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The singular change of the order parameter at the first order martensitic transformation (MT) temperature restricts the caloric response to a narrow temperature range. Here the MT is tuned into a sluggish strain glass transition by defect doping and a large elastocaloric effect appears in a wide temperature range. Moreover, an inverse elastocaloric effect is observed in the strain glass alloy with history of zero-field cooling and is attributed to the slow dynamics of the nanodomains in response to the external stress. This study offers a design recipe to expand the temperature range for good elastocaloric effect.

Acoustic emission (AE) of 316L stainless steel with of low Ni content shows, under tension, simultaneously three avalanche processes. One avalanche process relates to the movement of dislocations, the others to martensitic transformations and detwinning/twinning. Detwinning/twinning occurs predominantly at the early stage of the plastic deformation while martensitic transformations only become observable after large plastic deformation. All processes coincide during deformation with variable effect on AE. An excellent fingerprint for the detection of the coincidence between the several mechanisms is the observation of multivalued E ~ A2 correlations. AE signals from moving dislocations decay more slowly (~ 7×10−3s) and show long avalanche durations. In contrast, AE signals during martensitic transformations and detwinning/twinning decay rapidly (

As a new ferroelastic state, strain glass, has attracted a lot of recent attentions and, most importantly, strain glass transitions (SGTs) could underpin many phenomena that have puzzled the physics community for decades, including the quasi-linear superelasticity and Invar and Elinvar anomalies. However, there has been a lack of fundamental understanding at the atomistic level beyond the phenomenological Landau theory. In this paper, we propose a way to obtain quantitatively the continuous strain/stress fields distribution caused by doped point defects through molecular statics calculations by incorporating a Gaussian probability distribution function. By using the quantitative strain/stress fields distribution to inform phase field simulations, we reproduce quantitatively the experimentally observed critical defect concentrations separating the normal martensitic phase transition from strain glass transition at different temperatures and critical temperatures for spontaneous strain glass to martensitic transition at different defect concentrations. Based on percolation theory, we demonstrate how the strain network created by point defects with a critical concentration regulates nucleation and growth of martensitic domains, suppresses autocatalysis by strain frustration, and changes the sharp first-order martensitic transformation into a continuous SGT. A general temperature- and defect-concentration-dependent percolation criterion is formulated for accurate prediction of SGT, which could enable high throughput computations for systematic search of new strain glass systems using simply molecular static calculations.

Ferroelectric (FE) materials, especially ABO3 FE perovskite oxides, have been extensively studied for their important applications in memory devices, electronics and sensors. However, the integration of FE perovskite oxides into miniaturized memory and electronic devices has been impeded by the critical thickness limitation, as out-of-plane ferroelectricity in most FE perovskite oxides will disappear when the oxide thin film thickness is below a critical value. On the other side, CuInP2S6 (CIPS) nano-flake, a prototypical two-dimensional (2D) FE material, has recently been demonstrated to display stable out-of-plane ferroelectricity at the atomic layer thickness by experiment, which offers a new candidate for developing FE devices in the 2D nanoscale regime. Herein, after investigation of the structural and ferroelectric properties of 2D CIPS layers, especially the interactions between out-of-plane polarization and the corresponding depolarization field using first-principles calculations, we reveal that out-of-plane ferroelectricity can even persist in the CIPS monolayer, which is only 3.4 A in thickness. Moreover, in order to explore the potential application of 2D FE CIPS layers as minimized FE devices, we design an ultrathin ferroelectric tunneling junction (FTJ) composed of a graphene/CIPS monolayer/graphene vertical van der Waals (vdW) heterostructure. Our transport simulations based on the non-equilibrium Green's function formalism predict that such an ultrathin FTJ device can still exhibit the typical tunneling electroresistance (TER) effect, where tunneling current strongly depends on the direction of FE polarization. Our work not only elucidates the origin of stable out-of-plane ferroelectricity appearing in 2D CIPS layers, but also demonstrates the practical application of a CIPS based 2D FTJ as a miniaturized, multi-functional and low-power consumption memory device for modern electronics.

The 1T′ phase of transition metal dichalcogenides (TMDs) is a low symmetry charge density wave (CDW) phase, which can be viewed as a periodically distorted structure (Peierls distortion) of the high symmetry 1T phase. In this paper, using density functional theory (DFT) calculations, we report that the positive charge (hole) injection is an effective method to modulate the Peierls distortion of MoS2 1T′ for a new CDW phase and superior electromechanical properties. A new stable CDW phase is discovered at a hole doping level of 0.10 h+ per atom, named 1T′t. Hole charging and discharging can induce a reversible phase transition of MoS2 among the three phases, 1T, 1T′ and 1T′t. Such a reversible phase transition leads to superior electromechanical properties including a strain output as high as −5.8% with a small hysteresis loop, multi-step super-elasticity, and multi-shape memory effect, which are valuable in active electromechanical device designs at the nanoscale. In-depth analysis of the change of the electronic structure under hole doping was performed to understand the new CDW phase and the observed phase transition. Using charge doping to modulate the Peierls distortion in two-dimensional materials can serve as a general concept for nano-active material designs.

Electric switching of non-polar bulk crystals is shown to occur when domain walls are polar in ferroelastic materials and when rough surfaces with steps on an atomic scale promote domain switching. All domains emerging from surface nuclei possess polar domain walls. The progression of domains is then driven by the interaction of the electric field with the polarity of domain boundaries. In contrast, smooth surfaces with higher activation barriers prohibit effective domain nucleation. We demonstrate the existence of an electrically driven ferroelectric hysteresis loop in a non-ferroelectric, ferroelastic bulk material.

Strain glass is a glassy state with frozen ferroelastic/martensitic nanodomains in shape memory alloys, yet its nature remains unclear. Here, we report a glassy feature in strain glass that was thought to be only present in structural glasses. An abnormal hump is observed in strain glass around 10 K upon normalizing the specific heat by cubed temperature, similar to the boson peak in metallic glass. The simulation studies show that this boson-peak-like anomaly is caused by the phonon softening of the non-transforming matrix surrounding martensitic domains, which occurs in a transverse acoustic branch not associated with the martensitic transformation displacements. Therefore, this anomaly neither is a relic of van Hove singularity nor can be explained by other theories relying on structural disorder, while it verifies a recent theoretical model without any assumptions of disorder. This work might provide fresh insights in understanding the nature of glassy states and associated vibrational properties., Strain glass is a new glassy state characterized by frozen ferroelastic nanodomains. Here, the authors discover a low-temperature feature in the specific heat of a strain glass, which is similar to the well-known boson peak anomaly of structural glasses, but cannot be explained by existing mechanisms.

Ferroelastic twin boundaries often have properties that do not exist in bulk, such as superconductivity, polarity etc. Designing and optimizing domain walls can hence functionalize ferroelastic materials. Using atomistic simulations, we report that moving domain walls have magnetic properties even when there is no magnetic element in the material. The origin of a robust magnetic signal lies in polar vortex structures induced by moving domain walls, e.g., near the tips of needle domains and near domain wall kinks. These vortices generate displacement currents, which are the origin of magnetic moments perpendicular to the vortex plane. This phenomenon is universal for ionic crystals and holds for all ferroelastic domain boundaries containing dipolar moments. The magnetic moment depends on the speed of the domain boundary, which can reach the speed of sound under strong mechanical forcing. We estimate that the magnetic moment can reach several tens of Bohr magnetons for a collective thin film of 1000 lattice planes and movements of the vortex by the speed of sound. The predicted magnetic fields in thin slabs are much larger than those observed experimentally in SrTiO3/LaAlO3 heterostructures, which may be due to weak (accidental) forcing and slow changes of the domain patterns during their experiments. The dynamical multiferroic properties of ferroelastic domain walls may have the potential to be used to construct localized magnetic memory devices in future.

Ripples are a class of native structural defects widely existing in 2D materials. They originate from the out-of-plane flexibility of 2D materials introducing spatially evolving electronic structure and friction behavior. However, the effect of ripples on 2D ferroics has not been reported. Here a molecular dynamics study of the effect of ripples on the temperature-induced ferroic phase transition and stress-induced ferroic domain switching in ferroelastic-ferroelectric monolayer GeSe is presented. Ripples stabilize the short-range ferroic orders in the high-temperature phase with stronger ferroicity and longer lifetime, thereby increasing the transition temperature upon cooling. In addition, ripples significantly affect the domain switching upon loading, changing it from a highly correlated process into a ripple-driven localized one where ripples act as source of dynamical random stress. These results reveal the fundamental role of ripples on 2D ferroicity and provide theoretical guidance for ripple engineering of controlled phase transition and domain switching with potential applications in flexible 2D electronics.

We present a combined experimental and theoretical study of the mineral atacamite ${\mathrm{Cu}}_{2}\mathrm{Cl}(\mathrm{OH}{)}_{3}$. Density-functional theory yields a Hamiltonian describing anisotropic sawtooth chains with weak 3D connections. Experimentally, we fully characterize the antiferromagnetically ordered state. Magnetic order shows a complex evolution with the magnetic field, while, starting at 31.5 T, we observe a plateaulike magnetization at about ${M}_{\mathrm{sat}}/2$. Based on complementary theoretical approaches, we show that the latter is unrelated to the known magnetization plateau of a sawtooth chain. Instead, we provide evidence that the magnetization process in atacamite is a field-driven canting of a 3D network of weakly coupled sawtooth chains that form giant moments.

Vacancies strongly interact with twin boundaries and often change dramatically the properties of the ferroelastic material. However, the understanding of this behavior at an atomic-level is still deficient. Here we study vacancy diffusion processes across very large length- and time-scales using a combination of molecular dynamics and Monte Carlo simulations. We find that vacancies reduce their energy by residing at twin boundaries, kinks inside domain boundaries, and junctions between domain boundaries. Vacancies have the largest binding energy inside junctions and co-migrate with the motion of the junctions. For the weaker trapping inside twin boundaries, a “ghost line” may be generated because vacancies do not necessarily diffuse with moving boundaries and are left behind, leaving a trace of a previous position of the domain boundary. Needle twins act as channels for fast diffusion with almost one order of magnitude higher vacancy diffusivity than in bulk. The relative concentration of vacancies at twin boundaries (ρVa) is a function of the average vacancy concentration (CVa) with ρVa ∼ CVaα and α = 0.61, in contrast to that of immobile vacancies case with α = 0.4. The concentration of vacancies at twin boundaries is enriched ca. 5 times at low temperatures. With increasing temperature, the enrichment drops as the trapping potential at the twin boundaries decreases (thermal release). The distribution of energy-drop upon twin pattern evolution follows a power law. The exponent e increases from ∼1.44 to 2.0 when the vacancy concentration increases. The power law exponent is the same in the athermal region, while Vogel-Fulcher behavior is found at high temperatures.

Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars subjected to compress along β in the Part I of this paper. In this Part II, a series of samples at different deformation stages were systematically analyzed by transmission electron microscopy (TEM) to understand the deformation mechanism for double size effects. Two distinct phase transformations were observed to take place at different stages of plastic deformation. In the initial stage, dislocation interaction with ω precipitates and deformation-induced ω variants transformation from one to another are predominant plastic deformation mechanisms when the total strain is less than 10%. As a result, high density of ω precipitates contributes to the weak size effect and the continuous stable strain-hardening behavior in the metastable β Ti-10V-2Fe-3Al alloy. In comparison, the martensitic transformation from β to α″ is induced when the critical strain reaches to 10%. The strain-induced martensitic transformation becomes primary deformation modes, which result in higher strain-hardening exponent and stronger size effect, in the second deformation stage. These results provide a new perspective of designing micro-electromechanical materials with an excellent combination of the enhanced yield strength and stable plasticity.

The effects of ω precipitates and orthorhombic (α″) martensitic transformation on stress-strain behavior were studied in metastable β Ti-10V-2Fe-3Al alloy single-crystal in submicron scales. We successfully distinguished the size effects induced by ω precipitates and α″ phase transformation through compressing the metastable β Ti-10V-2Fe-3Al along the orientation of β, which is unfavorable for α″ phase transformation. Two individual exponential curves express well the relationship between the strengthening exponents, which were regressed using the flow stresses at different ranges of plastic strains of 0.2–13% and the pillar widths, and the corresponding plastic strain. It is worth noting that the strengthening exponent jumps from 0.256 to 0.293 at 8% plastic strain. Double size effects are uncovered in metastable β Ti-10V-2Fe-3Al alloy single-crystal micropillars. The first (ω precipitate-related) size effect has been predicted with the proposed models, and the second (martensitic transformation-induced) size effect is interpreted with the modified Liu's equation.

Nanoribbons (NRs) of two-dimensional (2D) materials have attracted intensive research interests because of exotic physical properties at edges as well as tunable properties via width control. In this paper, using density functional theory (DFT) calculations, we discover sensitive dependence of magnetic properties of 1T'-MoS2 NRs, that is, periodic variation of magnetic moments between 0.1 and 1.2 μ B, on NR width (even or odd number of MoS2 units). Our results reveal that a special edge reconstruction, which is not recognized before, stabilizes the ferromagnetic (FM) ground state. Our results also suggest that the FM state could be stable under ambient condition. This study indicates a promising means to integrate multiple magnetic units for small-scale functional devices, such as information storage and spintronics, on a single piece of MoS2 NR by designing segments with different width.

Traditional strategies for improving piezoelectric properties have focused on phase boundary engineering through complex chemical alloying and phase control. Although they have been successfully employed in bulk materials, they have not been effective in thin films due to the severe deterioration in epitaxy, which is critical to film properties. Contending with the opposing effects of alloying and epitaxy in thin films has been a long-standing issue. Herein we demonstrate a new strategy in alkali niobate epitaxial films, utilizing alkali vacancies without alloying to form nanopillars enclosed with out-of-phase boundaries that can give rise to a giant electromechanical response. Both atomically resolved polarization mapping and phase field simulations show that the boundaries are strained and charged, manifesting as head-head and tail-tail polarization bound charges. Such charged boundaries produce a giant local depolarization field, which facilitates a steady polarization rotation between the matrix and nanopillars. The local elastic strain and charge manipulation at out-of-phase boundaries, demonstrated here, can be used as an effective pathway to obtain large electromechanical response with good temperature stability in similar perovskite oxides., Phase boundary engineering through chemical alloying and phase control is a traditional approach to enhancing piezoelectric properties. Here, the authors design a strategy in alkali niobate films, utilizing alkali vacancies without alloying to form nanopillars enclosed.

Multitudinous topological configurations spawn oases of many physical properties and phenomena in condensed-matter physics. Nano-sized ferroelectric bubble domains with various polar topologies (e.g., vortices, skyrmions) achieved in ferroelectric films present great potential for valuable physical properties. However, experimentally manipulating bubble domains has remained elusive especially in the bulk form. Here, in any bulk material, we achieve self-confined bubble domains with multiple polar topologies in bulk Bi0.5Na0.5TiO3 ferroelectrics, especially skyrmions, as validated by direct Z-contrast imaging. This phenomenon is driven by the interplay of bulk, elastic and electrostatic energies of coexisting modulated phases with strong and weak spontaneous polarizations. We demonstrate reversable and tip-voltage magnitude/time-dependent donut-like domain morphology evolution towards continuously and reversibly modulated high-density nonvolatile ferroelectric memories., Experimentally manipulating bubble domains remains elusive especially in the bulk form of ferroelectrics. Here, the authors achieve self-confined bubble domains with multiple polar topologies in bulk Bi0.5Na0.5TiO3 ferroelectrics, demonstrating reversible and donut-like domain morphology evolution.

Typical ferroelectrics possess a large spontaneous polarization Ps but simultaneously a large remnant polarization Pr as well, resulting in an inferior energy storage density.A mechanism that can reduce the Pr while maintain the Ps is demanded to enhance the energy storage property of ferroelectrics.In the present study, it is shown that after acceptor doping and aging treatment, the domain switching in ferroelectrics becomes reversible, giving rise to a pinched double hysteresis loop. The pinched loop with a large Ps and a small Pr thus results in an enhanced energy storage density. The physics behind is a defect induced internal field that provides a restoring force for the domains to switch back.The idea is demonstrated through a time-dependent Ginzburg-Landau simulation as well as experimental measurements in BaTiO$_3$ based single crystal and ceramics. The mechanism is general and can be applied to various ferroelectrics, especially the environment-friendly ones., under submission to RCS JMCA

Enhancing the electromechanical response by engineering domain boundaries in multiferroics has become a highly active research field in recent years. The starting point is the discovery that ferroelastic twin walls are polar inside a nonpolar matrix. The density of such twin walls is then greatly enhanced by forming complex twin patterns. Our computer simulations show that the interaction of nanocavities with differently charged configurations with twin boundaries generates strong piezoelectricity in ferroelastic (nonferroelectric) crystals. Cavity-induced domain patterns statistically break the inversion symmetry of a sample even when the cavities themselves obey inversion symmetry with relatively weak emerging piezoelectricity $(d\ensuremath{\sim}{10}^{\ensuremath{-}3}\phantom{\rule{0.16em}{0ex}}\mathrm{pm}/\mathrm{V})$ . Stronger piezoelectricity occurs in noncentrosymmetric charged cavity arrangements with a coefficient of $d\ensuremath{\sim}{10}^{\ensuremath{-}1}\mathrm{pm}/\mathrm{V}$. Structurally, the electric field polarizes and shifts the nanocavities by the displacement of trapped surface charges. The related strain fields interact with the ferroelastic domains, which act as soft bridges between the nanocavities. This leads to a significant deformation of the entire sample and hence to enhanced piezoelectricity. Our simulation results point to new directions for designing and enhancing electromechanical nanodevices based on ferroelastic templates even when the bulk material is structurally centrosymmetric.

A group theoretical model is proposed for linear/quadratic coupling between order parameters which arise from electronic and soft-mode instabilities in doped shape memory alloys, together with coupling to symmetry breaking shear strains. This model is tested by using resonant ultrasound spectroscopy (RUS) to follow the elastic and anelastic anomalies which accompany transitions from B2 to B19, 9R, and incommensurate structures in ${\mathrm{Ti}}_{50}{\mathrm{Pd}}_{50\ensuremath{-}x}{\mathrm{Cr}}_{x}$ alloy samples ($0\ensuremath{\le}x\ensuremath{\le}12$). The pure soft-mode transition gives rise to an incommensurate structure but without any associated changes in the shear modulus, implying that coupling with shear strains is weak. By way of contrast, the observed pattern of softening ahead of and stiffening below the martensitic transition is typical of pseudoproper ferroelastic behavior and confirms that there is strong bilinear coupling of the tetragonal shear strain to the order parameter associated with irrep ${\mathrm{\ensuremath{\Gamma}}}_{3}^{+}$ of the parent space group. The second order parameter has the symmetry properties of ${\mathrm{M}}_{5}^{\ensuremath{-}}$ in TiPd or of a point along the \ensuremath{\Sigma} line of the Brillouin zone for the 9R and incommensurate structures with high Cr contents. Comparison of shear modulus data for Cr-rich samples obtained by RUS at ${10}^{5}\ensuremath{-}{10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{Hz}$ with previously reported Young's modulus data obtained for different samples by dynamical mechanical analysis at \ensuremath{\sim}0.1--10 Hz has not revealed the dispersion with frequency that would be expected for a glass transition governed by Vogel-Fulcher dynamics. The two techniques differ in the magnitude of effective applied stress, however, and differences in chemical homogeneity between samples or decomposition during high-temperature measurements might also be a factor.

Significance This work demonstrates dislocation dynamics in body-centered-cubic (BCC) high-entropy alloys (HEAs). The local composition inhomogeneity in these concentrated solutions leads to an unconventional rate-controlling mechanism that governs dislocation mobility. The activated process becomes nanoscale detrapping, of kinks on screw dislocations and of trapped segments of edge dislocations, presenting an activation barrier of similar magnitude for both dislocation types. Their sluggish mobility explains the elevated strength and strain hardening in BCC HEAs., Atomistic simulations of dislocation mobility reveal that body-centered cubic (BCC) high-entropy alloys (HEAs) are distinctly different from traditional BCC metals. HEAs are concentrated solutions in which composition fluctuation is almost inevitable. The resultant inhomogeneities, while locally promoting kink nucleation on screw dislocations, trap them against propagation with an appreciable energy barrier, replacing kink nucleation as the rate-limiting mechanism. Edge dislocations encounter a similar activated process of nanoscale segment detrapping, with comparable activation barrier. As a result, the mobility of edge dislocations, and hence their contribution to strength, becomes comparable to screw dislocations.

The microstructural evolution of Mg–2.0Gd–1.2Y–1.0Zn–0.2Zr (GWZK) alloy during homogenization and subsequent aging heat-treatment and the tensile properties of the peak-aged sample at various temperatures have been investigated. The phase transformation during homogenization, static precipitation during aging, and deformation mechanisms at 250 °C are systematically analyzed through XRD, TEM and EBSD measurements. The peak-aged GWZK alloy has good combination of strength and ductility at elevated temperatures owing to its high thermal stability by the LPSO phases distributed along the grain boundaries. The change (19 MPa) in the yield strength of peak-aged samples between RT and 250 °C is mainly related to easy gliding of dislocations on the non-basal planes due to the decrease in critical resolved shear stress of slip. Two strengthening models are applied to quantitatively describe the effect of $${\left\{11\stackrel{-}{2}0\right\}}_{\alpha }$$ precipitate plates on Orowan strengthening corresponding to RT and 250 °C. Ignoring changes in solid solution strengthening and grain boundary strengthening of the peak-aged sample caused by increasing temperature, the calculated reduction value (16.5 MPa) in macroscopic yield strength from RT (based on the basal slip mode) to 250 °C (based on the pyramidal slip mode) is in good agreement with the measured reduction value (19 MPa).

Engineering the surface morphology is an effective way to obtain specific functionalities in applications of smart surfaces. Developing two-dimensional (2D) smart materials, which can undergo morphology changes under appropriate external stimuli, is a promising route to pursue for smart-surface design. In the work presented here, using density-functional-theory calculations, we systematically study the thermodynamic stability, crystal structure, and piezoelectric properties of black phosphorene (black $\mathrm{P}$) doped with metallic atoms on one side with different concentrations. Particularly, $\mathrm{Li}$-doped black $\mathrm{P}$ (${\mathrm{P}}_{4}{\mathrm{Li}}_{2}$) has a ${d}_{31}$ value of 6.28 pm/V, which is at least 4 times larger than that of any other 2D piezoelectric material. Via finite-element-method simulations, we show a ${\mathrm{P}}_{4}{\mathrm{Li}}_{2}$-based prototype design for obtaining a surface-morphology change under a vertical electric field. The surface swelling pattern can be detected by human fingers with an appropriate design of the geometry. This demonstrates the promise of 2D piezoelectric materials for use in smart-surface applications.

We present large-scale atomic simulations of shock-induced phase transition in Zr assisted by the machine learning method. The results indicate that there exists a critical piston velocity of ${U}_{\mathrm{p}}\ensuremath{\sim}0.85\phantom{\rule{0.16em}{0ex}}\mathrm{km}/\mathrm{s}$, above which the product phase has changed from \ensuremath{\omega} to bcc. Unlike the case in Fe, the shock-induced $\mathrm{hcp}\ensuremath{\rightarrow}\mathrm{bcc}$ nucleation mechanism in hcp-Zr single-crystal shows significant dependence on crystal orientation. For shock along the $[10\overline{1}0]$ direction, the hcp phase directly transforms into bcc as expected. However, for shock compression along [0001] and $[1\overline{2}10]$ directions, the $\mathrm{hcp}\ensuremath{\rightarrow}\mathrm{bcc}$ transformation occurs in quite a different manner, i.e., the Zr single crystal transforms into a disordered intermediate that subsequently exhibits ultrafast crystallization of the bcc phase within the timescales of subnanoseconds. We associate such presence of disordered intermediate structure with the sluggishness of shear stress relaxation, which leads to an elastic unstable condition of the crystal during the first few picoseconds of uniaxial compression, and suggests that the fewer possible shear planes (related to Burgers mechanism) for [0001] and $[1\overline{2}10]$ shock loading is an underlying factor for the orientation dependence.

Our early work showed that the evolution of the twin boundary pattern exhibits an avalanche behavior upon external loading of ferroelastic materials [Salje et al., Phys. Rev. B 83, 104109 (2011)]. The distribution of “jerks” (singularities of potential energy change) was found to follow a power law distribution below a Vogel–Fulcher temperature, mainly related to the movement of kinks in domain boundaries. We use molecular dynamics simulations to study the nucleation, scattering, and annihilation of phonons that are generated by the nucleation and propagation of such kinks. The interaction and scattering of phonons are correlated over a short time period and gradually become uncorrelated before annihilation at large temperature intervals. The movement and interaction of phonons show avalanche behavior. The probability of finding energy jerks follows a power law with exponents around 2.5–3. The distribution of waiting times between jerks also follows a power law. At temperatures above the Vogel–Fulcher temperature, scattering with thermal phonons becomes predominant and no phononic avalanches were observed.Our early work showed that the evolution of the twin boundary pattern exhibits an avalanche behavior upon external loading of ferroelastic materials [Salje et al., Phys. Rev. B 83, 104109 (2011)]. The distribution of “jerks” (singularities of potential energy change) was found to follow a power law distribution below a Vogel–Fulcher temperature, mainly related to the movement of kinks in domain boundaries. We use molecular dynamics simulations to study the nucleation, scattering, and annihilation of phonons that are generated by the nucleation and propagation of such kinks. The interaction and scattering of phonons are correlated over a short time period and gradually become uncorrelated before annihilation at large temperature intervals. The movement and interaction of phonons show avalanche behavior. The probability of finding energy jerks follows a power law with exponents around 2.5–3. The distribution of waiting times between jerks also follows a power law. At temperatures above the Vogel–Fulcher tem...

Using first-principles methods, we investigate the effect of Al on the generalized stacking fault energy of face-centered cubic (fcc) CrMnFeCoNi high-entropy alloy as a function of temperature. Upon Al addition or temperature increase, the intrinsic and extrinsic stacking fault energies increase, whereas the unstable stacking fault and unstable twinning fault energies decrease monotonously. The thermodynamic expression for the intrinsic stacking fault energy in combination with the theoretical Gibbs energy difference between the hexagonal close packed (hcp) and fcc lattices allows one to determine the so-called hcp-fcc interfacial energy. The results show that the interfacial energy is small and only weakly dependent on temperature and Al content. Two parameters are adopted to measure the nano-twinning ability of the present high-entropy alloys (HEAs). Both measures indicate that the twinability decreases with increasing temperature or Al content. The present study provides systematic theoretical plasticity parameters for modeling and designing high entropy alloys with specific mechanical properties.

Alpha-Fe nanowires are seeded with twin walls (TBs) with orientations perpendicular to the wire direction. Twisting the wire generates topological defects in the twin walls, namely new twin boundaries (kinks) inside the twin walls for small twist angles, and junctions between kinks for large twist angles. During twisting the kink motion is jerky and uncorrelated at small twisting angles. The probability density function (PDF) of jerk energies follows approximately a Gaussian distribution, indicating a mild deformation mode. The kink dynamics transforms from mild to wild at larger twist angles when complex twin patterns with a high density of junctions are generated. The collective motion of kinks now shows avalanche behavior with the energy being power-law distributed. The wildness, which measures the proportion of strain energy relaxed through such avalanches, is correlated with the junction density, and controlled by the external length scale (wire diameter) as well as an internal length scale (twin boundary spacing). Good strain-stress recoverability is achieved when unloading the wire before the formation of complex twin patterns. We correlate the evolution of twin patterns with a statistical analysis of jerk dynamics, which identifies the unique mechanical properties governed by twin boundary motion in nanowires.

International audience; We studied the torsion behavior of a-Fe nanowires seeded with twin boundaries (TBs) using moleculardynamics simulations. Twisting the wire generates topological defects in the twin walls, namely kinks insidethe twin walls for small twist angles, and junctions between kinks for large twist angles. During twisting thekink motion is jerky and uncorrelated at small twist angles. The probability density function (PDF) of jerksstrength follows approximately a Gaussian distribution, indicating a mild deformation mode. The kinkdynamics transforms from mild to wild at larger twist angles when complex twin patterns with a high densityof junctions are generated. The collective motion of kinks now shows avalanche behavior with theenergy being power-law distributed. The wildness, which measures the proportion of strain energy relaxedthrough such avalanches, is correlated with the junction density, and controlled by the external length scale(wire diameter) as well as an internal length scale (twin boundary spacing). Good strain-stress recoverabilityis achieved when unloading the wire before the formation of complex twin patterns. We correlate the evolutionof twin patterns with a statistical analysis of jerk dynamics, which identifies the unique mechanicalproperties governed by twin boundary motion in nanowires.

Ferroelastic domains generate polarity near domain walls via the flexoelectric effect. Applied electric fields change the wall dipoles and generate additional dipoles in the bulk. Molecular dynamics simulations show that the thickness of domain walls changes when an electric field is applied to the sample. Fields parallel to the walls lead to expansion of the wall thickness while fields perpendicular to the wall lead to shrinking of the wall thickness. The interactions between polar domain walls expand over more than 45 unit cells, the resulting forces change the wall-wall distances if pinning effects are small. The interaction increases nonlinearly with decreasing wall-wall distances favoring equal wall distances as the consequence of energy minimization under the constraints of a constant number of domain walls. Even for small groups of three walls the sequence of walls is locally periodic: assemblies of three parallel domain walls arrange themselves so that the intermediate domain wall is located exactly in the middle between the two outer walls. The driving force is appreciable if the distance between the outer domain walls is below approximately 30 lattice units. Pairs of domain walls often form needle domains where the shaft of the needle is ca. 3 lattice units wide. The movement of needle domains under applied electric field was simulated. The advancement and retraction of needles is larger in finite samples with charge-free surfaces than under periodic boundary conditions in the bulk. The needle tip moves even more freely when the sample surface is charged.

The damping and transformation behaviors of Ti50(Pd50−xCrx) shape memory alloys with x ranging from 4.0 to 5.0 are systematically investigated. The damping capacity (Q−1) at the martensitic transformation is found to be inversely proportional to the square root of frequency, i.e., Q−1∝ω−0.5. A relaxation peak or shoulder is observed slightly below the martensitic transformation damping peak for compositions within the compositional crossover region (4.5 ⩽ x ⩽ 4.8). Furthermore, the damping capacity at the martensitic transformation is smaller within the compositional crossover region (4.5 ⩽ x ⩽ 4.8), compared with that of compositions at both sides ( x = 4.0 and x = 5.0 ). These observations can be ascribed to the hysteretic motion of interfaces between different phases near the compositional crossover region.

A facile anodic treatment for magnesium alloy was proposed in a choline chloride-ethylene glycol based deep eutectic solvent (DES). Conversion films with interconnected porous networks and jagged nanorod arrays can be formed, which depends on the applied anodic current density. Crystallographic orientation of the substrate is accordingly changed. The conversion films are mainly composed of MgCO3 which results from the interaction between the Mg alloy and the released species from the decomposition of DES. A higher anodic current density produces a better corrosion resistant conversion film. Superhydrophobic and slippery surfaces can be achieved on the as-prepared conversion films to further improve the corrosion resistance.

The mechanism of radiation-induced detwinning is different from that of deformation detwinning as the former is dominated by supersaturated radiation-induced defects while the latter is usually triggered by global stress. In situ Kr ion irradiation was performed to study the detwinning mechanism of nanotwinned Cu films with various twin thicknesses. Two types of incoherent twin boundaries (ITBs), so-called fixed ITBs and free ITBs, are characterized based on their structural features, and the difference in their migration behavior is investigated. It is observed that detwinning during radiation is attributed to the frequent migration of free ITBs, while the migration of fixed ITBs is absent. Statistics shows that the migration distance of free ITBs is thickness and dose dependent. Potential migration mechanisms are discussed.

Using ab initio alloy theory, we investigate the lattice stability of paramagnetic AlxCrMnFeCoNi (0 ≤ x ≤ 5) high-entropy alloys considering the competing body-centered cubic (bcc) and face-centered cubic (fcc) crystal structures. The theoretical lattice constants increase with increasing x, in good agreement with experimental data. Upon Al addition, the crystal structure changes from fcc to bcc with a broad two-phase field region, in line with observations. The magnetic transition temperature for the bcc structure strongly decreases with x, whereas that for the fcc structure shows weak composition dependence. Within their own stability fields, both structures are predicted to be paramagnetic at ambient conditions. Bain path calculations support that within the duplex region both phases are dynamically stable. As compared to AlxCrFeCoNi, equiatomic Mn addition is found to shrink the stability range of the fcc phase and delay the appearance of the bcc phase in terms of Al content, thus favoring the duplex region in 3d-metals based high-entropy alloys.

Recent studies of complex concentrated alloys suggest unusual dislocation core structure and motion, thanks to a collective concentration/structural inhomogeneity. Less is known whether these effects also work in extreme conditions where the solid solution effect is overwhelmed by the large driving force. Here, we investigate the dislocation structure behind a shock-wave front in bcc high-entropy alloys (HEAs) using large-scale molecular dynamics (MD) simulations. In contrast to bcc elemental metals, we find anomalous “extended“ edge dislocation structure (6 ~ 8 Burgers vector) with high stability in shock compressed TiZrNb and NiCoFeTi HEAs. The unique dislocation structures can facilitate faster dislocation motion, thus deterring the early nucleation of deformation twins. Combined with continuum elasticity theory, we show that the “extended“ dislocation structure can be attributed to the presence of local structures with low elastic stability that are imparted by the nanoscale chemical heterogeneities in HEAs. We also show how the dislocation structures are affected by the interatomic potentials.

The edge states are of particular importance to understand fundamental properties of finite two-dimensional (2D) crystals. Based on first-principles calculations, we investigated on the bare zigzag boron nitride nanoribbons (zzBNNRs) with different spin-polarized states well localized at and extended along their edges. Our calculations examined the edge stress, which is sensitively dependent on the magnetic edge states, for either B-terminated edge or N-terminated edge. Moreover, we revealed that different magnetic configurations lead to a rich spectrum of electronic behaviors at edges. Using an uniaxial tensile strain, we proposed the magnetic phase transitions and thereby obtained the metallic to half-metallic (or reverse) phase transitions at edges. It suggests zzBNNR as a promising candidate for potential applications of non-metal spintronic devices.

This study systematically investigates the interface structure of three body-centered-cubic (BCC) metallic (Fe, V and Nb) films grown on MgO(100) substrates through experiments and simulations. Orientation relationships of their interfaces with the different lattice mismatches exhibit cube-on-cube configurations. The misfit dislocations at these three interfaces exhibit different characteristics. High resolution TEM (HRTEM), combined with first principle calculations, demonstrates the O-atop match type between metal atoms and MgO substrates for the first time. The fundamental mechanism in determining the interface configuration is discussed in terms of the work of separation and delocalization of atomic charge density.

Six-fold twinned nanowires sustain very large twists under torsion. The twist is generated by mobile twin boundaries. In contrast, torsion in un-twinned nanowires is carried by dislocations and leads to failure for small torsion angles. The twisted, twinned nanowire can be totally “untwisted” by the reverse motion of twin boundaries, leading to huge pseudo-elasticity. The movement of twin boundaries and dislocations is non-smooth. It progresses by jerks and avalanches. Molecular dynamics simulations show that twin boundary jerks are power law distributed and the size exponent is compatible with mean field theory.

Graphite and other lamellar materials are used as dry lubricants for macroscale metallic sliding components and high-pressure contacts. It has been shown experimentally that monolayer graphene exhibits higher friction than multilayer graphene and graphite, and that this friction increases with continued sliding, but the mechanism behind this remains subject to debate. It has long been conjectured that the true contact area between two rough bodies controls interfacial friction. The true contact area, defined for example by the number of atoms within the range of interatomic forces, is difficult to visualize directly but characterizes the quantity of contact. However, there is emerging evidence that, for a given pair of materials, the quality of the contact can change, and that this can also strongly affect interfacial friction. Recently, it has been found that the frictional behaviour of two-dimensional materials exhibits traits unlike those of conventional bulk materials. This includes the abovementioned finding that for few-layer two-dimensional materials the static friction force gradually strengthens for a few initial atomic periods before reaching a constant value. Such transient behaviour, and the associated enhancement of steady-state friction, diminishes as the number of two-dimensional layers increases, and was observed only when the two-dimensional material was loosely adhering to a substrate. This layer-dependent transient phenomenon has not been captured by any simulations. Here, using atomistic simulations, we reproduce the experimental observations of layer-dependent friction and transient frictional strengthening on graphene. Atomic force analysis reveals that the evolution of static friction is a manifestation of the natural tendency for thinner and less-constrained graphene to re-adjust its configuration as a direct consequence of its greater flexibility. That is, the tip atoms become more strongly pinned, and show greater synchrony in their stick-slip behaviour. While the quantity of atomic-scale contacts (true contact area) evolves, the quality (in this case, the local pinning state of individual atoms and the overall commensurability) also evolves in frictional sliding on graphene. Moreover, the effects can be tuned by pre-wrinkling. The evolving contact quality is critical for explaining the time-dependent friction of configurationally flexible interfaces.

Short-range strain-ordered strain glass (STG) appears when the kinetic limitation (local stress field produced by random defects) is strong enough to suppress thermodynamic driven long-range strain-ordered martensitic transition. The point defects and precipitates have been proven to be effective to suppress the martensitic transition and trap the system in a frozen STG state. The dislocations can also introduce the random stress field down to atomic scale, however the effectiveness of which on the crossover from martensite to STG transition remains unknown. Here we report the first experimental finding of the dislocation induced STG transition in a Ti50Ni45Fe5 martensitic alloy when the sample was cold compressed by over 20% plastically. Being different from the point defects and precipitates, the dislocations do not change the chemical composition of the matrix and consequently do not vary the transition temperature too much. Our finding verifies the effectiveness of local randomness on the crossover from martensite to STG transition and suggests that the global effect of defects is not a necessary condition for the occurrence of STG. The present finding may promote the application of STG alloy by assisting on the tuning of functional temperature range in STG alloy through adding the randomness of dislocations.

Ferroelectrics are usually inflexible oxides that undergo brittle deformation. We synthesized freestanding single-crystalline ferroelectric barium titanate (BaTiO3) membranes with a damage-free lifting-off process. Our BaTiO3 membranes can undergo a ~180° folding during an in situ bending test, demonstrating a super-elasticity and ultraflexibility. We found that the origin of the super-elasticity was from the dynamic evolution of ferroelectric nanodomains. High stresses modulate the energy landscape markedly and allow the dipoles to rotate continuously between the a and c nanodomains. A continuous transition zone is formed to accommodate the variant strain and avoid high mismatch stress that usually causes fracture. The phenomenon should be possible in other ferroelectrics systems through domain engineering. The ultraflexible epitaxial ferroelectric membranes could enable many applications such as flexible sensors, memories, and electronic skins.

We identified heterogeneous Mg-Ho alloys as an ideal material to measure the most extensive acoustic emission spectra available. Mg-Ho alloys are porous and show a high density of dislocations, which slide under external tension and compression. These dislocations nucleate near numerous heterogeneities. Two mechanisms compete under external forcing in the structural collapse, namely collapsing holes and the movements of dislocations. Their respective fingerprints in acoustic emission (AE) measurements are very different and relate to their individual signal strengths. Porous collapse generates very strong AE signals while dislocation movements create more but weaker AE signals. This allows the separation of the two processes even though they almost always coincide temporarily. The porous collapse follows approximately mean-field behavior (ε = 1.4, τ’ = 1.82, α = 2.56, x = 1.93, χ = 1.95) with mean field scaling fulfilled. The exponents for dislocation movement are greater (ε = 1.92, τ’ = 2.44, α = 3.0, x = 1.7, χ = 1.42) and follows approximately the force integrated mean-field predictions. The Omori scaling is similar for both mechanisms. The Bath’s law is well fulfilled for the porous collapse but not for the dislocation movements. We suggest that such ‘complex’ mixing behavior is dominant in many other complex materials such as (multi-) ferroics, entropic alloys and porous ferroelastics, and, potentially, homogeneous materials with the simultaneous appearance of different collapse mechanisms.

Ferroelectric-field switching in $\mathrm{BaTi}{\mathrm{O}}_{3}$ generates ``Barkhausen noise'' when domain walls are displaced. We show by acoustic emission spectroscopy that electric-field switching of ${90}^{\ensuremath{\circ}}$ boundaries generates large strain fields, which emit acoustic phonons during ferroelectric hysteresis measurements. We use highly sensitive receivers (microphones) to measure the time sequences of noise in close analogy to noise patterns in ferroelastic and magnetic materials. Domain-wall interactions and jamming generate the ``crackling noise'' that follows scale invariant avalanche dynamics: the avalanche energy and amplitude probability distribution functions follow power laws with exponents $\ensuremath{\varepsilon}=1.65$ (energy) and \ensuremath{\tau}\ensuremath{'} = 2.25 (amplitudes). Aftershocks are very common and follow Omori law with probability $\ensuremath{\sim}{t}^{\ensuremath{-}p}$ where $t$ is the time elapsed after the main shock and $p$ is the Omori exponent $p\ensuremath{\sim}1$. The interevent times follow a double power-law distribution with exponents 0.9 for small times and 2.2 for the larger times. The scaling behavior is consistent with predictions of mean field theory.