Elastic Softening Research Articles

Amorphous martensite in \u03b2-Ti alloys

Martensitic transformations originate from a rigidity instability, which causes a crystal to change its lattice in a displacive manner. Here, we report that the martensitic transformation on cooling in Ti–Zr–Cu–Fe alloys yields an amorphous phase instead. Metastable β-Ti partially transforms into an intragranular amorphous phase due to local lattice shear and distortion. The lenticular amorphous plates, which very much resemble α′/α″ martensite in conventional Ti alloys, have a well-defined orientation relationship with the surrounding β-Ti crystal. The present solid-state amorphization process is reversible, largely cooling rate independent and constitutes a rare case of congruent inverse melting. The observed combination of elastic softening and local lattice shear, thus, is the unifying mechanism underlying both martensitic transformations and catastrophic (inverse) melting. Not only do we reveal an alternative mechanism for solid-state amorphization but also establish an explicit experimental link between martensitic transformations and catastrophic melting.

Ferroquadrupolar Ordering due to the Quasi-degenerate Quartet in the Trigonal Chiral Structure of DyNi3Ga9

To clarify the origin of the phase transition at TN = 10 K in the trigonal chiral compound DyNi3Ga9, we performed ultrasonic measurements on single-crystalline samples. We found a significant elastic softening of the transverse modulus C66 toward TN. By analyzing the data using the strain susceptibility, the positive sign of the quadrupole–quadrupole coupling constant for C66 was obtained. These results strongly suggest that ferroquadrupolar ordering will be dominant for the phase transition at TN. Below TN, crystal symmetry will lower from the trigonal structure. Elastic softening below TN occurs in the other elastic moduli, reflecting that some kind of instability is still alive below TN. Elastic anomalies detected below TN suggest successive phase transitions.

Revealing the deformation mechanism of amorphous polyethylene subjected to cycle loading via molecular dynamics simulations.

Despite tremendous efforts being devoted to the study of the deformation behavior of polyethylene, the deformation mechanism of an amorphous polyethylene polymer under cycle shear-loading remains largely unknown. Here, we report the cycle shear deformation mechanism of an amorphous polyethylene polymer using molecular dynamics (MD) simulations. The stress–strain behaviors, including the elastic, yield, strain hardening, and strain softening regions, are qualitatively in agreement with the previous results. The values of the yield stress, Young's modulus and ultimate strength obtained from MD simulations are consistent with the previous data. The effects of the shear strain rate, temperature, and cycle shear-loading number on the stress–strain behaviors are investigated. Higher strain rate and a lower temperature result in a higher strength in the amorphous polyethylene polymer, attributed to the agglomeration of high local strains. With the increase of the cycle shear-loading number, the high strain region gradually expands from the upper and lower surface to the interior of the polyethylene polymer matrix, which provides the origin of crack initiation. The energy contributions are used in elucidating the inherent deformation mechanisms within the elastic, yielding, strain hardening, and strain softening regions, and the variation trend of energy is consistent with the stress–strain response.

Pressure-induced anomalies and structural instability in compressed β-Sb2O3.

Here, we report a high-pressure study of orthorhombic structured β-Sb2O3 (valentinite) by the combination of synchrotron in situ X-ray diffraction and first-principles theoretical calculations at pressures up to 40.5 GPa. Our results reveal that the metastable β-Sb2O3 undergoes an isostructural phase transition at high pressure, yielding a distorted β phase at 7-15 GPa through symmetry breaking and structural distortion as inferred from our XRD analyses and DFT theoretical calculations where pressure-induced elasticity softening is observed at pressures of 7-15 GPa. At pressures higher than 15 GPa, a new high-pressure monoclinic phase is discovered from the current synchrotron X-ray diffraction data. Upon further compression up to ∼33 GPa, the monoclinic Sb2O3 starts to lose its long-range order and forms an amorphous component coexisting with the monoclinic one. To further explore the structural instability and understand the origin of pressure-induced phase transitions in β-Sb2O3 upon compression, we have performed first-principles calculations to track the evolution of its phonon velocities, density of states and phonon dispersion curves under high pressure. Our results may play an important role in determining the local structures as well as their structural relationship among sesquioxides.

Stretching and compression of DNA by external forces under nanochannel confinement.

Mechanical deformation of dsDNA molecules inside square nanochannels is investigated using simulations based on a coarse-grained model of DNA. The combined action of confinement and weak external forces is explored in a variety of confinement regimes, including the transition zone relevant to nanofluidic experiments. The computed free energy and force profiles are markedly affected by the channel size. Effective elastic softening of confined DNA molecules relative to the bulk DNA is observed in the channels of intermediate widths. The extension of DNA from its bulk equilibrium length in nanofluidic devices is resolved into contributions from the passive extension due to confinement and from the active stretching induced by force. Potential implications of the very different energy costs computed for the two extension modes (extension by confinement takes much more free energy than stretching by force) for behavior of DNA in nanofluidic chips are indicated.

Measurement of O and Ti atom displacements in TiO2 during flash sintering experiments

AbstractIn‐situ flash experiments on rutile TiO2 were performed at the synchrotron at the Brookhaven National Laboratory. Pair distribution function analysis of total X‐ray scattering measurements yielded mean‐square atomic displacements of oxygen and titanium atoms during the progression of the 3 stages of flash. The displacements are measured to be far greater for oxygen atoms than for titanium atoms. These large displacements may signal an “elastic softening” of the lattice, which, recently, has been predicted as a precursor to the onset of flash.

First-principles investigation of elastic anomalies in niobium at high pressure and temperature

Niobium does not show any structure transition up to very high pressures. Nonetheless, by using density functional theory, we demonstrate in this work that it exhibits striking softening in elastic moduli C44 and C′ at a pressure from 20 to 150 GPa. A novel anomaly softening in C44 from 275 to 400 GPa is also predicted. The physics behind these two anomalies is elaborated by electronic structure calculations, which revealed that they are actually different—first one directly relates to an underlying rhombohedral distortion whereas the latter originates in an electronic topological transition. The large magnitude of the softening leads to a remarkable elastic anisotropy in both the shear and the Young's moduli of Nb. Further investigation shows that thermo-electrons have an important role in these anomalies. This effect has not been noticed before. With increased electronic temperature, it is found that all anomalies (both the elastic softening and anisotropy) in Nb are gradually diminished, effectively giving rise to a temperature-induced hardening phenomenon.

Softening of twist elasticity in the swollen smectic C liquid crystal

We investigated the swelling effect in the thermotropic smectic C liquid crystal phase by dilution of the solvent. We found that the twist elastic constant monotonically decreases with increasing swelling ratio, eventually becoming an order of magnitude smaller than its original value at the swelling limit. Our conclusion is that intercalated solvent molecules prevent direct collisions between liquid crystal molecules, which weakens the steric interaction to produce the orientation order across the smectic layers. This is the main mechanism for the softening of the twist elastic constant upon swelling.

Pressure-induced magnetic moment abnormal increase in Mn2FeAl and non-continuing decrease in Fe2MnAl via first principles

The magnetism of Fe2MnAl and Mn2FeAl compounds are studied by first principles. Evolutions of magnetic moment of Fe2MnAl display distinct variation trends under pressure, showing three different slopes at different pressure intervals, 0~100 GPa, 100~250 GPa, 250–400 GPa, respectively, and the moment collapses finally at 450 GPa. The magnetic moment of Mn2FeAl shows an increasing tendency below 40 GPa and decreases subsequently with pressure, and collapses ultimately at about 175 GPa. Such non-continuing decrease of Fe2MnAl originates from the unusual charge transfer of Fe and Mn and bond populations rearrangement of Fe-Fe and Mn-Fe, whereas the distinct moment evolution of Mn2FeAl is attributed to the complicated distributions of bond populations. The half-metallicity of the compounds can be maintained at low pressure, below about 100 GPa in Fe2MnAl and 50 GPa in Mn2FeAl. The magnetic moment collapse process didn’t induce volume and bond length anomalies in the two compounds, the unique anomaly is the elastic softening behaviour in elastic constant c44 and shear (G) and Young’s (E) moduli of Fe2MnAl at 270 GPa, where the second moment collapse occurs.

Elastic Properties of Few Nanometers Thick Polycrystalline MoS2 Membranes: A Nondestructive Study.

The performance gain-oriented nanostructurization has opened a new pathway for tuning mechanical features of solid matter vital for application and maintained performance. Simultaneously, the mechanical evaluation has been pushed down to dimensions way below 1 μm. To date, the most standard technique to study the mechanical properties of suspended 2D materials is based on nanoindentation experiments. In this work, by means of micro-Brillouin light scattering we determine the mechanical properties, that is, Young modulus and residual stress, of polycrystalline few nanometers thick MoS2 membranes in a simple, contact-less, nondestructive manner. The results show huge elastic softening compared to bulk MoS2, which is correlated with the sample morphology and the residual stress.

Understanding the Lithiation of the Sn Anode for High-Performance Li-Ion Batteries with Exploration of Novel Li-Sn Compounds at Ambient and Moderately High Pressure.

Volume expansion and elastic softening of the Sn anode on lithiation result in mechanical degradation and pulverization of Sn, affecting the overall performance of Li-Sn batteries. It can, however, be overcome with the help of void space engineering by using a LixSn phase as the prelithiated anode, where an optimal value for x is desired. Currently, Li4.25Sn is known as the most lithiated Li-Sn compound, but recent studies have shown that at high pressure, several exotic and unusual stoichiometries can be obtained that may even survive decompression from high-to-ambient pressure with improved mechanical properties. With a belief that hydrostatic pressure may help in realizing Li-richer (x > 4.25) Li-Sn compounds as well, we performed extensive calculations using an evolutionary algorithm and density functional theory to explore all stable and low-energy metastable Li-Sn compositions at pressures ranging from 1 atm to 20 GPa. This not only helped us in enriching the chemistry of a Li-Sn system, in general, but also in improving our understanding of the reaction mechanism in Li-Sn batteries, in particular, and guiding a route to improve the performance of Li-ion batteries through synthesis of Li-rich phases. Besides the experimentally known Li-Sn compounds, our study reveals the existence of five unreported stoichiometries (Li8Sn3, Li3Sn1, Li4Sn1, Li5Sn1, and Li7Sn1) and their associated crystal structures at ambient and high pressure. Although Li8Sn3 has been identified as one of the most stable Li-Sn compound in the entire pressure range (1 atm-20 GPa) with R3̅m symmetry, the Li-rich compounds like Li3Sn1-P2/m, Li4Sn1-R3̅m, Li5Sn1-C2/m, and Li7Sn1-C2/m are predicted to be metastable at ambient pressure and found to get thermodynamically stable at high pressure. Here, the discovery of Li5Sn1 and Li7Sn1 opens up the possibility to integrate them as a prelithiated anode for efficiently preventing electrochemical pulverization, as compared to the experimentally known highest lithiated compound, Li17Sn4.

Molecular dynamics simulations of thermally activated edge dislocation unpinning from voids inα-Fe

In this study, thermal unpinning of edge dislocations from voids in $\ensuremath{\alpha}$-Fe is investigated by means of molecular dynamics simulations. The activation energy as a function of shear stress and temperature is systematically determined. Simulations with a constant applied stress are compared with dynamic simulations with a constant strain rate. We found that a constant applied stress results in a temperature-dependent activation energy. The temperature dependence is attributed to the elastic softening of iron. If the stress is normalized with the softening of the specific shear modulus, the activation energy is shown to be temperature-independent. From the dynamic simulations, the activation energy as a function of critical shear stress was determined using previously developed methods. The results from the dynamic simulations are in good agreement with the constant stress simulations, after the normalization. This indicates that the computationally more efficient dynamic method can be used to obtain the activation energy as a function of stress and temperature. The obtained relation between stress, temperature, and activation energy can be used to introduce a stochastic unpinning event in larger-scale simulation methods, such as discrete dislocation dynamics.

Elastic moduli of the distorted Kagome-lattice ferromagnet Nd3Ru4Al12

The distorted kagome-lattice compound Nd3Ru4Al12 has the hexagonal structure. This compound is reported as a ferromagnet in which spins are aligned along the c-axis with the Curie temperature TC=39K. The nature of localized f-electrons is expected in Nd3Ru4Al12, and magnetic anisotropy can be attributed to a crystal electric field (CEF) effect. We performed ultrasonic measurements on a Nd3Ru4Al12 single-crystalline sample in order to investigate the phase transition at TC and the CEF effect. All longitudinal and transverse elastic moduli increase monotonically with decreasing temperature, and no clear elastic softening due to a quadrupole interaction is detected under the hexagonal CEF. This result is in contrast to an isomorphic compound Dy3Ru4Al12 with a remarkable elastic softening of the transverse modulus C44. At the ferromagnetic phase transition, the moduli show obvious elastic anomalies, suggesting characteristic couplings between a strain and a magnetic order parameter.

Elastic softening due to the quadrupole interaction and anomalous magnetic phase diagram under the magnetic field in HoRu2Al10

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Development of a finite element model to study the effect of temperature on erosion resistance of polyurethane elastomers

In this study, a numerical model based on the finite element technique was developed to study the effect of temperature on the erosion mechanism of material removal during solid particle impact on polyurethane elastomers. The hyperelastic, isotropic hardening, and Mullins damage criteria were chosen as the material model formulations to account for elastic, plastic, and stress softening behavior of the elastomer, respectively. The model inputs were determined experimentally by tensile testing and cyclic loading. In the finite element modeling approach that was developed the impact of ten erodant particles at a single location on the substrate elastomer at controlled temperatures of 22°C, 60°C, and 100°C was simulated. Erosion testing experiments were conducted to provide data for model verification. The results obtained from the finite element model showed that the final elongation at break and associated ultimate stress have the most significant influence on the erosion rate in cases where the stresses produced exceeded the failure stress during impact. This was the case for PU at 100°C in which the material had the lowest failure stress. The model also successfully simulated the mechanism of material removal by accumulating residual strains such that detachment of larger pieces of material occurred at the surface. The plastic deformation and Mullins stress softening were found as parameters affecting this type of wear. The Mullins damage caused by the impact of particles facilitated the localized detachment of small fragments from the surface upon impact of subsequent particles. Overall, although the model allowed for the study of the wear mechanisms and was capable of predicting the morphology of the eroded surfaces of the PU, the model failed to predict accurately the erosion rates, most probably due to the assumptions that were made to simplify the model.

Nanomechanical probing of thin-film dielectric elastomer transducers

Dielectric elastomer transducers (DETs) have attracted interest as generators, actuators, sensors, and even as self-sensing actuators for applications in medicine, soft robotics, and microfluidics. Their performance crucially depends on the elastic properties of the electrode-elastomer sandwich structure. The compressive displacement of a single-layer DET can be easily measured using atomic force microscopy (AFM) in the contact mode. While polymers used as dielectric elastomers are known to exhibit significant mechanical stiffening for large strains, their mechanical properties when subjected to voltages are not well understood. To examine this effect, we measured the depths of 400 nanoindentations as a function of the applied electric field using a spherical AFM probe with a radius of (522 ± 4) nm. Employing a field as low as 20 V/μm, the indentation depths increased by 42% at a load of 100 nN with respect to the field-free condition, implying an electromechanically driven elastic softening of the DET. This at-a-glance surprising experimental result agrees with related nonlinear, dynamic finite element model simulations. Furthermore, the pull-off forces rose from (23.0 ± 0.4) to (49.0 ± 0.7) nN implying a nanoindentation imprint after unloading. This embossing effect is explained by the remaining charges at the indentation site. The root-mean-square roughness of the Au electrode raised by 11% upon increasing the field from zero to 12 V/μm, demonstrating that the electrode's morphology change is an undervalued factor in the fabrication of DET structures.

Tunable thermal conductivity in mesoporous silicon by slight porosity change

We report the thermal conductivity of photoelectrochemically synthesized mesoporous silicon (MPS), with ∼20-nm diameter pores and 52%–58% porosity. The thermal conductivity of MPS samples with a thickness of a few microns was measured using the three omega (3ω) differential technique. We experimentally demonstrated that the thermal conductivity of MPS varies between 3 and 7 W/m K at room temperature and is dependent on the photoelectrochemical etching times used during the MPS synthesis, which induces a slight change in the MPS porosity. Calculations were conducted using the Boltzmann transport equation in the relaxation time approximation, with the results suggesting that the large thermal conductivity reduction in the MPSs was not entirely explained by the pore boundary scattering. Our findings indicate that elastic softening in the mesoporous structure may be responsible for the reduction in the thermal conductivity.

Ferroelectric, elastic, piezoelectric, and dielectric properties of Ba(Ti0.7Zr0.3)O3-x(Ba0.82Ca0.18)TiO3 Pb-free ceramics

We designed and synthesized a pseudo-binary Pb-free system, Ba(Ti0.7Zr0.3)O3-x(Ba0.82Ca0.18)TiO3, by combining a rhombohedral end (with only cubic to rhombohedral ferroelectric phase transition) and a tetragonal end (with only cubic to tetragonal ferroelectric phase transition). The established composition-temperature phase diagram is characterized by a tricritical point type morphotropic phase boundary (MPB), and the MPB composition has better ferroelectric, piezoelectric, and dielectric properties than the compositions deviating from MPB. Moreover, a full set of material constants (including elastic stiffness constants, elastic compliance constants, piezoelectric constants, dielectric constants, and electromechanical coupling factors) of the MPB composition are determined using a resonance method. The good piezoelectric performance of the MPB composition can be ascribed to the high dielectric constants, elastic softening, and large electromechanical coupling factor.

Negative Thermal Expansion of Ultrathin Metal Nanowires: A Computational Study.

Most materials expand upon heating because the coefficient of thermal expansion (CTE), the fundamental property of materials characterizing the mechanical response of the materials to heating, is positive. There have been some reports of materials that exhibit negative thermal expansion (NTE), but most of these have been in complex alloys, where NTE originates from the transverse vibrations of the materials. Here, we show using molecular dynamics simulations that some single crystal monatomic FCC metal nanowires can exhibit NTE along the length direction due to a novel thermomechanical coupling. We develop an analytic model for the CTE in nanowires that is a function of the surface stress, elastic modulus, and nanowire size. The model suggests that the CTE of nanowires can be reduced due to elastic softening of the materials and also due to surface stress. For the nanowires, the model predicts that the CTE reduction can lead to NTE if the nanowire Young's modulus is sufficiently reduced while the nanowire surface stress remains sufficiently large, which is in excellent agreement with the molecular dynamics simulation results. Overall, we find a "smaller is smaller" trend for the CTE of nanowires, leading to this unexpected, surface-stress-driven mechanism for NTE in nanoscale materials.

22aAU-4 超音波計測によるマルチフェロイック物質Ba_2CoGe_2O_7の四極子歪み相互作用の解明

22aAU-4 超音波計測によるマルチフェロイック物質Ba_2CoGe_2O_7の四極子歪み相互作用の解明