Drop Tube Technique Research Articles

Dendrite growth kinetics and microstructure evolution mechanism of Ni2TiAl intermetallic compound within substantially undercooled liquid Ni64Al19Ti17 alloy

The dendrite growth kinetics and microstructure evolution mechanism of the Ni2TiAl intermetallic compound within substantially undercooled liquid Ni64Al19Ti17 alloy were investigated using electrostatic levitation (ESL) and drop tube (DT) techniques. The growth velocity of primary Ni2TiAl dendrites displayed a power-law relation with liquid undercooling, achieving a maximum of 80 mm/s at the undercooling of 254 K (0.16 TL). The solidification pathway transformed from primary Ni2TiAl growth plus subsequent (Ni3Ti+Ni2TiAl) eutectic growth into the successive growth of Ni2TiAl and Ni3Ti phases as the alloy undercooling increased sufficiently. The high cooling rates of freely falling alloy droplets effectively suppressed the martensitic transformation observed for primary Ni2TiAl intermetallic compound under typical ESL radiative cooling condition. Both nanoindentation and Vickers hardness performances of the Ni2TiAl phase formed under these two conditions exhibited conspicuous enhancement due to grain refinement effects. The microstructures with martensitic transformation under ESL condition showed higher hardness than those under DT condition despite their larger grain size.

Complete solute trapping of rapidly growing nickel dendrites within liquid Ni72Mo28 hypoeutectic alloy

The rapid dendrite growth in highly undercooled liquid Ni72Mo28 hypoeutectic alloy was accomplished by containerless processing via electromagnetic levitation and drop tube techniques. The (Ni) dendrites achieved a high growth velocity of 26 m/s at the maximum undercooling of 226 K (0.14 TL) under levitated state. Remarkable dendritic structure refinement and Mo solubility extension were observed with the increase in undercooling. For freely falling alloy droplets, the largest undercooling was enhanced to 246 K (0.15 TL), which resulted in thorough solute trapping and almost segregationless solidification. A microstructure transition from columnar dendrite to equiaxed dendrite took place once alloy undercooling exceeded a threshold about 74–79 K. In addition, the Vickers hardness of primary (Ni) dendrite was significantly improved, which was caused by the extension of Mo solubility and microstructure refinement.

In situ solidification of eutectic Al-33wt%Cu droplets

Al-33wt%Cu eutectic droplets were rapidly solidified using Impulse Atomization, a drop tube technique. The samples were then processed in situ in an SEM using an HTN-0101 MEMS heating chip manufactured by Norcada. The temperature of the chip, as well as the heating and cooling rates, can be easily set using the chip’s control software, allowing for heating and cooling rates as fast as 1000°C/second. The droplets were heated above the eutectic melting temperature. Thanks to the oxide skin, the droplets retained their spherical shape, which allowed in situ solidification experiments. The live feed of the surface of the droplet obtained with the SEM was recorded during the experiments alongside the temperature-time profile. Various cooling rates were imposed and the liquid samples were shown to undercool prior to solidification. The resulting eutectic morphologies and spacings were then analyzed as a function of the cooling rates and undercoolings.

Promotion or Suppression of Eutectic and Peri-Eutectic Growth in Containerlessly Processed Fe–Ni–Ti Alloys

The competitive growth mechanisms of ternary eutectic and peri-eutectic transitions of liquid Fe–Ni–Ti alloys were investigated by drop tube technique. As liquid undercooling and cooling rate increased, the microstructure of eutectic Ni40.6Fe36.4Ti23 alloy transformed from the mixture of primary Ni3Ti phase and ternary eutectic into complete ternary eutectic. The predominant growth mode of independently grown Fe2Ti intermetallic phase varied from nonfaceted to faceted growth, while the γ-Fe(Ni) and Ni3Ti phases kept cooperative growth and formed a lamellar structure owing to the coherent interface along \((1 \overline{1}1)_{\gamma }\) and \((0001)_{{{\text{Ni}}_{{\text{3}}} {\text{Ti}}}} \). The peri-eutectic transformation in Fe66.5Ni17.6Ti15.9 alloy was suppressed due to the preferential growth of the primary α′-Fe phase as droplet size decreased. A needle-like Fe2Ti phase precipitated from the supersaturated α′-Fe phase during rapid solidification, and the α-Fe/Fe2Ti interface was characterized as semicoherent. The wear rate and friction coefficient of the ternary eutectic alloy firstly increased and then decreased as controlled by the combined factors of the microstructural transition and grain refinement. For the peri-eutectic alloy, the refinement of microstructure significantly improved the friction performance after rapid solidification. Moreover, the ternary eutectic alloy had a better wear resistance than peri-eutectic alloy, ascribed to the hardening intermetallic phases and the refined microstructure.

Solid solubility extension and nano-mechanical properties of rapidly solidified Fe-Zr eutectic alloys under free fall condition

The liquid-solid phase transitions and solidified microstructures of highly undercooled Fe-Zr eutectic melts were investigated by the drop tube technique. The quantitative relationships among droplet diameter (D), undercooling (ΔT) and cooling rate (Rc) were calculated for three types of eutectic alloys (Fe-9.8 at. % Zr, Fe-6 at. % Zr and Fe-16 at. % Zr alloys). Furthermore, the evolutions of solidified microstructure with droplet diameter were studied. The results show that the eutectic microstructure for Fe-9.8 at. % Zr droplet transforms from regularly lamellar shape to anomalous shape as the diameter reduces. The α-Fe dendrite in Fe-6 at. % Zr droplet transforms from coarse dendrite to refined dendrite with the decrease of droplet diameter, while the Fe2Zr dendrite in Fe-16 at. % Zr droplet transforms from faceted dendrite to non-faceted grain. Besides, a remarkable solid solubility extension was found in both the α-Fe dendrites of Fe-6 at. % Zr alloy and the Fe2Zr dendrites of Fe-16 at. % Zr alloy. The solid solubility of the α-Fe dendrite increases from 3 at. % to 5.6 at. % when the droplet diameter reduces from 1069 to 212 µm, while it decreases from 31 to 22 at. % in the Fe2Zr dendrite as the diameter reduces from 991 to 228 µm. The microstructural morphology and solid solubility are directly related to the nano-hardness and Young’s elastic modulus of alloys. The nano-mechanical properties of the eutectic gradually increase with the refinement of the layers, and gradually decrease as the irregularity of the anomalous eutectic increases. With the reduction of droplet diameter, the nano-hardness rises and Young’s elastic modulus continuously diminishes for the α-Fe dendrite due to the increase in the solute content of Zr, while these two mechanical properties gradually decrease for the Fe2Zr dendrite because of the reduction in the solute content of Zr.

Live record of vortex-shaped flow by microgravity solidification

A unique type of vortex-shaped microstructure reflecting the flow pattern during the microgravity solidification formed in Al70Ag20Ge10 alloy droplets using drop tube technique. A 2D axisymmetric model coupling heat transfer, fluid flow with phase change was established to simulate the fluid flow variation of alloy droplets during microgravity solidification. The Stokes motion was suppressed and the Marangoni motion was prominent in driving the vortex-shaped flow. The melt flow affected the phase selection, consequently the Ag2Al phase nucleated primarily instead of (Al) phase under near-equilibrium solidification, and the flow pattern was preserved during the rapid growth of the metastable Ag2Al dendrites. The Marangoni effect was weakened with the decrease of droplet size, and anomalous (Al) + Ag2Al and intergranular (Al) + (Ge) eutectics formed owing to the enhanced nucleation and growth competition among phases. In comparison, Al70Ag20Ge10 alloy was solidified under gravity condition by DSC and vacuum arc melting methods respectively. No vortex-shaped microstructure formed in DSC sample ascribed to the suppressed natural convection at small cooling rate. During arc melting, the electromagnetic force and enhanced natural convection caused strong turbulent flow, the metastable Ag2Al dendrites rapidly formed with increased vortex density.

Dendritic Growth of Rapid-Solidified Eutectic High-Entropy Alloy

Mould casting and drop-tube techniques were used to solidify a AlCoCrFeNi2.1 eutectic high-entropy alloy under conditions of high cooling rate. The samples obtained from two different methods present the same phase constituent, FCC and B2 phases. During mould casting experiments the alloy almost solidified into the eutectic structure consisting of lamellar and anomalous morphology, with a tiny fraction of cellular and dendrite morphology being observed at certain sites of the sample surface due to the corresponding high cooling rate. Instead, during drop-tube experiments a typical, coarse dendrite structure of FCC single phase was formed across the entire 106-150 μm particle. The cellular structure can also be formed directly from the melt. The rest region solidified into the general eutectic morphology as was observed in the casting rods. The results clearly indicate the transition from coupled eutectic growth to single-phase dendrite growth with increasing departures from equilibrium for the multi-component AlCoCrFeNi2.1 eutectic high-entropy alloy.

Structure and Property Modulations of 13Cr Stainless Steel Through Microgravity Solidification and Minor Ce Addition

Both rare‐earth doping and microgravity solidification processing bring significant influences upon the structure formation and mechanical properties of various alloys. Herein, the microstructure and performance of 13Cr martensitic stainless steel are modulated by adding minor Ce content and microgravity solidification by means of drop tube technique. The lattice structure of constituent phase α(Fe) is deformed evidently by microgravity solidification, and the lattice parameter variation is 1.8 times of that caused by Ce addition. The microstructures of this steel processed by arc melting consist of lath‐shaped martensites and a few feather‐shaped bainites whose volume fraction increases with the Ce content. The bainite is replaced by ferrite in the rapidly solidified microstructure of 13Cr steel under microgravity condition. Although both microgravity solidification and Ce addition induce the microstructure refinement and inclusion spheroidization, the former effect plays the dominant role. The compressive properties and microhardness decrease moderately with Ce addition due to the slight increase in bainite, however, the microgravity solidification brought the hardening effect and thus the microhardness of the steel droplets increases with undercooling.

Microstructural evolution and magnetic property of rapidly solidified Co62Mo38 hypereutectic alloy

To investigate the microstructural dependence of magnetic property for hypereutectic Co62Mo38 alloy, it was sufficiently undercooled by electromagnetic levitation (EML) and drop tube (DT) techniques. The maximum liquid undercoolings attained as 232 K (0.13 TL) at levitated state and 313 K (0.18 TL) under microgravity condition, respectively. At small undercoolings, the final microstructure consisted of lath-shaped primary ε-Co7Mo6 dendrites and interdendritic (α-Co + ε-Co7Mo6) eutectics. As the undercooling increased, the volume fraction of α-Co phase decreased continuously, leading to a reduction in saturation magnetization. Meanwhile, the coercive force exhibited a downward trend due to the coarsening of α-Co grain. Once the undercooling exceeded 271 K, the equiaxed growth of ε-Co7Mo6 compound was induced. In this case, the value of saturation magnetization abruptly increased with the increase of alloy undercooling, whereas the coercive force was further reduced.

Rapid growth kinetics and microstructure modulation of intermetallic compounds within containerlessly solidifying Mo-Co refractory alloys

To explore an innovative approach to prepare high-performance Mo-based refractory alloys, the rapid solidification mechanisms of undercooled Mo-40%Co hyperperitectic alloy were investigated by both electromagnetic levitation (EML) and drop tube (DT) techniques. During EML experiments, typical peritectic solidification still prevailed even at the maximum undercooling of 252 K (0.14 TL). The dendrite growth velocity of primary σ-Mo3Co2 compound varied with alloy undercooling by a power relation, which attained 24.3 mm s−1. As the bulk undercooling increased, the conspicuous dendrite fragmentation of primary phase took place, resulting in its grain refinement and volume fraction reduction. Meanwhile, the second recalescence degree exhibited a decreasing tendency, which indicated that the peritectic reaction was remarkably suppressed in the undercooled state. Under free fall condition, two critical diameters were determined for tiny alloy droplets and the corresponding undercooling thresholds were calculated as 361 and 443 K, respectively. When the alloy droplet was larger than 707 μm, the typical peritectic solidification proceeded and the final microstructure was composed of primary σ-Mo3Co2 dendrites and interdendritic ε-Mo6Co7 peritectic phase. With the reduction of droplet diameter, the direct nucleation of peritectic phase was induced. In this case, the ε-Mo6Co7 intermetallic compound regions appeared preferentially at the periphery of alloy droplets, whereas the typical peritectic microstructure filled in the center area. Once the droplet diameter decreased to 93 μm, the peritectic solidification characteristics disappeared completely and the metastable ε-Mo6Co7 single phase became the unique growth morphology.

Microstructure evolution and nano-hardness modulation of rapidly solidified Ti–Al–Nb alloy

Microstructure evolution, the formation of B2 (ordered BCC) phase and its relation to the nano-mechanical properties of the rapidly solidified Ti 68 Al 32 , Ti 63 Al 32 Nb 5 and Ti 58 Al 32 Nb 10 alloys were investigated by the drop tube technique. For the Ti 68 Al 32 alloy droplets, the solidified microstructures are composed of α 2 -Ti 3 Al phase, when the droplet diameters (D) are ranging from 226 to 1100 μm. With the decrease of droplet diameters, the microstructural characteristics of α 2 phase transform from coarse dendrites to the fully equiaxed grains. Notably, at D < 226 μm, the formation of α 2 phase is suppressed, resulting in the retention of metastable α-Ti phase. For the Ti 68-x Al 32 Nb x (x = 5, 10) alloys, the increasing of Nb contents promotes the formation of B2 phase and diminishes the size of the α 2 grains. Upon further decreasing the droplet diameter, the microstructure evolves from dendrite dendrites to equiaxed dendrites, and the B2 phase precipitates from the core of α 2 dendrites to α 2 grain boundaries, which ascribes to Nb-segregation from the dendrite core area to the grain boundary via L→β and β→α, respectively. Nanoindentation test reveals that the hardness of α 2 phase first increases and then decreases with the decrease of droplet diameters, which corresponds to the microstructure morphology, phase constitution, grain refinement of the alloys. Meanwhile, the additions of Nb exhibit enhanced properties of α 2 phase compared with those with low or without Nb additions, which can effectively modulate the hardness of these alloys. • TEM images confirmed that the formation of α 2 phase is suppressed, resulting in the retention of metastable α-Ti phase. • The increasing of Nb contents promotes the formation of B2 phase and diminishes the size of the α 2 grains. • B2 phase precipitates from the core of α 2 dendrites to α 2 grain boundaries with the decreases of droplet diameter • The addition of Nb can effectively modulate the hardness of Ti 3 Al–Nb alloys.

Microstructural refinement and anomalous eutectic structure induced by containerless solidification for high-entropy Fe–Co–Ni–Si–B alloys

The mechanical properties of high-entropy alloys (HEAs) strongly depend on their microstructural features. In this work, the phase formation and morphologies of Fe26.7Co26.7Ni26.7Si8.9B11.0 HEAs fabricated under different undercooling levels by the drop tube technique together with subsequent quenching were investigated in detail. It was found when the droplet size decreases (i.e. the increase of the undercooling level), the grain refinement is easily achieved without any phase transformation for the present hypoeutectic HEA which is composed of the fcc dendrites and eutectic structure. After quenching under large undercooling levels, the liquid should be deeply undercooled into an asymmetric couple zone, leading to the primary formation of eutectic structure and the subsequent precipitation of the fcc dendrites. The direct product of rapid solidification after deep undercooling and subsequent recalescence may induce the decoupled eutectic growth, resulting in a transition from the regular into anomalous eutectic structures. Furthermore, the volume fractions of the eutectic structures and the fcc dendrites increase and decrease with decreasing droplet size, respectively. As a result, the Vickers hardness gradually increases with decreasing droplet size, which exhibits a linear relationship with the volume fraction of eutectic matrix. The present study may provide a simple way to tailor microstructures and mechanical properties of HEAs.

Transition from Crystal to Metallic Glass and Micromechanical Property Change of Fe-B-Si Alloy During Rapid Solidification

The effects of high undercooling and a large cooling rate can be achieved by the use of a containerless drop tube technique, which is conducive to rapid solidification and formation of a metastable phase. Here, the rapid solidification of Fe78Si13B9 (S1) and Fe78Si9B13 (S2) alloys was completed under microgravity condition. Based on theoretical calculations, a maximum undercooling of 433 K (0.29 TL) and 412 K (0.28 TL) was obtained, respectively. The microstructure evolution and the formation of an amorphous-nanocrystalline structure for the two alloys were compared and analyzed. The results show that S2 alloy has better amorphous forming ability and higher hardness. During the solidification of S1 alloy, the primary phase α-Fe grows by the manner of dendrites, and the secondary dendrite arm spacing decreases exponentially with increased undercooling. An amorphous-nanocrystalline structure is developed when the undercooling is increased up to 388 K; S2 alloy forms an amorphous-nanocrystalline structure at an undercooling of 275 K and is completely amorphized after exceeding an undercooling of 402 K. In addition, the hardness and elastic modulus are acquired by nanoindentation technology under different degrees of undercooling. The phase constitution, morphology, distribution, and grain refinement of the alloys have important effects on the micromechanical properties of these alloys.

Microstructural evolution and magnetic property of a rapidly solidified ternary Fe37.5Cu37.5Sn25 peritectic-type alloy

The liquid Fe37.5Cu37.5Sn25 peritectic-type alloy was rapidly solidified by glass fluxing, drop tube, and melt spinning techniques. The solidification structures consisted of three phases, including the αFe solid solution and the Cu3Sn and Cu6Sn5 intermetallic compounds under different conditions. In this work, the bulk undercoding ΔT equals the liquidus temperature TL minus the nucleation temperature TN of the primary solid phase. During the bulk undercooling process, a maximum liquid undercooling of 272 K (0.18TL) was achieved. Metastable liquid phase separation was induced if the undercooling exceeded 177 K. Within a moderate undercooling range from 8 to 177 K, the solidified structures showed coarse dendrites, and the dendritic growth velocity increased to 41.5 mm/s following a power relation. As the drop tube technique provided a containerless state, a maximum surface cooling rate of 3.5 × 104 K/s was achieved. The alloy droplets displayed metastable liquid phase separation even at the largest droplet diameter of 944 μm. The cooling rate and thermal Marangoni migration were found to greatly influence the phase-separated patterns of the alloy droplets. Under the melt spinning condition, the microstructures of the Fe37.5Cu37.5Sn25 alloy ribbons were significantly refined and displayed soft magnetic characteristics. Furthermore, the experimental results demonstrated that the grain size of the αFe phase affected the coercivity of the alloy ribbons.

Rapid solidification mechanism of liquid quinary Ni-Zr-Ti-Al-Cu alloy investigated by high-speed cinematography

&lt;sec&gt; The ability to undercool and solidification mechanism of liquid quinary Ni&lt;sub&gt;40&lt;/sub&gt;Zr&lt;sub&gt;28.5&lt;/sub&gt;Ti&lt;sub&gt;16.5&lt;/sub&gt;Al&lt;sub&gt;10&lt;/sub&gt;Cu&lt;sub&gt;5&lt;/sub&gt; alloy are investigated by electromagnetic levitation (EML) and drop tube (DT) technique. Under the EML condition, the maximum undercooling of levitated alloy can reach up to 290 K (0.21&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;L&lt;/sub&gt;). Under the DT condition, the alloy achieves higher undercooling than EML, and solidifies finally into metallic glass. At lower undercooling, the solidification structure of the alloy is composed of primary Ni&lt;sub&gt;3&lt;/sub&gt;Ti phase, secondary Ni&lt;sub&gt;10&lt;/sub&gt;Zr&lt;sub&gt;7&lt;/sub&gt; phase and eutectic (Ni&lt;sub&gt;10&lt;/sub&gt;Zr&lt;sub&gt;7&lt;/sub&gt;+Ni&lt;sub&gt;21&lt;/sub&gt;Zr&lt;sub&gt;8&lt;/sub&gt;) phase. With the rise of undercooling, the solidification structure displays the following evolution events: phase morphology refinement, primary phase inhibition, phase number reduction, and amorphous phase formation.&lt;/sec&gt;&lt;sec&gt; By using the high-speed cinematography technique, three nucleation modes are distinctly observed on the levitated alloy melt surface at the beginning of solidification, that is, single-point nucleation, multi-point nucleation and annular nucleation. The levitation state corresponding to single-point mode nucleation is relatively stable, and the alloy undercooling is also relatively low. The annular nucleation only occursin the case with high rotation speed, and the undercooling is greater than 208 K. The discrepancy between nucleation modes is due to the He gas flow for forced cooling. &lt;/sec&gt;&lt;sec&gt; The theoretical calculations indicate that the alloy droplets achieve high undercoolingand large cooling rate under the DT condition. The experimental results show that when the droplet diameter decreases to 498 μm, the amorphous phase begins to appear in the alloy particles. It is noteworthy that the amorphous phase is preferentially formed inside the droplet, but not on the outer surface. The morphology of solidification structure reveals that different regions of the droplet have various local undercoolings, which result in the distribution characteristics of amorphous phase. The volume fraction of amorphous phase increases linearly with the decrease of particle diameter. When the droplet diameter decreases to 275 μm, the alloy droplets are completely frozen into glassy particles.&lt;/sec&gt;&lt;sec&gt; The average eutecticspacing values are also measured at different alloy undercoolings. Compared with the classical binary eutectic growth model, the experimental eutectic growth law exhibits a large deviation in index. This indicates that the eutectic growth in multicomponent alloys displays more complex kinetic characteristics.&lt;/sec&gt;

Primary dendrite growth of Co3Sn2 intermetallic compound in rapidly solidified ternary Co35Cu35Sn30 alloy

The rapid solidification of ternary Co35Cu35Sn30 alloy droplets with 20–785 µm diameter was realized by drop tube technique, aiming to investigate the dendrite growth mechanism of primary Co3Sn2 intermetallic compound at high cooling rate. As alloy droplet diameter decreases, the surface segregation occurs prior to the solidification process with an increased probability, and the segregation shell becomes more apparent. The dendrite growth of primary Co3Sn2 compound dominates the droplet solidification process, which evolves from equiaxed dendrite to spherical grain and is accompanied by remarkable grain refinement with the decrease of droplet diameter. Meanwhile, a peculiar zone near the droplet surface in which the primary Co3Sn2 compound grows directionally is observed. This directional structure becomes more evident as droplet diameter decreases, which is attributed to the temperature gradient along the radial direction inside droplet. In each alloy droplet, owing to the increased cooling rate from the center to the surface, the grain size decreases accordingly.

Rapid dendrite growth mechanism and solute distribution in liquid ternary Fe-Cr-Ni alloys

Stainless steels with excellent hardness and corrosion resistance performance have been widely used in industrial production. Ternary Fe-Cr-Ni alloys, as a model alloy of nickel chromium stainless steels, are of great importance in the fields of material science. Under non-equilibrium solidification condition, alloys may have new microstructure and improved performance. In this paper, two liquid ternary Fe-Cr-Ni alloys are deeply undercooled and rapidly solidified in a 3-m drop tube to investigate the microstructure evolution and solute distribution of alloy droplets with different sizes. In the drop tube experiments, the Fe-Cr-Ni alloy samples with a mass of 1.5 g are placed in a φ16 m mm×150 mm quartz tube with a 0.5-mm-diameter orifice at its bottom and heated by induction heating device in a high vacuum chamber. Then the samples are melted and overheated to 200 K above their liquidus temperatures for several seconds. The alloy melt is ejected out of the small orifice and dispersed into numerous droplets after adding high pressure helium gas flow. The alloy droplets with diameters ranging from 68 μm to 1124 μm are achieved. After experiments, the alloy droplets with different sizes are mounted respectively. Then they are polished and etched. The drop tube technique provides an efficient way to study the rapid solidification mechanism of alloys. Besides the experiments, the dendrite growth velocities of primary phase in two Fe-Cr-Ni alloys are calculated theoretically using the modified LKT/BCT model. As droplet size decreases, both cooling rate and undercooling increase exponentially and the morphologies of two alloys become well refined. Under the near-equilibrium solidification condition with a cooling rate of 10 K/min, both alloys consist of coarse lath-like α phase. After rapid solidification of Fe81.4Cr13.9Ni4.7 alloy droplets during free fall, the microstructure presents a lath-like α phase, resulting from the solid-solid phase transition. As undercooling increases, the primary δ phase is converted from the coarse dendrite with long trunk into equiaxed grain. For Fe81.4Cr4.7Ni13.9 alloy, the microstructure is composed of α phase grains. The transition of primary γ phase from coarse dendrite with long trunk to refined equiaxed grain occurs as the undercooling increases. Meanwhile, both dendrite trunk length and secondary dendrite arm spacing decrease drastically, suggesting that the rapid solidification is the main reason for grain refinement. Moreover, the relative segregation degree of solute Cr and Ni inside α phase grain also decreases obviously with the increase of undercooling, and the microsegregation of Ni is more remarkable than that of Cr. This suggests that the high cooling rate and undercooling cause the solute to be distributed evenly. Compared with that of γ phase, the dendrite growth velocity of δ phase is large and its dendrite tip radius is small. The two phase transform from solute diffusion controlled growth into thermal diffusion controlled growth as undercooling increases to 8 K. When undercooling is larger than 8 K and within the experimental undercooling range, the dendrite growth of both Fe-Cr-Ni alloys is controlled by thermal diffusion.

Experimental investigations and phase-field simulations of triple-phase-separation kinetics within liquid ternary Co-Cu-Pb immiscible alloys.

The phase-separation kinetics and microstructure evolution mechanisms of liquid ternary Co_{43}Cu_{40}Pb_{17} immiscible alloys are investigated by both the drop tube technique and phase-field method. Two successive phase separations take place during droplet falling and lead to the formation of a three-phase three-layer core-shell structure composed of a Co-rich core, a Cu-rich middle layer, and a Pb-rich shell. The Pb-rich shell becomes more and more conspicuous as droplet diameter decreases. Meanwhile, the Co-rich core center gradually moves away from the core-shell center. Theoretical analyses show that a larger temperature gradient inside a smaller alloy droplet induces the accelerated growth of the surface segregation shell during triple-phase separation. The residual Stokes motion and the asymmetric Marangoni convection result in the appearance of an eccentric Co-rich core and the core deviation degree is closely related to the droplet size and initial velocity. A three-dimensional phase-field model of ternary immiscible alloys, which considers the successive phase separations under the combined effects of Marangoni convection and surface segregation, is proposed to explore the formation mechanisms of three-phase core-shell structures. The simulated core-shell morphologies are consistent with the experimental observations, which verifies the model's validity in reproducing the core-shell dynamic evolution. Numerical results reveal that the development of three-phase three-layer core-shell structures can be attributed to the primary and then secondary phase separations dominated simultaneously by Marangoni convection and surface segregation. Furthermore, the effects of droplet temperature gradient on the growth kinetics of the surface segregation shell are analyzed in the light of phase-field theory.

Solidification pathways of ternary Cu62.5Fe27.5Sn10 alloy modulated through liquid undercooling and containerless processing

The active control of microstructure evolution is still a challenging factor for the development of advanced immiscible alloys. Here, we make an attempt to modulate the solidification pathways of undercooled Cu62.5Fe27.5Sn10 alloy by glass fluxing and drop tube techniques. Through regulating the liquid undercooling, three types of microstructures, dendrite, dispersive structure and macrosegregation pattern, were formed under the normal gravity condition. Below the first critical undercooling of 15 K, the alloy melt displayed the normal peritectic solidification. At moderate undercoolings above 15 K, the metastable liquid phase separation took place and the solidified microstructure appeared as homogeneously dispersed structure. If undercooling further overtook the second threshold of 107 K, macrosegregation occurred and the bulk alloy separated into an Fe-rich zone and a Cu-rich zone. Under the free fall condition, the alloy droplets with the droplet diameter beyond 805 μm showed the equilibrium peritectic solidification. If the droplet diameter decreased below 805 μm, the metastable liquid phase separation was induced and the microstructural morphology of Cu62.5Fe27.5Sn10 alloy droplet evolved from dendrite into dispersive structure. Furthermore, experimental and simulated results revealed that the temperature gradient had great influence on the size distribution of Fe-rich globules.

Metastable coupled-growth kinetics between primary and peritectic phases of undercooled hypoperitectic Fe54.5Ti45.5 alloy

The metastable coupled-growth kinetics between the primary Fe2Ti and peritectic FeTi phases of undercooled Fe54.5Ti45.5 alloy was systematically investigated by both electromagnetic levitation and drop tube techniques. Employing a high-speed camera, the rapid crystallization processes of levitated bulk alloy were recorded in the undercooling range of 34–187 K. In small undercooling regime below 143 K, peritectic solidification proceeded and the dependence of primary Fe2Ti dendritic growth velocity V on the bulk undercooling ΔT satisfied a power relation of V = 2.43 × 10−14 × ΔT7.72 (mm s−1). Once liquid undercooling increased beyond 143 K, the metastable coupled-growth was induced and the microstructure was characterized by the Fe2Ti rods embedded in FeTi phase. Furthermore, the coupled-growth velocity decreased linearly with the rise in undercooling according to V = 1.47 × 103-7.44ΔT (mm s−1). In drop tube experiment, peritectic solidification characteristics of small alloy droplets disappeared and the primary and peritectic phases directly nucleated from undercooled liquid and grew cooperatively to form spherical coupled-growth cells if droplet diameter decreased below 481 μm.