Temperature Gradient Zone Melting Research Articles

Investigation on melt concentration during thermal stabilization before directional growth in a Mg–Li eutectic alloy in presence of element volatilization

The initial melt concentration C for following directional solidification has been confirmed to be controlled by temperature gradient zone melting (TGZM) during thermal stabilization in previous studies. However, the accurate prediction of C has not been reported in the case of element volatilization before. In the present work, the hypoeutectic Mg-7.3Li alloy is selected to investigate the relationship between the melt concentration C and thermal stabilization process in presence of lithium volatilization. The increase of C has been shown to be interrupted when the thermal stabilization time is long, which is distinct from other alloys where no element volatilization exists. The theoretical analysis has been performed to predict C after thermal stabilization, and both the TGZM effect and Li loss caused by lithium volatilization during thermal stabilization are taken into account. The present prediction shows excellent agreement with the experimental result in Mg-7.3Li alloy, confirming the influence of lithium volatilization. Thus, this work can shed light on the prediction of C during directional solidification of alloys in presence of element volatilization.

Hypo-peritectic TRIS–NPG in a stationary temperature gradient: Thermodynamics, grain boundary migration and phase identification

At a stationary temperature gradient, the grain boundaries comprising residual melt can migrate. Their migration is governed by liquid diffusion and, in multiphase materials, depends highly on the phases present. Therefore, this phenomenon can be used for phase identification if data on metastable extensions of the corresponding phase diagram are available. Thus, we re-optimised the thermodynamic description of Tris(hydroxymethyl)aminomethane-Neopentylglycol (TRIS–NPG), a transparent peritectic alloy often used as a model alloy for metallic solidification, and used the predicted metastable liquidus and solidus curves to evaluate the grain boundary migration observations. As we found temperature gradient zone melting (TGZM) at low temperatures, the presence of the peritectic phase could be excluded even though a near-peritectic alloy had been processed. The liquid diffusivity, as a function of the position/temperature, was estimated from the TGZM velocity measurements. The data suggest that the diffusion coefficient deep in the mush is one order of magnitude smaller than that close to the liquidus temperature. This may be typical for non-dilute alloys, where the concentration of the intergranular liquid changes considerably.

Phase field modeling of partial remelting during reheating of a multiphase peritectic solidification microstructure

While they are known for years to be the main source of equiaxed grains in metallurgical processes such as casting, melting and remelting phenomena currently receive an increasing attention owing to the developments of additive manufacturing. In multiphase alloys, i.e. having a eutectic or peritectic microstructure, these phenomena still remain poorly documented. In this work, the solidification and subsequent remelting of peritectic alloys have been studied using phase field simulations, with emphasis on the influence of the peritectic equilibrium, that yields the peritectic reaction L+β→α below the peritectic temperature and the reverse peritectic reaction α→L+β above. During solidification, the growth of the peritectic α phase has little influence on the growth of the pro-peritectic β phase. On the other hand, the evolution during remelting of a previously solidified peritectic microstructure is more complex and involves temperature gradient zone melting and liquid film migration phenomena. Close to the peritectic temperature, the reverse peritectic reaction is driven by liquid film migration, and some scenarii are evidenced depending on whether the growth takes place without triple junction from a pure α phase or with a triple junction along the α/β interface. At the scale of the mushy zone, the Temperature Gradient Zone Melting phenomenon also occurs but remains incomplete at the time scale investigated here. Finally, a new mechanism for fragmentation, induced by the reverse peritectic reaction during remelting, has been identified.

Influence of hexagonal to orthorhombic phase transformation on diffusion-controlled dendrite evolution in directionally solidified Sn–Ni peritectic alloy

The hexagonal to orthorhombic (HO) transformation from β-Ni3Sn2 (hexagonal) phase to α’-Ni3Sn2 (orthorhombic) phase was confirmed in directionally solidified Sn–Ni peritectic alloys. It is shown that the remelting/resolidification process which is caused by both the temperature gradient zone melting (TGZM) and Gibbs−Thomson (G−T) effects can take place on secondary dendrites. Besides, the intersection angle between the primary dendrite stem and secondary branch (θ) is found to increase from π/3 to π/2 as the solidification proceeds. This is the morphological feature of the HO transformation, which can change the diffusion distance of the remelting/ resolidification process. Thus, a diffusion-based analytical model is established to describe this process through the specific surface area (SV) of dendrites. The theoretical prediction demonstrates that the remelting/resolidification process is restricted when the HO transformation occurs during peritectic solidification. In addition, the slope of the prediction curves is changed, indicating the variation of the local remelting/resolidification rates.

Equiaxed grain structure formation during directional solidification of a refined Al-20wt.%Cu alloy: In situ analysis of temperature gradient effects

Three series of directional solidification experiments on refined Al-20wt.%Cu alloys have been carried out with different temperature gradients, and for each of them a wide range of cooling rates were applied. The experiments were performed in horizontal configuration to minimize the impact of gravity-driven phenomena and characterized in situ and in real-time by using the X-radiography technique. The influence of the temperature gradient on the microstructure formation (impact of Temperature Gradient Zone Melting: TGZM), the nucleation distance, the average grain size and morphology (elongation factor and grain orientation) have been analysed quantitatively. The experimental results are discussed with current theoretical models and similar experimental works.

Analysis of temperature gradient zone melting and the thermal migration of liquid particles through a solid

Analytical solutions are derived to describe the migration of a liquid particle or droplet, enriched in one component, through a solid under a thermal gradient, a behavior also known as temperature gradient zone melting or TGZM. Such a particle migrates in the direction of increasing temperature and is driven by different temperatures and equilibrium compositions that produce dissolution of the solid matrix at the leading end, regrowth at the trailing end, and diffusion of the excess component across the droplet. While simple, linear concentration and temperature profiles arise within the liquid particle during migration, both droplet velocity and size exhibit nonlinear growth with time. Conditions for the validity of the analytical TGZM solutions are identified, and suggestions are put forth for experimental application and further improvements.

Modelling of liquid film migration in Al-Cu alloys.

Microstructural evolution in the presence of liquid film migration (LFM) is simulated for Al-Cu alloys using a cellular automaton (CA) model. Simulations are performed for the microstructural evolution and concentration distribution in an Al-4 wt.%Cu alloy with initially equiaxed grain structures holding in a temperature gradient. A slight deviation from local equilibrium, estimated from experimental data, is considered to be the driving force for LFM. The direction of LFM is triggered by concentration fluctuations setting a concentration gradient as a further driving force. The simulation successfully reproduces the experimentally observed microstructures generated by LFM accompanied by a particle free zone behind the liquid film. The solid concentration in the particle free zone is found to be the equilibrium solid concentration. The simulated concentration profile across the migrating liquid film agrees well with experimental measurements. The simulated grain structure becomes coarser and highly elongated after holding in the temperature gradient. The results reveal that the increase in transversal grain width is mainly controlled by LFM, while the grain elongation in longitudinal direction is attributed to both LFM and temperature gradient zone melting. The solid concentration decreases from the initial (supersaturated) composition to the local equilibrium solid concentration corresponding to the local temperature. This article is part of the theme issue 'Transport phenomena in complex systems (part 2)'.

Morphology evolution of abnormal tertiary dendrite by diffusion- controlled remelting/resolidification in directionally solidified Sn–Mn peritectic alloy

Although the growth of tertiary dendrite is also important in influencing the properties of alloys, the analysis on it is relatively scarce since the growth of it is always assumed to catch up to the tips of growing primary dendrite. Distinct from this, the abnormal tertiary dendrites which grow towards lower temperature has been observed on thicker secondary branches during upward directional solidification of Sn–40at.%Mn peritectic alloy. The experimental result shows that the dendritic structure is influenced by not only the temperature gradient zone melting (TGZM) but also the Gibbs-Thomson (G-T) effects which can induce diffusion-controlled remelting/resolidification processes. Further examination on this abnormal tertiary dendrite arm shows its gradual decomposition during directional solidification. Analysis indicates that the decomposition of it is closely related with these remelting/resolidification processes. The remelting rate at the back surface of the thicker secondary branch is found to be lower than that at the tip of abnormal tertiary dendrite. However, the remelting process is first finished on the back edge of thicker secondary branch where no abnormal tertiary dendrite exist due to the shorter length there. A diffusion-controlled analytical model incorporating peritectic reaction and both effects is established to analyze the decomposition process, and the Ivantsov's equation-based solutions have been introduced, which agrees reasonably well with the experimental observation.

Ce Filling Limit and Its Influence on Thermoelectric Performance of Fe3CoSb12-Based Skutterudite Grown by a Temperature Gradient Zone Melting Method.

CoSb3-based skutterudite is a promising mid-temperature thermoelectric material. However, the high lattice thermal conductivity limits its further application. Filling is one of the most effective methods to reduce the lattice thermal conductivity. In this study, we investigate the Ce filling limit and its influence on thermoelectric properties of p-type Fe3CoSb12-based skutterudites grown by a temperature gradient zone melting (TGZM) method. Crystal structure and composition characterization suggests that a maximum filling fraction of Ce reaches 0.73 in a composition of Ce0.73Fe2.73Co1.18Sb12 prepared by the TGZM method. The Ce filling reduces the carrier concentration to 1.03 × 1020 cm−3 in the Ce1.25Fe3CoSb12, leading to an increased Seebeck coefficient. Density functional theory (DFT) calculation indicates that the Ce-filling introduces an impurity level near the Fermi level. Moreover, the rattling effect of the Ce fillers strengthens the short-wavelength phonon scattering and reduces the lattice thermal conductivity to 0.91 W m−1 K−1. These effects induce a maximum Seebeck coefficient of 168 μV K−1 and a lowest κ of 1.52 W m−1 K−1 at 693 K in the Ce1.25Fe3CoSb12, leading to a peak zT value of 0.65, which is 9 times higher than that of the unfilled Fe3CoSb12.

Impurity Removal Leading to High-Performance CoSb3-Based Skutterudites with Synergistic Carrier Concentration Optimization and Thermal Conductivity Reduction.

Thermoelectric properties of CoSb3-based skutterudites are greatly determined by the removal of detrimental impurities, such as (Fe/Co)Sb2, (Fe/Co)Sb, and Sb. In this study, we use a facile temperature gradient zone melting (TGZM) method to synthesize high-performance CoSb3-based skutterudites by impurity removal. After removing metallic or semimetallic impurities (Fe/Co)Sb, (Fe/Co)Sb2, and Sb, the carrier concentration of TGZM-Ce0.75Fe3CoSb12 can be reduced to 1.21 × 1020 cm-3 and the electronic thermal conductivity dramatically reduced to 0.7 W m-1 K-1 at 693 K. Additionally, removing these impurities also effectively reduces the lattice thermal conductivity from 7.2 W m-1 K-1 of cast-Ce0.75Fe3CoSb12 to 1.02 W m-1 K-1 of TGZM-Ce0.75Fe3CoSb12 at 693 K. As a consequence, TGZM-Ce0.75Fe3CoSb12 approaches a high power factor of 11.7 μW cm-1 K-2 and low thermal conductivity of 1.72 W m-1 K-1 at 693 K, leading to a peak zT of 0.48 at 693 K, which is 10 times higher than that of cast-Ce0.75Fe3CoSb12. This study indicates that our facile TGZM method can effectively synthesize high-performance CoSb3-based skutterudites by impurity removal and set up a solid foundation for further development.

Macrosegregation and thermosolutal convection-related freckle formation in directionally solidified Sn–Ni peritectic alloy in crucibles with different diameters

Different from other alloys, the observation in this work on the dendritic mushy zone shows that the freckles are formed in two different regions before and after peritectic reaction in directional solidification of Sn–Ni peritectic alloys. In addition, the experimental results demonstrate that the dendritic morphology is influenced by the temperature gradient zone melting and Gibbs–Thomson effects. A new Rayleigh number (RaP) is proposed in consideration of both effects and peritectic reaction. The prediction of RaP confirms the freckle formation in two regions during peritectic solidification. Besides, heavier thermosolutal convection in samples with larger diameter is also demonstrated.

Superheater deposits and corrosion in temperature gradient – Laboratory studies into effects of flue gas composition, initial deposit structure, and exposure time

The heterogeneous nature of the ash chemistry of biomass fuels gives rise to challenges in predicting the deposit melting, sintering, and enrichment of corrosive ash species. An experimental method has been developed to study the evolution of ash deposit chemistry and morphology in temperature gradients simulating the conditions of real superheater deposits. The method is based on applying synthetic ash mixtures on an air-cooled corrosion probe, which is inserted into a tube furnace. The focus has been on how the melting behavior of alkali salt-rich deposits, i.e., KCl–K2SO4–NaCl–Na2SO4 mixtures, affects the chemistry and morphology. Intradeposit vaporization-condensation of alkali chlorides has been of interest. The interaction of reactive gas components (H2O + SO2), with the deposits, was also studied. The vaporization-condensation mechanism leads to enrichment of alkali chlorides in crevices and voids within deposits, leading also to build-up of chlorides on the steel surface, which causes accelerated corrosion, due to the formation of low-melting FeCl2 mixtures. Liquid phase sintering and temperature gradient zone melting (TGZM) were the main mechanisms for the supersolidus sintering of the deposits. Iron and nickel oxides were found within the deposits and at the outer edge of deposits, due to the TGZM mechanism.

Temperature gradient induced changes within superheater ash deposits high in chlorine

Cross-sections of kraft recovery boiler superheater deposits were analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). The observed deposit morphology can be explained by temperature gradient induced time-dependent processes such as diffusional transport of alkali chloride vapours, temperature gradient zone melting, formation of melt enriched in Cl and K, and movement of this enriched melt towards the steel. These processes have recently been identified on a laboratory scale under well-controlled conditions, and are now for the first time identified to take place also in actual boiler superheater deposits. The identified processes alter the local deposit composition and melting behaviour close to the steel. The local first melting temperature (T0) close to the steel is lower by 30 °C compared to that of the deposit bulk T0. The observations made in this work give new insight into the melting and ageing behaviour of superheater deposits, relevant for superheater corrosion.

Thermosolutal convection-induced freckle formation in steady and unsteady directional solidification: Analysis in Sn-Ni peritectic alloy

Peritectic alloys have attracted more and more attention during past decades due to their growing applications. However, different from the current analysis on other alloys, freckle formation is witnessed in two regions in Sn–Ni peritectic alloy during directional solidification: the former (latter) one before (after) peritectic reaction. Besides, compared with the steady growth condition, fewer freckles can be observed in deceleration growth while more freckles are formed in acceleration growth. In addition, it has been demonstrated that the dendritic morphology is greatly influenced by the Gibbs-Thomson (G-T) and temperature gradient zone melting (TGZM) effects. A Rayleigh number RaP in consideration of both effects was proposed. The prediction of RaP in the mushy zone shows two maximums during peritectic solidification, confirming the freckle formation in two regions in this work. Furthermore, the influence of unsteady growth condition on the freckle formation can also be successfully predicted by RaP.

Liquid morphology during diffusion-controlled liquid migration in thermal stabilization of directional solidification

Thermal stabilization is crucial in directional solidification since the required melt concentration for the following directional growth is obtained through it. In the thermal stabilization stage, a mushy zone where the microstructure changes as thermals stabilization proceeds is formed. The melt concentration difference in liquid droplets in it can lead to diffusion-determined remelting/resolidification phenomena in the mushy zone in the imposed temperature gradient. This is also known as liquid migration by temperature gradient zone melting (TGZM) which can influence the melt concentration at the initial directional growth interface. However, in addition to the axial temperature gradient, a radial one is always present, especially in a high axial temperature gradient. Besides, although the liquid droplet was assumed to be elongated along the axial direction due to the difference in the remelting and resolidification rates, it has not been confirmed yet. Thus, the liquid migration in the presence of both axial and radial temperature gradients was analyzed in the Sn–Ni peritectic system (L + Ni3Sn2→Ni3Sn4) when an axial temperature gradient Ga of 40K/mm is imposed in this work. A diffusion-controlled analytical model was built to describe the liquid migration in the presence of both temperature gradients. Both the analytical prediction and experimental observation confirm that the morphology change of liquid droplets through its migration is not obvious by the TGZM effect. Instead, the growth of liquid droplets with increasing temperatures during their migration is more attributed to the conglomeration of them due to solute enrichment at the solid-liquid interface.

Effect of hot pressing on the microstructure and thermoelectric properties of TGZM-grown YbFe-doped CoSb3 skutterudite

Skutterudites based on CoSb3 are considered potential candidates in thermoelectric applications in the field of mid-temperature power generation. The annealing process necessitates more than a week to complete the peritectic phase transition for traditional melting-quenching-annealing-sintering methods. Herein, a new strategy combining the temperature gradient zone melting process with hot pressing (TGZM-HP) was successfully employed to synthesize YbFe-doped p-type CoSb3 in a shorter time. The resulting samples show enhanced thermoelectric properties compared to TGZM-grown ones, especially in the case of 1 at.% YbFe-doped CoSb3. This compound exhibits a maximal Seebeck coefficient of 166.3 μVK−1 and a power factor of 1.8 mWm−1K−2 at 723 K, showing an increase in these values of 25% and 51%, respectively. Moreover, phonon scattering appears to be enhanced due to the significant reduction in the grain size of the TGZM-HP samples, resulting in a minimum thermal conductivity of 2.71 Wm−1K−1. A dimensionless figure of merit (ZT) of 0.48 was obtained for 1 at.% YbFe-doped CoSb3. This value is 1.6 times larger than that measured for TGZM-grown CoSb3. This work provides an alternative strategy to fabricate skutterudite materials and foster their development.

Efficiently synthesized n-type CoSb3 thermoelectric alloys under TGZM effect

Skutterudite compounds, the structure of "phonon glass electron crystal" (PGEC), possess excellent thermoelectric properties. Here, temperature gradient zone melting combined with hot pressing technique (TGZM-HP) was employed to synthesize n-type skutterudite CexNi1.5Co2.5Sb12 thermoelectric material. By doping element Ce, the carrier concentration of the alloy was optimized efficiently and the total thermal conductivity declined. For the TGZM-HP Ce0.6Ni1.5Co2.5Sb12 alloy, the power factor PF reaches to 1350 μW/(mK2), and the lattice thermal conductivity is dropped to 1.62 Wm−1K1 at 850 K, corresponding to an improved figure of merit ZT of 0.45. Importantly, compared with the traditional synthesis methods, this technique can precisely control the formation of thermometric phase and the preparation time is reduced remarkably.

Macrosegregation and thermosolutal convection-induced freckle formation in dendritic mushy zone of directionally solidified Sn-Ni peritectic alloy

Compared with the growing applications of peritectic alloy, none research on the freckle formation during peritectic solidification has been reported before. Observation on the dendritic mushy zone of Sn-36 at.%Ni peritectic alloy during directional solidification at different growth velocities shows that the freckles are formed in two different regions: region I before peritectic reaction and region II after peritectic reaction. In addition, more freckles can be observed at lower growth velocities. Examination on the experimental results demonstrates that both the temperature gradient zone melting (TGZM) and Gibbs-Thomson (G–T) effects have obvious influences on the morphology of dendritic network during directional solidification. The current theories onKI Rayleigh number Ra characterizing the thermosolutal convection of dendritic mushy zone to predict freckle formation through the maximum of Ra can only explain the existence of region I while the appearance of region II after peritectic reaction cannot be predicted. Thus, a new Rayleigh number RaP is proposed in consideration of evolution of dendritic mushy zone by both effects and peritectic reaction. Theoretical prediction of RaP also shows a maximum after peritectic reaction in addition to that before peritectic reaction, thus, agreeing well with the freckle formation in region II. In addition, more severe thermosolutal convection can be predicted by the new Rayleigh number RaP at lower growth velocities, which further demonstrates the reliability of RaP in describing the dependence of freckle formation on growth velocity.

Analysis on fluid permeability of dendritic mushy zone during peritectic solidification in a temperature gradient

Compared with the growing applications of peritectic alloys, none research on the fluid permeability K of dendritic network during peritectic solidification has been reported before. The fluid permeability K of dendritic network in the mushy zone during directional solidification of Sn-Ni peritectic alloy was investigated in this study. Examination on the experimental results demonstrates that both the temperature gradient zone melting (TGZM) and Gibbs-Thomson (G–T) effects have obvious influences on the morphology of dendritic network during directional solidification. This is realized through different stages of liquid diffusion within dendritic mushy zone by these effects during directional solidification. The TGZM effect is demonstrated to play a more important role as compared with the G–T effect during directional solidification. Besides, it is shown that the evolution of dendrite network is more complex during peritectic solidification due to the involvement of the peritectic phase. Through the specific surface SV, analytical expression based on the Carman–Kozeny model was proposed to analyze the fluid permeability of dendritic mushy zone in directionally solidified peritectic alloys. In addition, it is interesting to find a rise in permeability K after peritectic reaction in both theoretical predication and experimental results, which is different from that in other alloys. The theoretical predictions show that this rise in fluid permeability K after peritectic reaction is caused by the remelting/resolidification process on dendritic structure by the TGZM and G–T effects during peritectic solidification.

Investigation on morphology evolution of secondary branch during peritectic solidification in a temperature gradient through morphology characteristic: Experimental measurement and prediction

Despite the obvious influence of the secondary branch on final microstructure and properties of alloy, investigation on the morphology variation of it during solidification is not enough. On the basis of a parameter ϕ, the morphology evolution of it during peritectic solidification was analyzed. Experimental measurements on two peritectic alloys: Sn–Ni and Sn–Mn show that ϕ first decreases then turns to increase with solidification time. The minimum of ϕ during peritectic solidification is found after peritectic reaction. Further examination shows that ϕ first decrease because it is mainly determined by the growth of secondary branch before peritectic reaction. Then, it is mainly dependent on the radius of secondary branches since the growth space for them is obviously restricted after peritectic reaction. Further analysis shows that the continuous increase of ϕ with solidification time after peritectic reaction is caused by the remelting/resolidification process by both the Gibbs-Thomson (G-T) and temperature gradient zone melting (TGZM) effects on secondary branch. It was also confirmed that the remelting/resolidification process by the G-T effect is less important as compared with that by the TGZM effect. As a result, resolidification on both the front and back edges of thicker secondary branches leads to the “V”-type variation of ϕ during peritectic solidification. The analytical prediction in which the influences of these effects are taken into account shows reasonably agreement with the experimental measurement.