Rapid Solidification Experiments Research Articles

Investigation on nonequilibrium crystallization of highly undercooled Cu–Ni–Co alloys

Deep undercooling rapid solidification experiments of Cu–Ni–Co alloys were carried out using the molten glass purification cycle superheat method. 130 K, 177 K and 277 K undercooling degrees were obtained and analysed for Cu55Ni43Co2 alloy. The crystallographic information obtained by EBSD and the dynamics of the solidification front taken by high speed photography were analysed and it was concluded that both the formation of relatively rounded equiaxed grains during the first refinement and the subsequent formation of finer subcrystals were due to the release of a large amount of solidification energy during the solidification process. The re-glow stage releases a large amount of latent heat of solidification resulting in the remelting of the dendrites. TEM tests were carried out on Cu55Ni43Co2 alloy with a maximum undercooling of 277 K. It was found that part of the substructure showed a high density dislocation network. By analysing the effect of increasing undercooling on the microstructure evolution, combined with the EBSD and TEM characterisation data, it was found that the stress-induced recrystallization mechanism was the dominant factor in the grain refinement mechanism at large undercooling.

Orientation Gradients in Rapidly Solidified Pure Aluminum Thin Films: Comparison of Experiments and Phase-Field Crystal Simulations.

Rapid solidification experiments on thin film aluminum samples reveal the presence of lattice orientation gradients within crystallizing grains. To study this phenomenon, a single-component phase-field crystal (PFC) model that captures the properties of solid, liquid, and vapor phases is proposed to model pure aluminium quantitatively. A coarse-grained amplitude representation of this model is used to simulate solidification in samples approaching micrometer scales. The simulations reproduce the experimentally observed orientation gradients within crystallizing grains when grown at experimentally relevant rapid quenches. We propose a causal connection between defect formation and orientation gradients.

Rapid solidification of ternary Nb‐Y‐Ni immiscible alloys

AbstractThe rapid solidification experiments have been carried out for the Nb(45‐x)YxNi55 (x = 9, 19, and 27) alloys. The microstructure characterization shows the occurrence of the liquid‐liquid phase separation into the Niobium‐rich and Yttrium‐rich liquids. The Niobium‐rich spherical particles were embedded in the glassy Yttrium‐rich matrix for the alloy Nb36Y9Ni55, while glassy Yttrium‐rich ones were located in the glassy Niobium‐rich matrix for the alloy Nb18Y27Ni55. A structure of hierarchical separation was detected in the as‐quenched alloy Nb26Y19Ni55. The size distribution of the spherical particles in the as‐quenched Nb(45‐x)YxNi55 (x = 9 and 27) alloys ranges from several nanometers to sub micrometer and is close to the Gaussian distribution. The average diameters of the Niobium‐rich particles in the as‐quenched Nb36Y9Ni55 alloy and the Yttrium‐rich particles in the Nb18Y27Ni55 alloy are 0.23 μm and 0.15 μm, respectively. A dual‐glass structure in the as‐quenched Nb18Y27Ni55 alloy ribbon was determined by X‐ray diffraction and transmission electron microscopy.

In Situ and Ex Situ Characterization of the Microstructure Formation in Ni-Cr-Si Alloys during Rapid Solidification-Toward Alloy Design for Laser Additive Manufacturing.

Laser beam-based deposition methods such as laser cladding or additive manufacturing of metals promises improved properties, performance, and reliability of the materials and therefore rely heavily on understanding the relationship between chemical composition, rapid solidification processing conditions, and resulting microstructural features. In this work, the phase formation of four Ni-Cr-Si alloys was studied as a function of cooling rate and chemical composition using a liquid droplet rapid solidification technique. Post mortem x-ray diffraction, scanning electron microscopy, and in situ synchrotron microbeam X-ray diffraction shows the present and evolution of the rapidly solidified microstructures. Furthermore, the obtained results were compared to standard laser deposition tests. In situ microbeam diffraction revealed that due to rapid cooling and an increasing amount of Cr and Si, metastable high-temperature silicides remain in the final microstructure. Due to more sluggish interface kinetics of intermetallic compounds than that of disorder solid solution, an anomalous eutectic structure becomes dominant over the regular lamellar microstructure at high cooling rates. The rapid solidification experiments produced a microstructure similar to the one generated in laser coating thus confirming that this rapid solidification test allows a rapid pre-screening of alloys suitable for laser beam-based processing techniques.

Investigation of amorphous phase formation in Fe-Co-Si-B-P - Thermodynamic analysis and comparison between mechanical alloying and rapid solidification experiments

In recent years, amorphous alloys have received a considerable attraction because of their physical, mechanical and magnetic properties. Fe-Co alloy system has good soft magnetic properties (high magnetic saturation and high Curie point) which makes it suitable for high temperature magnetic applications. In this paper, the effect of different elements on glass forming ability (GFA) of Fe-Co system was evaluated based on Meidema semi-empirical model and the results were compared to mechanical alloying (MA) experiments. Si, B and P seem to be good elements to be added to Fe-Co alloys with respect to thermodynamics and kinetics requirements. Calculations based on extended Miedema model showed that enthalpy and Gibbs free energy changes for solid state amorphisation of Fe70Co7Si8B8P7 were −179 (kJ mol−1) and −181 (kJ mol−1) respectively, but MA experiments in three different routes did not lead to amorphisation in this system. Also comparison between MA and melt spinning (MS) techniques showed GFA in both techniques are completely different.

Rapid solidification mechanism of highly undercooled ternary Cu40Sn45Sb15 alloy

The rapid solidification of ternary Cu40Sn45Sb15 peri-eutectic type alloy was realized by glass fluxing and drop tube methods, and the corresponding maximum undercoolings are 185 K (0.22T L) and 321 K (0.39T L), respectively. The phase constitution of Cu40Sn45Sb15 alloy in these two rapid solidification experiments deviates from the two equilibrium phases (Sn + Cu6Sn5). In glass fluxing method, the structural morphology of Cu40Sn45Sb15 alloy is mainly characterized by a three-layer lamellar structure, which is comprised by an inner layer of long strips of primary e(Cu3Sn) phase, an intermediate layer of η(Cu6Sn5) phase and an outer layer of β(SnSb) phase. As undercooling rises, this lamellar structure is remarkably refined. When small alloy droplets are containerlessly solidified during free fall in drop tube, the primary e(Cu3Sn) phase grows by non-faceted mode into dendrites as droplet diameter decreases. Especially, solidification path alters in the smallest droplet with 50 μm diameter, in which η(Cu6Sn5) and Sn3Sb2 phases form directly from the metastable liquid phase by suppressing the primary e phase formation and the following peri-eutectic transformation.

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.

Analysis and Prevention of Dent Defects Formed during Strip Casting of Twin-Induced Plasticity Steels

Rapid-solidification experiments were conducted for understanding dent defects formed during strip casting of twin-induced plasticity (TWIP) steels. The rapid-solidification experiments reproduced the dent defects formed on these steels, which were generally located at valleys of the shot-blasted roughness on the substrate. The rapid-solidification experiment results reveal that the number of dips, the Mn content of the steel, and the surface roughness of the substrate affect the depth and size of dents formed on the solidified-shell surfaces, while the composition of the atmosphere gases and the carbon content of the steel are not factors. The formation of dents was attributed to the entrapment of gases inside the roughness valleys of the substrate surface and their volume expansion due to the temperature of the steel melt and the latent heat. The dents could be prevented when the thermal expansion of gases was suppressed by making longitudinal grooves on the substrate surface, which allowed the entrapped gases to escape. Sound solidified shells were obtained by optimizing the width and depth of the longitudinal grooves and by controlling the shot-blasting conditions.

Thermodynamics and kinetics of metallic amorphous phases in the framework of the CALPHAD approach

Amorphous alloys are metastable phases which have generated considerable interest in recent years because of their promising properties. This review considers thermodynamic and kinetic aspects in the modelling of metallic amorphous phases. The relevant literature on the glass transition and the thermodynamics of amorphous phases is discussed in detail. The CALPHAD approach is used for thermodynamic modelling: it employs collected experimental data to obtain a reliable assessment of properties and metastable phase diagrams. Thermodynamic modelling of Short Range Order (SRO) in the liquid and glass transition is described and critically discussed with regard to rapid solidification experiments. Solid state amorphization and amorphization by means of deposition are also discussed. The extension to kinetics is then described, involving a number of modelling parameters. Predictions can thus be carried out of the Glass Forming Range (GFR), i.e. the composition range where glass formation is expected, according to different levels of approximation and for various processes. The importance of model parameters (thermodynamic driving force, interfacial energy, transport properties) is also assessed and some case studies (Cu–Zr, Fe–B and Cu–Mg–Y) are discussed. Recent results on the simulation of nanocrystals growth in amorphous materials are reported.

Formation of the Microstructure in a Rapidly Solidified Cu-Co Alloy

A model is developed to describe the microstructure evolution in a Cu-Co drop during cooling in the metastable miscibility gap in the liquid state. Rapid solidification experiments have been performed with Cu84Co16 alloy in a drop tube. The kinetic details of the liquid-liquid phase transformation in the Cu-Co drops have been calculated. The results demonstrate that the microstructure development is dominated by the common action of the nucleation and the diffusional growth of the minority phase precipitates under rapid solidification conditions. The calculated size distribution of the Co precipitates in powders and their average radius as a function of cooling rate are compared with the experimental results. The agreement is satisfactory.

Experimental study on rapid solidification process using a novel ultrasound technique

Solidification rate is a crucial parameter affecting the microstructure formation in rapid contact solidification processes. A novel ultrasound technique has been developed for measuring the phase-change interface movement during a rapid contact solidification process. This new method utilized the echoes from two reflection interfaces for tracking the solidification front. It is therefore possible to eliminate the temperature effect on the sound speed in metals. The rapid solidification experiments were conducted by impacting an aluminum substrate onto a shallow liquid alloy 158. Echo signals were detected successfully and the phase change front position was determined with high resolution. A theoretical resolution of 4.5 μm can be achieved under current conditions. Experiments were conducted to study the effects of initial metal temperature, liquid pool thickness and impact height on a rapid contact solidification process. The experimental results show that with an increasing impact height, the solidification rate increases which means the thermal contact resistance is decreased. Therefore, the newly developed method can provide useful data for correlating the thermal contact resistance in rapid contact solidification processes.

Cellular growth of Zn-rich Zn–Ag alloys processed by rapid solidification

Abstract Rapid solidification experiments were carried out on single-phase Zn-rich Zn–Ag alloys processed by laser remelting and melt-spinning techniques. For comparison, Bridgman solidification was performed as well. Three solidification morphologies were mainly identified with increasing growth velocity from 0.02 to 300 mm/s: regular cells or dendrites of η, plate-like cells of η, and plane front of η. Plate-like and regular cells were found to be velocity-dependent and occurred in high-velocity and low-velocity domains, respectively. The development of solidification interface morphologies in the present Zn–Ag alloys was proposed accordingly over a wide range of growth velocities from the limit of constitutional supercooling to the limit of absolute stability. Four microstructure transitions were identified: low-velocity plane front of η→low-velocity plate-like cells of η→regular cells or dendrites of η→high-velocity plate-like cells of η→high-velocity plane front of η. It implies that a plate-like cell would be the favorable morphology at growth conditions approaching a stable planar solid/liquid interface in comparison with the regular one.

Morphological evolution of banding structures at high solidification velocity

Laser rapid solidification experiments were performed on a Ti–53at.%Al hypoperitectic alloy to investigate the dendritic growth behavior near the limit of high velocity absolute stability. By adopting an improved sampling method of TEM, the growth morphology evolution of the laser-resolidified layer was observed directly. High velocity banding structures was firstly detected in Ti–Al peritectic alloys. The high velocity banding structures are parallel to the solid–liquid interface and made of the oscillation structures grown alternatively in modes of cell and plane morphologies. In the light bands with cellular growth mode, all dislocation assembles parallel to the growth direction and forms cell boundaries; while all dislocation distributes randomly in the dark bands. The determined growth velocity range for the appearance of banding structures is 0.5–1.1 m/s and the origin of the banding structure agrees well with the prediction of the CGZK phenomenological model.

Formation of Y xNd 1− xBa 2Cu 3O 7− δ (0≤ x≤0.9) superconductors from an undercooled melt via aero–acoustic levitation

This paper presents the results of rapid solidification experiments performed on the copper oxide superconductors Y x Nd 1− x Ba 2Cu 3O 7− δ (0≤ x≤0.9). Spherical rare earth (RE) 123 specimens were levitated in O 2 using aero–acoustic levitation (AAL), melted with a laser, undercooled, and solidified. The peritectic transformation temperature for the reaction RE 2BaCuO 5+liquid→REBa 2Cu 3O 7− δ corresponding to the maximum recalescence temperature during solidification was determined. RE123 was formed directly from the melt for Y–Nd binary alloy compositions with Nd concentration greater than 20% (Y concentration less than 80%). A minimum in the peritectic transformation temperature for the Nd/Y123 system corresponding to a composition Y 0.3Nd 0.7123 was determined at 66 °C below the peritectic of pure Nd123.

High-velocity banding structure in the laser-resolidified hypoperitectic Ti 47Al 53 alloy

Stability analysis of a growing solid/liquid interface is the fundamental concept of modern solidification theory. Here, serial laser rapid solidification experiments were performed on a hypoperitectic Ti 47Al 53 alloy to explore the dendritic growth behavior near the limit of high-velocity absolute stability. SEM and TEM techniques were carried out to investigate the microstructure and identify the phase composition. By adopting an improved sampling method of TEM, the growth morphology evolution of the laser-resolidified layer was observed directly and high-velocity banding structure was firstly detected in Ti–Al peritectic alloys. The high-velocity banding structures are parallel to the solid/liquid interface (normal to the growth direction) and made of the oscillation structures grown alternatively in modes of cell and plane morphologies. In light bands with cellular growth mode, all dislocation assembles are parallel to the growth direction and forms the cell boundaries, while all dislocation distributes randomly in dark bands. The determined growth velocity range for the appearance of high-velocity banding structures is about 0.5∼1.1 m s −1 according to the rapid solidification experiments, and the origin of the banding agrees well with the prediction of the CGZK phenomenological model (Acta Metal. Mater. 40 (1992) 983).

Rapid solidification behavior of Zn-rich Zn–Ag peritectic alloys

Rapid solidification experiments, including laser remelting, melt-spinning and wedge casting, were carried out to investigate the rapid solidification behavior of Zn-rich Zn–Ag peritectic alloys containing up to 9.0 at% Ag. For comparison, Bridgman solidification experiments of the same alloys were also carried out for growth velocities ranging from 0.02 to 4.82 mm/s, which were lower than that of 12–54.5 mm/s for laser remelting and that in the order of 10 2 mm/s for melt-spun samples. Optical images and transmission electron microscopy (TEM) showed that instead of the typical structure consisting of primary dendrites of ϵ surrounded by peritectic η, a two-phase plate-like η+ ϵ with (or without) primary dendrites of ϵ was observed in Zn–3.1, 4.4, 6.3 and 9.0 at% Ag alloys when the growth velocity was higher than a critical value. It was found that the higher was the alloy concentration, the higher was the critical growth velocity for the formation of fully two-phase plate-like η+ ϵ. From the TEM micrographs, the volume fraction of ϵ in the fully two-phase plate-like η+ ϵ increased from 0.09 to 0.50 with increase in alloy concentration from 3.1 to 6.3 at% Ag. A plausible analysis was proposed to interpret the dependence of microstructural transitions on the growth velocity in Zn–3.1 to 9.0 at% Ag alloys, that is, primary dendrites of ϵ in a matrix of peritectic η→two-phase plate-like η+ ϵ with primary dendrites of ϵ→ fully two-phase plate-like η+ ϵ.

Microstructure evolution of Cu–Mn alloy under laser rapid solidification conditions

Laser rapid solidification experiments were performed on a Cu–Mn alloy to investigate its microstructure evolution as a function of growth rate. By taking transverse and longitudinal sections of the laser traces, the resulting morphology and the corresponding growth rate were quantitatively measured by scanning electron microscopy (SEM) and image analysis. As the growth rate increased, the morphological transition from dendrite→fine cellular crystal→banded structure→fully cell-free microstructure, involving planar front growth, was clearly observed in the laser molten pool. By determining these transition conditions and comparing them with the predictions of theoretical models, we found that the origin of the banding can be well elucidated by the current Carrard–Gremaud–Zimmermann–Kurz (CGZK) phenomenological model, while the critical condition of planar front growth was in reasonable agreement with Mullins–Sekerka (M–S) stability theory.

Interface dynamics, instabilities, and solute bands in rapid directional solidification

In rapid solidification experiments on metallic alloys structures have been observed which are periodic along the growth direction. The origin of these banded structures has been ascribed to an oscillatory instability of the solid-liquid interface characterized by large variations of the interface velocity; this instability was predicted by several authors incorporating nonequilibrium effects into the classic Mullins-Sekerka analysis. In this paper the rapid solidification of a binary alloy, directed by a moving temperature field, is studied with the phase-field model; in a region of the parameter space an oscillatory instability is evidenced, which reflects in alternating low and high concentration solute bands. The equations of the model are numerically solved to show under what conditions (i.e., isotherm velocity and temperature gradient) the banded structure can be observed. In many respects the results agree with the linear stability analysis of the free-boundary equations performed by Merchant and Davis [G. J. Merchant and S. H. Davis, Acta Metall. Mater. 38, 2683 (1990)]; we detected also significant deviations which trace their roots to the diffuse solid-liquid interface characteristic of the phase-field model, opposed to the zero dimension interface of the free-boundary model.

Melting Temperature, Transformation and Metastable Phase Diagram of Rapidly Solidified Ag–Cu Alloys

Cu-Ag samples produced during rapid solidification experiments using a Planar Flow Melt Spinning Process or a levitation casting technique were rapidly heated in a mirror furnace to a variety of temperatures at rates between 100 and 1.6 K/s. The surface of each sample was observed in a confocal laser scanning microscope. Metallographic analyses were performed on the samples by optical and scanning electron microscopy. The rapidly solidified samples melted far below the equilibrium melting point. The melted and resolidified samples mostly consisted of structures similar to those seen in alloys with a miscible gap. It is proposed that this type of behaviour is a result of lattice defects formed during the rapid solidification process. The effect of these defects on the thermodynamic properties was investigated. A metastable phase diagram with a miscibility gap can be constructed below the equilibrium melting point. This could explain the observed phase structure and the melting of the alloy below the equilibrium melting point.

Oscillatory instabilities in rapid directional solidification

The rapid directional solidification of a binary alloy is studied with the phase-field model. An oscillatory instability is evidenced which originates alternating low and high concentration solute bands. This instability, which is related to the banded structure observed in rapid solidification experiments, is predicted by previous studies conducted within the free-boundary formulation of the problem, incorporating nonequilibrium effects as solute trapping. In the present paper the governing equations of the model are numerically solved to show under what conditions (i.e., isotherm velocity and temperature gradient) the banded structure can be observed.