicene occurs via homogeneous local rearrangements of atoms in the glassy matrix. Atomic mechanism of crystallization and subsequent melting of the obtained crystalline t-silicene is studied and the role of liquid-like atoms in this phase transition is clarified.

Dilatancy signatures of amorphous plasticity probed by X-ray synchrotron radiation

The dilatancy signatures associated with the plastic deformation of a typical Zr-based amorphous alloy are captured by using synchrotron radiation X-ray diffraction (XRD) and small angle X-ray scattering (SAXS) techniques. XRD demonstrates that the right-subpeak atoms of the second coordination shell accommodate the local dilatation, which occurs prior to the plastic yielding and during the softening-to-flow transition process. Mediated by the local dilatation, local atomic rearrangements can operate at the left-subpeak of the second shell and/or at the fourth shell with a characteristic lengthscale of about 1 nm. The obtained dilatancy signatures of amorphous plasticity are further confirmed by SAXS in terms of the nano-scale structural heterogeneity.

Recrystallization mechanism and activation energies of severely-deformed magnesium during annealing process

This study investigated the annealing process of severely-deformed magnesium through experimental testing and theoretical modeling to uncover the recrystallization mechanism and derive the corresponding nucleation and grain growth activation energies. The fine grain structure in magnesium was acquired through one type of severe plastic deformation techniques, i.e. equal channel angular pressing (ECAP). The recrystallization was achieved through annealing the ECAPed magnesium samples at various elevated temperatures (i.e., 300 °C, 350 °C, and 400 °C) and durations (i.e., 30, 40, and 60 min). The ECAP processing produced fully twinned samples that possessed heterogeneous microstructure with finer microstructure in a sample's periphery region and coarser microstructure in a sample's center region. The microstructure analysis disclosed that twin-free recrystallized grains primarily nucleated at twin boundaries through the novel integration of the bulging mechanism and the twinning-type local atomic rearrangement. The recrystallization nucleation activation energy (Qn) and Avrami exponent (n) for the ECAPed magnesium were determined to be 22.2 kJ/mol and 5, respectively. This Qn is significantly different from the literature-suggested self-diffusion activation energy and should be the twinning formation activation energy. This study also found that the grain growth activation energies varied with the annealing temperature and the corresponding values were deduced from the experimental data.

Double twin-like crystalline reorientations in Mg single crystals: Molecular dynamics simulations

Two types of double twin-like crystalline reorientations have been observed in the tension of Mg single crystals by molecular dynamics method. One can be approximately described as a typical {101¯1}-{101¯2} double twin, while the interface between the primary {101¯1} twin and the secondary {101¯2} twin shows a zigzag appearance consisting of co-existing {101¯2} twin boundary and basal/prismatic interface. The other is formed by a prior reoriented crystal with a rugged boundary, and a following {101¯1} twin, while the basal/prismatic interface dominates the boundary between the reoriented crystals. These double twin-like crystalline reorientations with misorientation angles of about 66° randomly occur under the tension along or near the [12¯10]-axis. The migration of basal/prismatic interface and twin boundary is sensitive to the relationship of the crystalline orientation and the loading direction, which causes the reoriented crystal annihilating or growing. Moreover, the migration of special rugged grain boundary between the reoriented crystal and the matrix is revealed to be fulfilled by the local atomic rearrangements, which contributes to the growth of double twin-like reoriented crystals.

The mechanism of plastic deformation in intact and irradiated GaN during indentation: A molecular dynamics study

A series of molecular dynamics simulations are carried out to study the mechanical properties of GaN. Firstly, indentation simulations are performed on the c-plane and m-plane GaN. Combined with the evolution of defects in the GaN substrate, the anisotropic mechanical responses are discussed in detail. It is found that plastic deformation in an intact c-plane GaN starts with generation and extension of planar defects. These planar defects eventually gather together and transform into a disordered region with a growing size. While for the m-plane GaN, the deformation during an indentation is dominated by the nucleation and propagation of dislocations. Secondly, the irradiation effect on the mechanical properties of GaN is investigated. Irradiated models, which subjected to different irradiation dose ranging from 5 eV/atom to 50 eV/atom, are adopted in the indentation simulations. A hardening phenomenon is found for the low-dose irradiated m-plane GaN, and the hardnesses of c-plane and m-plane GaN decrease significantly when the irradiation dose exceeds 30 eV/atom. In addition, the indentations conducted on the irradiated models induce dramatically different deformation behaviors. In particular, with the increase of irradiation dose, the deformation mechanism transforms from the plastic activities on slip systems to local rearrangements of atoms in disordered regions.

Nuclear magnetic resonance analysis and activation energy spectrum of the irreversible structural relaxation of amorphous zirconium tungstate

Zirconium tungstate undergoes a sequence of phase transitions from cubic ($\ensuremath{\alpha}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$) to orthorhombic ($\ensuremath{\gamma}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$) to amorphous ($a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$) upon increasing pressure at room temperature. The amorphous phase is known to undergo anomalous endothermic recrystallization into a high-temperature $\ensuremath{\beta}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ phase above ${600}^{\phantom{\rule{0.16em}{0ex}}\ensuremath{\circ}}\mathrm{C}$ at ambient pressure (and back to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ when brought to room temperature). The endothermic recrystallization of $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is preceded by an irreversible exothermic structural relaxation. New W-O bonds are formed upon amorphization, continuing a tendency of increasing W coordination number in going from $\ensuremath{\alpha}$ to $\ensuremath{\gamma}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$. In fact, contrarily to $\ensuremath{\alpha}\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$, in which one-quarter of the oxygen atoms are bonded only to one W (terminal oxygens), previous works found no evidence of single-bonded oxygen atoms in $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$. It thus could be argued that the irreversible character of the structural relaxation of $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is due to W-O bond breaking upon annealing of the amorphous phase. To test this hypothesis, x-ray diffraction, $^{17}\mathrm{O}$ magic-angle spinning NMR, Raman, and far-infrared analyses were performed on samples of amorphous zirconium tungstate previously annealed to increasingly higher temperatures, looking for any evidence of features that could be assigned to the presence of terminal oxygen atoms. No evidence of single-bonded oxygen was found before the onset of recrystallization. Furthermore, the kinetics of the structural relaxation of $a\ensuremath{-}{\mathrm{ZrW}}_{2}{\mathrm{O}}_{8}$ is consistent with a continuous spectrum of activation energy, spanning all the range from 1 to $2.5\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$. These findings suggest that the structural relaxation of amorphous zirconium tungstate, however irreversible, is not accompanied by W-O bond breaking, but most probably characterized by a succession of (mostly) irreversible local atomic rearrangements.

Formation and evolution of crystal and local structures in nanostructured Ln2Ti2O7 (Ln = Gd–Dy)

The lanthanide titanates Ln2Ti2O7 (Ln = lanthanide) demonstrate a very broad range of structural, chemical and physical properties. We have studied the whole process of the crystallization and local atomic structure rearrangement in Ln2Ti2O7 (Ln = Gd, Tb, Dy) samples synthesized by combination of sol-gel and coprecipitation methods with the subsequent calcination of Ln-Ti precursors. X-ray absorption spectroscopy (XAFS), X-ray powder diffraction, infrared spectroscopy, Raman spectroscopy, scanning electron microscopy with energy dispersive X-ray spectroscopy and simultaneous thermal analysis were used for the complex analysis of the structural properties. It was shown that crystallization of amorphous precursors directly resulted in the formation of nanocrystalline powders with the pyrochlore structure which is formed in process of ordering of both cationic and anionic sublattices. We have demonstrated that both XAFS and Raman spectroscopy ensure the particularly sensitive markers of changes in local electronic and crystal structure of Ln2Ti2O7 depending on the type of Ln element and the preparation procedure. It was established that splitting of the first Ln-O shell in the FT modulus of L3-Ln EXAFS spectra appears to be the reliable indicator of the ordered pyrochlore structure formation, while the changes in the pre-edge region of the K-Ti XANES spectra reflect the process of the Ln2Ti2O7 electronic structure formation.

Mechanical properties of amorphous Ge2Sb2Te5 thin layers

The working principle of a Phase Change Memory (PCM) cell exploits the repeated reversible transition between a crystalline and an amorphous phase of chalcogenide alloys typically Ge2Sb2Te5, that are characterized respectively by a high (SET) and a low (RESET) conductive state. The change in density between the two phases (6%) induces a very high compressive stress to the active amorphous region by the surrounding crystalline materials. Moreover, the physical iterative transformation between crystalline and amorphous phase transformation introduces a swelling and deswelling effect. This is one of the key failure mechanisms that are limiting the reliability of the final integration of the PCM system. Knowledge of the mechanical properties of the amorphous phase is then an important factor. Amorphous structure, i. e. its short-range order, depends on the adopted formation procedure. In this paper we analyze the mechanical characteristics of sputtered amorphous Ge2Sb2Te5 thin layers and the modification introduced by ion irradiation, a procedure adopted to simulate the amorphous state produced by melt quenching. Measurements of Young's Modulus and Hardness were performed using Ultra High-Nano Indentation on plane samples. The values of both quantities increase of about 10–20% in the 30 keV Ge+ irradiated samples. This trend is due to the reduction of homopolar wrong bonds (GeGe and TeTe) present in the as deposited film. Thermal spikes associated to the impinging ion cause a local atomic rearrangement that results in a structure similar to that of the crystalline phase. The investigation was extended to cantilevers of length in the range 10–200 μm, with a layer of 100 nm Ge2Sb2Te5 deposited on 280 nm thick SiN. The cantilever modal analysis and the out of plane deflection measurements were correlated using a Finite Element modeling, that makes use of the mechanical values measured by Ultra high Nano Indentation. After deposition the amorphous Ge2Sb2Te5 layer is subject to a compressive mechanical shrinkage, this internal stress is released after ion implantation.

Electronic Transport of (SiC)n (n = 1–4) Molecular Chains via First-Principles Calculations

The rapid development of miniaturized electronic devices has aroused intense interest in one-dimensional (1D) nanostructures research. Silicon carbide (SiC) owns abundant 1D geometrical structures, but little is known about their electronic transport properties. Hence, we conduct the systematic study of the electronic transport behavior of (SiC)n (n = 1–4) molecular linear chain, based on the density functional theory and nonequilibrium Green’s function formalism. Results reveal that the conductance of SiC chain is highly sensitive to the local atomic rearrangement, and n is also an important factor to affect their transport properties: the longer the SiC chain, the higher localization of the frontier molecular orbitals and the deeper suppression in transmission spectrum which gives the lower equilibrium conductance and the current. Current–voltage curve of SiC chains exhibit nonlinearity, implying their semiconductor characteristics. The rectifying ratios increase with n which reflect their asymmetric bu...