Crystal nucleation in deeply undercooled melts of bulk metallic glass forming systems

Crystal nucleation in metallic glass forming systems is explored by means of a non-classical model assuming a diffuse solid/liquid interface. The model predictions are demonstrated initially for an example system with general hypothetical properties. The predicted nucleation rate matches that obtained from the classical nucleation theory for shallow undercoolings, and deviates from it with increasing undercooling. For certain system properties, a physical spinodal is predicted, i.e. the calculated energy barrier to crystal nucleation vanishes at a finite critical undercooling. The numerical results for the example system are then fitted into a simple formula for nucleation frequency, in which the critical undercooling is taken as an adjustable parameter. This formula fits well into the nucleation data of the bulk metallic glass forming system Zr₄₁Ti₁₄Cu₁₂Ni₁₀Be₂₃, when a critical undercooling of 440 K is taken. The existence of a physical spinodal in this system is shown to be consistent with various other experimental observations. It is also shown that the nucleation behaviour in such systems cannot be explained by classical nucleation theory, even if phase separation in the liquid is taken into account.     

Effect of nonlinear liquidus and solidus on dendrite growth in bulk undercooled melts

On the base of nonlinear liquidus and solidus,an extended model for dendrite growth in bulk undercooled melts was developed under local non-equilibrium conditions both at the interface and in the bulk liquid.In terms of thermodynamic calculations of the phase diagram,the model predictions are relatively realistic physically,since few fitting parameters are used in the model predictions.Adopting three characteristic velocities,i.e.the critical velocity of absolute solute stability(VC*),the velocity of maxima...     

Modeling dendrite growth in undercooled concentrated multi-component alloys

Most theoretical work on dendrite growth has focused on dilute binary alloys, while most industrial alloys are concentrated multi-component systems. By incorporating the local non-equilibrium effects both at the interface and in the bulk liquid, the thermodynamic database and diffusional interaction, a model was developed for dendrite growth in undercooled concentrated multi-component alloys. An experimental study of dendrite growth in undercooled Ni–18 at.% Cu–18 at.% Co melts was carried out and the measured interface velocities (V) were well predicted by the present model over the whole undercooling range (ΔT = 30–313 K). During dendrite growth the partition coefficients change non-monotonically due to interaction between the species and changes in the dendrite tip radius. Interaction between the species also leads to a lower interface velocity and larger ΔT and V as the ΔT–V relation plateaus. The previous definition of constitutional undercooling, i.e. the sum of the contributions of each solute, is not applicable to concentrated multi-component alloys. The controlling mechanisms during dendrite growth are discussed with respect to the results of the calculations.     

溶质截留对过冷共晶生长过程的影响

以过冷凝固条件下已有的共晶生长理论模型为基础,考虑了快速非平衡凝固条件下会出现溶质截留现象,探讨溶质截留对生长过程的影响.发现溶质截留的引入,增大了共品生长速度,减小了层片间距和枝晶尖端半径.初始平衡溶质分配系数越小,溶质截留对过冷共晶生长过程的影响越显著.     

The nucleation and growth of metastable pitting on pure iron

The nucleation and growth of metastable pitting on pure iron surface in NaNO₂ + NaCl solution were investigated by potentiodynamic tests. The current fluctuations appear as cluster and overlapped. The active sites for nucleation depend mainly on the surface geometry. The growth behaviour of pure iron is different from stainless steels and carbon steels. On pure iron surface, many pits pile up together to form huge damage area. Pits are shallow, do not grow deeply and pits array one by one along the direction of abrasion grooves. The growth of stable pitting is different from metastable pitting.     

In situ observation of Si faceted dendrite growth from low-degree-of-undercooling melts

We directly observed the transition of crystal growth behavior of Si in a low undercooling region. We succeeded in observing the initiation of faceted dendrite growth from a portion of parallel twins with increasing degrees of undercooling. The critical undercooling for growing a faceted dendrite was experimentally determined to be ΔT = 10 K. We also confirmed that parallel twins associated with faceted dendrite growth were formed between grain boundaries and not at grain boundaries during melt growth. The parallel-twin formation was explained in terms of a model of twin formation on the {1 1 1} facet plane at the growth interface.     

Nucleation barrier height in undercooled metallic melts

The phase-field model of a liquid-to-solid transition was constructed where the model parameters were linked quantitatively to the interfacial properties,and the variation of nucleation barrier height in undercooled metallic melts with respect to undercooling was studied respectively based on two kinds of forms of local free energy density.The calculation results show that,with the increase of undercooling,the critical nucleus does not show bulk properties,and the nucleation barrier height decreases gradually and deviates more and more from that predicted by the classical nucleation theory in both cases.The physical spinodal occurs for a specific form of the local free energy density,where the nucleation barrier height vanishes when the undercooling reaches a critical value and the reduced nucleation barrier height can be expressed by a function of the ratio of undercooling to critical undercooling.     

Thermal recalescence and mushy zone coarsening in undercooled melts

Precision adiabatic recalescence experiments on both pure and binary undercooled melts have been conducted. The adiabatic constraint limits the amount of net phase transformation from the metastable under-cooled state (creating a mushy zone) and also sets bounds on the microstructural parameters of the mushy zone, namely the volume fraction of the phases, their length scales, and the interfacial compositions. As many as three distinct kinetic time scales are observed in these experiments: a short time scale (less than ～1 s) of rapidly increasing temperature from the initial nucleation temperature, associated with the propagation of the dendrites and release of latent heat; an intermediate time scale (～103–104 s) of slowly rising temperature, associated with coarsening of the mushy zone; and a long time scale (days) of steadily falling temperature, associated with solid-state diffusional adjustments near the solid-liquid interface.     

On occurrence of multiple-site crystallization in undercooled mullite melts

Using a high-speed video, an interesting multiple-site crystallization phenomenon in undercooled mullite oxide is observed in situ in an aero-acoustic levitation apparatus when a laser heating system is incorporated. This crystallization behavior is discussed considering nucleation theory and growth analysis by defining conditions under which multiple-site crystallization can occur.     

Phase selection in highly undercooled Fe-B eutectic alloy melts

The high undercooling technique by molten glass slag purification and cyclical superheating in Ar atmosphere was applied to bulk Fe-B alloy melts. A hypercooling was achieved which suppressed the formation of stable phase and consequently promoted the nucleation of metastable phase. Fe-17%B and Fe-20%B alloys were investigated, respectively. TEM and X-ray powder diffraction analyses verify the formation of metastable phase in the highly tmdercooled Fe-B alloy melts. Besides, the critical nucleation work of Fe2B and Fe3B phases was calculated to predict phase selection in the undercooled melts. The results show that the metastable phase formation is a function of the undercooling achieved prior to nucleation. And the amount of undercooling is an important factor in determining microstructural development by controlling phase selection in the undercooled melts.     

Containerless processing of the undercooled metallic melts — overview

Brief overview of current containerless electrostatic levitation processing technique and research progress of the area of bulk metallic glass formation is introduced. Undercooling behavior during solidification of the bulk metallic glass forming Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 alloy has been studied using the containerless electrostatic levitation processing technique. The melt is successfully undercooled to the glass transition temperature forming the amorphous phase with the proper thermal treatment. Differential scanning calorimetry (DSC) is used to determine the Gibbs free energy difference between the crystal and the undercooled liquid. The results indicate that the Gibbs free energy difference between the metastable undercooled liquid and the crystalline solid of the Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 alloy is relatively small compared to that of conventional metallic glass forming binary alloys even for large undercoolings. The hemispherical total emissivity of undercooled liquid is measured in the whole region of undercooled liquid state. Due to the combining effects of excellent thermal stability of the undercooled liquid in the Zr_41.2Ti_13.8Cu_12.5Ni_10.0Be_22.5 alloy with unique experimental technique of the containerless electrostatic levitation processing, it is possible to construct the complete time-temperature-transformation (TTT) diagram. The measured TTT diagram exhibiting the expected “C” shape can not be satisfactorily explained by the existing models due to the complex crystallization mechanisms.     

Computation of metastable phases in tungsten-carbon system

Metastable phase equilibria in the W-C system are presented in the vicinity of the metastable reactions involving W₂C, WC_1−x , and WC. Metastable phase boundaries were obtained by reproducing the stable boundaries using optimized Gibbs energy formulations and extrapolating them into regions of metastability. Four metastable reactions were obtained: a metastable congruent melting reaction of WC at 3106 K, a metastable eutectic reaction between WC_1−x and graphite at 2995 K, a metastable eutectic reaction between W₂C and WC at 2976 K, and a metastable eutectic reaction between W₂C and graphite at 2925 K. The reaction enthalpies and entropies associated with these transitions are also computed using the available Gibbs energy data. Furthermore, possible kinetic paths that could lead to metastability are discussed.     

Computation of metastable phases in tungsten-carbon system

Metastable phase equilibria in the W-C system are presented in the vicinity of the metastable reactions involving W₂C, WC_1−x , and WC. Metastable phase boundaries were obtained by reproducing the stable boundaries using optimized Gibbs energy formulations and extrapolating them into regions of metastability. Four metastable reactions were obtained: a metastable congruent melting reaction of WC at 3106 K, a metastable eutectic reaction between WC_1−x and graphite at 2995 K, a metastable eutectic reaction between W₂C and WC at 2976 K, and a metastable eutectic reaction between W₂C and graphite at 2925 K. The reaction enthalpies and entropies associated with these transitions are also computed using the available Gibbs energy data. Furthermore, possible kinetic paths that could lead to metastability are discussed.     

Metastable phase formation in Ti–Al–Nb undercooled melts

Metastable phase formation processes in ternary Ti–Al–Nb alloys were studied by containerless electromagnetic levitation for melt undercooling up to 300 K below the liquidus temperature. Dendrite growth velocities of 15–25 m s⁻¹ for highly undercooled Ti–Al–Nb melts were consistent with primary β-phase formation, which is promoted by Nb addition. From double-recalescence events in Ti₄₀Al₅₀Nb₁₀ and Ti₄₅Al₅₀Nb₅ melts beyond a critical undercooling a subsequent β to α phase transformation in the semi-solid state was inferred. A second recalescence near 1300 °C, which was attributed to an α to γ solid state transformation, was observed in the pyrometer trace for the Ti₄₅Al₅₀Nb₅ and Ti₄₀Al₅₀Nb₁₀ alloys. The γ phase formation was suppressed in favour of a homogeneous α₂ phase in undercooled Ti₄₅Al₄₅Nb₁₀ samples quenched onto a chill substrate, whereas in Ti₄₀Al₅₀Nb₁₀ high undercooling enabled a direct γ phase solidification.     

Non-equilibrium solidification of undercooled Ni-31.44%Pb monotectic alloy melts

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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.     

Novel criterion for splitting of plate-like crystal growing in undercooled silicon melts

An undercooled drop of silicon was grown to crystals in containerless states with electromagnetic and electrostatic levitators. A high-speed video camera was used to monitor the growth rate and observe the crystal–melt interface as a function of undercooling. The morphology of the growing crystal changed from a mono-plate crystal to a multi-plate crystal, and then to faceted dendrite with increasing undercooling. The mono-plate and multi-plate crystals observed at undercooling of less than 100 K were shaped by a faceted planer interface and wavy-edge plane. The critical undercooling for the transition from mono-plate to multi-plate depends on the sample size; it was 50 and 80 K for samples of 5 and 1.7 mm in diameter, respectively. A novel criterion for the transition from mono-plate to multi-plate based on the instability of the wavy-edge plane is proposed.     

深过冷Ni-31.44%Pb偏晶合金快速凝固行为

采用熔融玻璃净化和循环过热相结合的方法研究了Ni-31．44％Pb偏晶合金宽过冷区间的凝固组织演化规律；从形核热力学和动力学两方面分析过冷熔体中稳定相(α)和亚稳相(L2)两相的竞争形核规律。研究结果表明：过冷Ni-31．44％Pb偏晶合金在快速凝固阶段本质上是以枝晶方式生长，首先形成α枝晶骨架，再辉重熔后分布于枝晶间的残余掖相按照正常凝固模式进行分相／偏晶等后续反应。