Bulk Undercooling Research Articles

Rapid solidification structure refinement mechanism in highly undercooled Cu based ternary alloys

Applying cycle superheating and molten glass purification technologies, Cu60Ni35Co5 alloys were deeply undercooled to maximum undercoolings of 284 K. The microstructure characteristics and its evolution varying with bulk undercooling during the rapidly solidified process were analyzed. It was found that, after minor addition of element Co, crystal refinement mechanisms of the undercooled Cu60Ni35Co5 system was similar to those of the Cu–Ni sytem. Crystal refined microstructure appearing at the low undercoolings was result of dendrite remelting mechanism; the dominant factor of grain refinement microstructure appearing at high undercooling regime was resultant from the recrystallization process. Driving force of stress induced strain energy for recrystallization could be divided into two parts, one was the thermal stress generated by the solidification latent releasing in the recalescence process, and the other was accumulated stress and plastic strain resultant from interaction between primary dendrite and liquid flow driven by rapid solidification.

Microstructural Evolution and Applied Performance of a Highly Undercooled Bulk Ni-5% Cu Alloy

Microstructural Evolution and Applied Performance of a Highly Undercooled Bulk Ni-5% Cu Alloy

Rapid dendritic solidification and structure hardening mechanism of substantially undercooled quaternary nickel alloys

The high undercooling and rapid solidification of liquid quaternary Ni-5%Cu-5%Sn-5%Si and Ni-5%Cu-5%Sn-5%Ge alloys were achieved by glass-fluxing technique to explore their dendritic growth kinetics, microstructural evolution and hardening mechanisms. The dendrite growth velocity of α-Ni phase was found to increase with the rise of bulk undercooling, which could be described by power law functions for both alloys. With the increase of liquid alloy undercooling, the morphology of α-Ni phase transformed from developed dendrites into equiaxed grains. Both the fine grain hardening and solute hardening effects resulted in the elevated Vickers hardness for these two alloys. Nano-sized Ni3Si precipitations were detected within the first alloy, which distributed homogeneously in the α-Ni phase matrix and transformed from faceted to nonfaceted morphology at sufficiently large undercoolings. The precipitate hardening effect dominated the Vickers hardness enhancement for this Si containing alloy. Rapid solidification could modulate the formation of ordered precipitate phase and thus improve the hardening effect by changing dislocation motion conditions.

Metastable coupled growth kinetics between primary and peritectic intermetallic compounds within the liquid Mo-37 wt% Co refractory alloy

The metastable coupled growth between primary σ-Mo3Co2 and peritectic ε-Co7Mo6 phases for hypoperitectic Mo-37 wt% Co refractory alloy was investigated by electromagnetic levitation (EML) and drop tube (DT) methods. The highest alloy undercooling attained 261 K (0.14 TL) under EML conditions, which reached up to 311 K (0.17 TL) in DT experiments. At small undercoolings, the typical peritectic solidification prevailed and the corresponding double recalescence processes were in-situ recorded for levitated alloys. In this case, the primary σ-Mo3Co2 phase preferentially nucleated from liquid alloy and its growth velocity monotonously increased as a power function with the rise of undercooling. When the liquid undercooling increased beyond the threshold of 162 K, the competitive nucleation between σ-Mo3Co2 and ε-Co7Mo6 compounds was facilitated, leading to a novel lamellar eutectic-like microstructure for peritectic-type alloys. Meanwhile, the measured growth velocity exhibited a sudden drop at first and then slightly increased with the bulk undercooling. Once the alloy undercooling exceeded 295 K (D = 226 µm), only the metastable coupled growth mechanism could occur within the liquid alloy. The final microstructure of hypoperitectic Mo-37 wt% Co alloy even transformed from lamellar to anomalous eutectic-like morphology in a sufficiently undercooled state. Furthermore, as the undercooling increased, the random orientation relationship between σ-Mo3Co2 and ε-Co7Mo6 phases in typical peritectic solidification mode was replaced by the parallel orientation, i.e. {111}σ//{101̅0}ε for metastable coupled growth mechanism.

Effect of bulk undercooling on microstructure transformation mechanism of rapidly solidified nickel alloys

The microstructural evolution and mechanism of grain refinement during rapid solidification of Ni–Cu alloys were studied by molten glass purification and cyclic superheating. The maximum undercooling has achieved 320 K for Ni65Cu35, 295 K for Ni55Cu45 and 280 K for Ni50Cu50 alloys. Two mechanisms were found to be responsible for the grain refinements under small undercooling and large undercooling, respectively. The grain refinement at small undercooling is mainly caused by dendrite superheating and remelting, whereas that for large undercooling is mainly due to recrystallization occurred at high angle grain boundary and high proportion of twin boundaries. Meanwhile, almost no dislocations and other defects were observed in the substructure of the alloy with large undercooling.The average microhardness was found a sharp decrease in the alloy with large undercooling, which indicates that the plastic strain accumulated in the microstructure formed under large undercooling is completely dissipated in post-recalescence, and the deformation energy can offer the driving force for recrystallization in the microstructure.

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.

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.

Observation of dendrite growth velocity and microstructure transition in highly undercooled single phase alloys

Dendrite growth velocity as a function of initial bulk undercooling was systematically studied in situ using high speed thermal imaging of highly undercooled Ni90Cu10 and Ni95Cu5 single phase alloy melts. For these two non-dilute concentrated alloys, the dendrite growth velocity first increased continuously at small and mediate undercooling regimes and then discontinuously to a maximum value at the high undercooling range. A clear plateau appeared in the growth velocity-undercooling curve revealed a gradual transition from mainly solute-controlled growth to mainly thermal-controlled growth, which was very similar to the solidification characteristics of a dilute alloy. The discontinuity appeared in the growth velocity-undercooling relationship was suggested to be induced by the finite solute diffusion speed in the bulk melt ahead of the advancing solid-liquid interface. Microstructure, substructure and microtexture of the two undercooled alloys were characterized using EBSD technique, and the fundamental mechanism behind “spontaneous grain refinement” is elucidated. The microstructure evolution indicated two kinds of grain refinements: one was dendrite remelting induced grain refinement and the other was recrystallization induced grain refinement. It was found that the texture of the as-solidified microstructure was getting more and more random as the initial bulk undercooling increased, which was due to the increased stress and recrystallization in the as-solidified microstructure. New evidence of grain refinement was found in the as-solidified samples. We found that the samples undercooled beyond the critical undercooling had a grain refined microstructure and that this corresponded with a clear discontinuity in the dendrite growth velocity-undercooling curve. Microstructural data suggested that in Ni-Cu alloys grain refinement at a high undercooling was a consequence of a recrystallization process, which contrasted with conventional grain refined alloys where grain refinement appeared to be the result of remelting-induced dendritic fragmentation.

The growth velocity-undercooling relationship of highly undercooled Ni and Ni-Cu melts

ABSTRACTSolidification velocities as a function of initial bulk undercoolings were systematically investigated using high speed thermal imaging of highly undercooled pure Ni and Ni-Cu alloy samples. For pure Ni, the dendrite growth velocity increased continuously to a maximum value of about 52 m s−1 at a maximum undercooling of 320 K. For Ni-Cu alloy, the dendrite growth velocity first increased continuously and then discontinuously to a maximum value of about 55 m s−1 at 300 K. A clear discontinuity appeared in the growth velocity-undercooling curve is considered to be caused by the finite speed of solute diffusion in the bulk liquid ahead of the migrating solid–liquid interface.

Solute redistribution and micromechanical properties of rapidly solidified multicomponent Ni-based alloys

The solute trapping effect and microhardness enhancement of quaternary Ni-5%Cu-5%Ag-5%Sn and quinary Ni-5%Cu-5%Ag-5%Sn-5%Ge alloys during rapid dendritic solidification are investigated by glass fluxing technique. In these two alloys, the experimental maximum undercoolings of 310 K (0.18TL) and 220 K (0.13TL) have been achieved and the rapidly solidified microstructures are composed of α(Ni) and (Ag) solid solution phases. The morphological transition from coarse dendrite into equiaxed structure is observed for α(Ni) phase with the increase of undercooling. The dendritic growth velocity of α(Ni) phase in quaternary Ni-5%Cu-5%Ag-5%Sn alloy is larger than that in quinary Ni-5%Cu-5%Ag-5%Sn-5%Ge alloy, which increases firstly and then decreases with the enhancement of bulk undercooling. The Vickers hardness of α(Ni) phase in these two alloys is enhanced with the increase of undercooling, which is attributed mainly to the grain refinement effect. Meanwhile, the solute trapping effect of Cu, Sn and Ge elements in the α(Ni) phase also contributes to the microhardness enhancement under large undercoolings. The addition of Ge element effectively increases the microhardness of α(Ni) phase due to solute strengthening mechanism.

Growth velocity-undercooling relationship and structure refinement mechanism of undercooled Ni-Cu alloys

The growth velocity-undercooling relationship as a function of bulk undercoolings was systematically measured using high speed thermal imaging of highly undercooled Ni-Cu alloy samples. It was found that the growth velocity of Ni80Cu20 alloy was almost coincident with that of Ni95Cu5 alloy and the growth velocity of Ni85Cu15 alloy was almost coincident with that of Ni90Cu10 alloy, which was a new result that was unreported previously for concentrated solid solution alloys. It was also found that the solidification velocity increased smoothly with undercooling up to the maximum value. Growth velocity can be adequately represented by the current dendritic growth model. A clear discontinuity in the growth velocity-undercooling curve was observed at a critical undercooling. This discontinuity in the velocity-undercooling curve can be attributed to the finite speed of solute diffusion in the bulk liquid ahead of the growing dendrite. Microstructural evidences in these samples indicative the occurrence of dendritic fragmentation. Microstructural analysis using light microscopy and EBSD analysis reveals that recrystallization accompanies the grain refinement at high undercoolings. Furthermore, at high undercoolings, a high density of annealing twins in the as solidified samples indicates the occurrence of recrystallization in the solidification microstructure and a high density of subgrains in the quenched samples are observed within the microstructure, which indicates the occurrence of extensive recovery, a result previously unreported in samples solidified from highly undercooled melts.

Dendrite growth and Vickers microhardness of Co7Mo6 intermetallic compound under large undercooling condition

The dendritic growth process and Vickers microhardness enhancement of primary Co7Mo6 phase in undercooled liquid Co-50%Mo hypereutectic alloy are systematically investigated by using electromagnetic levitation and drop tube. It is found that the rapid solidification microstructures are mainly characterized by primary Co7Mo6 dendrites plus interdendritic (Co7Mo6+Co) eutectic irrespective of experimental conditions. In electromagnetic levitation experiment, the obtained maximum undercooling reaches 203 K (0.12TL). With the rise in bulk undercooling, primary Co7Mo6 dendrite growth velocity monotonically increases according to a power function and reaches 22.5 mm-1 at the highest undercooling. The secondary dendrite spacing decreases from 45.8 to 13.6 m, while Co content in primary dendrites shows an increasing trend. This indicates that an evident grain refinement and solute trapping take place for primary Co7Mo6 dendrites during rapid solidification. The dependence of Vickers microhardness on Co content follows an exponential function. Moreover, the variation of Vickers microhardness with the grain size also satisfies an exponential relationship. In addition, Lipton-Kurz-Trivedi/Boettinger-Coriel-Trivedi model is used to analyze the growth kinetics of primary Co7Mo6 dendrites. In the experimental undercooling range, the growth process of primary Co7Mo6 dendrites is controlled mainly by solute diffusion and they grow sluggishly. Under free fall condition, liquid Co-50%Mo alloy is subdivided into many droplets inside a drop tube and their diameters range from 1379 to 139 m. With alloy droplet size decreasing, both droplet undercooling and cooling rate increase rapidly. In a large droplet-diameter regime above 392 m, primary Co7Mo6 phase displays faceted-growth characteristics. Furthermore, primary Co7Mo6 dendrites are refined greatly and their solute solubility is significantly extended as droplet size becomes smaller. Once the alloy droplet diameter decreases to a value below this threshold value, the faceted-growth characteristics start to disappear gradually, which is accompanied with a conspicuous grain refinement and a solute solubility extension. Both the solute solubility enhancement and grain size refinement contribute significantly to the exponential improvement in microhardness if primary Co7Mo6 phase grows in a faceted way. Otherwise, the solute solubility enhancement and grain size refinement result in the linear increase of Vickers microhardness. Theoretical analyses demonstrate that the primary phase microhardness is strongly dependent on its solute content and morphology characteristic.

Reexamination of crystal growth theory of graphite in iron-carbon alloys

Most analysis of graphite morphology in cast iron-carbon alloys is performed on samples cooled to room temperature. This raises the concern that the crystallization of graphite is obscured by subsequent recrystallization and growth in solid state. To bring clarity to this issue, the authors used Field Emission Gun Scanning Electron Microscopy on deep-etched interrupted solidification (quenched) specimens to reveal the morphology of graphite growing in contact with the liquid at the very beginning of solidification.To understand the complexity of graphite crystallization in iron alloys, the analysis included evidence from the crystallization of materials with analogous hexagonal structure, such as of snowflakes and metamorphic graphite, and from the crystallization of diamond cubic structure silicon crystals in aluminum-based alloys. Information from research discussing graphite produced through gas-solid (chemical vapor deposition) and solid-solid (graphite in steel) transformations was also exploited.The large variety of graphite solidification morphologies described in this and earlier papers derives from the complexities of its faceted growth during crystallization, a diffusion-limited crystal growth process, in the presence of anisotropic surface energy and anisotropic attachment kinetics. It was confirmed that the basic building blocks of the graphite aggregates are hexagonal faceted graphite platelets generated through the growth of graphene layers. As solidification advances, the platelets thicken through layer growth through two-dimensional or screw dislocation nucleation. Depending on bulk composition, local supersaturation and undercooling, the platelets aggregate through a variety of mechanisms including tiled-roof and foliated crystals and dendrites, curved-circumferential, cone-helix, helical (macro-spiral), and polyhedral pyramidal (or conical) sectors growth. The final graphite shape of graphite spheroids is affected by the crystallography of the nucleus, as it affects the initial growth of the graphite platelets.

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.

Relationship between microstructure and mechanical properties of undercooled K4169 superalloy

Various undercoolings 14-232 K of bulk K4169 superalloys were obtained by the method of molten glass fluxing combined with superheating cycling and the mechanical properties of undercooled K4169 with as-solidified state were tested. Microstructures and phases composition in undercooled bulk K4169 superalloy were identified by transmission electron microscope (TEM), scanning electron microscope (SEM) and optical microscopy (OM). The morphology of dendrites, grain size and intergranular phase all change with the increased undercooling. Meanwhile, the relationship between microstructure of undercooled K4169 superalloy and tensile properties was investigated. The experimental results show that the uniform distribution of Laves phase and the decrease of grain size and intergranular phase content are favorable for the improvement of mechanical properties. The maximum tensile strength and elongation obtained at undercooling of 232 K are 932.2 MPa and 6.5 %, respectively.

Dendritic Growth Characteristics of Cu-Rich Zone within Phase Separated Fe<sub>50</sub>Cu<sub>50</sub> Alloy

The undercooled Fe50Cu50 alloy experiences a metastable liquid phase separation and separates into a Fe-rich zone and a Cu-rich zone within the gravity field. The growth characteristics of the Cu-rich zone were investigated by the glass fluxing method, and the achieved undercooling range was 20−261 K. The volume fraction of the Cu-rich zone decreases with the enhancement of the bulk undercooling. The microstructural morphologies of the Cu-rich zone are similar at all the undercooling conditions, that is, αFe dendrites and particles are distributed inside (Cu) phase matrix. The secondary dendritic arm spacing of αFe dendrites decreases with the increase in bulk undercooling. The growth mechanism of αFe dendrites was analyzed by using the LKT/BCT dendritic growth theory. The dendritic growth in the Cu-rich zone is mainly controlled by solute diffusion so that the dendritic growth velocity is only several millimeters per second. Besides, the calculated results indicate that there is only inconspicuous solute trapping during the solidification of Cu-rich zone.

Microstructural characteristics and mechanical properties of peritectic Cu–Sn alloy solidified within ultrasonic field

Abstract The dynamic solidification of peritectic Cu–70%Sn alloy was accomplished within a 20 kHz ultrasonic field under the power range up to 440 W. The ultrasound promotes the nucleation of primary e(Cu3Sn) phase and prevents the bulk undercooling of liquid alloy. As sound power increases, the ultrasonic field brings about a striking size refinement effect to the primary e intermetallic compound by more than one order of magnitude. Meanwhile, it facilitates or even completes the usual peritectic transformation (L + e → η) which occurs only to a very limited extent during static solidification. This is mainly because the refinement of primary e phase induced by ultrasound greatly reduces the characteristic length of peritectic microstructure, which increases solid state Fourier number by more than one order of magnitude. The dramatic increase in the volume fraction of peritectic phase and the prominent grain refinement effect due to ultrasound lead to the remarkable improvement of mechanical properties for Cu–70%Sn alloy, whose compressive strength and microhardness are increased by factors of 4.8 and 1.45, respectively.

Liquid phase separation and rapid dendritic growth of highly undercooled ternary Fe62.5Cu27.5Sn10 alloy

The phase separation and dendritic growth characteristics of undercooled liquid Fe62.5Cu27.5Sn10 alloy have been investigated by glass fluxing and drop tube techniques. Three critical bulk undercoolings of microstructure evolution are experimentally determined as 7, 65, and 142 K. Equilibrium peritectic solidification proceeds in the small undercooling regime below 7 K. Metastable liquid phase separation takes place if bulk undercooling increases above 65 K. Remarkable macroscopic phase separation is induced providing that bulk undercooling overtakes the third threshold of 142 K. With the continuous increase of bulk undercooling, the solidified microstructure initially appears as well-branched dendrites, then displays microscale segregation morphology, and finally evolves into macrosegregation patterns. If alloy undercooling is smaller than 142 K, the dendritic growth velocity of γFe phase varies with undercooling according to a power function relationship. Once bulk undercooling exceeds 142 K, its dendritic growth velocity increases exponentially with undercooling, which reaches 30.4 m/s at the maximum undercooling of 360 K (0.21TL). As a comparative study, the liquid phase separation of Fe62.5Cu27.5Sn10 alloy droplets is also explored under the free fall condition. Theoretical calculations reveal that the thermal and solutal Marangoni migrations are the dynamic mechanisms responsible for the development of core-shell structure.

Microstructural evolution of ternary Ag33Cu42Ge25 eutectic alloy inside ultrasonic field

Ultrasonic field with a frequency of 20kHz is introduced into the solidification process of ternary Ag33Cu42Ge25 eutectic alloy from the sample bottom to its top. The ultrasound stimulates the nucleation of alloy melt and prevents its bulk undercooling. At low ultrasound power of 250W, the primary ε2 phase in the whole alloy sample grows into non-faceted equiaxed grains, which differs to its faceted morphology of long strip under static condition. The pseudobinary (Ag+ε2) eutectic transits from dendrite shape grain composed of rod type eutectic to equiaxed chrysanthemus shape formed by lamellar structure. By contrast, the ultrasound produces no obvious variation in the morphology of ternary (Ag+Ge+ε2) eutectic except a coarsening effect. When ultrasound power rises to 500W, divorced ternary (Ag+Ge+ε2) eutectic forms at the sample bottom. However, in the upper part, the ultrasonic energy weakens, and it only brings about prominent refining effect to primary ε2 phase. The microstructural evolution mechanism is investigated on the cavitation, acoustic streaming and acoustic attenuation.

Solute redistribution profiles during rapid solidification of undercooled ternary Co-Cu-Pb alloy

The solute redistribution and phase separation of liquid ternary Co-35%Cu-32.5%Pb immiscible alloy have been investigated using glass fluxing method. A bulk undercooling of 125 K was achieved and the macrosegregation pattern was characterized by a top Co-rich zone and a bottom Cu-rich zone. The average solute contents of the two separated zones decreased with the increase of undercooling, except for the solute Pb in Cu-rich zone. With the enhancement of undercooling, a morphological transition from dendrites into equaxied grains occurred to the primary α(Co) phase in Co-rich zone. The solute redistribution of Cu in primary α(Co) phase was found to depend upon both the undercooling and composition of Co-rich zone. Stokes migration is shown to be the main dynamic mechanism of droplet movement during liquid phase separation.