Local Lattice Distortion Research Articles

Effect of silicon addition on the microstructures, mechanical properties and helium irradiation resistance of NiCoCr-based medium-entropy alloys

A series of NiCoCrSix (x = 0, 0.1, 0.2, 0.3) medium-entropy alloys were synthesized to investigate the effect of Si addition on the microstructures, mechanical properties and helium irradiation resistance of the NiCoCr-based alloys. The microstructures and the phase evolution of these alloys were examined using electron microscopy as well as the existing models. The results show that the alloys exhibit a single face-centered-cubic structure when the Si content is in the range of 0–6.25 at.%, whereas phase decomposition occurs as the Si content further increases (∼9 at.%). Comparing with NiCoCr, NiCoCrSi0.2 exhibited improved strength without sacrificing the ductility. The yield strength and ultimate tensile strength increased from 438 to 599 MPa and 921 to 1032 MPa, respectively, whilst the ductility kept as high as ∼50%. Our analysis indicates that the increased strength is mainly ascribed to the grain boundary strengthening. The large ductility is primarily due to the low stacking fault energy and shear modulus of the NiCoCrSi0.2 alloy, which makes mechanical twinning in small grains accessible and provides a steady strain-hardening rate in a wide strain regime. Besides, the enhanced compositional complexity and local lattice distortion induced by the introduction of Si can improve the helium irradiation resistance of NiCoCr, resulting in reduced average bubble size.

Structural and luminescent properties of Eu-ion-doped CaHfO3

We investigated the structural and luminescent properties of Eu3+-doped CaHfO3, which was synthesized using the solid-state reaction method. The crystal structures of the samples were analyzed by applying the Rietveld refinement technique on their X-ray diffraction patterns. Because of cation vacancies and/or interstitial oxygens, the unit-cell volumes of the samples increased, and the local lattice distortions were alleviated when the Eu3+ concentration increased. For near-ultraviolet photoexcitation, the emission spectra of our samples showed a series of narrow bands, which were assigned to the energy transitions of 5D0 → 7FJ (J = 0, 1, 2, 3, and 4) in the Eu3+ ions. The luminescence behaviors of the samples were evaluated using Judd–Ofelt intensity parameters, which provided useful information regarding the local structure around the Eu3+ ions.

Unravelling the role of Bi-substitution on vibronic ferromagnetism in La2−xBixMnCoO6 (x = 0, 0.5 and 1)

The crucial role of high Bi-content on magnetic properties of ambient pressure synthesized double perovskite La2−xBixMnCoO6 (x = 0, 0.5 and 1) has been investigated. Structural characterization was carried out using X-ray and electron diffraction analyses as well as HAADF-STEM imaging, which confirm the monoclinic structure with P21/n space group for all compositions. A comprehensive dc-magnetization and ac-susceptibility studies unravel the complex magnetic features of parent La2MnCoO6 and its associated changes upon substitution of La by Bi. Both the ferromagnetic TC and reduced magnetization with increase in Bi-content, originated from weakening of ordered Mn4+—O—Co2+ superexchange interactions. Bi-substitution also leads to suppression in magnetic disordering, which should strengthen the ferromagnetic state. These antagonist effects i.e. suppression of disordering and lowering of TC can be related to the evolution of Mn3+—O—Co3+ (e1—O—e1) vibronic ferromagnetism of smaller exchange stabilization. The local lattice distortion due to stereoactive 6s2-electron pairs of Bi3+ possibly results in cooperative lattice distortion favouring the stabilization of Jahn-Teller active intermediate-spin state Co3+ and high spin Mn3+. It is suggested that Bi-substitution favours the reaction Co2+ + Mn4+ = Co3+ + Mn3+ to the right side, which is otherwise biased to left by 0.2 eV.

Irradiation damage in (Zr0.25Ta0.25Nb0.25Ti0.25)C high-entropy carbide ceramics

This research revealed the mechanisms of irradiation damage in the novel high entropy ceramic materials. (Zr0.25Ta0.25Nb0.25Ti0.25)C high-entropy carbide ceramics (HECC) with a single-phase rock-salt structure was synthesized by spark plasma sintering, which was irradiated by 3 MeV Zr ions to 20 dpa at 25, 300, and 500 °C. X-ray diffraction analysis showed that (Zr0.25Ta0.25Nb0.25Ti0.25)C maintained a high phase stability without phase transformation after irradiation. About 0.2% lattice parameter expansion was revealed. The irradiation-induced microstructures were comprised of defect clusters with diameters of several nanometers, without void formation or radiation-induced segregation. The defect clusters were characterized by transmission electron microscopy as two types of dislocation loops, including perfect loops with Burgers vectors of b = a/2<1 1 0> and faulted Frank loops with Burgers vectors of b = a/3<1 1 1>. The growth of dislocation loops may be suppressed by the strong local lattice distortion. Nanoindentation tests showed irradiation-induced hardness increase, which was possibly caused by dislocation loops and lattice strain. Overall, the high irradiation resistance, along with other excellent physical properties makes HECC promising structural materials for advanced reactor designs.

Dynamical processes of interstitial diffusion in a two-dimensional colloidal crystal

In two-dimensional (2D) solids, point defects, i.e., vacancies and interstitials, are bound states of topological defects of edge dislocations and disclinations. They are expected to play an important role in the thermodynamics of the system. Yet very little is known about the detailed dynamical processes of these defects. Two-dimensional colloidal crystals of submicrometer microspheres provide a convenient model solid system in which the microscopic dynamics of these defects can be studied in real time using video microscopy. Here we report a study of the dynamical processes of interstitials in a 2D colloidal crystal. The diffusion constants of both mono- and diinterstitials are measured and found to be significantly larger than those of vacancies. Diinterstitials are clearly slower than monointerstitials. We found that, by plotting the accumulative positions of five- and sevenfold disclinations relative to the center-of-mass position of the defect, a sixfold symmetric pattern emerges for monointerstitials. This is indicative of an equilibrium behavior that satisfies local detailed balance that the lattice remains elastic and can be thermally excited between lattice configurations reversibly. However, for diinterstitials the sixfold symmetry is not observed in the same time window, and the local lattice distortions are too severe to recover quickly. This observation suggests a possible route to creating local melting of a lattice (similarly one can create local melting by creating divacancies). This work opens up an avenue for microscopic studies of the dynamics of melting in colloidal model systems.

Short-range chemical order and local lattice distortion in a compositionally complex alloy

This work presents an X-ray absorption spectroscopy study on a single-phase state of the Al8Cr17Co17Cu8Fe17Ni33 compositionally complex alloy, focused on the local crystal structure around each alloying element. The comparison of 1st shell bond lengths, obtained by the analysis of extended X-ray absorption fine structure (EXAFS) measured at the K-edges of each alloying element, indicates that Al8Cr17Co17Cu8Fe17Ni33 crystallizes in a distorted arrangement of an fcc lattice. A modest bond length dependence of the alloying elements with increasing atomic number is observed, with minima and maxima at Cr/Co, and Al/Cu, respectively. 1st shell bond lengths spread over ~0.03 Å; consequently, such variations cannot result in lattice distortions greater than ~0.04 Å. EXAFS results clearly indicate short-range order in the alloy: pairing of Al with Ni and Cu is favored, correlating well with a γ’ precipitate composition (Al-Ni-Cu rich) reported in previous work, while Al-Cr bonding is unfavored and no Al-Al pairs are observed. Electronic structure information was obtained through comparison between near-edge regions of alloying elements and corresponding pure metals. Intensity comparison of K-edge features agree with a charge variation of p states in Al8Cr17Co17Cu8Fe17Ni33, where Ni and Cu act as p states electron acceptors, suggesting an orbital hybridization with Al, responsible for a shrinkage in Al metallic radius in the alloy by ∼0.17 Å.

Novel Al-X alloys with improved hardness

In this study, our goal is to design solid solution strengthened aluminum alloys for manufacturing technologies that involve high cooling rates. This investigation starts with an analysis of solid solution strengthening using first principles calculations to determine elastic property changes and local lattice distortions from the introduction of different elements into a host aluminum lattice. These results, coupled with both equilibrium and non-equilibrium solubility data, leads to the selection of cerium and cobalt as the primary candidate alloying elements. Alloys of AlCe and AlCo at concentrations of 0.5, 1.0, and 3.0 at. % are then synthesized and subjected to laser glazing to produce non-equilibrium microstructures. The microstructure and solid solution characteristics are determined using a combination of scanning electron microscopy and transmission electron microscopy. Furthermore, nanoindentation is used to measure the hardness showing that both candidate systems harden significantly after glazing. In addition, Al-1.0Co at. % achieves a hardness comparable to Al6061-T6. These results conclusively show that cerium and cobalt are promising elements in the next generation aluminum alloys which make use of non-equilibrium processing conditions such as additive manufacturing.

Genesis of the periodic lattice distortions in the charge density wave state of 2H−TaSe2

Using high-energy x-ray diffraction and large-scale three-dimensonal structure modeling, we reveal the evolution of local lattice distortions in the archetypal charge density wave (CDW) system $2H\ensuremath{-}\mathrm{TaS}{\mathrm{e}}_{2}$ as it is cooled from its normal state above room temperature down to temperature where Ta atoms form a long-range ordered superstructure. In particular, we find that the structure is formed via a gradual in-plane clustering of Ta atoms exhibiting unusually short bonding distances already near room temperature, and not via a cooperative distortion of Ta atomic planes taking place at the critical ordering temperature. Our findings clarify the debated local symmetry and magnitude of the periodic lattice distortions in the CDW state of $2H\ensuremath{-}\mathrm{TaS}{\mathrm{e}}_{2}$, and emphasize the role of locally correlated lattice distortions as a precursor of CDWs in low-dimensional solids. Also, we demonstrate an efficient experimental approach to study them.

Imaging phonon dynamics with ultrafast electron microscopy: Kinematical and dynamical simulations.

Ultrafast x-ray and electron scattering techniques have proven to be useful for probing the transient elastic lattice deformations associated with photoexcited coherent acoustic phonons. Indeed, femtosecond electron imaging using an ultrafast electron microscope (UEM) has been used to directly image the influence of nanoscale structural and morphological discontinuities on the emergence, propagation, dispersion, and decay behaviors in a variety of materials. Here, we describe our progress toward the development of methods ultimately aimed at quantifying acoustic-phonon properties from real-space UEM images via conventional image simulation methods extended to the associated strain-wave lattice deformation symmetries and extents. Using a model system consisting of pristine single-crystal Ge and a single, symmetric Lamb-type guided-wave mode, we calculate the transient strain profiles excited in a wedge specimen and then apply both kinematical- and dynamical-scattering methods to simulate the resulting UEM bright-field images. While measurable contrast strengths arising from the phonon wavetrains are found for optimally oriented specimens using both approaches, incorporation of dynamical scattering effects via a multi-slice method returns better qualitative agreement with experimental observations. Contrast strengths arising solely from phonon-induced local lattice deformations are increased by nearly an order of magnitude when incorporating multiple electron scattering effects. We also explicitly demonstrate the effects of changes in global specimen orientation on the observed contrast strength, and we discuss the implications for increasing the sophistication of the model with respect to quantification of phonon properties from UEM images.

Correlation analysis of strongly fluctuating atomic volumes, charges, and stresses in body-centered cubic refractory high-entropy alloys

Local lattice distortions in a series of body-centered cubic alloys, including refractory high-entropy alloys, are investigated by means of atomic volumes, atomic charges, and atomic stresses defined by the Bader charge analysis based on first-principles calculations. Analyzing the extensive data sets, we find large distributions of these atomic properties for each element in each alloy, indicating a large impact of the varying local chemical environments. We show that these local-environment effects can be well understood and captured already by the first and the second nearest neighbor shells. Based on this insight, we employ linear regression models up to the second nearest neighbor shell to accurately predict these atomic properties. Finally, we find that the elementwise-averaged values of the atomic properties correlate linearly with the averaged valence-electron concentration of the considered alloys.

RF-MBE and MOVPE InxGa1-xN films over the entire composition range: A study on growth method dependence

Abstract This paper reports the influence of different growth methods on the InxGa1-xN ternary alloys over the entire composition range. We have successfully fabricated the single crystalline InxGa1-xN alloys without phase separation and metal indium (In) inclusion over the entire In composition region by the RF-MBE and MOVPE methods. The fluctuation of the c-axis is found smaller in the RF-MBE InxGa1-xN alloys while the fluctuation of a-axis is found smaller in the MOVPE InxGa1-xN alloys. We show that the Raman inactive B1 (High) mode has appeared in addition with the E2 (High) and A1 (LO) modes in this ternary alloy. The behavior of E2 (High) and B1 (High) modes of the InxGa1-xN alloys greatly depends on the composition and growth method. It is also found that the growth method dependence behavior of the FWHM of E2 (High) mode is similar to the a-axis fluctuation behavior in the XRC analysis. Our results spectacle that the local lattice distortion in the MOVPE grown InxGa1-xN alloys in the intermediate In composition region is very large compared with the RF-MBE samples. These findings are very important to better understand the fabrication of high-quality InxGa1-xN alloys aimed at the further development of InGaN-based high-performance optoelectronics and solar cells.

Disorder-driven glasslike thermal conductivity in colusite Cu26V2Sn6S32 investigated by Mössbauer spectroscopy and inelastic neutron scattering

The influence of structural disorder on the thermal transport in the colusite $\mathrm{C}{\mathrm{u}}_{26}{\mathrm{V}}_{2}\mathrm{S}{\mathrm{n}}_{6}{\mathrm{S}}_{32}$ has been investigated by means of low-temperature thermal conductivity and specific heat measurements (2--300 K), $^{119}\mathrm{Sn}$ M\"ossbauer spectroscopy and temperature-dependent powder inelastic neutron scattering (INS). Variations in the high-temperature synthesis conditions act as a key parameter for tuning the degree of disorder in colusite compounds. Intriguingly, we find that all synthesized samples are disordered, the degree of which varies with the synthesis conditions used. M\"ossbauer data clearly evidence that Sn atoms do not solely occupy the $6c$ site of the crystal lattice but are present on possibly both the Cu and V sites, leading to a random distribution of these three cations within the unit cell. Increasing the disorder in these materials tends to lead to a smearing out of the main features in the phonon density of states measured by INS. Although the evolution of the inelastic signal upon warming is well described by a quasiharmonic approximation, elastic properties calculations indicate large average Gr\"uneisen parameters, consistent with those determined experimentally from thermodynamic data. Increasing the level of disorder results in a decreased average Gr\"uneisen parameter suggesting that the lowered lattice thermal conductivity is not driven by enhanced anharmonicity. These results provide experimental evidence to support that the remarkable changeover in the lattice thermal conductivity from crystalline to glasslike is solely driven by enhanced disorder accompanied by local lattice distortions.

Local lattice distortions and dynamics in extremely overdoped superconducting YSr2Cu2.75Mo0.25O7.54

A common characteristic of many "overdoped" cuprates prepared with high-pressure oxygen is Tc values ≥ 50 K that often exceed that of optimally doped parent compounds, despite O stoichiometries that place the materials at the edge or outside of the conventional boundary between superconducting and normal Fermi liquid states. X-ray absorption fine-structure (XAFS) measurements at 52 K on samples of high-pressure oxygen (HPO) YSr2Cu2.75Mo0.25O7.54, Tc = 84 K show that the Mo is in the (VI) valence in an unusually undistorted octahedral geometry with predominantly Mo neighbors that is consistent with its assigned substitution for Cu in the chain sites of the structure. Perturbations of the Cu environments are minimal, although the Cu X-ray absorption near-edge structure (XANES) differs from that in other cuprates. The primary deviation from the crystal structure is therefore nanophase separation into Mo- and Cu-enriched domains. There are, however, indications that the dynamical attributes of the structure are altered relative to YBa2Cu3O7, including a shift of the Cu-apical O two-site distribution from the chain to the plane Cu sites. Another effect that would influence Tc is the possibility of multiple bands at the Fermi surface caused by the presence of the second phase and the lowering of the Fermi level.

Structural, dielectric, ferroelectric, and magnetic properties of lead-free (1-x)BiFe0.95Ga0.03Mn0.02O3−xBaTiO3 ceramics

We report the structural, dielectric, ferroelectric and magnetic properties of (1-x)BiFe0.95Ga0.03Mn0.02O3−xBaTiO3 (BFGMO−xBTO) lead-free ceramics. Structural characterizations demonstrate that the BFGMO forms single-phase solid solutions with the BTO within the doping level of x = 0.3–0.65. With increasing the content of BTO the dielectric constant and ferroelectric Curie temperature shift towards lower temperatures, exhibiting relaxor-like ferroelectric behaviors. Moreover, the remnant polarization reaches the maximum value of 2Pr ~27.67 μC/cm2 at x = 0.35 near the morphotropic phase boundary. At room temperature, remnant magnetization increases - to the maximal value of 2Mr ~0.584 emu/g at x = 0.45, which is attributed to local lattice distortion and destruction of the spiral antiferromagnetic spin structure upon the doping of BTO. The coexistence of ferroelectric and ferromagnetic performance at room temperature makes the environment-friendly BFGMO−xBTO ceramics potential candidate for multiferroic device applications.

A compromise between piezoelectricity and transparency in KNN-based ceramics: The dual functions of Li2O addition

Transparent ceramics with good electrical performance have recently drawn broad interest as promising multifunctional materials. Here, we report that a superior transmittance (T = 75 % at 2000 nm) and good piezoelectricity (d33 ∼ 150 pC/N) can be simultaneously realized in 0.93K0.5Na0.5NbO3-0.07SrZrO3 (KNN-SZ) ceramics by Li2O regulation. The effect of Li2O regulation has two parts: first, the presence of Li2O facilitates the grain growth of KNN-SZ, considering that it melts at a relatively low temperature as a proper sintering aid; second, the introduced Li+ causes local lattice distortion, resulting in the coexistence of orthogonal and tetragonal (O–T) phases. The enlarged grains reduce the light scattering by grain boundaries for a higher optical transmittance; meanwhile, large grains stand as a prerequisite for the macroscopic domain structure favoured for decent piezoelectricity, which could also be partly caused by the coexistence of O–T phases. We believe that these findings might make KNN-based ceramics a preferable candidate for optoelectronic devices.

Defect properties in a VTaCrW equiatomic high entropy alloy (HEA) with the body centered cubic (bcc) structure

We report first-principles results of the point defect properties in a V-Ta-Cr-W high-entropy alloy (HEA) with the body-centered cubic (bcc) structure. Different from the widely-investigated face-centered cubic (fcc) HEAs, the local lattice distortion is more pronounced in bcc ones, which has a strong influence on the defect properties and defect evolution under irradiation. Due to the large size of Ta, the exchange between vacancies and Ta exhibits lower energy barriers. On the other hand, interstitial dumbbells containing V and Cr possess lower formation energies. These defect energetics predicts an enrichment of V and Cr and a depletion of Ta and W in the vicinity of defect sinks. Besides, we find that interstitial dumbbells favor the [110] orientation in the HEA, instead of [111] direction in most nonmagnetic bcc metals, which helps to slow down interstitial diffusion significantly. Consequently, the distribution of migration energies for vacancies and interstitials exhibit much larger overlap regions in the bcc HEA compared to fcc HEAs, leading to the good irradiation resistance by enhancing defect recombination. Our results suggest that HEAs with the bcc structure may bear excellent irradiation tolerance due to the particular defect properties.

Unconventional Non-Uniform Local Lattice Distortion in Dilute Ti-Mo Solid Solution

The substitutional solute atom induced local lattice distortion (LLD) in dilute metal solid solution was believed to be uniform because the solute atom was supposed to occupy the high symmetry lattice site without breaking the point group symmetry of the crystal lattice. Contrary to this conventional picture, we report in this paper that substitutional Mo atom in Ti-Mo solid solution occupies an off-center position instead of the high symmetry lattice site and leads to highly non-uniform LLD, as evidenced by our first principles calculations. The physics underlying the off-center occupation and non-uniform LLD are shown to be the Jahn-Teller splitting of the degenerated d states of Mo atom. With which, the solid-solutions suffering from non-uniform LLD are predicted. The non-uniform LLD challenges the application of classical solid solution hardening model based on uniform LLD and spherical stress tensor assumption to this kind of solid solutions. This work also questions the traditional view that elementary substitutional solute atoms in metal solid solutions should not induce anelastic relaxation and internal friction.

Local Lattice Distortion and Chemical Short Range Order in High Entropy Alloys

Local Lattice Distortion and Chemical Short Range Order in High Entropy Alloys

Influence of NiFe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; nanoparticle doping on properties of single-domain YBCO bulk superconductors

NiFe&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; (NFO) nanoparticle doped YBCO bulk superconductors are fabricated by using a novel top-seed infiltration growth (TSIG) technique. The growth morphology, microstructure and superconducting properties are investigated. The results show that at low doping levels, the normal growth of YBCO single domain is not affected by the NFO doping, but at high doping levels, obvious random nucleation appears at the edge of the sample. The measurement of levitation force indicates that the maximum levitation force on the sample first increases and then decreases with the increase of the NFO doping amount, and the largest levitation force is obtained to be 33.93 N for the sample with a doping level of 0.2% (weight percent). Low-temperature magnetization measurement shows that the YBCO sample exhibits that &lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; value decreases with NFO amount increasing, and the superconducting transition width (Δ&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt;) also broadens gradually. The sample with the optimal doping (0.2% weight percent) presents an enhanced zero-field &lt;i&gt;J&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; value of 8.68 × 10&lt;sup&gt;4&lt;/sup&gt; A/cm&lt;sup&gt;2&lt;/sup&gt;, which is 31% higher than the sample without dopant. In addition, a more obvious secondary peak of 4.37 × 10&lt;sup&gt;4&lt;/sup&gt; A/cm&lt;sup&gt;2&lt;/sup&gt; at a field of 1 T is observed for the 0.2 wt.% NFO doped sample, which indicates the existence of enhanced &lt;italic/&gt;δ&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; pinning in the bulk. The SEM measurement shows that two types of particles are trapped in the Y-123 matrix for YBCO sample doped with 0.2% weight percent NFO: one is the large particle with a size mainly ranging from 0.5 μm to 2.0 μm, and the other is small nano-inclusion mainly ranging from dozens of nanometers to about several hundreds of nanometers. Such a microstructure is very similar to the microstructure of the undoped sample we reported earlier. So whether the NFO nanoparticles exist in the microstructure cannot be judged just from the morphology of the nano-inclusions. The electron probe microarea analysis (EPMA) result shows that different concentration distributions of Ni and Fe elements are observed in the sample doped with 0.2% weight percent NFO, which indicates the separation of NFO nanoparticles in the heat treatment process, and the dissolved Ni and Fe ions finally exist in the form of element substitutions in the YBCO bulk. Such element substitutions can introduce local lattice distortions and weak-superconducting regions into the superconducting matrix, which can act as effective flux pinning centers, and hence improving the properties of the samples.

Re‐entrant dipole glass‐like behavior and lattice dynamics of 0.65Bi(Mg1/2Ti1/2)O3‐0.35PbTiO3

AbstractThe 0.65Bi(Mg1/2Ti1/2)O3‐0.35PbTiO3 (0.65BMT‐0.35PT) ceramic, nearby morphotropic phase boundary (MPB), between tetragonal and rhombohedral was prepared. Dielectric permittivity dependence of temperature exhibits a re‐entrance dipole glass‐like behavior near room temperature. The behavior was described by the Vogel‐Fulcher law and the new glass model. Furthermore, a phenomenological statistical model was employed to determine the distribution of clusters dependence of temperature. The lattice dynamics of the 0.65BMT‐0.35PT ceramic was estimated by Raman spectroscopy from 298‐723 K. A disruption of ferroelectric long‐range order and an occurrence of local lattice distortion were revealed. The relaxor‐ferroelectric (r‐FE) nature of the 0.65BMT‐0.35PT was characterized by polarization‐electric field P(E). A close relation between the distribution of clusters and ferroelectric properties was established.