Thermodynamic anomaly of the sub-T(g) relaxation in hyperquenched metallic glasses

NiTi2, a New Liquid Glass

Many endothermic liquid-liquid transitions, occurring at a temperature Tn+ above the melting temperature Tm, are related to previous exothermic transitions, occurring at a temperature Tx after glass formation below Tg, with or without attached crystallization and predicted by the nonclassical homogenous nucleation equation. A new thermodynamic phase composed of broken bonds (configurons), driven by percolation thresholds, varying from ~0.145 to Δε, is formed at Tx, with a constant enthalpy up to Tn+. The liquid fraction Δε is a liquid glass up to Tn+. The solid phase contains glass and crystals. Molecular dynamics simulations are used to induce, in NiTi2, a reversible first-order transition by varying the temperature between 300 and 1000 K under a pressure of 1000 GPa. Cooling to 300 K, without applied pressure, shows the liquid glass presence with Δε = 0.22335 as memory effect and Tn+ = 2120 K for Tm = 1257 K.

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.

Building and Breaking Bonds by Homogenous Nucleation in Glass-Forming Melts Leading to Transitions in Three Liquid States

The thermal history of melts leads to three liquid states above the melting temperatures T_m containing clusters-bound colloids with two opposite values of enthalpy +Δε_lg × ΔH_m and -Δε_lg × ΔH_m and zero. All colloid bonds disconnect at T_n+ > T_m and give rise in congruent materials, through a first-order transition at T_LL = T_n+, forming a homogeneous liquid, containing tiny superatoms, built by short-range order. In non-congruent materials, (T_n+) and (T_LL) are separated, T_n+ being the temperature of a second order and T_LL the temperature of a first-order phase transition. (T_n+) and (T_LL) are predicted from the knowledge of solidus and liquidus temperatures using non-classical homogenous nucleation. The first-order transition at T_LL gives rise by cooling to a new liquid state containing colloids. Each colloid is a superatom, melted by homogeneous disintegration of nuclei instead of surface melting, and with a Gibbs free energy equal to that of a liquid droplet containing the same magic atom number. Internal and external bond number of colloids increases at T_n+ or from T_n+ to T_g. These liquid enthalpies reveal the natural presence of colloid-colloid bonding and antibonding in glass-forming melts. The Mpemba effect and its inverse exist in all melts and is due to the presence of these three liquid states.

β-Relaxation and Crystallization Behaviors in a Pulse-Current-Thermoplastic-Formed La-Based Bulk Metallic Glass

We use the pulse current thermoplastic forming technique based on joule heating to rejuvenate the atomic structure of a La₆₂Al₁₄Ag_2.34Ni_10.83Co_10.83 bulk metallic glass (BMG). The pulse-formed sample exhibits more pronounced β-relaxation than the as-cast one due to the increased free volume. Instead, the sub-T_g annealing clearly weakens the β-relaxation and also makes it more isolated from the α-relaxation, showing contributions from free volume and preferred structure. However, both treatments exhibit little influence on the following α-relaxation and high temperature crystallization kinetics. Our results open an effective way to rejuvenate the structure of BMGs and provide an in-depth understanding of the relationship between structural relaxations and crystallization kinetics of BMGs.

Shadow glass transition as a thermodynamic signature of β relaxation in hyper-quenched metallic glasses

One puzzling phenomenon in glass physics is the so-called 'shadow glass transition' which is an anomalous heat-absorbing process below the real glass transition and influences glass properties. However, it has yet to be entirely characterized, let alone fundamentally understood. Conventional calorimetry detects it in limited heating rates. Here, with the chip-based fast scanning calorimetry, we study the dynamics of the shadow glass transition over four orders of magnitude in heating rates for 24 different hyper-quenched metallic glasses. We present evidence that the shadow glass transition correlates with the secondary (β) relaxation: (i) The shadow glass transition and the β relaxation follow the same temperature-time dependence, and both merge with the primary relaxation at high temperature. (ii) The shadow glass transition is more obvious in glasses with pronounced β relaxation, and vice versa; their magnitudes are proportional to each other. Our findings suggest that the shadow glass transition signals the thermodynamics of β relaxation in hyper-quenched metallic glasses.

Switching between Crystallization from the Glassy and the Undercooled Liquid Phase in Phase Change Material Ge2 Sb2 Te5

Controlling crystallization kinetics is key to overcome the temperature-time dilemma in phase change materials employed for data storage. While the amorphous phase must be preserved for more than 10 years at slightly above room temperature to ensure data integrity, it has to crystallize on a timescale of several nanoseconds following a moderate temperature increase to near 2/3 T_m to compete with other memory devices such as dynamic random access memory (DRAM). Here, a calorimetric demonstration that this striking variation in kinetics involves crystallization occurring either from the glassy or from the undercooled liquid state is provided. Measurements of crystallization kinetics of Ge₂ Sb₂ Te₅ with heating rates spanning over six orders of magnitude reveal a fourfold decrease in Kissinger activation energy for crystallization upon the glass transition. This enables rapid crystallization above the glass transition temperature T_g . Moreover, highly unusual for glass-forming systems, crystallization at conventional heating rates is observed more than 50 °C below T_g , where the atomic mobility should be vanishingly small.

Mechanism of Biomineralization Induced by Bacillus subtilis J2 and Characteristics of the Biominerals

Abstract: Biomineralization induced by microorganisms has become a hot spot in the field of carbonate sedimentology; however, the mechanisms involved still need to be explored. In this study, the bacterium Bacillus subtilis J2 (GenBank MG575432) was used to induce the precipitation of calcium carbonate minerals at Mg/Ca molar ratios of 0, 3, 6, 9, and 12. Bacillus subtilis J2 bacteria released ammonia to increase pH, but the ammonia released only made the pH increase to 8.25. Carbonic anhydrase was also produced to catalyze the hydration of carbon dioxide, and this process released carbonate and bicarbonate ions that not only increased pH but also elevated carbonate supersaturation. The biominerals formed at a Mg/Ca molar ratio of 0 were spherulitic, elongated, dumbbell-shaped, and irregularly rhombohedral calcite; at a Mg/Ca molar ratio of 3, the biominerals were calcite and aragonite, the weight ratio of calcite decreased from 26.7% to 15.6%, and that of aragonite increased from 73.3% to 84.4% with increasing incubation time. At higher Mg/Ca molar ratios, the biominerals were aragonite, and the crystallinity and thermal stability of aragonite decreased with increasing Mg/Ca molar ratios. FTIR results showed that many organic functional groups were present on/within the biominerals, such as C–O–C, N–H, C=O, O–H, and C–H. HRTEM-SAED examination of the ultra-thin slices of B. subtilis J2 bacteria showed that nano-sized minerals with poor crystal structure had grown or been adsorbed on the EPS coating. The EPS of the B. subtilis J2 strain contained abundant glutamic acid and aspartic acid, which could be deprotonated in an alkaline condition to adsorb Ca2+ and Mg2+ ions; this made EPS act as the nucleation sites. This study may provide some references for further understanding of the mechanism of biomineralization induced by microorganisms.

Hidden amorphous phase and reentrant supercooled liquid in Pd-Ni-P metallic glasses

An anomaly in differential scanning calorimetry has been reported in a number of metallic glass materials in which a broad exothermal peak was observed between the glass and crystallization temperatures. The mystery surrounding this calorimetric anomaly is epitomized by four decades long studies of Pd-Ni-P metallic glasses, arguably the best glass-forming alloys. Here we show, using a suite of in situ experimental techniques, that Pd-Ni-P alloys have a hidden amorphous phase in the supercooled liquid region. The anomalous exothermal peak is the consequence of a polyamorphous phase transition between two supercooled liquids, involving a change in the packing of atomic clusters over medium-range length scales as large as 18 Å. With further temperature increase, the alloy reenters the supercooled liquid phase, which forms the room-temperature glass phase on quenching. The outcome of this study raises a possibility to manipulate the structure and hence the stability of metallic glasses through heat treatment.

The role of liquid–liquid transition in glass formation of CuZr alloys

The structure evolution during LLTs is beneficial to the glass forming ability (GFA) of Cu–Zr systems.

The β-relaxation in metallic glasses

Focusing on metallic glasses as model systems, we review the features and mechanisms of the β-relaxations, which are intrinsic and universal to supercooled liquids and glasses, and demonstrate their importance in understanding many crucial unresolved issues in glassy physics and materials science, including glass transition phenomena, mechanical properties, shear-banding dynamics and deformation mechanisms, diffusion and the breakdown of the Stokes–Einstein relation as well as crystallization and stability of glasses. We illustrate that it is an attractive prospect to incorporate these insights into the design of new glassy materials with extraordinary properties. We also outline important questions regarding the nature of β-relaxations and highlight some emerging research directions in this still-evolving field.

A Direct Link between the Fragile-to-Strong Transition and Relaxation in Supercooled Liquids

It is known that both the fragile-to-strong (F-S) transition and relaxation processes occur in numerous supercooled liquids upon cooling toward the glass transition temperature. The key question is whether and how these two dynamic processes are correlated. Here, we show a direct link between the two processes for both metallic glass-forming liquids (MGFLs) with different fragilities and also for nonmetallic glass-forming liquids. By comparing the F-S transition extent parameter f with the parameter r that characterizes the competition between the α and the slow β relaxations, we have discovered a negative exponential connection between the two parameters of supercooled liquids. The finding indicates that the slow β relaxation plays a dominant role in the F-S transition. This work provides new insight into the microscopic mechanism of the F-S transition and creates a strong basis for predicting whether and to what extent the F-S transition occurs in supercooled liquids.