Search

Through the use of purification and recirculation superheating techniques on molten glass, the Ni65Cu33Co2 alloy was successfully undercooled to a maximum temperature of 292 K. High-speed photography was employed to capture the process of interface migration of the alloy liquid, allowing for an analysis of the relationship between the morphological characteristics of the alloy liquid solidification front and the degree of undercooling. Additionally, the microstructure of the alloy was examined using metallographic microscopy, leading to a systematic study of the microscopic morphological characteristics and evolution laws of the refined structure during rapid solidification. The research reveals that the grain refining mechanism of the Ni-Cu-Co ternary alloy is consistent with that of the binary alloy (Ni-Cu). Specifically, under low undercooling conditions, intense dendritic remelting was found to cause grain refinement, while under high undercooling conditions, recrystallization driven by accumulated stress and plastic strain resulting from the interaction between the liquid flow and the primary dendrites caused by rapid solidification was identified as the main factor contributing to grain refinement. Furthermore, the study highlights the significant role of the Co element in influencing the solidification rate and reheat effect of the alloy. The addition of Co was also found to facilitate the formation of non-segregated solidification structure, indicating its importance in the overall solidification process. [ABSTRACT FROM AUTHOR]

The experiment employed the use of melt purification and cyclic superheating technique to achieve maximum undercooling of Ni65Cu31Co4 alloy at 300K. Simultaneously, high-speed photography techniques were used to capture the process of alloy liquid phase interface migration, and analyzed the relationship between the shape characteristics of the front end of alloy solidification and undercooling. The microstructure of the alloy was observed through metallographic microscopy, and the micro-morphological characteristics and evolution of the rapidly solidified microstructure were systematically studied. It is found that the grain refinement mechanism of Ni-Cu-Co ternary alloy is similar to that of Ni-Cu binary alloy. Grain refinement at low undercooling is caused by intense dendritic remelting, while grain refinement at high undercooling is attributed to recrystallization, driven by the stress and plastic strain accumulated from the interaction of liquid flow and primary dendrites caused by rapid solidification. It also shows that the addition of the third element Co plays a significant role in solidification rate and re-ignition effect. [ABSTRACT FROM AUTHOR]

This study was designed as a preliminary experiment for missions at the electrostatic levitation furnace in the International Space Station (ISS-ELF) such as Hetero-3D. The objective of this study was to employ a high-speed camera to determine the recalescence time of undercooled Ti6Al4V alloy with TiC heterogeneous nucleation site particles more accurately compared to measurements using a pyrometer. The sample was melted and solidified in the ground-based electrostatic levitation (ESL) furnace. The changes in the luminescence emitted from the sample surface due to recalescence were recorded using a high-speed camera, and the intensity at the center of each captured image was analyzed with MATLAB® software. As a result, the intensity of the undercooled samples increased significantly during recalescence. The maximum change in the intensity was 101 per 256 gray levels in the recording at 7,200 frames per second (fps) and the noise was at most only 3. Therefore, the recalescence time could be obtained using the high-speed camera. At 15,000 fps, the recalescence time was within 6.7 × 10-5 s, which was much more accurate than the time (0.11 s) measured using a pyrometer at a sampling rate of 120 Hz in the ESL furnace. For enhanced measurement accuracy, it is crucial to establish an appropriate exposure time to prevent the intensity from reaching the lower limit of 0 just before recalescence and the upper limit of 255 immediately after that. This study suggests that the high-speed camera newly installed in the ISS-ELF in 2023 may have the potential to achieve a more accurate recalescence time than the pyrometer at a sampling rate of 100 Hz in the ISS-ELF. [ABSTRACT FROM AUTHOR]

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.

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.

Finite element simulations of welding, additive manufacturing and other thermomechanical processes require numerical algorithms both robust and economical in CPU time and memory resources. This paper focusses on thermal and metallurgical features of such simulations, leaving aside mechanical aspects. Two topics are envisaged: the evolution of the proportions of the various phases, resulting from anisothermal metallurgical transformations, and that of the temperature, depending on transformation latent heats. The simulation of phase changes uses Leblond and Devaux (1984)'s classical kinetic model for anisothermal transformations. With the hypothesis of modest variations of temperature over each discretized time-interval, the numerical algorithm proposed provides a quasi-exact solution within each such interval. The time-step may then be defined in reference to the sole characteristic time of the process itself, without considering those tied to the kinetic constants. The heat diffusion equation including latent heat effects is solved using a finite element technique suggested by Feulvarch et al. (2009), wherein the specific enthalpy is used as principal nodal unknown instead of the temperature. In this approach the temperature is considered as a well-defined function of the enthalpy (account being taken of the metallurgical transformations). This permits to use a robust and accurate algorithm for the integration in time, even in the presence of recalescence. Numerical examples are finally provided in order to illustrate the efficiency of the algorithms proposed. [ABSTRACT FROM AUTHOR]

The maximum 320 K undercooling of Ni65Cu35 alloy was obtained by means of cyclic superheating in the state of molten glass. The solidification structuresat different undercoolings were systematically studied. There are two grain refinementsat low and high undercoolings. It was generally believed that the grain refinement atlowundercooling was caused by dendrite superheat remelting. Electron backscattering diffraction was used to characterize the solidification structure athighundercooling, and obvious random orientation, high-angle grain boundaries and twin boundaries were found, indicating grain refinement was caused by recrystallization under highundercooling. There were almost no visible dislocation defects in the TEM bright-field images at high undercooling, and the microstructure hardness decreased obviously after grain refinement. This further illustrated the rapid accumulation of stress and defects in recalescenceare completely abreacted in recrystallization during post-recalescence period, at the same time as the driving force to promote the second grain refinement in the microstructure. [ABSTRACT FROM AUTHOR]

High-speed camera (HSC) is an advanced technology, which can be applied to observe the solid–liquid interface directly during solidification. It has never been reported that the temperature evolutions of solidification that was monitored by using cameras and their images. In this study, a new application of HSC technology was developed for the temperature evolution investigation. With this technology, the effects of the measured position, the thermocouple size and thermocouple sensitivity (delay) can be demonstrated in temperature measurement. It is important that the higher accuracy of temperature profiles can be obtained by the HSC technology. This method can be expected to analyze the temperature profiles of small melt-pool and subsequent rapid solidification process in additive manufacturing. In the end, we discussed the relationship of the recalescence rate and the solidification interface. [ABSTRACT FROM AUTHOR]

• Long anisotropic carbon fibers work as effective heat tunnels with dissipated radial heat flux along their axes in PCMs. • For inorganic PCM, long, anisotropic CF inserts enhanced both charging and discharging processes. • For organic PCM, the enhancement is valid in charging process, but is less in discharging process with reduced convection. In thermal energy storage systems, phase change materials (PCMs) are widely used for thermal energy management. Most PCMs have low thermal conductivities, which limits the heat transfer rate within PCM and thus makes the phase-changing process very slow. However, thermal conductivities of PCMs can be altered by inserting targeted additives. We hypothesize that the size and shape of these additive inserts play a key role in thermal management efficiency. To this end, the impacts of carbon fiber (CF) inserts on the phase change behavior and consequent heat transfer efficiencies of inorganic and organic PCMs were investigated using experiments and simulations. Long, anisotropic CFs with high thermal conductivities formed continuous fast heat flux tunnels inside PCMs to enhance the heat transfer. Such CFs could extend the phase change fronts from the limited container-shaped interface to the larger surface of numerous CF inserts inside the PCM. These special CF inserts work with a new heat transfer mechanism, different from conventional small additives or long isotropic CF inserts. The thermal energy release rate increased by 2.5 times with 1 wt.% anisotropic CF inserts in inorganic PCM. However, CF inserts in liquid organic PCM hindered the natural convection and compromised the improved heat conduction. The lab-scale multiphysics simulations support these experimental observations and indicate that CF inserts have potential to enhance heat transfer in inorganic PCMs, but they are less effective in organic PCMs. [ABSTRACT FROM AUTHOR]

To capture CO 2 crystal nucleation during the desublimation, we conducted a molecular dynamics study on the desublimation process of CO 2 on a low temperature plate. The desublimation process is characterized by three models corresponding to different degrees of supercooling. When the plate temperature is below 82 K, the solid CO 2 with a glassy state structure forms after CO 2 gas molecules adsorb onto the cool plate. In the temperature range of 83–113 K, CO 2 crystal nucleus forms within the disordered solid CO 2 , and subsequently the nucleus grows into Pa3-type crystal. For the plate temperature between 114 and 128 K, a liquid CO 2 phase initially forms, the nucleus then generates and grows to develop into Pa3 crystal. The CO 2 crystal nucleation is accompanied by the reduction of molecular potential energy and the increase of molecular kinetic energy. Additionally, the recalescence phenomenon occurs during CO 2 crystal nucleation. It is crucial that the molecular kinetic energy and molecular distance fall within a specific range to enable the formation of CO 2 nucleus and regular Pa3 crystal. Consequently, the CO 2 crystal nucleation can only occur within a specific range of plate temperatures. • The CO 2 desublimation is characterized by three models that correspond to different degrees of supercooling. • The transformations from solid CO 2 with a glassy state or liquid CO 2 to Pa3-type crystal are observed. • The CO 2 nucleation is marked bya reduction in molecular potential energy and an increase in molecular kinetic energy. • The molecular kinetic energy and molecular distance fall within a specific range to enable the formation of CO 2 nucleus. [ABSTRACT FROM AUTHOR]

In this article we present a thermal coupling to a recently published two-time scale phase field crystal model (MPFC) to describe latent heat contributions of systems undergoing rapid phase transformations. We call this formulation Temperature Field Crystal (TFC), which permits the study of coupled density, vacancy and temperature field dynamics at the length-scale of atomic ordering. Following a derivation of the new thermo-density coupled MPFC model, several physical properties of the model are demonstrated. We reproduce the thermo-density interface profiles encountered in the steady state solidification growth limit. We further illustrate the existence of a vacancy concentration inherent in the phase field crystal amplitude. It is shown that with a frozen thermal gradient, of the form often used in directional solidification studies, a vacancy gradient is established along with an associated thermal stress. In particular, we show that within the isochoric limit of the model, effects of thermal expansion are incorporated through the change of amplitude with respect to temperature, rather than by increasing the equilibrium lattice length. The TFC model is then applied to the study of select solidification processes. It is shown that the release of latent heat during recalescence is accompanied by a change in the average thermal pressure. Further, we illustrate, to our knowledge for the first time, that modulations of the recalescence curve can be indicative of plastic deformation, dislocation activity, and phonon scattering. Notably, the temperature evolution may be used as a marker for the grain distribution attained from nucleation following a system quench. [Display omitted] [ABSTRACT FROM AUTHOR]

Applying cycle superheating and molten glass purification technologies, Cu55Ni45 and Cu55Ni43Co2 alloys were deeply undercooled to maximum undercoolings of 284 K and 250 K respectively. The microstructure characteristics and its evolution varying with bulk undercooling during the rapidly solidified process were analyzed. It was found that, after minor addition of element Co, refinement mechanisms of dendritic grain of the undercooled Cu-Ni-Co alloys was the same as those of the Cu Ni alloys. Dendritic grain refinement occurring at low undercooling range was induced by dendrite breakup due to remelting, while the dominant factor of the grain refinement at high undercooling range was the recrystallization process. The driving force (i.e. stress induced strain energy) for recrystallization could be divided into two parts, one was the thermal stress generated by the releasing of solidification latent heat during the recalescence process, and the other was the accumulated stress and plastic strain caused by the interaction of liquid flow and primary dendrites caused by rapid solidification. • The maximum undercoolings were achieved respectively by using the deep undercooling technology. • The recalescence behavior and microstructure refinement were systematically studied. • The relationship between solute composition and solidification process was established. • High-angle grain boundaries and twin boundaries exist in the microstructure at high undercooling. [ABSTRACT FROM AUTHOR]

Super austenitic stainless steel (SASS) is widely used in extreme environments due to its excellent corrosion resistance and mechanical properties, and a feature of this grade is its relatively high nitrogen content. As valuable tools to study the solidification process and phase transformation, high temperature confocal microscopy (HTCM) and thermal analysis methods, such as differential thermal analysis (DTA) and differential scanning calorimetry (DSC), have been widely reported in the investigation of SASS. However, the inherent limitations of these two techniques make it difficult to explain the complex solidification behaviors of SASS, which involves multiple phases. Here, a unique design of HTCM-DTA was employed to investigate the nucleation and subsequent phase transformation of SASS. This in-house build apparatus enables contemporaneous in-situ observation of the surface and thermal analysis of the bulk thermal events. Using the novel HTCM-DTA device, we systematically revealed the four solidification modes of SASS and analyzed them in detail through thermodynamic arguments and multiple sets of experimental data under different conditions: (a) bimodal solidification with solid-state phase transformation, (b) bimodal solidification with liquid-state phase transformation, (c) pseudo-bimodal solidification, and (d) unimodal solidification. By comparing with the findings of earlier studies, the limitation of spatiotemporal separation employing HTCM and thermal analysis methods was pointed out. The results suggest that both the shielding gas and the time of retaining in the liquid state affect the N content in the melt, leading to a change of the primary nucleation phase. The cooling rate not only affects the undercooling and microstructure but also the transformation stage of δ-ferrite to γ-austenite, and when the phase transformation occurs in the liquid state it is usually accompanied by recalescence. These insights contribute understanding required for adjusting the continuous casting process of high nitrogen alloys. • In-situ observations on surface and thermal analysis of the bulk in parallel. • Solidification mode and phase transformation highly depend on shielding gas. • Four different solidification modes of S31254 were found and analyzed in detail. • The recalescence phenomenon was observed and analyzed frame by frame. • C.R. and t L influence the solidification behavior. [ABSTRACT FROM AUTHOR]

• Development of a well-controlled experimental system for ice-water phase change to elucidate five stages of solidification. • Formulation and validation of a two-dimensional semi-analytical solution to capture solidification at micro- and macro-scales. • Parametric study for key operational parameters in cold thermal energy storage (CTES). Cold thermal energy storage (CTES) is a process that supplies cold thermal energy to a medium for storage and extracts it whenever is needed. The storage medium is phase change material (PCM), which makes great use of the large quantity of latent heat released during solidification or melting. However, some key fundamental and applied issues have not yet been resolved: how do we overcome the thermal interference in PCMs from unstable microscopic crystallization and external mechanical forces? Are there any unified and robust models to predict the whole time evolution of phase change accurately and rapidly? To fulfill these research gaps, we firstly establish a novel, well-controlled experimental system for PCMs that mitigates the thermal disturbance over a medium to large volume during solidification, capable of measuring transient temperature data and characterizing freezing stages at both macro- and micro-scale. We also develop in detail a unified semi-analytical mathematical framework to model the solidification of PCMs, consisting of five subsequent stages: liquid supercooling, nucleation, recalescence, equilibrium freezing, and solid subcooling. The modeling results yield a good agreement with our experimental data in several scenarios, particularly the nucleation temperature and time as well as the total freezing time are accurately predicted. Lastly, we extend our study to investigate the thermal effects of various radial positions, geometries, initial temperatures, heat transfer fluid temperature, and heat transfer coefficients in the context of CTES system. [ABSTRACT FROM AUTHOR]