Near-equilibrium Solidification Research Articles

Morphological Heredity of Intermetallic Nb5Si3 Dendrites in Hypereutectic Nb-Si Based Alloys via Non-Equilibrium Solidification

For hypereutectic Nb-Si based alloys, primary Nb5Si3 phases typically grow in a faceted mode during equilibrium or near-equilibrium solidification, which damages the ductility and toughness. To address this issue, here we artificially manipulate the growth morphology of Nb5Si3 using electron beam surface melting (EBSM) and subsequent annealing treatments. Results show that such a non-equilibrium solidification pathway enables the transition from faceted growth to non-faceted dendritic growth of Nb5Si3, along with evident microstructure refinement, generation of metastable β-Nb5Si3 phases and elimination of chemical segregation. The transformation from β-Nb5Si3 to α-Nb5Si3 and Nb solid solution (Nbss) particles is triggered by the annealing treatment at 1450 °C for 5 h. Also, we find the annealing-mediated formation of inherited Nb5Si3 dendrites that maintain the dendritic morphology of the original as-solidified β-Nb5Si3 dendrites. This work thus provides a feasible routine to obtain thermally stable and refined α-Nb5Si3 dendrites in hypereutectic Nb-Si based alloys.

X-ray Imaging of Alloy Solidification: Crystal Formation, Growth, Instability and Defects.

Synchrotron and laboratory-based X-ray imaging techniques have been increasingly used for in situ investigations of alloy solidification and other metal processes. Several reviews have been published in recent years that have focused on the development of in situ X-ray imaging techniques for metal solidification studies. Instead, this work provides a comprehensive review of knowledge provided by in situ X-ray imaging for improved understanding of solidification theories and emerging metal processing technologies. We first review insights related to crystal nucleation and growth mechanisms gained by in situ X-ray imaging, including solute suppressed nucleation theory of -Al and intermetallic compound crystals, dendritic growth of -Al and the twin plane re-entrant growth mechanism of faceted Fe-rich intermetallics. Second, we discuss the contribution of in situ X-ray studies in understanding microstructural instability, including dendrite fragmentation induced by solute-driven, dendrite root re-melting, instability of a planar solid/liquid interface, the cellular-to-dendritic transition and the columnar-to-equiaxed transition. Third, we review investigations of defect formation mechanisms during near-equilibrium solidification, including porosity and hot tear formation, and the associated liquid metal flow. Then, we discuss how X-ray imaging is being applied to the understanding and development of emerging metal processes that operate further from equilibrium, such as additive manufacturing. Finally, the outlook for future research opportunities and challenges is presented.

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.

Effect of Al Content on Hot-Tearing Susceptibility of Mg-10Zn-xAl Alloys

The hot-tearing susceptibility of Mg-10Zn-xAl (x = 0, 2, 5, and 7) alloys was studied using constrained rod casting installed with a load cell, thermocouple, and data-management system. Addition of Al content reduced the freezing range and increased the liquid fraction at the end of the primary stage of solidification, during the vulnerable period of alloy solidification. Eutectic healing was observed in alloys containing Al cast at a higher initial mold temperature due to near-equilibrium solidification. Grain refining was confirmed at the microstructural level due to addition of Al. The results also suggested that increasing Al content enabled hot-tearing resistance of Mg-10Zn alloy by facilitating liquid feed for the strain developed during the mid-stage of solidification (0.65Fs to 0.74Fs). Experimental results were compared with Pandat’s simulation results to predict the occurrence of hot tearing in Mg-10Zn-xAl alloys. This indicated that Pandat’s solidification curve could be used to determine the hot-tearing nature of Mg-Zn alloys. The recently proposed hot-tearing criteria of Kou and Clyne–Davie roughly agreed with the experimental results, but did not perfectly predict the influence of different casting conditions.

Controlled Solidification to Obtain Equilibrium Microstructure in DC Cast Rolling Ingot

In 6xxx Al alloys, most of the secondary constituent phases that are formed at the primary aluminum (α-Al) grain boundary or interdendritic region during direct chill casting are brittle “metastable” phases, which complicates the downstream processes of the cast ingot. To overcome this, the cast ingot is usually homogenized and heat-treated before hot rolling. In the current study, we have shown the possibility of casting 6xxx Al alloy ingot with near-equilibrium solidification structure, thereby eliminating the homogenization process. This in turn increases productivity by reducing downstream processing steps. Novel jet stirring and in situ homogenization techniques control the solidifying microstructure, and the microstructural evidences show that the cast ingot has equilibrium structures. The thermal history of the solidifying ingots is found to play a significant role in producing equilibrium microstructure. The setup of a modified DC casting process with a mechanism for the observed microstructure changes as compared to the standard process has been discussed.

A method based on the centroid of segregation points: A Voronoi polygon application to solidification of alloys

A new method for controlling segregation delicately based on the centroid of independent segregation points in an alloy macrostructure is established using Voronoi polygon method. Voronoi polygon area is found to nearly obey the log-normal distribution, which shows that a polygon may reflect a new solidifying unit. The relationship between Voronoi polygon characteristics and carbon segregation extent (segregation ratio) is investigated at different locations. In a pure columnar grain zone, the segregation extent worsens as the Voronoi polygon size decreases; in an equiaxed grain zone, this extent worsens as the size increases. A near-equilibrium solidification model for polygons under different conditions is studied, and the results are consistent with the aforementioned finding of segregation ratio. Compared with a secondary arm spacing, the fractal dimension of the solidification structure may be a better method to describe the characteristics of the solidification structure and the size change of Voronoi polygons as to relevant segregation. Thus, this study proposes a new experimental method that depicts the macrostructure from a different perspective, i.e., the segregation point instead of the solidification structure. The formation of Voronoi polygons created by segregation points during solidifying should be divided into two stages, i.e., appearance of the segregation point and determination of the polygon's outline. The segregation extent can be controlled by altering the size of the relevant Voronoi polygon, which is a unique alternative strategy for improving the uniformity of element distribution rather than simultaneously considering nucleus formation, solidification structure growth, and liquid flow as to the complex process of segregation formation.

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.

Effects of initial undercooling on microstructure formation and recrystallisation of undercooled melts

ABSTRACTThe critical undercoolings for the two grain refinement events and the onset of recrystallisation event are determined by detailed analysis of the microstructure evolution of bulk undercooled Ni–20 at.-%Cu alloy melts. The first grain refinement event occurred in the low undercooling range was explained by dendrite remelting. The second grain refinement event occurred in the high undercooling range was due to the combined effects of dendrite remelting stress-induced dendrite breakup during recalescence and recrystallisation during the near-equilibrium solidification stage after recalescence. The micro-stress induced by the solidification contraction during recalescence in the so called ‘first mushy zone’ would lead to distortion and breakup of primary dendrites. The stress-induced broken-up dendrites have sufficient driving force for recrystallisation.

Phase selection and solidification path transition of Ti–48Al–xNb alloys with different cooling rates

Ti–48Al–xNb alloys were solidified by containerless electromagnetic levitation with quenching system of the conical copper mold. The influence of cooling rates on phase selection of Ti–48Al–xNb alloys was investigated. In near-equilibrium solidification condition, the dendrite β phase is observed as the leading phase. No other metastable phase (e.g., α phase) is observed. In contrast, in rapid solidification condition, the metastable α phase is observed in as-quenched Ti–48Al–2Nb alloy. Furthermore, the metastable α phase is replaced by the primary β phase with Nb addition increasing. For Ti–48Al–(x = 4, 6, 8)Nb alloys, increasing cooling rate results in a solidification path transition. The peritectic reaction (L + β → α) is therefore significantly suppressed. The relationships between primary dendrite arm spacing (λ1) and cooling rate (τ) can be described.

Microstructure and Selection of Grain Boundary Phase of Cu-Ni-Si Ternary Alloys

Abstract Cu-Ni-Si alloys with 90wt% Cu and 10wt% (Ni+Si) of different Ni/Si atomic ratios were prepared under a near-equilibrium solidification condition. The microstructures were observed by scanning electron microscope (SEM). The phase composition was identified by X-ray diffraction (XRD). The phase transformations during the solidification process were analyzed by differential scanning calorimetry (DSC) under an Ar atmosphere. Results show that the effects of Ni/Si atomic ratios on the microstructures of Cu-Ni-Si alloys can be mainly ascribed to the variation of the amount of δ-Ni2Si phase formed in the inner of α-Cu grains and the selection of grain boundary phase between Cu-Si and Ni-Si phases. The former is controlled by the Ni and Si concentrations in the primary α-Cu grains and the latter is a result of the competition between Cu-Si and Ni-Si intermetallics. With the increase of Ni/Si atomic ratio, Ni-Si intermetallics gradually win from the competition due to its lower heat of formation and the increase of Ni content in the inter-granular residual liquid. As a result, grain boundary phase changes from Cu5Si phase to Ni3Si phase gradually.

Nano-eutectic growth in Co-17.8 wt%Gd alloy ribbons and the magnetostrictive properties at different wheel speeds.

Under near-equilibrium solidification conditions, the Co-17.8 wt%Gd eutectic alloy forms rod-like eutectic microstructure of (αCo) solid solution and Co17Gd2 compound. When the solidification condition is far from the equilibrium, the rapid growth of nano-eutectic in Co-17.8 wt%Gd alloy ribbons is realized by the single-roller techniques. The average granular size (d) of nano-eutectic in the center of ribbons varies with the increase of wheel speed (V), d = 510.36-25.51 V+0.44 V2. XRD results of ribbons at different wheel speeds indicate that, with the rise of wheel speed, the main peak of Co17Gd2 compound becomes more and more notable, whereas the main peak of (αCo) solid solution tends to reduce. Along the length direction, the Co-17.8 wt%Gd alloy ribbons have the negative magnetostrictive strain. The magnetostrictive strain enhances with the increase of wheel speed. At the wheel speed of 40 m/s, the magnetostrictive coefficient of ribbons is measured to be - 733 ppm at the magnetic field of 6 kOe. The influence of the wheel speed and the magnetic field on the maanetostrictive coefficient is discussed.

Thermophysical properties and rapid solidification of Al-Cu-Ge alloys

Al-Cu-Ge alloy system, a typical ternary eutectic alloy system, has been used widely in the industries. Our research is helpful for better understanding its thermophysical properties and improving its structural performance. In this paper, the specific heat values and thermal expansion coefficients of Al55Cu10Ge35, Al70Cu10Ge20 and Al80Cu10Ge10 alloys are investigated. The microstructural characteristics and the solidification paths of these alloys under near-equilibrium solidification and rapid solidification conditions are studied comparatively. Their specific heat values increase as Al content increases and Ge content decreases. The softening temperature is 666 K, and the thermal expansion coefficient fluctuates around 1.5×10-5 K-1 in the temperature range of 370–650 K. Under a near-equilibrium solidification condition, the last formation microstructure is (Al)+(Ge) pseudobinary eutectic instead of ternary eutectic. This means that (Al), (Ge), and CuAl2 phases are difficult to nucleate simultaneously or grow cooperatively. In comparison, during rapid solidification, the nucleation of primary phase is depressed, pseudobinary eutectic and ternary eutectic are much easier to form in these alloys.

Effects of liquid separation on the microstructure formation and hardness behavior of undercooled Cu–Co alloy

Applying a melt-fluxing method, a substantial degree of undercooling up to 220 K has been obtained for Cu50Co50 immiscible alloy and the non-equilibrium solidification microstructure is investigated considering the successive processes of undercooling, liquid separation, non-equilibrium and near-equilibrium solidification. For samples undercooled by 62, 79 and 105 K, grain refinement occurs and becomes more serious without the occurrence of liquid separation and peritectic transformation. In contrast, metastable liquid demixing occurs upon the undercooling reaching 125 K, and it proceeds completely with the further increase of undercooling to 188 and 220 K, which can be ascribed to the sufficient time through the immiscible region. After liquid separation, versatile solidification microstructure for Co-rich and Cu-rich regions can be observed due to the different non-equilibrium effect. Moreover, second liquid separation becomes more serious for higher undercooling because of the enhanced immiscible effect. Finally, the solute content of primary Co-rich phase was analyzed with the consideration of solute trapping and liquid immiscible effects, which gives an excellent explanation for the variation of hardness behavior.

Research on the Optimization of Ni/Si Atomic Ratio for Cu-Ni-Si Alloys with High Strength and High Electrical Conductivity

Two sets of Cu-Ni-Si alloys with different Cu contents and Ni/Si atomic ratios were fabricated under the state of near-equilibrium solidification. The microstructures were observed by SEM and phase compositions were identified by XRD. The electrical conductivity and hardness were tested by Eddy-Conductivity Apparatus (ECA) and Digital Rockwell Hardmeter, respectively. The experimental results show that all the researched Cu-Ni-Si alloys are consisted of three phases, i.e. α-Cu(Ni,Si), Ni3Si and δ-Ni2Si. With the increase of Ni/Si atomic ratio, the amount of Ni3Si decreases persistently but that of δ-Ni2Si and electrical conductivity increases firstly and then decreases, the hardness decreases firstly and then increases following by a decrease finally. Both the electrical conductivity and hardness reach a relativity highest value when the Ni/Si atomic ratios are 2.6:1 and 2.4:1 for Cu-Ni-Si alloys with 95wt.% and 90wt.% Cu content respectively.

Use of recalescence behavior analysis for the prediction of grain refinement in undercooled Cu–Ni alloy

Considering non-equilibrium solidification and its influence on subsequent near-equilibrium solidification together, the description of recalescence behavior in bulk undercooled Cu70Ni30 alloy was adopted to predict the corresponding microstructure transition. The thermal plateau time for near-equilibrium solidification can be deduced directly with the calculation of non-equilibrium solid fraction formed in recalescence. On the basis of quantitative description for recalescence behavior, the non-equilibrium solid fraction, residual liquid fraction, dendrite broken-up time, and thermal plateau time can be determined as functions of initial undercooling. Then, a simple and accurate application of dendrite fragmentation model was performed as the grain refinement at both low- and high-undercooling originates from dendrite breakup. The predicted undercooling regions for the double grain refinement agree well with the experimental observation. Moreover, the change of grain morphology for the second grain refinement can be ascribed to the occurrence of recrystallization produced by the enhanced residual stress upon highly undercooled solidification.

Modeling the overall solidification kinetics for undercooled single-phase solid-solution alloys. II. Model application

An overall solidification kinetic model was applied to undercooled Ni–15 at.% Cu alloy. A good agreement between the model predictions and the measured cooling curves was obtained by adopting a “phenomenological” heat boundary condition. Applying numerical calculations, it was demonstrated that solute is uniformly distributed at the purely thermal-controlled growth stage; a transition from non-equilibrium to near-equilibrium solidification occurs at the mainly thermal-controlled growth stage only if sufficiently high initial undercooling is available; and a transition from recalescence to post-recalescence occurs at the solutal-controlled growth stage wherein the current model reduces to Scheil’s equation. The volume fraction solidified during recalescence is the same as the ratio of the initial undercooling to the hypercooling limit, and solidification should end generally at a temperature between the prediction of Scheil’s equation and that of the Lever rule if back diffusion is considered.

Microstructure and solidification behavior of Cu–Ni–Si alloys

The microstructure and solidification behavior of Cu–Ni–Si alloys with four different Cu contents was studied systematically under near-equilibrium solidification conditions. The microstructures of these Cu–Ni–Si alloys were characterized by SEM and the phase composition was identified by XRD analysis. The phase transition during the solidification process was studied by DTA under an Ar atmosphere. The results show that the microstructure and solidification behavior is closely related to the composition of Cu–Ni–Si alloys. The microstructure of Cu–Ni–Si alloys with higher than 40% Cu content consists of primary phase α-Cu(Ni, Si) and eutectic phase (β 1-Ni 3Si + α-Cu(Ni,Si).When the Cu content is about 40%, only the eutectic phase (β 1-Ni 3Si + α-Cu(Ni,Si)) is present. DTA analysis shows there are three phase transitions during every cooling cycle of alloys with higher than 40% Cu content, but only one for 40% Cu content. Cu–Ni–Si alloy with 40% Cu solidifies by a eutectic reaction, but Cu–Ni–Si alloys with higher than 40% Cu content solidify as a hypoeutectic reaction.

Non-Equilibrium and Near-Equilibrium Solidification of Undercooled Melts of Ni- and Al-based Alloys

Atomization and levitation are applied to undercool metallic melts and alloys prior to solidification. Heterogeneous nucleation on crucible walls is completely avoided giving access to large undercoolings. An undercooled melt possesses an enhanced free energy so that it can select different pathways to metastable solids. In this work dendrite growth and microstructure formation are investigated during solidification of undercooled melts of Ni- and Al-based alloys with respect to non-equilibrium solidification during recalescence and near equilibrium solidification during post-recalescence period

Metastable phase formation in undercooled Fe-Co melts under terrestrial and parabolic flight conditions

Soft magnetic Fe-Co alloys display primary fcc phase solidification for>19,5 at% Co in conventional near-equilibrium solidification processes. Undercooled Fe-Co melt drops within the composition range of 30 to 50 at% Co have been investigated with the electromagnetic levitation technique. The solidification kinetics was measured in situ using a high-resolution Siphotodiode. Melt drops were undercooled up to 263 K below the liquidus temperature and subsequently quenched onto a chill substrate in order to characterize the solidification sequence and microstructure. The transition from stable fcc phase to metastable bcc primary phase solidification has been observed after reaching a critical undercooling level. The critical undercooling increases with rising Co content. The growth velocity drops obviously after transition to metastable bcc phase formation. Parabolic flight experiments were performed in order to study the phase selection under reduced gravity conditions. Under microgravity conditions, a much smaller critical undercooling and an increased life time of the metastable bcc phase were obtained. This result was validated with TEM investigations. The appearance of Fe-O particles gives an indirect hint for an intermediate fcc phase formation from the metastable bcc phase at elevated temperature.

Metastable phase formation in undercooled Fe–Co melts

Abstract Soft magnetic Fe–Co alloys display the primary fcc phase solidification for >18 at.% Co in conventional near-equilibrium solidification processes. Undercooled Fe–Co melt drops within the composition range of 70–50 at.% Fe have been investigated with the electromagnetic levitation technique. The solidification kinetics was measured in situ using a high-resolving Si photodiode. The melt drops were undercooled up to 263 K below the liquidus temperature and subsequently quenched onto a chill substrate in order to characterise the solidification sequence and the microstructure. The transition from stable fcc phase to primary metastable bcc phase has been observed after reaching a critical undercooling level which increases with rising Co content. The growth velocity drops obviously after transition to metastable bcc phase formation. Moreover, parabolic flight experiments were performed in order to study the phase selection under reduced gravity conditions. The investigation of the solidification events yielded a much smaller critical undercooling and an increased life time of the metastable bcc phase.