Undercooling Solidification Research Articles

Evading strength−ductility trade-off of GH605 alloy using magnetic field-assisted undercooling treatment

Undercooling solidification under a magnetic field (UMF) is an effective way to tailor the microstructure and properties of Co-based alloys. In this study, by attributing to the UMF treatment, the strength−ductility trade-off dilemma in GH605 superalloy is successfully overcome. The UMF treatment can effectively refine the grains and increase the solid solubility, leading to the high yield strength. The main deformation mechanism in the as-forged alloy is dislocation slipping. By contrast, multiple deformation mechanisms, including stacking faults, twining, dislocation slipping, and their strong interactions are activated in the UMF-treated sample during compression deformation, which enhances the strength and ductility simultaneously. In addition, the precipitation of hard Laves phases along the grain boundaries can be obtained after UMF treatment, hindering crack propagation during compression deformation.

Understanding the influence of boron in additively manufactured GammaPrint®-700 CoNi-based superalloy

Boron is commonly added to superalloys in small amounts to enhance creep resistance, but can lead to cracking at high concentrations, especially during the additive manufacturing process. Two variants of CoNi-based GammaPrint®-700 superalloy with different B contents (0.08 at% vs 0.16 at%) were printed via laser powder bed fusion (LPBF) with the same printing parameters, with only the high B alloy exhibiting solidification cracking. Atom probe tomography (APT) revealed stronger segregation behaviors in the high B alloy compared to the low B alloy at both the inter-dendritic regions and grain boundaries (GBs). The segregation behavior at inter-dendritic regions was well captured with Scheil simulation and can correlate with the existing cracking susceptibility index (CSI) on cracking tendencies, although high angle GBs are where cracking occurs according to electron backscatter diffraction (EBSD) measurements. Additionally, the extent of GB segregation was compared between the high B and low B alloy. Higher B additions led to significantly more GB B segregation in the high B alloy compared to the low B alloy. For the high B alloy, the cracked region of one GB exhibited higher levels of B compared to the uncracked region of the same GB. However, much higher B contents were also found in two other uncracked GBs in the high B alloy, which demonstrates that higher GB B concentrations are not fully responsible for the cracking. A much larger variance in GB B segregation content was found in the high B alloy compared to the low B alloy. These phenomena were explained with a solidification model with the GB segregation content expressed explicitly by a modified Langmuir-McLean equation. This model linked the GB segregation content with solidification undercooling, which can be used as quantitative cracking criteria for future builds.

Evolution mechanism of rapid solidification microstructure of an undercooled copper-based single phase alloy

Using undercooling technology combined with melt purification of glass and recirculation overheating, different undercoolings were successfully obtained for Cu60Ni38Co2 alloy samples. Applying high speed photography technique, the physical process of undercooled solidification was observed and relationship between undercooling and solidification rate was analyzed. Experimental results indicate the critical undercoolings exist in the structure evolution process. Electron backscatter diffraction (EBSD) analysis was applied to characterize the Cu60Ni38Co2 alloy sample with a maximum undercooling of 255K, revealing grain orientation and the corresponding orientation difference distribution. In addition, transmission electron microscopy (TEM) showed high density dislocation networks. Combining EBSD and TEM data analysis, an important conclusion was drawn: recrystallization is the main factor for grain refinement at high undercooling. This finding is of great significance for a deeper understanding of the structure evolution in the physical process of undercooled alloys.

Rapid Solidification of Invar Alloy.

The Invar alloy has excellent properties, such as a low coefficient of thermal expansion, but there are few reports about the rapid solidification of this alloy. In this study, Invar alloy solidification at different undercooling (ΔT) was investigated via glass melt-flux techniques. The sample with the highest undercooling of ΔT = 231 K (recalescence height 140 K) was obtained. The thermal history curve, microstructure, hardness, grain number, and sample density of the alloy were analyzed. The results show that with the increase in solidification undercooling, the XRD peak of the sample shifted to the left, indicating that the lattice constant increased and the solid solubility increased. As the solidification of undercooling increases, the microstructure changes from large dendrites to small columnar grains and then to fine equiaxed grains. At the same time, the number of grains also increases with the increase in the undercooling. The hardness of the sample increases with increasing undercooling. If ΔT ≥ 181 K (128 K), the grain number and the hardness do not increase with undercooling.

Microstructure refinement mechanism upon deeply undercooled solidification of a Ni88Cu6Co6 alloy

Applying molten glass purification and cyclic superheat methods, deep undercooling treatment was performed on a Ni88Cu6Co6 alloy system. The alloy was undercooled to an undercooling as high as 300 K and the corresponding microstructure characteristics were analyzed systematically. Compared with the Ni88Cu12 system, the addition of a minor amount of Co had a pronounced impact on the grain refinement mechanism of Ni88Cu6Co6 system. Grain refinement observed at low undercooling was attributed to the dendritic melting mechanism. On the other hand, in the high undercooling region, the recrystallization process played a dominant role in refining the microstructure. Through the present analysis, the characteristic undercoolings of microstructure evolution had been determined. In addition, BCT model was applied to study the free growth of an undercooled Ni88Cu12 alloy, and by comparing Ni88Cu12 and Ni88Cu6Co6 alloys, we aimed to find the effect of Co on the microstructure refinement mechanisms of deeply undercooled Ni–Cu alloys.

Microstructure refinement mechanisms in undercooled solidification of binary and ternary nickel based alloys

Molten glass purification and cycle superheating technologies were used to make Ni65Cu35, Ni65Cu33Co2 and Ni65Cu31Co4 alloys obtain maximum undercoolings of 320 K, 292 K and 300 K respectively. In order to analyze the relationship between morphological characteristics of solidification front and undercooling change during migration of solid–liquid interface, a high-speed camera was used to capture pictures of the recalescence process. Observing the microstructure of the undercooled alloys using metallographic microscope, the characteristics and evolution of microstructure during rapid solidification process of undercooled liquids were analyzed. It was found that grain refinement mechanisms of highly undercooled Ni–Cu–Co alloys was the same as those of the Ni–Cu alloys. Dendrite remelting leads to the grain refinement at low undercooling, while the dominant factor of grain refinement at high undercooling is recrystallization process induced by stress. The internal driving force can be divided into two parts: one is the thermal stress generated by the releasing of solidification latent heat during recalescence process, and the other is the stress and strain accumulated by interaction of liquid flow and primary dendrite during rapid solidification. We also found that addition of third element Co not only played an important role in solidification rate and recalescence effect, but also significantly improved the average hardness of grain refined microstructure, which was about 80% higher than that of as cast alloy. The addition of trace Co was also conducive to the formation of non-segregation solidification microstructure.

Microstructure, precipitation behavior, and properties of quaternary Cu–Be–Co–Ni alloy under electromagnetic levitation

Electromagnetic levitation (EML) can provide a containerless environment and, consequently, large undercooling solidification for various metals. In this study, the microstructures, precipitated phase characteristics, properties, and strengthening mechanisms of Cu–0.2Be–1.6Co–1.6Ni alloy with different undercooling conditions were investigated using the EML method. The achieved undercooling range was 76–315 K. The microstructure of the alloy was mainly composed of dendrites and slender-strip precipitates. Under EML, the length and width of the precipitates decreased by an order of magnitude with increasing undercooling. The precipitates transformed from a α(Co) phase with a face-centered cubic structure to a metastable γ′ phase with a body-centered tetragonal structure. The microhardness increased almost linearly from 120 to 154 HV with enhanced undercooling. The electrical conductivity was higher than that of the master alloy and increased slightly with enhanced undercooling. Quantitative analyses of the strengthening mechanisms revealed the vital roles of solution and precipitation strengthening in the mechanical properties of EML alloys, whereby the former was weakened and the latter was strengthened as undercooling progressed. The simultaneous enhancement of the mechanical and electrical properties of Cu–0.2Be–1.6Co–1.6Ni alloy can be attributed to the increased solute precipitation and refined precipitates due to deep undercooling.

Correlation between Differential Fast Scanning Calorimetry and Additive Manufacturing Results of Aluminium Alloys.

High-strength aluminium alloy powders modified with different nanoparticles by ball milling (7075/TiC, 2024/CaB6, 6061/YSZ) have been investigated in-situ during rapid solidification by differential fast scanning calorimetry (DFSC). Solidification undercooling has been evaluated and was found to decrease with an increasing number of nanoparticles, as the particles act as nuclei for solidification. Lower solidification undercooling of individual powder particles correlates with less hot cracking and smaller grains in the material produced by powder bed fusion of metals by a laser beam (PBF-LB/M). Quantitatively, solidification undercooling less than about 10–15 K correlates with almost crack-free PBF-LB/M components and grain sizes less than about 3 µm. This correlation shall be used for future purposeful powder material design on small quantities before performing extensive PBF-LB/M studies.

The Evolution of Growth Morphology of Co3sn2 Phase in Undercooled Solidification of Co-Sn Alloy

The Evolution of Growth Morphology of Co3sn2 Phase in Undercooled Solidification of Co-Sn Alloy

Solidification Pattern of Si-Alloyed, Inoculated Ductile Cast Irons, Evaluated by Thermal Analysis

The solidification cooling curve itself as well as its first derivative, and related temperatures, reported to the calculated equilibrium temperatures in stable and metastable solidification systems, are used to predict the solidification characteristics of the cast iron. Silicon, as the most representative cast iron element, and inoculation, as graphitizing metallurgical treatment, have a major influence on the transition from the liquid to the solid state. Six experimental programs are performed, with Si content typically for non-alloyed (<3.0% Si), low (3.0–3.5% Si) and medium alloyed (4.5–5.5% Si) ductile cast irons, as Si-content increasing, and inoculation simultaneous effects. Silicon is an important influencing factor, but the base and minor elements also affect the equilibrium eutectic temperatures, much more in the Fe-C-Si-Xi stable system (15–20 °C) than in the metastable system (5–10 °C), comparing with their calculation based only on a Si effect (Fe-C-Si system). The highest positive effect of inoculation is visible in non-Si alloyed cast irons (2.5% Si): 9–15 °C for the eutectic reaction and 3 to 4 times increased at the end of solidification (37–47 °C). Increased Si content decreases inoculation power to 7–9 °C for low alloying grade (up to 3.5% Si), with the lowest contribution at more than 4.5% Si (0.3–2.0 °C). 2.5–3.5% Si ductile cast irons are more sensitive to high solidification undercooling, especially at the end of solidification (but with a higher efficiency of inoculation), compared to 4.5–5.5% Si ductile cast irons, at a lower undercooling level, and at lower inoculation contribution, as well.

Assessment of AlZnMgCu alloy powder modification for crack-free laser powder bed fusion by differential fast scanning calorimetry

Additive manufacturing, e.g. by laser powder bed fusion (LPBF), is very attractive for lightweight constructions, as complex and stress-optimised structures integrating multiple functions can be produced within one process. Unfortunately, high strength AlZnMgCu alloys tend to hot cracking during LPBF and thus have not so far been applicable. In this work the melting and solidification behaviour of AlZnMgCu alloy powder variants with particle surface inoculation was analysed by Differential Fast Scanning Calorimetry. The aim is to establish a method that makes it possible to assess powder modifications in terms of their suitability for LPBF on a laboratory scale requiring only small amounts of powder.Therefore, solidification undercooling is evaluated at cooling rates relevant for LPBF. A method for the temperature correction and normalisation of the DFSC results is proposed. Two ways of powder modification were tested for the powder particles surface inoculation by titanium carbide (TiC) nanoparticles: via wet-chemical deposition and via mechanical mixing.A low undercooling from DFSC correlates with a low number of cracks of LPBF-manufactured cubes. It appears that a reduced undercooling combined with reduced solidification onset scatter indicates the possibility of crack-free LPBF of alloys that otherwise tend to hot cracking.

In situ and ex situ studies of anomalous eutectic formation in undercooled Ni–Sn alloys

Anomalous eutectic formation in undercooled Ni–Sn alloys was investigated by in situ X-ray diffraction and ex situ remelting and annealing experiments. Dynamic recrystallization and partial remelting of primary solids followed by repeated nucleation and growth of eutectic grains in the mushy zone were revealed by time-resolved X-ray diffraction. Ex situ experiments demonstrated that partial remelting of near-equilibrium solidified alloys of eutectic or near-eutectic composition can convert regular lamellar eutectic into anomalous eutectic, whereas high-temperature annealing of splat-quenched alloys of similar composition can convert eutectic or two-phase dendrites into anomalous eutectic. It is concluded that compared to ripening in solid-states, partial remelting of eutectic or two-phase dendrites in a mushy zone provides a more realistic mechanism for anomalous eutectic formation in undercooled solidification of Ni–Sn eutectic alloys.

Sulfur and Oxygen Effects on High-Si Ductile Iron Casting Skin Formation

The main objective of the present paper is to evaluate by thermal (cooling curve) analysis the solidification pattern and the occurrence of the layer of degenerate graphite at the surface of ductile iron castings (3.15% Si, typically as 450-18 grade, ISO 1563/2011), with or without a mold coating, including S or O, and different agents (carbonic material, iron powder), supposed to act to block their diffusion into the iron melt. It is found that the mold coating materials temperately influence the parameters of the solidification cooling curves and, more visibly, the occurrence and the thickness of the undesired skin layer. Different graphite morphologies comparable to the casting body are present, at a large range of thicknesses, from 50 up to 200 µm. The sulfur presence in the mold coating will promote a higher skin thickness compared to oxygen (up to 50% by oxygen and 2.5–3.3 times for sulfur action), despite the fact that in the casting body, the graphite nodularity undergoes a limited decrease (from 85% up to 82%–83% level). Carbonic material or iron powder supplementary addition decreases these undesired effects, but the solidification undercooling compared to the equilibrium system is increased. It is found that carbonic material is more efficient at limiting oxygen than iron powder is at limiting the negative effects of sulfur on the casting skin thickness. More experiments are necessary to quantify their capacity to block the oxygen or sulfur transfer into the iron melt.

Correlation between Solidified Microstructure Evolution and Undercooling of Au-12 Wt.%Ge Eutectic Alloy

Highly undercooled solidification experiments were carried out by melt purification combined with cyclic superheating method on Au-12 wt.%Ge eutectic alloy. The solidification structures of Au-12 wt.%Ge eutectic alloy under different undercoolings were also analyzed by using the scanning electron microscope (SEM). The experimental results revealed that the maximum undercooling could reach up to 102 K. The microstructure analysis showed that the coarse bulk eutectic existed in the solidification structure when the undercooling was less than 34 K. When the undercooling was larger than 34 K and less than 56 K, the solidification structure transformed into cellular eutectic. The coarse primary (α-Au) phase precipitated from the undercooled alloy melt when the undercooling was larger than 56 K. The volume fraction of the primary (α-Au) phase gradually increased with the increase of undercooling. In this paper, a method to regulate the solidification structure of Au-12 wt.%Ge eutectic alloy is proposed, which provides a new way to improve the solidification structure and has important guiding significance for the processing and forming process of Au-12 wt.%Ge eutectic alloy.

Ultra-rapid cooling of aluminum: Homogeneous solidification to anisotropic nanocrystals

Aluminum fcc-crystal surfaces (110) are irradiated by series of ≈15 ns-long pulses of laser light. Each pulse is calculated to produce an ≈2 µm thick surface layer of liquid and quasi-liquid whose temperature decays rapidly, becomes supercooled liquid until ≈168 K below the nominal melting temperature, and then freezes homogeneously into fcc nanocrystals and amorphous atoms. The cooling rate is ≈1.2 × 109 K s−1 in the undercooled solidification region, which we call ultra-rapid because it is faster than that in experiments involving splat-cooling or melt-spinning. However, it is slower than those in a molecular-dynamics simulation with a million aluminum atoms, which was described by Mahata et al. [Model. Simul. Mater. Sci. Eng. 26, 025007 (2018)]. Standard θ/2θ x-ray diffraction is applied to the resulting solid. The magnitude and location of the diffraction peaks yield estimates of the anisotropy and the sizes of the nanocrystals. The sizes, between about 4 nm and 50 nm, are on the order of “critical” as defined in classical nucleation theory. The anisotropy is caused by a difference in growth rates among various crystal faces, which is in qualitative agreement with theoretical predictions. For example, the loosely packed (311) face grows much faster than that of the close packed (111).

Simultaneous thermal and contraction / expansion curves analysis for solidification control of cast irons

The first part of the paper summarizes the performance of two mould devices, illustrating by representative shrinkage tendency results in ductile cast iron as affected by mould rigidity (green and furan resin sand moulds) and inoculant type (FeSi-based alloys). Less rigid green sand moulds encourage the formation of contraction defects, not only because of the high initial expansion values (εdi)max, but also because of the increased solidification undercooling. A high inoculation efficiency means not only lowering the carbides formation sensitivity and increasing the nodule count, but also a prolonged graphitization through to the end of the eutectic freezing, as observed by the high population of small late forming nodules, which leads to minimizing the tendency for shrinkage. The second part of the paper illustrates an application of this equipment to commercial foundry use. It conducts thermal analysis and volume change measurements in a single ceramic cup with cast iron quality as a variable. La-bearing FeSi inoculant appears to be more effective than RE (Rare Earth) -FeSi alloy in FeSiMgCa treated irons (no RE), in terms of reducing eutectic undercooling and (εdi)max, favourable for lower sensitivity to shrinkage formation. Experiments also compared solidification patterns for white [WI], grey [GI] and ductile [DI] irons, to correlate the most important events between the cooling curves and contraction curves, to evaluate the sensitivity to shrinkage formation. All of the irons have similar values for initial expansion up to the start of eutectic freezing, but, after that, the graphite formation promotes expansion (more than 5 times for nodular graphite), resulting in a difference in maximum expansion (2 times higher for DI). The graphitic expansion has two contrary effects. Increased graphitic expansion (force) leads to a higher shrinkage sensitivity during the first part of the eutectic reaction, but also to a decrease of shrinkage at the end of solidification, due to forcing the last liquid iron to occupy the previous formed cavities. Consequently, strong graphitization process promotion at the end of solidification favours the castings' soundness.

The recalescence rate of cooling curve for undercooled solidification

Recalescence rate (R) in cooling curve is well known that affected by undercooling in solidification, but the accurate relationship of them is not clear yet. In this paper, based on the undercooled solidification of Fe-B alloy, the factor affected on recalescence process was investigated. The relationship R = VΔT/D was first found, where V is the growth velocity, ΔT the recalescence degree (approximate the undercooling), D the focus region diameter dependent on the distance of the pyrometer. With this result the solidification interface growth velocity can be predicted from recalescence of cooling curve, vice versa. In addition, an approximate relation between growth velocity and the size of the critical nucleus was shown.

Effect of the Third Element Ni on the Solidification Microstructure of Undercooled Cu-40 wt.% Pb Monotectic Alloy Melt

In this paper, the evolution of solidification microstructure of Cu-40 wt.% Pb monotectic alloy of the third element Ni pair under deep undercooling conditions was studied. By comparing the phenomena of liquid phase separation during deep undercooling and rapid solidification of Cu-40 wt.% Pb monotectic alloy, the melt of the alloy increases with the undercooling, and the solidification structure appears uneven or even stratified. With the addition of the third element Ni, the liquid phase separation can be effectively inhibited by the change of interfacial energy. The solidified structure undergoes the transformation from coarse dendrite to the first kind of granular and refined dendrite in a wide undercooling range. When the undercooling reaches 143 K, the structure begins to show an inhomogeneous trend.

Application of thermal analysis in solidification pattern control of La-inoculated grey cast irons

The new generation of acid-lined medium-frequency coreless induction furnaces revolutionized the iron foundries, resulting higher overheating and lower S and Al content iron melt, characterized by higher eutectic undercooling solidification and consequently high chill [carbides] and unfavourable undercooled graphite morphologies sensitiveness. Having established that Al, Zr, Ti, Ca, Ba and La appear to have a key role in graphite nucleation in grey cast iron, thermal [cooling curves] investigations are undertaken to explore their effects on solidification parameters. The experimental heats are obtained in acid-lined coreless induction melting. Some important active elements are added to FeSi-based alloy, usually used as conventional inoculant in cast iron foundry industry, resulting complex inoculating systems [Al, Ca–La; Al, Ca–La, Zr; Al, Ca–La, Ba; Al, Ca–La, Zr, Ba; Al, Ca–La, Zr, Ti and Al, Ca–La, Zr, Ba, Ti]. In-mould [ceramic cup, cooling modulus of approximately 0.75 cm] inoculation is applied directly in the Quik-cup™ cooling curve analysis equipment, for the same alloy addition rate. The La effects are evaluated in low S and Al, high overheated electric grey iron solidification pattern with and without any other oxide-sulphides forming elements contribution. Study is focused on the representative temperatures during eutectic reaction and at the end of solidification, and it results in an undercooling referring to equilibrium eutectic temperature in the stable and metastable solidification system. As primary evaluation for general application, La, Ca, Al–FeSi alloy have a high efficiency in low S and Al and higher carbon equivalent grey cast irons, electrically melting, without any other active elements contribution. For specific applications, more complex La-bearing alloys are recommended, such as for higher dendritic austenite amount promotion [LaBaZrTi–FeSi alloy] or for lower eutectic recalescence [LaBaZr–FeSi alloy].

Relation between undercooled solidification and solid-state grain growth accompanying dynamic segregation

Understanding the interrelations between nonequilibrium solidification and subsequent solid-state grain growth plays a more significant role in material preparation. Using the molten glass purification combined with the cycle superheating methods to obtain high vacuum condition, a remarkable refined grain was achieved in undercooled Ni-B and Ni-Pb alloys, then the inherent linkage of rapid solidification and grain growth has been studied. For two stages of grain growth, i.e., alternating growth and stabilization, a decreasing relation between the final grain size and the initial melt undercooling is found. Applying nonequilibrium solidification models and thermo-kinetic grain-growth models accompanying dynamic segregation demonstrates that the calculations are consistent with the experimental results in solidification, grain growth, and their inherent linkage, respectively. It is concluded that the alternating growth is controlled by the competition of grain boundary energy (thermodynamics) and drag (kinetics). The final microstructure is determined by both the initial undercooled solidification (grain refinement and solute trapping) and the subsequent solid-state grain growth and stabilization (solute segregation and precipitation).