Bulk Undercooling Research Articles

Correlated process of phase separation and microstructure evolution of ternary Co-Cu-Pb alloy

The phase separation and rapid solidification of liquid ternary Co45Cu42Pb13 immiscible alloy have been investigated under both bulk undercooling and containerless processing conditions. The undercooled bulk alloy is solidified as a vertical two-layer structure, whereas the containerlessly solidified alloy droplet is characterized by core-shell structures. The dendritic growth velocity of primary α(Co) phase shows a power-law relation to undercooling and achieves a maximum of 1.52 m/s at the undercooling of 112 K. The Pb content is always enriched in Cu-rich zone and depleted in Co-rich zone. Numerical analyses indicate that the Stokes motion, solutal Marangoni convection, thermal Marangoni convection, and interfacial energy play the main roles in the correlated process of macrosegregation evolution and microstructure formation.

Dynamic solidification mechanism of ternary Ag-Cu-Ge eutectic alloy under ultrasonic condition

The dynamic solidification of ternary Ag38.5Cu33.4Ge28.1 eutectic alloy within a 35 kHz ultrasonic field is investigated and compared with both its equilibrium solidification by DSC method and its rapid solidification in drop tube. The volume fractions of the primary (Ge) phase and pseudobinary (Ag+ɛ2) eutectic solidified within ultrasonic field are larger than those formed under equilibrium state, whereas that of ternary (Ag+ɛ2+Ge) eutectic exhibits the reverse trend. During rapid solidification, the liquid alloy droplet directly solidifies into ternary (Ag+ɛ2+Ge) eutectic if its diameter is smaller than 350 μm. The ultrasound stimulates the nucleation of alloy melt and prevents the bulk undercooling. With the increase of sound intensity, the primary (Ge) phase transfers from faceted dendrites to nonfaceted blocks with blunt edges, and its grain size is remarkably reduced. Both pseudobinary (Ag+ ɛ2) and ternary (Ag+ɛ2+Ge) eutectics experience a morphological transition from regular to anomalous structures. This indicates that their cooperative growth mode is replaced by independent growth of eutectic phases under the combined effects of cavitation and acoustic streaming. The ultrasound also shows a prominent coarsening effect to the pseudobinary (Ag+ɛ2) and ternary (Ag+ɛ2 +Ge) eutectics.

Dual solidification mechanisms of liquid ternary Fe-Cu-Sn alloy

Liquid ternary Fe47.5Cu47.5Sn5 alloy displayed dual solidification mechanisms when it was undercooled by up to 329 K (0.19TL). Below a critical undercooling of about 196 K, it solidified just like a normal peritectic alloy, even though metastable phase separation occurred to a microscopic extent. Once bulk undercooling exceeds 196 K, macroscopic segregation played a dominant role in solidification. In both cases, the solidification process was always characterized by two successive peritectic transformations: firstly primary γFe dendrites reacted with liquid phase to form (Cu) phase, and subsequently the (Cu) phase reacted with residual liquid phase to yield β-Cu5.6Sn intermetallic compound. The primary γFe dendrites achieved a maximum growth velocity of 400 mm/s and experienced a growth kinetics transition as a result of macrosegregation. Since the (Cu) phase was both the product phase of the first peritectic transformation and also the reactant phase for the second peritectic transformation, it appeared as two layers in solidification microstructures due to the microsegregation of Sn solute. The boundary continuity between the macroscopically separated Fe-rich and Cu-rich zones was enhanced with the increase of undercooling.

Crystal Growth in Al72.9Ge27.1 Alloy Melt under Acoustic Levitation Conditions

The nonequilibrium solidification of liquid Al72.9Ge27.1 hypoeutectic alloy is accomplished by using single-axis acoustic levitation. A maximum undercooling of 112K (0.16TL) is obtained for the alloy melt at a cooling rate of 50 K/s. The primary (Al) phase displays a morphological transition from coarse dendrite under a normal conditions to equiaxed grain under acoustic levitation. In the (Al)+(Ge) eutectic, the (Ge) phase exhibits a conspicuous branched growth morphology. Both the primary (Al) dendrites and (Al)+(Ge) eutectics are well refined and the solute content of the primary (Al) phase is extended under acoustic levitation. The calculated and experimental results indicate that the solute trapping effect becomes more intensive with the enhancement of bulk undercooling.

Microstructure evolution of the undercooled Co–Ni–Ga magnetic shape memory alloy

Microstructure evolution of the martensite Co 50Ni 22Ga 28 magnetic shape memory alloy was investigated by the method of glass fluxing combined with superheating cycling. A maximum bulk undercooling of 220 K is achieved and a mass of sub-grains are introduced by the undercooling. A number of dislocations and large internal stress, due to the undercooling rapid solidification, give rise to the production of sub-grains during the recovery process. These sub-grains, as well as martensite variants, are remarkably refined with the increase of undercooling. The recrystallization and growth of sub-grains during the following annealing are also discussed.

Microstructure evolution and non-equilibrium solidification of undercooled Ni-29.8at% Si eutectic alloy melts

Microstructure formation and transition of undercooled bulk Ni70.2Si29.8 eutectic alloy melt were investigated by melt fluxing, cyclical overheating and cooling under high-frequency vacuum melting. The maximum undercooling of the alloy melt amounted to 428 K. Scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDS) and optical microscopy techniques (OM) were adopted to investigate the microstructure and identify the phase composition. The cooling curves of eutectic alloys upon solidification which were subjected to different undercoolings were described and compared. The complex microstructure evolution was observed in the as-solidified samples with the increase of undercooling. Surprisingly, an extremely fine microstructure was achieved at the max undercooling of 428 K, and the lamellar distance of about 50–100 nm was observed. Based on the solution entropy of eutectic phases, the microstructure transition with the undercooling was analyzed. Calculated results showed that the microstructure transition process was ascribed to solution entropy of transition, i.e., the complex microstructure evolution was attributed to a transition from faceted-faceted (FF) → faceted-nonfaceted (FN) → nonfaceted-nonfaceted (NN) eutectic systems concurring with increased undercooling.

Refinement of the γ phase with melt undercooling in a two-phase Co 2NiGa magnetic shape memory alloy

The effect of melt undercooling on the γ phase is investigated in a two-phase Co 2NiGa magnetic shape memory alloy by employing the method of glass fluxing combined with superheating cycling. A maximum bulk undercooling of 230 K is achieved. Microstructural evolution of the Co 2NiGa alloy is investigated by optical microscopy with respect to different degrees of undercooling. Over the whole range of undercooling, it is found that both the size and the volume fraction of the γ phase are drastically influenced by undercooling. Especially, when the undercooling is in the range of 190–230 K, the alloy has an obvious preferred orientation.

Undercooling and solidification of Ni77P23 alloy in a 52-m drop tube

This paper applies techniques of containerless processing, drop tube and glass fluxing, to undercool and solidify Ni77P23 alloys. Different diameter spheres were collected at the bottom of a 52-m long drop tube. Both crystalline and amorphous phase were formed in various size specimens due to the different cooling rate. The variation of partial undercooling with bulk undercooling is calculated for the Ni77P23 alloys. The deep undercooling and rapid solidification behaviour of Ni77P23 melts has been analysed with respect to microstructure formation and transition during fluxing and 52-m drop process of undercooled melts.

TWO-STATE STRUCTURE OF NANOMETER-SIZED Cu PARTICLES

Molecular dynamics simulations have been employed to investigate the static and dynamic properties of unsupported spherical Cu particles with size ranging between 1 and 10 nm. The potential energy, the structural arrangement, and the mobility of atomic species were studied for each nanometer-sized particle within the temperature range between 300 K and the melting point. Two distinct regions, namely an internal domain and a surface layer, can be identified within each nanoparticle on the basis of the radial profiles of such quantities. The atomic species belonging to the interior of the particle display a bulk-like behavior. By contrast, the surface layer is characterized by an excess potential energy associated with a disordered structure. At relatively low temperatures, the surface atoms possess structural and energetic features intermediate between the ones of a superheated bulk solid and of an undercooled bulk liquid. Pre-melting processes at the surface are also evident at temperatures close to the melting point. The nanometer-sized particles can be thus regarded as heterogeneous two-state systems consisting of roughly distinguishable bulk-like and surface regions.

Effect of denucleating glass composition on undercooling of Fe83Ga17 alloy melts

Abstract Adopting glass fluxing combined with superheating cycling method, the undercooling and its stability of Fe 83 Ga 17 alloy melts were investigated using different kinds of denucleating glass: B 2 O 3 , 90% NaSiCa + 10% B 2 O 3 (simplified as Na–Si–Ca–Al–B) and 70% Na–Si–Ca–Al–B + 30% Na 2 B 7 O 4 . The results showed that different glass has different denucleating mechanism. The purification of B 2 O 3 glass is only a physical process, by which the stable bulk undercooling cannot be obtained during superheating–cooling cycles. While taking Na–Si–Ca–Al–B glass as purifying agent, its denucleating mechanism is a comprehensively physicochemical process. But the stability of undercooling is still undesirable because of the separation between melt and glass during cooling process in superheating cycling. A stable bulk undercooling can be obtained by physicochemical denucleating process in the case of 70% Na–Si–Ca–Al–B + 30% Na 2 B 7 O 4 molten glass owing to its suitable viscosity.

Observations of nucleation catalysis effects during solidification of SnAgCuX solder joints

While modification of a strong (high Cu) Sn-Ag-Cu (SAC) solder alloy with a substitutional alloy addition (X=Co, Fe, Zn, and Ni) for Cu has been demonstrated to enhance solder joint strength and ductility after aging at 150°C for 1,000 h, control of the as-solidified SAC+X solder joint microstructure is also needed to inhibit under cooling and nucleation of brittle pro-eutectic phases (e.g., Ag3Sn). Bulk undercooling measurements of SAC+X alloys and microstructural analysis of SAC+X solder joints were used to rank the effectiveness and consistency of low-level (X < 0.15 wt.%) substitutional additions to a base SAC composition, Sn-3.5Ag-0.95Cu (wt.%). This SAC composition was selected to favor thermodynamically the nucleation of pro-eutectic Cu6Sn5 over that of Ag3Sn and the formation of an enhanced ternary eutectic fraction in the joint microstructure, while retaining a pasty range that is only 3°C. Using differential scanning calorimetry with sample pans that serve as either inert (aluminum) or actively wetting (copper) substrates, reflow cycles were studied that simulated surface mount (1.5°C/s) and ball-grid array (0.17°C/s) cooling rates. Of the SAC+X solders tested with copper pans, X = Zn appeared to be most effective and consistent, providing catalytic enhancement of the nucleation temperature for even the minimum concentration (0.05 wt.%) and lowest cooling rate.

Solidification characteristics of Pb-Sb hypereutectic alloy within ultrasonic field

The solidification of Pb-16%Sb hypereutectic alloy is investigated within ultrasonic field with a frequency of 15 kHz. It is found that the ultrasonic field promotes crystal nucleation and terminates the further bulk undercooling of the alloy melt. Theoretical analysis shows that the cavitation effect and the forced bulk vibration are the main factors that reduce the undercooling level. With the increase of ultrasound intensity, the primary (Sb) phase experiences a growth mode transition from faceted to non-faceted branched growth, and the macrosegregation of primary (Sb) phase is gradually suppressed. In addition, the microstructures of Pb-Sb eutectic exhibit a conspicuous coarsening with increasing ultrasound intensity, and a structural transition of “lamellar eutectic—anomalous eutectic” occurs when ultrasound intensity rises up to 1.6 W/cm2. The ultrasonic field also changes the solute distribution adjacent to the solidification front, which lowers the Pb contents in primary (Sb) phase.

Rapid solidification of highly undercooled bulk liquid superalloy: recent developments, future directions

The status of rapid solidification of highly undercooled bulk liquid superalloys is critically evaluated and future directions of research in this technologically important area are proposed. First, consideration is given to purification mechanisms and preparation of a novel SiO2–ZrO2–B2O3 (i.e. Si–Zr–B) compound mould coating, for multi-component superalloy melts, with an emphasis on DD3 alloy. On this basis, thermodynamics of the undercooled multi-component melt and the nucleation inhibiting effect of the Si–Zr–B coating are discussed. Consideration is also given to evolution of microstructure and sub-structure within a wide undercooling regime, in particular, the mechanism of dendrite growth and the characteristics of rapid solidification. Growth mechanisms for the two types of grain morphology, at low and high undercoolings, are reviewed, as is precipitation of the strengthening phase, γ' (Ni3Al(Ti)), in the as solidified superalloy subjected to different melt undercoolings. Finally, directional rapid solidification of the undercooled superalloy melt in Si–Zr–B coated moulds is considered, which leads on to discussion of the mechanical properties of the ultra-fine directional dendritic structures produced at different melt undercoolings.

Undercooling and solidification behavior of magnetostrictive Fe–20 at.% Ga alloys

Using electromagnetic levitation melting combining fluxing method, the Fe–20at.% Ga alloy was undercooled through heating–cooling cycles. The samples, after recalescence, cooled down to about 1173K, and then quenched into ice–water mixtures to maintain the microstructure of rapid solidification. A maximum bulk undercooling of 337K was achieved. Within the undercooling range of 0–337K, the microstructure evolution of bulk undercooled Fe–20at.% Ga was investigated. Over the whole range of undercooling, the solidification microstructures were equiaxed grains. When the undercooling was less than 221K, the grain size decreased smoothly with increasing undercooling and had homogeneous distribution. When the undercooling reached the range of 230–315K, the fine equiaxed grains and abnormal large grains coexisted, and the area fraction of abnormal large grains increased with increasing undercooling. When the undercooling exceeded 320K, only large grains existed. Especially, when the undercooling was in the range of 173–221K, the grain had preferred orientation. The Fe–20at.% Ga alloy rods with [001] preferred orientation was prepared by exciting the undercooled Fe–20at.% Ga melt at undercooling 205K and the [001] preferred orientation was approximately 11.42° away from the rod axis.

Phase selection in undercooled Nd–Fe–Co–B alloy droplets

Abstract Nd14Fe69Co10B7 alloys were levitated, melted and solidified using the electromagnetic levitation technique for the purpose of studying the effect of cobalt addition on phase selection in undercooled Nd–Fe–B alloy melts. Bulk undercoolings of up to 80 K were achieved in the levitated melt droplets. Similar to cobalt-free alloys, γ-(Fe, Co) solid solution, φ-Nd2(Fe, Co)14B compound, and the metastable χ-Nd2(Fe, Co)17Bx compound (x~1) were solidified primarily in sequence of the increasing bulk undercooling. The critical undercoolings for the primary formation of the φ-phase and of the χ-phase were determined to be 50 and 65 K, respectively. Compositional analysis showed that the γ-phase has a lower cobalt concentration than that of the bulk alloys, whereas the φ- and χ-phases have a cobalt concentration close to that of the bulk alloys. However, the cobalt concentrations of all the three phases do not change significantly with the bulk undercooling of the samples.

Solidification velocity of undercooled Ni–Cu alloys

The solidification velocity of Ni–Cu alloys was measured as a function of bulk undercooling using high-speed thermal imaging of electromagnetically levitated samples. Two departures from power law growth (approximating plateaus) in the velocity versus undercooling data were observed: the first occurred at intermediate undercoolings and is attributed to copper solute, while the second occurred at high undercoolings and is hypothesized to be an effect of oxygen. The Ivantsov solution with marginal stability arguments (IMS model) is a widely used model that relates dendrite growth velocity to total undercooling for dilute alloy systems. However, the model does not predict a plateau at intermediate undercoolings for alloys with a large equilibrium partition coefficient, k E. Satisfactory agreement between the model and experimental results can be obtained by using a value of k E that is smaller than the alloy’s actual value, but this is physically unreasonable and causes disagreement with currently accepted kinetic models.

High undercooling of bulk water during acoustic levitation

The experiments on undercooling of acoustically levitated water drops with the radius of 5–8 mm are carried out, and the maximum undercooling of 24 K is obtained in such a containerless state. Various factors influencing the undercoolability of water under acoustic levitation are synthetically analyzed. The experimental results indicate that impurities tend to decrease the undercooling level, whereas the dominant factor is the effect of ultrasound. The stirring and cavitation effects of ultrasound tend to stimulate the nucleation of water and prevent further bulk undercooling in experiments. The stirring effect provides some extra energy fluctuation to overcome the thermodynamic barrier for nucleation. The local high pressure caused by cavitation effect increases the local undercooling in water and stimulates nucleation before the achievement of a large bulk undercooling. According to the cooling curves, the dendrite growth velocity of ice is estimated, which is in good agreement with the theoretical prediction at the lower undercooling. The theoretical calculation predicts a dendrite growth velocity of 0.23 m/s corresponding to the maximum undercooling of 24 K, at which the rapid solidification of ice occurs.

Solidification of levitated Nd–Fe–B alloy droplets at significant bulk undercoolings

Nd–Fe–B alloys with composition of Nd 16Fe 76B 8, Nd 18Fe 73B 9 and Nd 22Fe 67B 11 were melted and solidified using an electromagnetic levitation technique. Two types of solidification behavior were observed depending on the bulk undercooling achieved prior to solidification. The samples with small undercoolings were solidified by the predominant primary formation of the Nd 2Fe 14B compound, whereas those with large undercoolings were solidified by the primary formation of the metastable Nd 2Fe 17B x compound ( x∼1) plus the subsequent formation of Nd 2Fe 14B. The critical undercooling for the primary Nd 2Fe 17B x formation was determined to be 40, 70 and 130 K in the three alloy compositions, respectively. The liquidus temperature of the metastable Nd 2Fe 17B x compound was estimated to be 1423, 1393 and 1283 K, respectively. The Nd 2Fe 17B x compound was found to decompose into a mixture of Fe plus Nd 2Fe 14B due to the slow cooling rates of the samples. It was suggested that the metastable Nd 2Fe 17B x compound may have a lower interfacial energy than that of the stable Nd 2Fe 14B compound, hence being favored in nucleation-controlled phase selection at large undercoolings.

Phase separation and structural evolution of undercooled Fe–Sn monotectic alloy

Abstract Fe–48.8wt.% Sn monotectic alloy was rapidly solidified both in a drop tube and with the glass fluxing method, and its phase separation characteristics and structural evolution are investigated. The regime of cooling rate of different size droplets in the drop tube is calculated from 7.0×102 to 2.8×104 K s−1 with a decrease of droplet diameter from 800 to 100 μm. The effect of Marangoni migration is more significant than that of Stokes motion with the decrease of gravitational acceleration, but it has a weak influence on the dispersion of the second liquid phase. The immiscibility gap is calculated and it shows that spinodal decomposition is difficult to occur because the necessary undercooling is quite large. Nucleation and growth show different characteristics when droplet size varies. Although the Sn-rich liquid phase separates from the undercooled melt, it cannot change significantly the composition of parent liquid phase, and the growth of terminal composition solid is suppressed, which is confirmed by the measured and calculated dendritic growth velocity by bulk undercooling experiments with the glass fluxing technique. Well-branched dendrites of the solid phase are difficult to form during monotectic transformation when droplets solidify in the drop tube owing to the very short solidification period.

Critical undercoolings for different primary phase formation in Nd/sub 15/Fe/sub 77.5/B/sub 7.5/ alloy

Nd/sub 15/Fe/sub 77.5/B/sub 7.5/ alloys were melted and solidified using the electromagnetic levitation technique. Bulk undercoolings of up to 78 K were achieved. Three kinds of primary phase formation were observed depending on the undercooling level. /spl gamma/-Fe solid solution was solidified as primary phase at low undercoolings. It was replaced by the peritectic /spl phi/-Nd/sub 2/Fe/sub 14/B compound upon reaching a critical undercooling of 40 K. The primary phase was changed further to the metastable /spl chi/-Nd/sub 2/Fe/sub 17/B/sub x/ (x = 0 - 1) compound above a second critical undercooling of 55 K. However, /spl gamma/-Fe formation reentered the samples with high undercoolings because of a strong recalescence. The /spl chi/-Nd/sub 2/Fe/sub 17/B/sub x/ compound was found to have decomposed during the post-solidification process, which also gave rise to a small amount of /spl gamma/-Fe in the solidified samples. Nevertheless, it was found that the total amount of the /spl gamma/-Fe phase in the samples with high undercoolings has been reduced due to the primary solidification of the two compound phases.