Ni Peritectic Alloy Research Articles

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.

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.

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.

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.

Diffusion-induced liquid migration during thermal stabilization in presence of temperature gradients with different directions

Abstract The initial growth conditions of directional solidification are obtained after the thermal stabilization stage during which a mushy zone is formed. The difference in melt concentrations within the liquid droplets in the mushy zone by the axial temperature gradient can induce simultaneous remelting/resolidification by temperature gradient zone melting (TGZM). This is known as the liquid migration which is closely related to the initial growth condition of directional solidification. However, a radial temperature gradient is always present in addition to the axial one, while the influence of it on liquid migration has not been discussed yet. Thus, the liquid migration in the presences of both axial and radial temperature gradients was investigated in Sn Ni peritectic alloy. A diffusion-controlled model was established to predict liquid migration when both the axial and radial temperature gradients are taken into consideration. Both analytical result and experimental measurement confirmed that the radial temperature gradient played an important role in liquid migration. Further analysis has shown that the influence of the radial gradient was different at different parts of the mushy zones.

Restriction or acceleration: Investigation on the remelting/resolidification process during Sn–Ni peritectic solidification in a temperature gradient

In this work, extensive remelting/resolidification process induced by diffusion on the secondary dendrite arms in the mushy zone was confirmed in a Sn–Ni peritectic alloy. Examination on the dependence of λ2 on local solidification time showed that the remelting/resolidification process on secondary dendrite arm by the Gibbs-Thomson effect was restricted at the initial of peritectic reaction. Then, this restriction turns to disappear as directional solidification proceeds. Experimental observation shows that the remelting/resolidification on secondary dendrite arm could be attributed to both the Gibbs-Thomson and temperature gradient zone melting (TGZM) effect. Further analysis showed that this remelting/resolidification process was complicated in peritectic alloys due to interaction between peritectic reaction and the TGZM effect by the imposed temperature gradient. It was found that the influence of peritectic reaction on the remelting/resolidification process was different at different stages of solidification. The restriction or acceleration of the Gibbs-Thomson effect by peritectic reaction at different stages of solidification was clarified by diffusion-controlled analytical model describing the remelting/resolidification process. The dependence of the remelting/resolidification process on secondary dendrite arm on solidification time can be attributed to the interaction between peritectic reaction, the Gibbs-Thomson and TGZM effects.

Determination of the liquid diffusion coefficient of the Sn-Ni peritectic alloy through temperature gradient zone melting

In the present work, a new method determining the liquid phase diffusion coefficients DL in a binary peritectic alloy is presented. This method is based on the evolution in both the microstructure and melt concentrations which can be linked through liquid droplet migration in the mushy zone during thermal stabilization. The thermal stabilization experiments of different time (2 to 8 h) are performed on Sn–Ni peritectic alloy (L+Ni3Sn2→Ni3Sn4) in a Bridgman-type directional solidification furnace. Two different mushy zones are formed during thermal stabilization of samples which are kept still in the furnace. The diffusion-controlled liquid droplet migration is caused by remelting/resolidification by temperature gradient zone melting (TGZM). It leads to not only the variation of volume fractions of solid(fS)/liquid(fL) phases in the mushy zone but also the increase of the melt concentration in the complete-liquid zone C. Thus, based on the conservation of mass during thermal stabilization, fS and fL can be correlated to not only the liquid droplet migration velocity vm but also the melt concentration in the complete-liquid zone C during thermal stabilization. Since fS and fL can be easily obtained through microstructure analysis, the experimental value of vm at specific location/temperature is given. Then, the expression of vm which is function of DL is provided by an analytical model describing the remelting/resolidification process during the liquid migration. Finally, DL and the activation energy of the Sn-Ni peritectic alloy are obtained by equating the experimental values of vm with the expression of vm by this analytical model.

Influence of radial convection on microstructure development in Sn-Ni peritectic system in a temperature gradient

Abstract The influence of radial convection on the microstructure development has been investigated in directionally solidified Sn-36at.%Ni peritectic alloys at a constant growth velocity with different sample diameters. Since the rejected Sn is denser than Ni, there existed no axial convection in directionally solidified Sn-36at.%Ni peritectic alloys, however, the radial convection still influenced the microstructures. It was demonstrated through the dendrite tip radius R that the diffusive growth condition could be obtained in this alloy when the sample diameter was reduced to 1 mm. Furthermore, it was confirmed that the radial convection could obviously promote the peritectic transformation process in Sn-Ni peritectic alloy.

Migration of liquid particle from mushy zone interface in temperature gradient

The thermal stabilization process after which directional growth of crystals initiates is crucial in obtaining the initial growth conditions of directional solidification. A mushy zone is formed during thermal stabilization due to the imposed temperature gradient. The difference in melt concentrations across the liquid particles in the mushy zone induces simultaneous remelting/resolidification by the temperature gradient zone melting (TGZM), and this phenomenon is called the migration of liquid particle. It is usually assumed that the migration of liquid particle initiates from the superheated solid phases in the mushy zone in single phase alloy. However, the migration of liquid particles from the mushy zone interface at peritectic temperature TP during thermal stabilization has been confirmed in a Sn–Ni peritectic alloy (Liquid + Ni3Sn2 → Ni3Sn4) in this work. This newly observed migration of the liquid particles from the mushy zone interface has been described to simultaneous remelting/resolidification at the leading/trailing interface of the liquid particles through a diffusion-controlled analytical model.

Determination of solid–liquid interfacial energy of Ni3Sn2 phase by grain boundary groove method in a temperature gradient

The Ni3Sn2 phase plays an important role in numerous fields including solder alloys and lithium storage. However, the precise measurements of many parameters which are essential in describing the physical properties of this phase, such the Gibbs-Thomson coefficient, solid–liquid interfacial energy, grain boundary energy and entropy of fusion per unit volume have not been carried out yet. In this work, the equilibrated grain boundary groove (GBG) shapes for a solid Ni3Sn2 phase in equilibrium with the liquid were observed from Sn-36 at.%Ni peritectic alloy in a linear temperature gradient by using a Bridgman type directional solidification apparatus after experiencing a thermal stabilization time of 8 h. The time required to obtain the GBG shapes in this alloy with a large freezing range was analyzed through the temperature gradient zone melting (TGZM) theory. The Gibbs-Thomson coefficient, solid-liquid interfacial energy and grain boundary energy for the solid Ni3Sn2 phase in equilibrium with liquid were determined in this work, which will help the growth and property control of the fields mentioned above.

General model on position of the solid/liquid interface during preparation of a directionally solidified peritectic alloy containing intermetallic compound phases

Abstract Based on our earlier work on thermal stabilization of Sn Ni peritectic alloy where both the primary and peritectic phases are intermetallic compound phases in an imposed temperature gradient, the migration of the solid/liquid interface and its position during thermal stabilization have been analyzed. The thermal stabilization after which the directional solidification takes place determines the initial growth condition of directional solidification, like the position (migration) of the solid/liquid interface. The migration of the solid/liquid interface is a rather interesting and complex phenomenon, which is directly determined by the relationship between three different melt concentrations: melt concentration of the boundary layer adjoining the solid/liquid interface CL, melt concentration of the complete-mixed zone Cm and the equilibrium melt concentration at the solid/liquid interface Ce. Both the downward migration by remelting of solid phases and upward migration by resolidification of solid phases have been discussed in this work. Downward/upward migration of the solid/liquid interface continues if CL is larger/smaller than Ce. During migration of the solid/liquid interface, the solute boundary layer is gradually destroyed. Analytical models have been established to describe the migration of the solid/liquid interface which is rather complex in peritectic systems.

Response and prediction of microstructures indicating deceleration growth condition in a Sn-Ni peritectic alloy

The Sn-36 at.%Ni peritectic alloy in which both the primary and peritectic phases are intermetallic compounds was directionally solidified attending different deceleration growth conditions. The cellular-dendrite growth of the primary Ni3Sn2 phase was studied in this analysis, and the influence of the deceleration rate on the microstructure length scales including primary/higher order dendrite arm spacing (λ1, λ2, λ3) and dendrite tip radius (R) were analyzed. Experimental results showed that although these length scales all decreased with the increasing deceleration rate, the responses of the secondary dendrite arm spacing λ2 and the tip radius of primary dendrite R are faster than that of λ1. This difference in response leads to a larger variation in the λ1/λ2 ratio with the increasing deceleration rates, as compared with the λ2/R ratio. Furthermore, the ratio of the thickness of primary dendrite stem d to the primary dendrite arm spacing λ1, d/λ1 was used to predict the melt concentration at the solid/liquid interface Cl. These length scales were related to the melt concentration Cl which is obtained through the d/λ1 ratio.

Migration of liquid phase from the primary/peritectic interface in a temperature gradient

The migration of the liquid droplets from the primary α/peritectic β interface at the peritectic temperature TP has been observed and analyzed in a Sn–Ni peritectic alloy. During the isothermal annealing stage of the interrupted directional solidification, a concentration gradient is established across the liquid droplets along the direction of the temperature gradient due to the temperature gradient zone melting. Simultaneous remelting/resolidification at the top/bottom of the liquid droplets by this concentration gradient have been confirmed to lead to migration of these droplets towards higher temperatures. The dependence of the migration distance of the liquid droplets on isothermal annealing time has been well predicted. Furthermore, since the lengths of the liquid droplet are not uniform along the direction of the temperature gradient, the remelting/resolidification rates which are dependent on the local morphology of liquid droplet are different at different local positions of the liquid droplets. It has been demonstrated that the morphology of the liquid droplet was also influenced by the morphologies of the liquid phase themselves. Therefore, the morphology of the liquid droplet itself changes from spherical to some kinds of irregular shapes during its migration.

Detachment of secondary dendrite arm in a directionally solidified Sn-Ni peritectic alloy under deceleration growth condition.

In order to better understand the detachment mechanism of secondary dendrite arm during peritectic solidification, the detachment of secondary dendrite arm from the primary dendrite arms in directionally solidified Sn-36at.%Ni peritectic alloys is investigated at different deceleration rates. Extensive detachment of secondary dendrite arms from primary stem is observed below peritectic reaction temperature TP. And an analytical model is established to characterize the detachment process in terms of the secondary dendrite arm spacing λ2, the root radius of detached arms and the specific surface area (SV) of dendrites. It is found that the detachment mechanism is caused by not only curvature difference between the tips and roots of secondary branches, but also that between the thicker secondary branches and the thinner ones. Besides, this detachment process is significantly accelerated by the temperature gradient zone melting (TGZM) effect during peritectic solidification. It is demonstrated that the reaction constant (f) which is used to characterize the kinetics of peritectic reaction is crucial for the determination of the detachment process. The value of f not only changes with growth rate but also with solidification time at a given deceleration rate. In conclusion, these findings help the better understanding of the detachment mechanism.

Response of primary dendrite distribution to deceleration growth conditions in a Sn[sbnd]Ni peritectic alloy

The primary dendrite arm spacing (PDAS) λ1 and its distribution of primary Ni3Sn2 phase has been examined in directionally solidified Sn-36 at.%Ni peritectic alloys during the transient conditions attending deceleration. Dependences of λ1 on both the deceleration rate and solidification time have been investigated through the statistical techniques including minimal spanning tree (MST) and Voronoi polygon. The existence of a range of PDAS is confirmed, and this range Δ is closely related with the deceleration rates. This range also increases with increasing solidification time at a fixed deceleration rate. Both the analyses by MST and Voronoi polygon show that the distribution of λ1 is greatly influenced by decelerating growth condition. The range of λ1, Δ can well reflect the history-dependent growth of primary dendrite arms. In addition, due to the solidification characteristic of SnNi peritectic system, λ1 does not response quickly to the variation in growth conditions.

On oscillatory microstructure during cellular growth of directionally solidified Sn-36at.%Ni peritectic alloy.

An oscillatory microstructure has been observed during deep-cellular growth of directionally solidified Sn–36at.%Ni hyperperitectic alloy containing intermetallic compounds with narrow solubility range. This oscillatory microstructure with a dimension of tens of micrometers has been observed for the first time. The morphology of this wave-like oscillatory structure is similar to secondary dendrite arms, and can be observed only in some local positions of the sample. Through analysis such as successive sectioning of the sample, it can be concluded that this oscillatory microstructure is caused by oscillatory convection of the mushy zone during solidification. And the influence of convection on this oscillatory microstructure was characterized through comparison between experimental and calculations results on the wavelength. Besides, the change in morphology of this oscillatory microstructure has been proved to be caused by peritectic transformation during solidification. Furthermore, the melt concentration increases continuously during solidification of intermetallic compounds with narrow solubility range, which helps formation of this oscillatory microstructure.

On migration of primary/peritectic interface during interrupted directional solidification of Sn-Ni peritectic alloy

The migration of the primary/peritectic interface in local isothermal condition is observed in dendritic structure of Sn–Ni peritectic alloy after experiencing interrupted directional solidification. It was observed that this migration of primary Ni3Sn2/peritectic Ni3Sn4 interface towards the primary Ni3Sn2 phase was accompanied by migration of liquid film located at this interface. The migration velocity of this interface was confirmed to be much faster than that of peritectic transformation, so this migration was mostly caused by superheating of primary Ni3Sn2 phase below TP, leading to nucleation and migration of liquid film at this interface. This migration can be classified as a kind of liquid film migration (LFM), and the migration velocity at the horizontal direction has been confirmed to be much faster than that along the direction of temperature gradient. Analytical prediction has shown that the migration of liquid film could be divided into two stages depending on whether primary phase exists below TP. If the isothermal annealing time is not long enough, both the liquid film and the primary/peritectic interface migrate towards the primary phase until the superheated primary phase has all been dissolved. Then, this migration process towards higher temperature is controlled by temperature gradient zone melting (TGZM).

Microsegregation of peritectic systems in a temperature gradient: Analysis in directionally solidified Sn–36at.%Ni peritectic alloy

Microsegregation is influenced by many factors during solicitation while the coupling effects of some important factors have been dismissed. During directional solidification of Sn–36at.%Ni peritectic alloys under high temperature gradient, melting/solidification of both the primary and peritectic phase caused by temperature gradient zone melting has been observed. An analytical model has been proposed to analyze the effect of temperature gradient zone melting on microsegregation of interdendritic melt in peritectic alloys. The coupling influences of peritectic reaction, coarsening process and solid state back-diffusion have all been included in this model. Both the experimental and theoretical analyses have demonstrated that melting/solidification of both the primary and peritectic phase could be accelerated by temperature gradient zone melting during peritectic solidification. Thus, microsegregation of interdendritic melt is restricted by temperature gradient zone melting during peritectic solidification. The analytical model in the present work shows inherent consistence with our previous models.

Composition-dependent phase substitution in directionally solidified Sn-22at.%Ni peritectic alloy

Growth substitution of the primary Ni3Sn2 phase by peritectic Ni3Sn4 phase during solidification has been observed for the first time in directionally solidified Sn-22at.%Ni peritectic alloy. Dependence of this phase substitution phenomenon on growth distance, and thus composition of liquid phase has been confirmed, which shows significant deviations from the present understanding that the growth should be distance-independent. This substitution process is shown to arise from the continuous variation in melt concentration at the solid/liquid interface during growth of primary Ni3Sn2/peritectic Ni3Sn4 phases which are intermetallic compounds with narrow solubility ranges. An analytical model based on solute conservation during solidification has been presented to describe this phase substitution process, which gives well agreement with the experimental results. Besides, the growth distance required for phase substitution is inversely proportional to the growth velocity. It has also been demonstrated that the steady-state melt concentration could not be achieved during growth of peritectic systems containing intermetallic compounds with narrow solubility range, such as Sn-Ni peritectic alloys. This indicated that steady-state growth can not be obtained during solidification this kind of alloys.

Influence of solutal convection on solute distribution of melt during preparation of directionally solidified Sn–36 at.%Ni peritectic alloy

Experiments consisting of melting followed by thermal stabilization on Sn–36at.%Ni peritectic alloy have been carried out in a Bridgman-type furnace. Due to imposed temperature gradient, a mushy zone is created between the complete liquid zone and the non-molten zone. Microstructure evolution and solute distribution in the melt during thermal stabilization have been characterized. As thermal stabilization time increases, the volume fraction of liquid in the mushy zone decreases. A Sn boundary layer which results from evacuation of liquid in the mushy zone is built up at the solid/liquid interface at the initial of thermal stabilization then gradually disappears. A model is proposed to describe this boundary layer which is influenced by solutal convection during thermal stabilization. It is found that solute segregation in the complete liquid zone can be destroyed by solutal convection. This solute boundary layer can lead to further downward migration of the solid/liquid interface during thermal stabilization.