Zr Melts Research Articles

Atomic structure insight into crystallization of undercooled liquid metal Zr during isothermal relaxation processes

ABSTRACTThe crystallization of undercooled liquid zirconium (Zr) was investigated via molecular dynamics simulations. The atomic structures were characterised and analysed by means of pair distribution function, angular distribution function, and the largest standard cluster analysis. At low temperature (T = 1000 K), it is found that the crystallization begins at the initial stage of relaxation and takes the pathway of undercooled liquid (UCL)→BCC→HCP instead of UCL→HCP. The HCP-type medium-range orders (MROs) are formed from the inside of the precursors formed by the BCC-type MROs, and growing at the cost of the precursors, in agreement with the Ostwald’s step rule. The number of MROs is found to play a key role in forming the crystal phases in this process. However, at high temperature (T = 1200 K) the time-scale to reach crystallization is two orders of magnitude bigger than that of T = 1000 K. No intermediate stage of crystallization is observed. The undercooled Zr melts are directly transformed into BCC crystal phases. The max size of BCC-MROs is intimately correlated with the crystallization.

Electrical Resistivity of Cu–Zr Melts

For the first time, the electrical resistivity of liquid Cu64.5Zr35.5, Cu50Zr50, and Cu33.3Zr66.7 alloys at temperatures up to 1600 K has been measured by the contactless method in a rotating magnetic field. The measurements were taken during cooling in a helium atmosphere with a purity of 99.995%. The error of the electrical resistivity determination did not exceed 5%. It has been found that the electrical resistivity of liquid Cu64.5Zr35.5, Cu50Zr50, and Cu33.3Zr66.7 alloys decreases linearly with increasing temperature, while it increases with temperature for liquid copper and zirconium. The concentration dependences of the electrical resistivity and its temperature coefficient have a maximum at 55 at % and a minimum at about 60 at % Zr, respectively. These concentration dependences are similar to the dependences obtained for amorphous alloys and are explained in the framework of the Ziman theory.

Diffusion in Ni–Zr Melts: Insights from Statistical Mechanics and Atomistic Modeling

AbstractAn accurate database of diffusion properties of Ni–Zr melts is generated within the framework of the molecular‐dynamics method in conjunction with a semi‐empirical many‐body interatomic potential. The reliability of the model description of Ni–Zr melts is confirmed via comparison of the simulation results with the existing experimental data on diffusion properties of Ni–Zr melts. A statistical mechanical formalism is employed to understand the behavior of the cross‐correlation between the interdiffusion flux and the force caused by the difference in the average random accelerations of atoms of different species in the short time limit . This theoretical description is exploited to analyze the simulation data on the diffusion properties of Ni–Zr melts. On this basis, it is found that in the composition range both single‐particle and collective diffusion dynamics slow down homogeneously upon undercooling of Ni–Zr melts. Furthermore, it is inferred that such homogeneous dynamical slowdown is related to the enhanced stability of undercooled melt against crystallization. As a consequence, Ni–Zr alloys within this composition range are identified as viable glass formers.

Effect of high pressure on the formation and evolution of clusters during the rapid solidification of zirconium melts

The formation and evolution of clusters during the rapid solidification of liquid zirconium (Zr) under the pressure of 0–80GPa were examined through molecular dynamics simulations. Local atomic structures were analyzed using pair-distribution function, Honeycutt–Anderson bond-pair index method, and developed cluster-type index method. Results revealed that Zr melts can be completely vitrified into a glassy state under 30–60GPa. (13 1-1441 10-1551 2-1661) Kasper clusters, instead of (12 12-1551) icosahedra, dominate the formation of Zr metallic glasses. The numbers of Kasper clusters increase with increasing pressure. Under ≤20GPa, the coexistence structures of the stable hexagonal close-packed (HCP) with meta-stable body-centered cubic (BCC) configurations are obtained with phase separation in the final solids. The fraction of BCC structures increases with increasing pressure. The evolution of basic clusters in the crystallization of Zr melts always takes the path of Kasper clusters→(14 6-1441 8-1661) BCC→(12 6-1421 6-1422) HCP. Crystallization under ≥70GPa lacks an intermediate stage. Instead, the supercooled liquid (scl) Zr is directly transformed into A15 phase (scl→A15), which is composed of (12 12-1551) icosahedra and (14 12-1551 2-1661) Kasper clusters. The A15 structure is perfected with increasing pressure. Competition between densification and atomic diffusion contributes to the non-monotonic effect of high pressures on the rapid cooling of Zr melts.

Viscosity of the liquid Al–6Mg–1Mn–0.2Sc–0.1Zr alloy

The microstructure and the phase composition of as-cast Al–Mg–Mn–Sc–Zr alloy samples are studied by electron microscopy and electron-probe microanalysis. The processes of solidification and melting of this alloy are described. The temperature dependence of the kinematic viscosity of the Al–Mg–Mn–Sc–Zr melts is studied during heating and subsequent cooling of the samples. The measurement results are used to determine the temperature at which inherited microheterogeneities in the melts are destroyed irreversibly.

Chemical interaction and thermodynamic properties of (Cu,Ni)-Zr glass-forming alloys

The model of ideal associative solutions was applied to analyze the influence of strong chemical interaction in (Cu,Ni)-Zr melts depending on their glass-forming ability. Within the model framework thermodynamic properties of both Ni-Zr and Cu-Zr systems in the liquid state were calculated. The formation enthalpies of intermetallic compounds of these systems were redefined by the matching procedure, taking into account the additive manifestation of chemical interaction. It was suggested that directed (covalent) interaction causes formation of associative complexes which impedes diffusion and slows down crystallization. The intensity of interparticle interaction in these alloys is found to have no decisive influence on their glass-forming ability.

The effect of pressure on the crystallization of rapidly supercooled zirconium melts.

Molecular dynamics simulations have been performed to explore the effect of pressure (P) on the crystallization of zirconium (Zr) under rapid cooling. The structural evolutions have been analysed in terms of the system energy, the pair distribution function and the largest standard cluster analysis. It was found that at the cooling rate of 1.0 × 1011 K s-1, which can crystallize Zr melts into hcp crystals via the bcc intermediate state under zero pressure, the critical pressure (Pc) for vitrification is about 28.75 GPa, and the larger the pressure, the higher the glass transition temperature Tg. At P < Pc the Ostwald's step rule is applied to Zr melts. Crystallization of rapidly super-cooled Zr melts under pressure always begins with the bcc phase and ends in the hcp crystal; the higher the pressure, the lower the onset temperature (Tc) of crystallization. Unlike the single-intermediate-state crystallization (SisC) under zero pressure, multiple-intermediate-state crystallization (MisC) is usually observed under pressure. Structural analysis reveals that if nucleation is essentially completed at the end of the first crystalline (bcc-dominated) stage, MisC will occur; otherwise, SisC occurs. The origin of such an observation is also discussed from the effect of pressure upon the thermodynamics and kinetics factors. These findings are useful for comprehensively understanding the solidification of metals under pressure.

Thermodynamic properties of Cu–Zr melts: The role of chemical interaction

General statistical model is applied to analyze the role of chemical interaction in associated systems. We show that, at certain conditions, chemical interaction between associates may be not essential above a distectic point and so the model of ideal associated solutions is a good approximation for describing high temperature properties of associated system with chemical interaction. Within the frames of such conception, we calculate thermodynamic properties of Cu–Zr system in liquid state. The enthalpies of formation of Cu–Zr intermetallic compounds were redefined by using matching procedure taking into account the additive manifestation of chemical interaction. We conclude that simple model which is free of adjusting parameters allows us to calculate thermodynamic properties of Cu–Zr melts with quite good accuracy.

Accurate determination of the Gibbs energy of Cu–Zr melts using the thermodynamic integration method in Monte Carlo simulations

The design of multicomponent alloys used in different applications based on specific thermo-physical properties determined experimentally or predicted from theoretical calculations is of major importance in many engineering applications. A procedure based on Monte Carlo simulations (MCS) and the thermodynamic integration (TI) method to improve the quality of the predicted thermodynamic properties calculated from classical thermodynamic calculations is presented in this study. The Gibbs energy function of the liquid phase of the Cu-Zr system at 1800 K has been determined based on this approach. The internal structure of Cu-Zr melts and amorphous alloys at different temperatures, as well as other physical properties were also obtained from MCS in which the phase trajectory was modeled by the modified embedded atom model formalism. A rigorous comparison between available experimental data and simulated thermo-physical properties obtained from our MCS is presented in this work. The modified quasichemical model in the pair approximation was parameterized using the internal structure data obtained from our MCS and the precise Gibbs energy function calculated at 1800 K from the TI method. The predicted activity of copper in Cu-Zr melts at 1499 K obtained from our thermodynamic optimization was corroborated by experimental data found in the literature. The validity of the amplitude of the entropy of mixing obtained from the in silico procedure presented in this work was analyzed based on the thermodynamic description of hard sphere mixtures.

Liquid phase separation in Gd–Ti and Gd–Zr melts

Liquid phase separation in two binary melt systems, Gd–Zr and Gd–Ti, related to phase separated metallic glasses are investigated by electromagnetic levitation experiments. Gd–Ti samples quenched onto a copper chill substrate from temperatures below the binodal line exhibit typical coarse phase separated microstructures, assigning melt immiscibility. The miscibility gap of Gd–Ti melts extends at least from 10 to 80 at.% Gd, much wider than reported previously. The critical temperature is about 1580 °C at Gd 20Ti 80. In the Gd–Zr system no indication of a stable liquid miscibility gap was detected. However, melt droplets quenched from the undercooled state exhibit concurrent coarse Gd and Zr primary phase constituents suggesting a metastable miscibility gap.

Correlation between local structure of melts and glass forming ability for Fe78M9B13 (M=Nb, Si, and Zr) alloys

The structures of Fe78M9B13 (M=Nb, Si, and Zr) melts have been studied by ab initio molecular dynamics simulation. The essential features of the structures are the same in these melts. Prism-type local atomic arrangements dominate around B atoms and icosahedral-like clusters are frequently found around Fe atoms. However, the structures of the three melts differ in the degree of structural distortion caused by introducing M atoms to the prisms. The structural and dynamical properties of these melts change with different M elements, suggesting that they are closely correlated with their glass formation. The degree of complexity of molten structure caused by the interactions among Fe, M, and B may play an important role in their glass forming ability.

Modelling of dendritic solidification in undercooled dilute Ni–Zr melts

The kinetics of dendritic solidification in undercooled Ni–Zr samples is analyzed using new experimental results and theoretical studies based on a sharp-interface model and phase-field simulations. The predictions of a sharp-interface model and of a diffuse-interface model describing the phase transition considering both thermal and solutal diffusion are compared with the new experimental results by evaluating the dendritic tip velocity in electromagnetically levitated Ni–Zr samples.

Characteristics of corium debris bed generated in large-scale fuel-coolant interaction experiments

The formation of corium debris as the result of fuel-coolant interaction (energetic or not) has been studied experimentally in the FARO and KROTOS facilities operated at JRC-Ispra between 1991 and 1999. Experiments were performed with 3–177 kg of UO 2–ZrO 2 and UO 2–ZrO 2–Zr melts, quenched in water at depth between 1 and 2 m, and pressure between 0.1 and 5.0 MPa. The effect of various parameters such as melt composition, system pressure, water depth and subcooling on the quenching processes, debris characteristics and thermal load on bottom head were investigated, thus, giving a large palette of data for realistic reactor situations. Available data related to debris coolability aspects in particular are: • Geometrical configuration of the collected debris. • Partition between loose and agglomerated (“cake”) debris. • Particle size distribution with and without energetic interaction. These data are synthesised in the present contribution.

Heterogeneous dynamics on the microsecond scale in simulatedNi0.5Zr0.5metallic melts far below the glass temperature

From molecular dynamics simulations for up to 2.5 micros, results are reported on structural dynamics in glass-forming metallic Ni(0.5) Zr(0.5) melts at 700 and 785 K. After elimination of high-frequency fluctuations, cascades of atoms and avalanches of atom chains, local in space and time, are identified as complex processes that govern the heterogeneous low-frequency dynamics in sufficiently aged structures near the glass temperature.

Thermodynamic Properties of Melts and Phase Equilibria in the Copper–Zirconium System

The vapor composition over and thermodynamic properties of Cu–Zr melts (5.1–85.0 at. % Zr) are studied by Knudsen cell mass spectrometry between 1191 and 1823 K. The data set obtained comprises more than 1100 activity values for various compositions and temperatures. The thermodynamic behavior of Cu–Zr melts is described in terms of the associated-solution approach with an accuracy no worse than the experimental accuracy. The melts are shown to contain two molecular species: CuZr and Cu2Zr. The contributions of different types of chemical bonding to the Gibbs energy and enthalpy of formation of Cu–Zr melts are asymmetrical and shifted from the equiatomic composition in opposite directions: the extremum of covalent bonding is shifted to the Cu-rich side, while metallic bonding is more significant in Zr-rich alloys. The rapid temperature variation of covalent bonding leads to a large excess heat capacity C p E of the melts and a negative excess entropy Δf S E, which rapidly drops with decreasing temperature. It is shown that not only C p E and Δf S E but also their temperature variations are governed by the parameters of association reactions and depend more strongly on the entropy than on the enthalpy of complex formation. This indicates that, in the general case, the glass-forming capabilities of melts are independent of the interparticle interaction and accounts for the pronounced tendency of Cu–Zr melts toward amorphization.