Drop Tube Technique Research Articles

Theoretical prediction and experimental observation for microstructural evolution of undercooled nickel–titanium eutectic type alloys

The high undercooling and rapid solidification of hypoeutectic Ni–13%Ti, eutectic Ni–14%Ti and hypereutectic Ni–16%Ti alloys were realized by electromagnetic levitation and drop tube techniques. The competitive growth of primary (Ni) dendrite and Ni3Ti compound with (Ni+Ni3Ti) eutectic caused significant microstructural variations beyond some critical undercoolings. For the hypoeutectic Ni–13%Ti alloy, the solidification microstructure was characterized by primary (Ni) dendrites plus interdendritic (Ni+Ni3Ti) eutectics in the moderate undercooling range below 196 K. When the alloy melt was undercooled beyond 196 K, complete solute trapping occurred and thus induced the formation of metastable single-phase microstructure. The microstructural morphology of eutectic Ni–14%Ti alloy appeared as (Ni+Ni3Ti) lamellar structures in the small undercooling regime below 40 K, whereas it displayed a hypoeutectic microstructure dominated by the primary (Ni) dendrites at larger undercoolings. In the case of hypereutectic Ni–16%Ti alloy, an irregular eutectic structure was formed at the undercoolings higher than 41 K and it was significantly refined under substantial undercooling conditions. These microstructural transitions were further analyzed by calculating the coupled zone of (Ni+Ni3Ti) lamellar eutectic, which leaned to the Ni3Ti phase side and covered a composition range from 13.1 to 19.0 wt%Ti.

Primary phase growth and microstructure evolution of rapidly solidifying ternary Ti-12Al-8V alloy

Liquid Ti-12Al-8V alloy was rapidly solidified by electromagnetic levitation and drop tube techniques. The levitation processing avoided the violent heterogeneous nucleation of primary β phase and led to an actual maximum undercooling of 212K (0.11TL). The primary β dendrites exhibited a high growth velocity up to 14.9m/s but subsequently transformed into basket weave structures. Drop tube experiments indicated that the final microstructure morphology was determined by the coupled influences of undercooling extent and cooling rate. If the cooling rate exceeded 3.31×103K/s, the primary β phase was retained metastably to ambient temperature.

Rapidly solidified Ag-Cu eutectics: A comparative study using drop-tube and melt fluxing techniques

A comparative study of rapid solidification of Ag-Cu eutectic alloy processed via melt fluxing and drop-tube techniques is presented. A computational model is used to estimate the cooling rate and undercooling of the free fall droplets as this cannot be determined directly. SEM micrographs show that both materials consist of lamellar and anomalous eutectic structures. However, below the critical undercooling the morphologies of each are different in respect of the distribution and volume of anomalous eutectic. The anomalous eutectic in flux- undercooled samples preferentially forms at cell boundaries around the lamellar eutectic in the cell body. In drop-tube processed samples it tends to distribute randomly inside the droplets and at much smaller volume fractions. That the formation of the anomalous eutectic can, at least in part, be suppressed in the drop-tube is strongly suggestive that the formation of anomalous eutectic occurs via remelting process, which is suppressed by rapid cooling during solidification.

Rapid solidification mechanism and magnetic property of ternary equiatomic Fe33.3Cu33.3Sn33.3 alloy

Rapid solidification is a typical non-equilibrium phase transition process, and the crystallization rate of liquid metal is larger than 1 cms-1. If the alloy is solidified in this case, the solute segregation is reduced or even eliminated and the solid solubility can be improved significantly. Rapid solidification technique can be used to refine the microstructures of alloys, which provides an effective method to prepare the novel metastable materials and improve their strengths, plasticities magnetic properties, etc. In this work, the rapid solidification mechanism and magnetic property of ternary equiatomic Fe33.3Cu33.3Sn33.3 alloy are investigated by drop tube and melt spinning techniques. It is known that Fe-Cu-Sn ternary alloy forms a typical immiscible system. However, the experimental results reveal that the liquid phase separation does not take place during the rapid solidification of ternary equiatomic Fe33.3Cu33.3Sn33.3 alloy. The solidification microstructures are all composed of primary Fe dendrites together with Cu3Sn and Cu6Sn5 phases. Under the free fall condition, as the drop tube technique provides microgravity and containerless states, the maximum surface cooling rate and maximum undercooling of alloy droplets are 1.3105 Ks-1and 283 K (0.19 TL), respectively. When the surface cooling rate reaches 1.9103 Ks-1, the primary Fe phase appears as coarse dendrites, and its maximum dendrite length is 41 m. Meanwhile, the Cu3Sn and Cu6Sn5 phases are distributed in the Fe interdendritic spacings. Once the surface cooling rate increases up to 3.3103 Ks-1, the morphology of the primary Fe phase transforms from coarse dendrites into broken dendrites. It is found that the cooling rate and undercooling greatly affect the solidification microstructure of alloy droplets. During the melt spinning experiments, since the large temperature gradient exists between the wheel surface and free surface, the solidification microstructure is subdivided into two crystal zones according to the different microstructure morphologies of Fe phase: fine grain (zone I) and coarse grain (zone II), where zone I is characterized by granular grains while zone II has some dendrites with secondary branch. Under the rapid cooling condition, the microstructures of ternary equiatomic Fe33.3Cu33.3Sn33.3 alloy ribbons are refined significantly and show soft magnetic characteristics. As the surface cooling rate increases from 8.9106 to 2.7107 Ks-1, the lattice constant of Fe solid solution rises rapidly and the coercivity increases from 93.7 to 255.6 Oe. Furthermore, the results indicate that the grain size of Fe phase is the main factor influencing the coercivity of alloy ribbons.

Substantial undercooling and rapid dendrite growth of liquid Ti-Al alloy

It is highly desirable to undercool titanium based alloy melts and modulate their dendritic solidification process due to the relevant applications in aerospace engineering. But the serious chemical reactivities of this category of alloys result in potent heterogeneous nucleation and suppress remarkable undercoolings in the course of normal material processing. This paper shows that such a challenge can be solved by containerless processing approach. Liquid Ti-25 wt.%Al alloy is highly undercooled and rapidly solidified under containerless state by both electromagnetic levitation and drop tube techniques. Its metastable undecoolability, crystal nucleation mechanism and dendrite growth process are examined experimentally and analyzed theoretically. Those heterogeneous nuclei with wetting angles above 60 are found to be quite difficult to eliminate even during levitation processing, thus reducing the undercoolability of this alloy. The maximum undercooling of bulk alloy melt reaches 210 K (0.11 TL). The thermodynamic driving force to initiate the nucleation of -Ti phase increases almost linearly with the enhancement of undercooling. The phase dendrite displays a growth velocity up to 11.2 m/s, indicating that the rapid solidification is realized at the relatively slow cooling rate of levitated alloy melt. With the increase of undercooling, phase dendrite experiences a kinetic transition from solute diffusion controlled to thermal diffusion controlled growth. Once undercooling exceeds 100 K, the nonequilibrium solute trapping effect brings about the practically desirable segregationless solidification. Nevertheless, the single condition of substantial undercooling is insufficient to suppress the solid state transformation of phase. It is decomposed into 2-Ti3Al phase plus a small amount of -TiAl compound after containerless solidification at levitated state. A more efficient approach to controlling and modulating the solidification microstructures is to utilize the coupled effects of high undercooling and rapid quenching, which proves to be feasible through the rapid solidification of alloy droplets inside drop tube. For those alloy droplets with diameters ranging from 77 to 1048 m, their cooling rates attain a maximum of 1.05105 K/s, and the predicted maximum undercooling is 227-778 K. In this case, phase dendrites are well refined and kept in a metastable state until ambient temperature. The heat transfer calculations indicate that the thermal radiation is the dominant cooling mechanism for the large alloy droplets above 690 m, whereas thermal convection becomes the major cooling mechanism for the small alloy droplets below 690 m. The microgravity condition during free falling does not show apparent effect on the microstructural formation of these alloy droplets.

Microstructure evolution and mechanical properties of drop-tube processed, rapidly solidified grey cast iron

The microstructure, phase composition and microhardness of rapidly solidified grey cast iron BS1452 Grade 250 are compared against the conventionally solidified alloy. Powder samples were prepared using containerless processing via the drop-tube technique. The rapidly cooled droplets were collected and sieved into size range from ≥850µm to ≤53µm diameters corresponding to estimated rates of 200–23,000Ks−1. Microstructure evaluations were made by optical and scanning electron microscopy, while XRD was used for identification and analysis of evolved phases. The control sample showed extensive graphite flake formation which was absent in virtually all the droplets samples. With decreasing particle size (increasing cooling rate) we observed an increase in the proportion of Fe3C present and the retention of γ-Fe in preference to α-Fe, with the proportion of retained austenite increasing with increasing cooling rate. At the highest cooling rates utilised a Martensitic or acicular ferrite structure was observed. Cooling rates of 200Ks−1 resulted in a doubling of the measured microhardness relative to the as-received (slowly cooled) material. Cooling at the highest rates achieved resulted in a further doubling of the measured microhardness.

Microstructural Evolution and Phase Formation in Rapidly Solidified Ni-25.3 At. Pct Si Alloy

The drop-tube technique was used to solidify droplets of the Ni-25.3 at. pct Si alloy at high cooling rates. XRD, SEM, and TEM analysis revealed that the metastable phase, Ni25Si9, formed as the dominant phase in all ranges of the droplets, with γ-Ni31Si12 and β 1-Ni3Si also being present. Three different microstructures were observed: the regular and anomalous eutectic structures and near single-phase structure containing small inclusions of a second phase, termed here as heteroclite structure. Both eutectic structures comprise alternating lamellae of Ni25Si9 and β 1-Ni3Si, which, we conjecture, is a consequence of an unobserved eutectic reaction between the Ni25Si9 and β 1-Ni3Si phases. The matrix of the heteroclite structure is also identified as the metastable phase Ni25Si9, in which twined growth is observed in the TEM. As the cooling rate is increased (particle size decreased), the proportion of droplets displaying the entire heteroclite structure tends to increase, with its fraction increasing from 13.91 pct (300 to 500 µm) to 40.10 pct (75 to 106 µm). The thermodynamic properties of the Ni25Si9 phase were also studied by in-situ heating during XRD analysis and by DTA. This showed the decomposition of Ni25Si9 to β 1 and γ-Ni31Si12 for temperatures in excess of 790 K (517 °C).

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.

Non-Equilibrium Processing of Ni-Si Alloys at High Undercooling and High Cooling Rates

Melt encasement (fluxing) and drop-tube techniques have been used to solidify a Ni-25 at.% Si alloy under conditions of high undercooling and high cooling rates respectively. During undercooling experiments a eutectic structure was observed, comprising alternating lamellae of single phase γ (Ni31Si12) and Ni-rich lamellae containing of a fine (200-400 nm) dispersion of β1-Ni3Si and α-Ni. This is contrary to the equilibrium phase diagram from which direct solidification to β-Ni3Si would be expected for undercoolings in excess of 53 K. Conversely, during drop-tube experiments a fine (50 nm) lamellar structure comprising alternating lamellae of the metastable phase Ni25Si9 and β1-Ni3Si is observed. This is also thought to be the result of primary eutectic solidification. Both observations would be consistent with the formation of the high temperature form of the β-phase (β2/β3) being suppressed from the melt.

Lamella structure formation in drop-tube processed Ni–25.3 at.% Si alloy

Rapid solidification experiments have been performed on an Ni–25.3at.% Si alloy using drop tube techniques. The dominant phase formed is found to be Ni25Si9, with Ni31Si12 and β1-Ni3Si also being present. SEM and TEM analysis revealed a novel eutectic structure consisting of lamellar of metastable Ni25Si9 and β1-Ni3Si, the width of these being around 200nm and 20nm respectively. This result indicates that there is a possible eutectic reaction for the Ni25Si9 and β1-Ni3Si phases existing in the metastable phase diagram.

Containerless solidification of Ag–Al and Ag–Cu eutectic alloys in a drop tube

Drop tube technique has been used to containerlessly solidify Ag–28wt.%Al and Ag–28wt.%Cu eutectic alloys. Inductively melted sample was forced by Ar pressure of 2bar through the small bore at the lower end of a quartz tube and a thin liquid jet is formed that disperses into small droplets of different size owing to the Rayleigh instability. The droplets cool, undercool and solidify during free fall in time of 1 s under the conditions of reduced gravity in He atmosphere. The cooling rates at the solidification temperature are computed as a function of droplet size by heat balance calculations. They are ranging between 0.5×103 and 3.0×104K/s for droplets in size range of 100–900μm diameters. The cooling rate at the solidification temperature decreases with the increase of droplet size. Nusselt, Reynolds and Prandtl numbers of the droplets were calculated. Microstructures of the alloys were analyzed and eutectic spacings are determined. Microhardness of the droplets with different sizes was measured revealing a dependence on the cooling rate with which the droplets are solidified.

Peri-eutectic solidification and complicated microstructural evolution of liquid undercooled Cu–Ag–Sb alloy

Rapid solidification of Cu46.4Ag38Sb15.6 alloy related with peri-eutectic transition was investigated in terms of microstructural evolution by means of glass fluxing and drop tube techniques. The microstructure formations of both bulk samples and alloy droplets are influenced by undercooling prior solidification, phase transition, cooling rate and volume fraction of the residual melt during different solidification stages. Two peri-eutectic transitions (L + (Cu) → (Ag) + Cu3Sb and L + Cu3Sb → (Ag) + Cu2Sb) are verified to be involved in the solidification of bulk samples. However, the second peri-eutectic transition is not reached in the solidification pathway of alloy droplets. The solidification product of peri-eutectic transition L + α → β + γ generates in three manners: the formation of (β + γ) eutectic by the reaction between L and α phases, the direct growth of anomalous (α + β + γ) structure or (β + γ) eutectic from the melt.

Primary dendrite growth of Ni3Sn intermetallic compound during rapid solidification of undercooled Ni-Sn-Ge alloy

Liquid Ni-31.7%Sn-2.5%Ge alloy was highly undercooled by up to 238 K (0.17TL) with glass fluxing and drop tube techniques. The dendritic growth velocity of primary Ni3Sn compound shows a power-law relation to undercooling and achieves a maximum velocity of 380 mm/s. The addition of Ge reduces its growth velocity as compared with the binary Ni75Sn25 alloy. A structural transition from coarse dendrites into equiaxed grains occurs once undercooling exceeds a critical value of about 125 K, which is accompanied by both grain refinement and solute trapping. The Ni3Sn intermetallic compound behaves like a normal solid solution phase showing nonfaceted growth during rapid solidification.

Rapid Growth and Magnetic Properties of Fe<sub>7</sub>CO<sub>3</sub> Intermetallic Compound

By drop tube and melt spinning techniques, rapid growth of Fe7Co3 intermetallic compound has been realized. The microstructures and nucleation characteristics under different cooling rates are investigated. With the increase of cooling rate, Fe7Co3 intermetallic compound grains are refined in both rapid growth manners. Experimental observation and theoretical analysis indicate that the nucleation rate in single-roller quenching is much greater than that in free-fall method when the cooling rate is under a same level. The comparison of magnetic property between the alloy ribbons and other dimensional magnetic material were made. The superior soft magnetic properties of Fe-Co alloy ribbons changes sensitively with the cooling rate, which is mainly decided by the wheel velocity.

Phase separation and rapid solidification of liquid Cu60Fe30Co10 ternary peritectic alloy

The metastable liquid phase separation and rapid solidification of Cu60Fe30Co10 ternary peritectic alloy were investigated by using the drop tube technique and the differential scanning calorimetry method. It was found that the critical temperature of metastable liquid phase separation in this alloy is 1623.5 K, and the two separated liquid phases solidify as Cu(Fe,Co) and Fe(Cu,Co) solid solutions, respectively. The undercooling and cooling rate of droplets processed in the drop tube increase with the decrease of their diameters. During the drop tube processing, the structural morphologies of undercooled droplets are strongly dependent on the cooling rate. With the increase of the cooling rate, Fe(Cu,Co) spheres are refined greatly and become uniformly dispersed in the Cu-rich matrix. The calculations of Marangoni migration velocity (V M) and Stokes motion velocity (V S) of Fe(Cu,Co) droplets indicated that Marangoni migration contributes more to the coarsening and congregation of the minor phase during free fall. At the same undercooling, the V M/V S ratio increases drastically as Fe(Cu,Co) droplet size decreases. On the other hand, a larger undercooling tends to increase the V M/V S value for Fe(Cu,Co) droplets with the same size.

Phase formation in undercooled Sm–Co alloy melts

Sm x Co 100− x alloy melts ( x = 10.5, 12.5 and 17 at.%) are processed containerlessly by electromagnetic levitation and drop-tube technique in order to get access to the metastable state of the undercooled melt. By dint of undercooling non-equilibrium phase formation is facilitated. The solidified specimen are subsequently analyzed by electron microscopy, X-ray diffraction and thermal magnetic analyses. The results show that different types of solidification behaviors are observed depending on alloy composition and undercooling level prior to solidification. Bulk samples processed by electromagnetic levitation technique are solidified by primary formation of either Sm 2Co 17 or SmCo 5, whereas primary phase formation of metastable SmCo 7 is detected in small particles attained by drop-tube processing where cooling rates lie in the order of 10 3–10 5 K/s.

Crystallization of Nd 2Fe 17B x from stoichiometric melt composition

Containerless solidification of gas-atomized Nd 10Fe 85B 5 melt droplets was carried out using the drop tube technique. The phase constituents and microstructure of the solidified samples were investigated by means of powder X-ray diffraction analysis, thermomagnetic analysis, and scanning electron microscopy. Besides α-Fe and Nd 2Fe 14B, non-equilibrium phases such as Nd 2Fe 17B x , ɛ-Nd, and Nd 1.1Fe 4B 4 were identified. The microstructure of the samples was categorized into three types, including a quasi-single phase one that consists mainly of Nd 2Fe 17B x dendrites. The lattice parameters and Curie temperature of Nd 2Fe 17B x were determined, and compared with those of the same type phase crystallized in Nd-rich Nd–Fe–B melt compositions. The results were discussed with respect to the stoichiometry, formation as well as potential application of Nd 2Fe 17B x .

Metastable phase separation and rapid solidification of undercooled Co-Cu alloy under different conditions

The metastable liquid phase separation and rapid solidification behaviours of Co61.8Cu38.2 alloy were investigated by using differential thermal analysis (DTA) in combination with glass fluxing, electromagnetic levitation (EML) and drop tube techniques. It is found that the liquid phase separation process and the solidification microstructures intensively depend on the experimental processing parameters, such as undercooling level, cooling rate, gravity level, liquid surface tension and the wetting state of crucible. Large undercooling and surface tension difference of the two liquids tend to facilitate further separation and cause severe macrosegregation. On the other hand, rapid cooling and low gravity effectively suppress the coalescence of the minority phase. Severe macrosegregation patterns are formed in the bulk samples processed by both DTA and EML. In contrast, disperse structures with fine spherical Cu-rich spheres homogeneously distributed in the matrix of Co-rich phase have been obtained in drop tube.

Rapid growth of FeAl intermetallic compound under high undercooling conditions

Fe-58at%Al alloy is undercooled up to 222 K (0.15TL) with the drop tube technique. It is found that there exists a critical undercooling about 185 K, beyond which a “dendrite-equiaxed” growth morphology transition occurs in FeAl intermetallic compound. This transition is characterized by sharp decrease of its grain size. Once the undercooling exceeds 215 K, the peritectic transformation is suppressed completely and a fibrous structure is formed, which results from the cooperative growth of FeAl and FeAl2 compounds.

A Transition from Eutectic Growth to Dendritic Growth Inducedby High Undercooling Conditions

Cu-8 wt.%Al eutectic alloy was undercooled by up to 187 K(0.14 TE) using a drop tube technique. The crystal growth andphase selection mechanisms were investigated during containerless rapidsolidification. It is found that the microstructural morphology ischaracterized by lamellar eutectic growth at small undercoolings.However, if the liquid alloy is undercooled by more than 25 K, eutecticgrowth will be suppressed completely and the dendritic growth of (Cu)solid solution dominates its solidification process. When theundercooling exceeds 153 K, a microstructural transition from coarsedendrite to equiaxed dendrite takes place.