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Medium-range ordering (MRO) and structural heterogeneity in Zr-Cu-Co-Al metallic glasses (MGs) are characterized and their influence on ductility is investigated. Angular correlation analysis and digital reconstruction of dark-field images of nanodiffraction patterns acquired using four-dimensional scanning transmission electron microscopy reveal structural symmetries of the MRO regions localized at the nanometer scale, as well as their size distribution. The type and size distribution of the MRO regions change as a function of MG composition, with some MRO types clearly resembling the symmetry of known intermetallic phases. The MRO appears to become more structurally frustrated (e.g., lack of sixfold symmetry) when compositional heterogeneity increases, which may be inherently connected to the observed increase in ductility. Based on this hypothesis, mesoscale deformation simulations incorporating the experimentally acquired MRO information (types and sizes) are performed to gain insights on potential MRO-ductility relationship. Different types of MRO are assumed to have different properties and their influences on the stress-strain curve, the largest connected-free-volume, and the total number of extreme strain value sites are investigated parametrically. The simulation results reveal that the degree of heterogeneity in the MRO structures, in terms of both type and size, correlates directly with the ductility of the MGs.

In experimentally measured temperature-composition ferroelectric phase diagrams of ${\mathrm{BaTiO}}_{3}$-based binary systems, a quadruple point where cubic (C), tetragonal (T), orthorhombic (O), and rhombohedral (R) phases converge has been frequently reported in previous work. More interestingly, the quadruple points are experimentally found to behave as a critical point with large enhancement in properties. However, it has remained a fundamental question as to whether a quadruple point in a binary ferroelectric system defies the thermodynamic phase rule and whether such a point necessarily goes critical. In this study, it is demonstrated by Landau theory that a C-T-O-R quadruple point in a binary ferroelectric system can only exist in the form of a unique type of critical point at which two first-order transition lines and two second-order ones meet, and such critical quadruple points do not defy the thermodynamic phase rule. It is further shown that at such a critical C-T-O-R quadruple point, the system exhibits infinitely large piezoelectric coefficients, which agrees with the high piezoelectricity observed at the C-T-O-R quadruple point in a number of ${\mathrm{BaTiO}}_{3}$-based binary ferroelectric systems and also helps to explain the large piezoelectricity obtained at the morphotropic phase boundaries of these quadruple point based systems.

Here, we report a new phenomenon of uniform and continuous transformation of a single polarization domain into alternating nanodomains of two polarization vectors with the same magnitude but different directions at ferroelectric morphotropic phase Boundary (MPB). The transformation is fully reversible and could enhance the piezoelectric coefficient ${d}_{33}$. Further free energy calculations illustrate that such a polarization ``decomposition'' process occurs within the region on the Landau free energy curve with respect to the polarization direction where the second derivative becomes negative, which is similar to spinodal instability in phase transformations such as spinodal ordering or isostructural phase separation (e.g., spinodal decomposition). This ``polarization spinodal'' uncovers a new mechanism of polarization switching that may account for the ultrahigh ahysterestic piezoelectric strain at the MPB. This work could shed light on the development of phase transition theory and the design of novel ferroelectric memory materials.

This work investigates two characteristic phenomena associated with crystalline defects, dimensionality reduction and self-organization, during a structural phase transition. The authors find that the combination of those two phenomena lead to a unique defect-free deformation mechanism in Ni-Ti alloys, which originates from the coupling of different types of broken symmetries.

As a classical structural phase transformation with a long history in physical metallurgy, the face-centered cubic (fcc) to body-centered cubic/tetragonal (bcc/bct) martensitic transformation in steels is one of the most studied crystal structure changes induced by either temperature or stress. However, it has yet to be realized that besides the well-known Bain path that leads to {112} twinning in the bct lattice, a hidden pathway could also be activated during the transformation, which leads to the formation of slip martensite and {110} twinning in the bct lattice. We show by symmetry and crystallographic analysis that activation of the hidden pathway is driven by internal or external stress coupled with short-range migration of interstitial carbon atoms. By considering both the hidden pathway and the Bain path, we construct a transformation pathway network for the fcc to bcc/bct transformation, which allows a systematic analysis of defects generation during transformation cycling. A new mechanism for the formation of slip martensite in steels is demonstrated through the analysis of symmetry breaking along both the hidden pathway and the Bain path, by which two distinctive types of martensites, slip and twinning martensites, are placed on an equal footing from a crystallographic point of view. The pathway network provides new insights into structural transformation mechanisms and the nature of transformation induced defects in steels.

In crystalline solids, grain boundaries (GBs) play a significant role in determining a large number of material properties. The design and synthesis of special GBs has been a long-standing challenge in materials science and engineering. Here we demonstrate a mechanism to produce special GBs. Unique multigrain structures can be obtained through cyclic, diffusionless phase transformations under external fields, with all GBs being coherent special GBs. The crystallographic character of the GBs produced in this way is dictated by the broken symmetry during the phase transformations, while the topology of the GB network is determined by the geometric compatibility and self-organization of the multigrain structures. Such a mechanism not only suggests an alternative method of GB engineering, but also reveals a fundamental relationship between special GBs and phase transformations from a crystallographic point of view.

A distinguished structural characteristic of martensite in Fe-C steels is its tetragonality originating from carbon atoms occupying only one set of the three available octahedral interstitial sites in the body-centered-cubic (bcc) Fe lattice. Such a body-centered-tetragonal (bct) structure is believed to be thermodynamically stable because of elastic interactions between the interstitial carbon atoms. For such phase stability, however, there has been a lack of direct experimental evidence despite extensive studies of phase transformations in steels over one century. In this Rapid Communication, we report that the martensite formed in a high carbon Fe-8Ni-1.26C (wt%) steel at room temperature induced by applied stress/strain has actually a bcc rather than a bct crystal structure. This finding not only challenges the existing theories on the stability of bcc vs bct martensite in high carbon steels, but also provides insights into the mechanism for martensitic transformation in ferrous alloys.

Microstructural evolution during thermally induced phase separation in a polymer solution with a highly asymmetrical miscibility gap is analyzed using Flory-Huggins thermodynamics and nonlinear Cahn-Hilliard kinetics. Computer simulation results demonstrate that, in contrast to systems with symmetric miscibility gaps, interesting morphological-evolution pathways (MEPs) including cluster-to-percolation and percolation-to-cluster transitions are identified. These MEPs are rationalized according to asynchronous evolution of the two product phases. For a highly asymmetric polymer system, the initial solution composition is also found to play a critical role in determining the MEPs. According to the simulation results a map of MEPs in terms of initial solution composition and aging time of phase separation is established to guide future microstructural design in asymmetrical polymer solutions.

We report a strain glass state (B19\ensuremath{'} strain glass) in a Ni-rich TiNi shape memory alloy produced by cold rolling. As compared to previously reported strain glasses, this strain glass state has outstanding properties including quasilinear superelasticity with a large recoverable strain (\ensuremath{\sim}4%), and slim hysteresis and high strength (\ensuremath{\sim}1.0 GPa) over a wide temperature range (\ensuremath{\sim}200 K). The existence of the B19\ensuremath{'} strain glass state is confirmed by (i) frequency dispersion of storage modulus, (ii) continuous decrease of electrical resistivity, and (iii) continuous growth of B19\ensuremath{'} nanodomains upon cooling. This study proves that the effect of defect strength on the creation of a strain glass state is in parallel to the effect of cooling rate on the creation of a structural glass, e.g., any strain crystal (i.e., martensite) can be turned into a strain glass if strong enough defects could be engineered.

The monoclinic (M) phase reported recently in ferroelectric solid solutions having compositions at the morphotropic phase boundary (MPB) between the tetragonal and rhombohedral phases could be responsible for the extraordinary piezoelectric properties of these materials, but its very existence is under serious debate. In this article we show by computer simulation that using a sixth order Landau free energy polynomial with small polarization anisotropy at the MPB and taking into account the electrostatic and elastic interactions, a hierarchical structure consisting of M phase nanodomains is formed within a composition or temperature range enclosing the MPB. It is found that the M phase, even though unstable according to the Landau free energy, is stabilized by the long-range elastic and electrostatic interactions. A new phase diagram containing the M phase is established based on the simulation results.

The very highly conducting state of polypyrrole is investigated. A Drude metallic response, similar to the behavior of usual metals, is determined, based on the coincidence of dc and microwave conductivities, measurement of a huge and negative microwave dielectric constant, and the optical determination of an unusual plasma frequency in the far ir. Only a fraction of the conduction electrons participate in the Drude behavior. The real part of the dielectric function shows a second plasma frequency in the near ir, attributed to the remaining conduction electrons, confined due to a lack of homogeneous, three-dimensional order.

We report a complete phase diagram that describes the relationships among all polar states in doped ferroelectrics, including the paraelectric (polar liquid), ferroelectric (polar crystal), relaxor (polar glass), and precursory states (partially frozen nanopolar domains). We employ a model that considers a randomly distributed local polarization field associated with point defects, which breaks the symmetry of the Landau free energy with respect to polarization. In the meantime, the model also takes into account the effect of point defects on the overall stability of the ferroelectric phase. Based on this model, the phase field simulations reproduce all the polar states and important characteristics associated with ferroelectric-glass (relaxor) transition observed in experiments, including rugged free energy, wide relaxation time, nanosized ferroelectric domain structure, ``diffuse'' transition, temperature dependence of third-order dielectric susceptibility, nonergodicity, frequency dependence of dielectric loss, and domain switching.

Drastically different two-phase microstructures have been reported for alloy epitaxial films, including self-organized nanoscale concentration modulations of vertical and lateral stripes. To understand the disparity of these microstructures, we study their formation mechanisms via spinodal decomposition during film deposition with the aid of computer simulations. Based on the simulation results, a microstructure map is established that describes relationships among the morphology of self-organized two-phase microstructure, initial alloy composition, and deposition rate relative to the phase separation kinetics in the film. Depending on the deposition rate relative to the kinetics of spinodal decomposition in the film, both laterally and vertically modulated microstructures could be obtained.

The interplay between diffusional and displacive atomic movements is a key to understanding deformation mechanisms and microstructure evolution in solids. The ability to handle the diffusional time scale and the structural complexity in these problems poses a general challenge to atomistic modeling. We present here an approach called diffusive molecular dynamics (DMD), which can capture the diffusional time scale while maintaining atomic resolution, by coarse-graining over atomic vibrations and evolving a smooth site-probability representation. The model is applied to nanoindentation and sintering, where intimate coupling between diffusional creep, displacive dislocation nucleation, and grain rotation are observed.

The formation of nanostructures during two-step quench in binary polymer systems having various types of liquid miscibility gaps are investigated systematically via computer simulations using the phase field method. Coupled liquid spinodal decomposition and fluid flow processes are considered by solving simultaneously the Cahn-Hilliard and Navier-Stokes equations. Various interesting phenomena and morphological patterns are predicted. It is found that the primary microstructures developed at the first quench and isothermal holding temperature greatly affect the secondary microstructures developed during the second quench and isothermal holding. Depending on the morphology and scale of the primary microstructure, either multicore and multishell or unicore and unishell structures are predicted. The breakup of annuluses in a core-shell structure in two dimensions is analyzed. The effects of viscosity on the formation of core-shell structure and on the growth and coarsening behaviors of bimodal droplets produced by the two-step quench in systems are also investigated.

The kinetics of multiparticle coarsening controlled by a long-range elastic interaction is considered. Without any a priori assumptions concerning the morphology of coarsening particles, it is shown that translational motion and reverse coarsening are two important mechanisms responsible for the morphological evolution of initially randomly distributed new phase particles into a regular array in an elastically anisotropic crystal. The general characteristic of the strain-induced coarsening in a multiparticle system is determined by the tendency to form a pseudoperiodical precipitate macrolattice which minimizes the elastic strain energy

We investigate the effect of anisotropy in grain boundary mobility on the dynamics, morphology, and topology of grain growth, using both analytical and numerical calculations on a generalized anisotropic phase field model. The dependence of the grain boundary mobility on both inclination and misorientation is included. In contrast to the isotropic case, where a single grain in a polycrystalline system grows linearly with time, it is found that the growth rate of a single n-sided grain in the anisotropic case is time dependent. The growth rate of the average area is also time dependent, except for the two limiting cases of textured and randomly oriented grain structures. However, strong grain shape anisotropy develops in the textured case, indicating that the grains grow in a non-self-similar manner. In the intermediate case, the deviation from a growth exponent of unity is of order 10%, while the size and edge distributions are similar to those of the isotropic system. Our results indicate that statistical self-similarity may not be required for linear growth kinetics and time-invariant size and edge distributions.