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The thermodynamic description of the fcc phase in the Al-Cu system has been revised, allowing for the prediction of metastable fcc/liquid phase equilibria to undercoolings of ΔT = 421 K below the eutectic temperature. Hypoeutectic Al-Cu alloys that are prone to pronounced microsegregation were solidified containerlessly in electromagnetic levitation. Solidus and liquidus concentrations were experimentally determined from highly undercooled samples employing energy-dispersive X-ray analysis. Solid concentrations at a rapidly propagating solid/liquid interface were additionally calculated using a sharp interface model that considers all undercoolings and is based on solvability theory. Modelling results (front velocity versus undercooling) were also corroborated by in situ observation with a high-speed camera. A newly established thermodynamic description of the fcc phase in Al-Cu is compatible with existing CALPHAD-type databases. Inconsistencies of previous descriptions such as a miscibility gap between Al-fcc and Cu-fcc on the Al-rich side, an unrealistic curvature of the solidus line in the same composition range or an azeotropic point near the melting point of Cu, are amended in the new description. The procedure to establish the description of phase equilibria at high undercoolings can be transferred to other alloy systems and is of a general nature. This article is part of the theme issue ’Transport phenomena in complex systems (part 2)’.

Jet rapid solidification (JRS) is a key process to obtain the ribbon of magnetic materials. Numerical simulation is an effective method to analyze real-time distribution of status parameters during the JRS process and optimize the process. In this paper, the numerical simulation research in two JRS processes, namely, planar flow casting (PFC) and melt spinning (MS), is reviewed. First, based on the principle of rapid solidification, the working principles of PFC and MS processes are summarized and distinguished. The theoretical models, analytical models, and research methods of PFC and MS processes are further introduced and compared in three main research aspects, denoted as melt puddle, cooling roller, and inclusions in the tundish. Regarding the melt puddle, the influence of the melt puddle on the thickness of rapidly solidified ribbons is analyzed. The flow behavior and heat transfer analysis of melt puddle are discussed. In terms of the cooling roller, numerical analyses of the cooling roller structure optimization, parameter optimization, and “fluid–solid-thermal” coupling are summarized. Moreover, the simulation of the movement of inclusions in the PFC tundish is compared. Lastly, the current challenges and future opportunities are also addressed. The development of high-quality JRS ribbons has significant theoretical, social, and economic significance because its quality and stability determine the final characteristics of magnets. Therefore, the application of numerical simulation technology in the JRS process is beneficial for optimizing process routes and system structures, achieving the visual tracking of process, and objectively guiding the development of new JRS equipment.

The additive manufacture of compositionally graded Al/Cu parts by laser engineered net shaping (LENS) is demonstrated. The use of a blue light build laser enabled deposition on a Cu substrate. The thermal gradient and rapid solidification inherent to selective laser melting enabled mass transport of Cu up to 4 mm from a Cu substrate through a pure Al deposition, providing a means of producing gradients with finer step sizes than the printed layer thicknesses. Divorcing gradient continuity from layer or particle size makes LENS a potentially enabling technology for the manufacture of graded density impactors for ramp compression experiments. Printing graded structures with pure Al, however, was prevented by the growth of Al 2 Cu 3 dendrites and acicular grains amid a matrix of Al 2 Cu. A combination of adding TiB 2 grain refining powder and actively varying print layer composition suppressed the dendritic growth mode and produced an equiaxed microstructure in a compositionally graded part. Material phase was characterized for crystal structure and nanoindentation hardness to enable a discussion of phase evolution in the rapidly solidifying melt pool of a LENS print.

A novel droplet solidification technique was developed to emulate sub-rapid solidification and facilitate the formation of deposited films during the strip casting of silicon steels (w(Si): 2.5 and 3.5 wt.%). With the increasing number of droplet ejection experiments, the peak heat fluxes between droplet and substrate decreased firstly (1rd–5th ejection), then increased (5th–7th ejection), and finally decreased again (> 7th ejection). In the first five experiments, the interfacial thermal resistance between the droplet and the substrate improved with increasing film thickness. However, at the onset of the 6th droplet ejection experiment, the deposited film initiated its melting process due to the accumulated thermal resistance, which has the potential to eradicate the cavity or air space existing between the droplet and the substrate. Consequently, the interfacial contact condition was improved gradually with the increasing melting area from 5th to 7th droplet ejection experiments, leading to an increase in heat fluxes. Increased SiO2 content in deposited films for 3.5 wt.% Si steel led to lower peak heat fluxes than for 2.5 wt.% Si steel. The solidification structure of the 2.5 wt.% Si steel droplet sample comprised a fine grain zone at the base, a columnar grain zone in the center, and an equiaxed grain zone at the top. However, the solidification structure of the 3.5 wt.% Si steel droplet only contained columnar grains and equiaxed grains, with a larger average grain size due to the lower interfacial heat flux.

The demand for Ni-based brazing filler metals with melting temperatures below 1000 °C and good corrosion resistance is increasing because lower temperatures allow for the use of continuous belt furnaces rather than batch vacuum furnaces and are compatible with base metals that cannot withstand temperatures over 1000 °C. Based on this background and to explore the possibility of manufacturing Ni-Cr-P-Si amorphous brazing foils, rapid solidification technology was used to investigate the effects of chromium, phosphorus, and silicon as alloying elements on the melting characteristics, amorphous forming ability and ductility of boron-free Ni-Cr-P-Si alloys. As a result, a novel boron-free Ni-25Cr-6P-3Si amorphous brazing foil with a melting temperature of 985 °C and good bending ductility was obtained.

In this study, commercial AZ31B magnesium alloy was used to compare the differences between the microstructure, texture, and mechanical properties of conventional solidification (as homogenized AZ31) and rapid solidification (as RS AZ31). The results demonstrate that a rapidly solidified microstructure leads to better performance after hot extrusion with a medium extrusion rate (6 m/min) and extrusion temperature (250 °C). The average grain size of as-homogenized AZ31 extruded rod is 100 μm after annealing and 4.6 μm after extrusion, respectively, but that of the as-RS AZ31 extruded rod is only about 5 μm and 1.1 μm, correspondingly. The as-RS AZ31 extruded rod attains a high average yield strength of 289.6 MPa, which is superior to the as-homogenized AZ31 extruded rod, and is improved by 81.3% in comparison. The as-RS AZ31 extruded rod shows a more random crystallographic orientation and has an unconventional weak texture component in <112¯1>/<202¯1> direction, which has not been reported yet, while the as-homogenized AZ31 extruded rod has an expected texture with prismatic <101¯0>/<1¯21¯0>//ED.

The strip casts of cobalt-free maraging steel were fabricated using a twin-roll strip casting simulator, and its characteristics of sub-rapid solidification were studied. Subsequently, the confocal laser scanning microscope (CLSM) was employed to in situ observe the phase transformation during the heat treatment of maraging steel strip cast such as austenitization, solution treatment, and aging processes. It was found that due to the high cooling rate during the twin-roll strip casting process, the sub-rapid solidified strip cast possessed a full lath martensitic structure, weak macrosegregation, and evident microsegregation with a dendritic morphology. During austenitization of strip cast, the austenite grain size increased with the austenitization temperature. After holding at 1250 °C for 250 s, the austenite grain size at the high temperature owned a high similarity to the prior austenite grain size of the strip cast, which effectively duplicates the microstructure of the strip cast after sub-rapid solidification. During the solution treatment process, the martensitic structure of the strip cast also underwent austenitic transformation, subsequently transformed into martensite again after quenching. Due to the low reheating temperature during solution treatment, the austenite grain size was refined, resulting in the fine martensitic microstructure after quenching. During the aging process of strip cast, some of martensite transformed into fine austenite, which was located in the interdendritic region and remained stable after air cooling, resulting in the dual-phase microstructure of martensite and austenite. The solute segregation of Ni and Mo elements during the sub-rapid solidification of strip cast caused the enrichment of Ni and Mo elements in the interdendritic region, which can expand the austenite phase region and thus enhance the stability of austenite, leading to the formation of austenite in the interdendritic region after aging treatment.

The complex producing procedures and high energy-consuming limit the large-scale production and application of advanced high-strength steels (AHSSs). In this study, the direct strip casting (DSC) technology with unique sub-rapid solidification characteristics and cost advantages was applied to the production of low-alloy Si–Mn steel with the help of quenching & partitioning (Q&P) concept to address these issues. Compared this method with the conventional compact strip production (CSP) process, the initial microstructure formed under different solidification conditions and the influence of heat treatment processes on the final mechanical properties were investigated. The results show that the initial structure of the DSC sample is a dual-phase structure composed of fine lath martensite and bainite, while the initial structure of the CSP sample consists of pearlite and ferrite. The volume fraction and carbon content of retained austenite (RA) in DSC samples are usually higher than those in CSP samples after the same Q&P treatment. DSC samples typically demonstrate better comprehensive mechanical properties than the CSP sample. The DSC sample partitioned at 300°C for 300 s (DSC-Pt300) achieves the best comprehensive mechanical properties, with yield strength (YS) of 1282 MPa, ultimate tensile strength (UTS) of 1501 MPa, total elongation (TE) of 21.5%, and product of strength and elongation (PSE) as high as 32.3 GPa·%. These results indicate that the excellent mechanical properties in low-alloy Si–Mn steel can be obtained through a simple process (DSC–Q&P), which also demonstrates the superiority of DSC technology in manufacturing AHSSs.

Sub-rapid solidification has the potential to enhance the columnar structure and the magnetic property of electrical steels. However, research on the hot deformation behavior of sub-rapid solidified non-oriented electrical steel, particularly at varying strain rates, has yet to be fully understood. The effect of thermal compression on the microstructure and mechanical properties of 3.15 wt.% Si non-oriented electrical steel strips produced through a strip casting simulator was systematically investigated. The findings reveal that increasing the deformation temperature enhances grain recrystallization, while the peak stress decreases with higher temperature. Furthermore, a lower strain rate favors dynamic recrystallization and reduces thermal stress. It can be seen that sub-rapid solidification can effectively reduce the thermal activation energy of non-oriented electrical steel, and the thermal activation energy is calculated to be 204.411 kJ/mol. In addition, the kinetic models for the dynamic recrystallization volume fraction of the studied 3.15 wt.% Si non-oriented electrical steel were established.

Enhancement of solid solubility in binary immiscible Cu-Co system due to thermodynamic contribution from ternary Mn addition and kinetic contribution from use of rapidly solidified ribbons as the precursor of ball milling has been investigated. The impact of ternary addition on the binary system’s free energy change at various concentrations and grain size has been estimated by employing the thermodynamic model proposed by Miedema incorporating appropriate modifications for ternary addition. The solid solubility results obtained from x-ray diffraction analysis of the rapidly solidified ribbons have been compared with the results obtained by the thermodynamic calculation, and an attempt has been made to identify the potential attributes contributing to the mechanically induced solid solubility in the immiscible systems. High-resolution transmission electron microscopy (HRTEM), differential thermal analyzers (DTA) and x-ray diffraction (XRD) have all been used to characterize the phase advancement during rapid solidification, mechanical alloying and isothermal annealing. Using a superconducting quantum interference device magnetometer (SQUID), magnetic characteristics have been investigated. Following annealing in the ball-milled Cu-Co-Mn alloy at 550 °C for 1 h, the ideal combination of magnetic characteristics was achieved. Graphical Abstract