Metallic Glacial Glass Formation by a First-Order Liquid-Liquid Transition

Approach to hyperuniformity in a metallic glass-forming material exhibiting a fragile to strong glass transition

We investigate a metallic glass-forming (GF) material (Al₉₀Sm₁₀) exhibiting a fragile-strong (FS) glass-formation by molecular dynamics simulation to better understand this highly distinctive pattern of glass-formation in which many of the usual phenomenological relations describing relaxation times and diffusion of ordinary GF liquids no longer apply, and where instead genuine thermodynamic features are observed in response functions and little thermodynamic signature is exhibited at the glass transition temperature, T_g. Given the many unexpected similarities between the thermodynamics and dynamics of this metallic GF material with water, we first focus on the anomalous static scattering in this liquid, following recent studies on water, silicon and other FS GF liquids. We quantify the "hyperuniformity index" H of our liquid, which provides a quantitative measure of molecular "jamming". To gain insight into the T-dependence and magnitude of H, we also estimate another more familiar measure of particle localization, the Debye-Waller parameter 〈u²〉 describing the mean-square particle displacement on a timescale on the order of the fast relaxation time, and we also calculate H and 〈u²〉 for heated crystalline Cu. This comparative analysis between H and 〈u²〉 for crystalline and metallic glass materials allows us to understand the critical value of H on the order of 10^-3 as being analogous to the Lindemann criterion for both the melting of crystals and the "softening" of glasses. We further interpret the emergence of FS GF and liquid-liquid phase separation in this class of liquids to arise from a cooperative self-assembly process in the GF liquid.

High-Temperature Liquid-Liquid Phase Transition in Glass-Forming Liquid Pd43Ni20Cu27P10

Liquid-liquid phase transition (LLPT) is a transition from one liquid state to another with the same composition but distinct structural change, which provides an opportunity to explore the relationships between structural transformation and thermodynamic/kinetic anomalies. Herein the abnormal endothermic LLPT in Pd₄₃Ni₂₀Cu₂₇P₁₀ glass-forming liquid was verified and studied by flash differential scanning calorimetry (FDSC) and ab initio molecular dynamics (AIMD) simulations. The results show that the change of the atomic local structure of the atoms around the Cu-P bond leads to the change in the number of specific clusters <0 2 8 0> and <1 2 5 3>, which leads to the change in the liquid structure. Our findings reveal the structural mechanisms that induce unusual heat-trapping phenomena in liquids and advance the understanding of LLPT.

High-entropy induced a glass-to-glass transition in a metallic glass

Glass-to-glass transitions are useful for us to understand the glass nature, but it remains difficult to tune the metallic glass into significantly different glass states. Here, we have demonstrated that the high-entropy can enhance the degree of disorder in an equiatomic high-entropy metallic glass NbNiZrTiCo and elevate it to a high-energy glass state. An unusual glass-to-glass phase transition is discovered during heating with an enormous heat release even larger than that of the following crystallization at higher temperatures. Dramatic atomic rearrangement with a short- and medium-range ordering is revealed by in-situ synchrotron X-ray diffraction analyses. This glass-to-glass transition leads to a significant improvement in the modulus, hardness, and thermal stability, all of which could promote their applications. Based on the proposed high-entropy effect, two high-entropy metallic glasses are developed and they show similar glass-to-glass transitions. These findings uncover a high-entropy effect in metallic glasses and create a pathway for tuning the glass states and properties.

Glass-to-glass transitions can help understanding the glass nature, but it remains difficult to tune metallic glasses into significantly different glass states. Here the authors demonstrate the high-entropy effects in glass-to-glass transitions of high-entropy metallic glasses.

Glassy or Amorphous? A Demonstration Using G-Phase Copper Containing a Fivefold Twinning Structure

The G-phase, a new metastable phase with its potential energy sitting right in the middle of the glass and crystal, was recently discovered in some simulations when the molten metallic liquid was quenched down to room temperature. In comparison with ordinary glass, the G-phase has a more ordered short-range structure but a similarly disordered long-range structure. The question is whether the G-phase can be termed a new type of glass. In this work, G-phase Cu is made in a molecular dynamics simulation using rapid quenching or isothermal annealing. Weak oscillations are found in the long-range atomic structure. The pseudo-fictive temperature is significantly lower than the Kauzmann temperature; fivefold twinning structures are distinguished in the G-phase whose constituent atoms are face-center-cubic or hexagonal-cubic-packed. This evidence suggests that G-phase Cu is not a glass. However, the G-phase is also metastable against crystallization. Therefore, G-phase Cu is neither a glass nor a crystal but belongs to a new mesophase.

The L-G phase transition in binary Cu-Zr metallic liquids

The authors recently reported that undercooled liquid Ag and Ag-Cu alloys both exhibit a first order phase transition from the homogeneous liquid (L-phase) to a heterogeneous solid-like G-phase under isothermal evolution. Here, we report a similar L-G transition and heterogenous G-phase in simulations of liquid Cu-Zr bulk glass. The thermodynamic description and kinetic features (viscosity) of the L-G-phase transition in Cu-Zr simulations suggest it corresponds to experimentally reported liquid-liquid phase transitions in Vitreloy 1 (Vit1) and other Cu-Zr-bearing bulk glass forming alloys. The Cu-Zr G-phase has icosahedrally ordered cores versus fcc/hcp core structures in Ag and Ag-Cu with a notably smaller heterogeneity length scale Λ. We propose the L-G transition is a phenomenon in metallic liquids associated with the emergence of elastic rigidity. The heterogeneous core-shell nano-composite structure likely results from accommodating strain mismatch of stiff core regions by more compliant intervening liquid-like medium.

Prediction of Second Melting Temperatures Already Observed in Pure Elements by Molecular Dynamics Simulations

A second melting temperature occurs at a temperature T_n+ higher than T_m in glass-forming melts after heating them from their glassy state. The melting entropy is reduced or increased depending on the thermal history and on the presence of antibonds or bonds up to T_n+. Recent MD simulations show full melting at T_n+ = 1.119T_m for Zr, 1.126T_m for Ag, 1.219T_m for Fe and 1.354T_m for Cu. The non-classical homogeneous nucleation model applied to liquid elements is based on the increase of the Lindemann coefficient with the heating rate. The glass transition at T_g and the nucleation temperatures T_nG of glacial phases are successfully predicted below and above T_m. The glass transition temperature T_g increases with the heating rate up to T_n+. Melting and crystallization of glacial phases occur with entropy and enthalpy reductions. A universal law relating T_n+ and T_nG around T_m shows that T_nG cannot be higher than 1.293T_m for T_n+= 1.47T_m. The enthalpies and entropies of glacial phases have singular values, corresponding to the increase of percolation thresholds with T_g and T_nG above the Scher and Zallen invariant at various heating and cooling rates. The G-phases are metastable up to T_n+ because the antibonds are broken by homogeneous nucleation of bonds.

Dynamic heterogeneity, cooperative motion, and Johari-Goldstein [Formula: see text]-relaxation in a metallic glass-forming material exhibiting a fragile-to-strong transition

We investigate the Johari-Goldstein (JG) [Formula: see text]-relaxation process in a model metallic glass-forming (GF) material ([Formula: see text]), previously studied extensively by both frequency-dependent mechanical measurements and simulation studies devoted to equilibrium properties, by molecular dynamics simulations based on validated and optimized interatomic potentials with the primary aim of better understanding the nature of this universal relaxation process from a dynamic heterogeneity (DH) perspective. The present relatively low temperature and long-time simulations reveal a direct correspondence between the JG [Formula: see text]-relaxation time [Formula: see text] and the lifetime of the mobile particle clusters [Formula: see text], defined as in previous DH studies, a relationship dual to the corresponding previously observed relationship between the [Formula: see text]-relaxation time [Formula: see text] and the lifetime of immobile particle clusters [Formula: see text]. Moreover, we find that the average diffusion coefficient D nearly coincides with [Formula: see text] of the smaller atomic species (Al) and that the 'hopping time' associated with D coincides with [Formula: see text] to within numerical uncertainty, both trends being in accord with experimental studies. This indicates that the JG [Formula: see text]-relaxation is dominated by the smaller atomic species and the observation of a direct relation between this relaxation process and rate of molecular diffusion in GF materials at low temperatures where the JG [Formula: see text]-relaxation becomes the prevalent mode of structural relaxation. As an unanticipated aspect of our study, we find that [Formula: see text] exhibits fragile-to-strong (FS) glass formation, as found in many other metallic GF liquids, but this fact does not greatly alter the geometrical nature of DH in this material and the relation of DH to dynamical properties. On the other hand, the temperature dependence of the DH and dynamical properties, such as the structural relaxation time, can be significantly altered from 'ordinary' GF liquids.