The effects of Mg additions on the Fe-containing intermetallic compounds and the heterogeneous nucleation of α-Al were investigated in Al-1.4Fe-0.7Mn-xMg alloys using scanning electron microscopy ...(SEM) and transmission electron microscopy (TEM). The experimental results show that the morphology of Al6(Fe,Mn) in eutectic structure was significantly modified from needle-like to plate-like and then Chinese Script with increasing Mg concentration. The effects of Mg addition on the composition and crystal structure of Al6(Fe,Mn) were also investigated. No Mg was detected in the Al6(Fe,Mn) compounds. However, clear interfacial segregation of Mg on the surface of the Al6(Fe,Mn) particles was observed. The grains of α-Al were refined with increased Mg addition. It is also found that the Mg addition in the Al alloys resulted in change to the major naturally formed oxides from Al2O3 to MgAl2O4, and eventually changed the heterogeneous nucleation of α-Al. The direct evidence of heterogeneous nucleation of α-Al on the naturally formed MgAl2O4 particles was observed.
•The morphology of BE-Al6(Fe,Mn) changed from needle-like to plate-like, and Chinese script with increasing Mg concentration.•Mg does not partition into the Al6(Fe,Mn) particles.•Mg segregated on the BE-Al6(Fe,Mn) surface.•The primary α-Al grains were refined by Mg addition in Al-1.4Fe-0.7Mn alloys.•An orientation relationship was defined: 1¯11¯ α-Al // 2¯22¯ MgAl2O4 and 1 1 0α-Al // 1 1 0 MgAl2O4.
Nanodroplets on a solid surface (i.e., surface nanodroplets) have practical implications for high-throughput chemical and biological analysis, lubrications, laboratory-on-chip devices, and near-field ...imaging techniques. Oil nanodroplets can be produced on a solid–liquid interface in a simple step of solvent exchange in which a good solvent of oil is displaced by a poor solvent. In this work, we experimentally and theoretically investigate the formation of nanodroplets by the solvent exchange process under well-controlled flow conditions. We find significant effects from the flow rate and the flow geometry on the droplet size. We develop a theoretical framework to account for these effects. The main idea is that the droplet nuclei are exposed to an oil oversaturation pulse during the exchange process. The analysis shows that the volume of the nanodroplets increases with the Peclet numberPeof the flow as ∝Pe
3/4, which is in good agreement with our experimental results. In addition, at fixed flow rate and thus fixed Peclet number, larger and less homogeneously distributed droplets formed at less-narrow channels, due to convection effects originating from the density difference between the two solutions of the solvent exchange. The understanding from this work provides valuable guidelines for producing surface nanodroplets with desired sizes by controlling the flow conditions.
Methane is condensed in a low-temperature heat exchanger, a critical step in the liquefaction of natural gas. Currently, the microscopic mechanism regarding the heterogeneous condensation of methane ...on the heat exchanger wall remains unclear. This lack of understanding is of scientific importance in advancing the development of natural gas liquefaction processes. Therefore, molecular dynamics simulation is used in this paper to study the process of methane heterogeneous nucleation and core growth during the early stage of condensation, and analyze the influence mechanism of wall energy and cold source temperature on nucleation kinetics. The results show that the high-energy wall can enhance the interaction between cold wall atoms and methane molecules, facilitate heat transfer, accrete methane molecules to condense and adsorb on the wall to form a core, thus increasing the condensation rate of methane. Whereas, the low-temperature cold source promotes the condensation nucleation and growth process by increasing the supersaturation of methane. This study investigates the process and kinetic characteristics of heterogeneous nucleation of methane from a microscale perspective, providing guidance for the development of natural gas liquefaction in low-temperature heat exchangers, with the aim of enhancing the diversity and reliability of energy supply.
•Heterogeneous condensation of methane gas on the wall was investigated.•The judgment of “liquid-like” clusters on different energy wall was studied.•Heterogeneous nucleation rate of methane gas was calculated.
The mechanism underlying the considerable refinement of primary Al3Ti intermetallic particles induced by ultrasonic treatment (UST) in an Al-0.4 wt% Ti alloy in the fully liquid state was ...investigated. Scanning electron microscopy, energy dispersive X-ray spectroscopy, focused ion beam 3D tomography and transmission electron microscopy were used to clearly identify that α-Al2O3 particles were located at or near the centres of primary Al3Ti particles in the samples solidified with and without UST. Crystallographic evaluation using the edge-to-edge matching model and experimental determination of orientation relationships between the α-Al2O3 and primary Al3Ti particles using the convergent beam Kikuchi line diffraction patterns confirmed the high potency of α-Al2O3 particles as nucleation sites for the Al3Ti phase. Based on the experimental results, the refining mechanism is discussed in terms of proposed hypotheses in the literature. It is suggested that the significant refinement of primary Al3Ti particles upon UST is due to the cavitation-induced deagglomeration and distribution of the α-Al2O3 particles and the cavitation-enhanced wetting of the α-Al2O3 particles by liquid aluminium.
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In this study, two Ferrium S53 steels with and without 1.5% Cu were prepared. The microscopic morphology and structure of these steels during aging treatment were analyzed by transmission electron ...microscopy (TEM). The average equivalent spherical radius (Rv), volume fraction (ϕv), and number density (Nv) of Cu-rich precipitates and M2C carbides during aging treatment were analyzed by atom probe tomography (APT), and the spatial distribution, interfacial elemental segregation, and one-dimensional concentration profiles of the two types precipitates were also characterized. Based on the distribution of elements, 5%Cu and 9% (C + Mo) isoconcentration surfaces were selected to obtain Cu-rich precipitates and M2C carbides, respectively. The results demonstrate that the Cu-rich precipitates formed before the M2C carbides. The segregation of Cr and Mo elements at the phase interface due to the nucleation and growth of Cu-rich precipitates can promote the formation of the M2C carbides. The two types of precipitates display adjacent distribution relationship in steel. The number density of M2C carbides in steel contain 1.5%Cu can be greatly increased when the Cu-rich precipitates serving as nucleation particles. The formation of needle-like M2C carbides inhibited the coarsening of Cu-rich precipitates. More dispersive and fine M2C carbides were obtained in the steel.
•The addition of Cu gives some more dispersive M2C carbides than in Ferrium S53 steel without Cu.•Cu-rich precipitates are formed first and their coarsening is inhibited.•Cu-rich precipitates can act as nucleation sites for the M2C carbides to refine the M2C carbides.
Fouling processes present significant challenges in many industries; understanding these processes is crucial for accurate prediction and planning effective mitigation protocols. Composite fouling is ...prevalent in many industrial applications, however, studies of these systems are scarce. We present, for the first time, new insights on the interaction between two common crystallization foulants: BaSO4 and CaCO3. We conducted a series of crystal growth experiments in a plug flow reactor. We used high resolution X-ray micro-computed tomography to visualize the crystal formation. In the first set of experiments, crystals (either BaSO4 or CaCO3) were grown on pristine steel surfaces; these pre-fouled surfaces were then used as substrates for the composite fouling studies. The chemical compositions of the substrates were different from that of the foulant, i.e., either BaSO4 on CaCO3 or CaCO3 on BaSO4. Growth rates were obtained through gravimetric analysis. We characterized surface properties, such as the crystal size distribution, surface coverage, and texture parameters through advanced image analysis. Our results reveal that deposition behaviour depends on complex substrate-foulant interactions. Whereas the net deposition of CaCO3 on a BaSO4 substrate increased with time, significant detachment processes were observed for the reverse case. The increased detachment of BaSO4 on a CaCO3 substrate can be attributed to either the weak interaction within the CaCO3 crystal structure or the CaCO3/steel interface. We also show how substrate properties govern crystal cluster sizes and distribution. The results from this work provide a basis for the development of more comprehensive prediction models.
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•Composite fouling systems were investigated•BaSO4 substrates nearly doubled the growth rate of CaCO3.•CaCO3 substrates promote BaSO4 detachment.•BaSO4 preferentially grew on top of CaCO3 resulting in smaller clusters.
•Attenuation of acoustic pressure is measured in liquid Al and water.•Acoustic streaming direction depends on the cavitation development.•Nucleation of primary phase on oxides is demonstrated in-situ ...and ex-situ.•Fragmentation of primary crystals occurs by fatigue-like mechanism.•De-agglomeration is a gradual process occurring from the surface of agglomerate.
Ultrasonic (cavitation) melt processing attracts considerable interest from both academic and industrial communities as a promising route to provide clean, environment friendly and energy efficient solutions for some of the core issues of the metal casting industry, such as improving melt quality and providing structure refinement. In the last 5 years, the authors undertook an extensive research programme into fundamental mechanisms of cavitation melt processing using state-of-the-art and unique facilities and methodologies. This overview summarises the recent results on the evaluation of acoustic pressure and melt flows in the treated melt, direct observations and quantitative analysis of cavitation in liquid aluminium alloys, in-situ and ex-situ studies of the nucleation, growth and fragmentation of intermetallics, and de-agglomeration of particles. These results provide valuable new insights and knowledge that are essential for upscaling ultrasonic melt processing to industrial level.
As a long-standing problem, the physics of freezing and melting temperature depressions of water in millimeter-sized pores remains elusive. We devised a series of experiments on the freezing and ...melting of water in glass tubes with varying tube diameters, water volumes, water-solid interfacial areas, and cooling rates. The results of the experiments indicate that the freezing temperature depression is highly correlated with the water-solid interfacial areas. It can be inferred that heterogeneous nucleation is the dominant physical origin for such freezing temperature depression. In contrast, melting temperature depression is orders of magnitude lower than that of freezing, and is independent of heating rate, water volume, water-solid interfacial area, and correlated with pore diameter, suggesting the governing role of capillarity. The recognized distinct physical origins of freezing and melting temperatures lead to the disparity between them, i.e., freezing and melting hysteresis. The findings resolve the controversy on the freezing mechanism of water in millimeter-sized pores, and can be used to better manipulate freezing and melting behaviors of pore water in various engineering applications, e.g., promoting or inhibiting freezing in biomaterials, and developing anti-freezing materials.
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Interfacial microbubbles widely exist and play important roles in nature and industries, while their growth process remained unknown in many aspects. In this study, the interfacial microbubbles are ...created in an air-supersaturated solution utilizing a sudden drop of ambient pressure, and the effects of different influential parameters on the growth process of interfacial microbubbles were studied by a self-made observation system. The investigations confirmed that the growth process of interfacial microbubbles under all conditions had experienced three stages, including the floating stage, transition stage and expansion stage. The experimental results also show that the magnitude of depressurization amplitude, surface wettability, surface roughness, dissolved air content, and solution surface tension have significant effects on the nucleation and growth of interfacial microbubbles. In addition, the modified diffusion theoretical model of dissolved air was successfully used to predict the changes in contact angle and bubble radius for interfacial bubbles during the growth process.
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•The growth characteristics of interfacial microbubbles were studied by a self-made observation system.•Interfacial microbubbles on the glass slide surfaces were generated by a sudden drop in ambient pressure.•Effects of solid surface properties and aqueous solutions on the interfacial microbubble growth were clarified.•An improved diffusion theoretical model was used for predicting the experimental data.
Large M7C3 carbides were refined by inoculation with small levels (0, 0.1, 0.5, and 1 wt%) of aluminum in chromium carbide overlay. Multiple cracks were observed in the large primary M7C3 carbides in ...0% aluminum sample, while less cracking was observed for refined M7C3 carbides, under similar scratch tests. The refinement mechanism is believed to involve the heterogeneous nucleation of carbides on Al2O3 that has formed prior to M7C3 nucleation. Aluminum is shown to shorten the growth time of primary M7C3 carbides by raising the eutectic temperature. The overlay is slightly softer in the 1% aluminum sample due to a decrease the amount of martensite and an increase of retained austenite in the overlay matrix. A new approach is introduced to refine the primary M7C3 carbides by using a small amount of Al addition without producing new hard phases.
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•Al decreased the size of primary M7C3 and refined the microstructure of the CCO.•Large primary M7C3 were more easily to be broken during wear tests.•The formation of Al2O3 acted as heterogeneous nuclei to refine microstructure.•The eutectic temperature was increased with Al addition.•Al suppressed the fraction of martensite in the matrix and softened the hardness.
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