Critical Nucleation Temperature Research Articles

Influence of V on the Microstructure and Precipitation Behavior of High-Carbon Hardline Steel during Continuous Cooling.

High-carbon hardline steels are primarily used for the manufacture of tire beads for both automobiles and aircraft, and vanadium (V) microalloying is an important means of adjusting the microstructure of high-carbon hardline steels. Using scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM), the microstructure and precipitation phases of continuous cooled high-carbon steels were characterized, and the vanadium content, carbon diffusion coefficient, and critical precipitation temperature were calculated. The results showed that as the V content increased to 0.06 wt.%, the interlamellar spacing (ILS) of the pearlite in the experimental steel decreased to 0.110 μm, and the carbon diffusion coefficient in the experimental steel decreased to 0.98 × 10-3 cm2·s-1. The pearlite content in the experimental steel with 0.02 wt.% V reached its maximum at a cooling rate of 5 °C·s-1, and a small amount of bainite was observed in the experimental steel at a cooling rate of 10 °C·s-1. The precipitated phase was VC with a diameter of ~24.73 nm, and the misfit between ferrite and VC was 5.02%, forming a semi-coherent interface between the two. Atoms gradually adjust their positions to allow the growth of VC along the ferrite direction. As the V content increased to 0.06 wt.%, the precipitation-temperature-time curve (PTT) shifted to the left, and the critical nucleation temperature for homogeneous nucleation, grain boundary nucleation, and dislocation line nucleation increased from 570.6, 676.9, and 692.4 °C to 634.6, 748.5, and 755.5 °C, respectively.

Revisiting the precipitation mechanisms of Guinier-Preston zones, η′, and η precipitates in Al-Zn-Mg alloys

Al-Zn-Mg alloys stand among the strongest Al alloys due to the interaction of Guinier-Preston (GP) zones and η′ precipitates with dislocations. Understanding the process of precipitate nucleation is critical to design age-hardenable Al-Zn-Mg alloys with improved mechanical properties. In this investigation, the structure and formation path of GP zones, η′ and η precipitates was studied by means of first principles calculations and statistical mechanics. The structures of potential GP configurations on the FCC convex hull were revealed as well as the presence in the convex hull of FCC MgZn2 clusters. They were found to be precursors of η′ precipitates because of the formation of stacking faults. The η′-to-η transformation was governed by the advance of the η′ interface towards the FCC MgZn2 cluster, so the equilibrium η precipitate is the result of the growth and coarsening of η′. Finally, the critical nucleation temperature for η′ precipitates was determined from the thermodynamic driving force of the transformation. These results rationalize the experimental data available in the literature and offer insights to enhance the strength of Al-Zn-Mg alloys.

Understanding the Effect of Nucleation in Amorphous Solid Dispersions through Time-Temperature Transformation.

In an earlier investigation, the critical cooling rate to prevent drug crystallization (CRcrit) during the preparation of nifedipine (NIF) amorphous solid dispersions (ASDs) was determined through a time-temperature transformation (TTT) diagram (Lalge et al. Mol. Pharmaceutics 2023, 20 (3), 1806-1817). The current study aims to use the TTT diagram to determine the critical cooling rate to prevent drug nucleation (CRcritN) during the preparation of ASDs. ASDs were prepared with each polyvinylpyrrolidone (PVP) and hydroxypropyl methylcellulose acetate succinate (HPMCAS). The dispersions were first stored under conditions promoting nucleation and then heated to the temperature that favors crystallization. The crystallization onset time (tC) was determined by differential scanning calorimetry and synchrotron X-ray diffractometry. TTT diagrams for nucleation were generated, which provided the critical nucleation temperature (50 °C) and the critical cooling rate to avoid nucleation (CRcritN). The strength of the drug-polymer interactions as well as the polymer concentration affected the CRcritN, with PVP having a stronger interaction than HPMCAS. The CRcrit of amorphous NIF was ∼17.5 °C/min. The addition of a 20% w/w polymer resulted in CRcrit of ∼0.05 and 0.2 °C/min and CRcritN of ∼4.1 and 8.1 °C/min for the dispersions prepared with PVP and HPMCAS, respectively.

Research on homogeneous nucleation and microstructure evolution of aluminium alloy melt.

In this paper, based on the embedded atom method (EAM) potential, molecular dynamics simulations of the solidification process of Al–4 at.%Cu alloy is carried out. The Al–Cu alloy melt is placed at different temperatures for isothermal solidification, and each stage of the entire solidification process is tracked, including homogeneous nucleation, nucleus growth, grain coarsening and microstructure evolution. In the nucleation stage, the transition from high temperature to low temperature manifests a change from spontaneous nucleation mode to divergent nucleation mode. The critical nucleation temperature of the Al–Cu alloy is determined to be about 0.42 Tm (Tm is the melting point of Al–4 at.%Cu) by calculating the nucleation rate and the crystal nucleus density. In the nucleus growth stage, two ways of growing up are observed, that is, a large crystal nucleus will absorb a smaller heterogeneous crystal nucleus, and two very close crystal nuclei will merge. In the microstructure evolution of the isothermally solidified Al–Cu alloy, it is emerged that the interior of all nanocrystalline grains are long-period stacking structure composed of face centred cubic (FCC) and hexagonal close-packed (HCP). These details provide important information for the production of Al–Cu binary alloy nano-polycrystalline products.

Kinetics and in situ observation of nonisothermal crystallization in Bayan Obo tailing-based nanocrystalline glass-ceramic

In recent years, the preparation of CMAS nanocrystalline glass-ceramics has shown potential as an application of secondary resourcing technology in utilizing Bayan Obo iron ore tailings containing rare earth elements. The crystallization mechanism for nanodiopside-type glass-ceramics was studied via an investigation of the nonisothermal crystallization kinetics of the glass system, combined with the in situ observation of softening and crystallization of the basic glass using a high-temperature laser confocal microscope. The results show that the activation energy of nucleation in the glass system is higher than that of crystal growth by using the Ozawa model. The crystallization mechanism changes as the crystallization fraction increases, that is, from the three-dimensional growth in which the nucleation rate increases with time in an interface-controlled manner (a > 1, b = 1, m = 3) at the initial stage of crystallization to a decreased nucleation rate in a diffusion-controlled growth (a = 0.5, b = 0.5, m = 3) at the middle and later stages. This process involves both surface crystallization and volume crystallization. The crystallization was observed in situ, and it was further confirmed that there exists a critical nucleation temperature between Tg and Tx, which is related to the interface free energy and critical Gibbs free energy difference. When the temperature exceeds the critical value of Tg + 55 K, the system begins to exhibit visible crystallization. With an increase in temperature, the basic glass softened considerably, while the crystal grew significantly. In addition, the surface roughness can be used to characterize the crystallization process, providing a new research method for crystal growth.

Single Crystal Castability and Undercoolability of PWA1483 Superalloy

Both of the single crystal (SX) castability and undercoolability of PWA1383 superalloy were investigated during the directional solidification and isothermal cooling. In all the six SX parts of a casting cluster, no stray grains were found, revealing a defect-free SX structure. This excellent SX castability of the superalloy was attributed to its good undercoolability. The melting point (TL) and the critical nucleation temperature (TN) of the alloy were measured to be 1327 °C and 1306 °C, respectively. The statistic average of the critical nucleation undercooling ΔTN = TL − TN of the alloy was determined to be about 21 K, exhibiting a relatively great capacity to be deeply cooled to a temperature below the melting point without the onset of solidification.

Columnar to equiaxed transition mechanism of pure copper induced by La

ABSTRACTThe solidification process of copper with and without La was observed by a high-temperature confocal laser scanning microscopy. The solidification microstructure was simulated by cellular-automaton and finite-element (CAFE) method and the source of equiaxed grains, the constitutional supercooling and nucleation rate were investigated. The results showed that some floats with regular shape appeared in copper melt with La addition, which is the source of equiaxed grains. The simulation results are in a good agreement with experimental results. The mechanism of columnar to equiaxed transition was that the spontaneous nucleation probability of copper melt was increased with La addition. The nucleation rate was increased due to the increase in critical nucleation temperature and constitutional supercooling, and the reduction of nucleation undercooling.

Effect of Two-Step Metal Organic Chemical Vapor Deposition Growth on Quality, Diameter and Density of InAs Nanowires on Si (111) Substrate

High-density (∼ 80/um2) vertical InAs nanowires (NWs) with small diameters (∼ 28 nm) were grown on bare Si (111) substrates by means of two-step metal organic chemical vapor deposition. There are two critical factors in the growth process: (1) a critical nucleation temperature for a specific In molar fraction (approximately 1.69 × 10−5 atm) is the key factor to reduce the size of the nuclei and hence the diameter of the InAs NWs, and (2) a critical V/III ratio during the 2nd step growth will greatly increase the density of the InAs NWs (from 45 μm−2 to 80 μm−2) and at the same time keep the diameter small. The high-resolution transmission electron microscopy and selected area diffraction patterns of InAs NWs grown on Si exhibit a Wurtzite structure and no stacking faults. The observed longitudinal optic peaks in the Raman spectra were explained in terms of the small surface charge region width due to the small NW diameter and the increase of the free electron concentration, which was consistent with the TCAD program simulation of small diameter (< 40 nm) InAs NWs.

Part II: Phase transformation-induced bloating behavior in diopside glass-ceramics for microwave dielectrics

Phase transformation and bloating behavior of CaMgSi2O6 glass-ceramics with ZrO2 nucleating agent using two-stage heat treatments were investigated as functions of annealing temperature. The specimens annealed below critical nucleation temperature (Tcn) and sintered at 950°C exhibit significant bloating phenomena. Kinetic study of CaO-MgO-SiO2-ZrO2 (CMSZ) glasses annealed below Tcn reveals high activation energy of crystallization for the ZrO2 nucleation, and XRD refinement results show the least ZrO2 contents. The formations of fleeing oxygen and bloating are likely associated with high energy barrier of ZrO2 phase and zirconia dissolution in the CMSZ glass. Specimens annealed above the Tcn and sintered at 950°C show significant reduction of activation energy of ZrO2 and enhanced densities. This work demonstrates that the bloating behavior of diopside glass-ceramics is attributed to the activation energy of nucleating agents.

Part Ι: Effects of two-stage heat treatment on densification, microstructural features and dielectric properties of CaO–MgO–SiO2 glass-ceramics with ZrO2 nucleating agents

Densification, microstructures and dielectric properties of CaO–MgO–SiO2 glass-ceramics were investigated with ZrO2 nucleating agent using two-stage heat treatment in the low temperature co-fired ceramic (LTCC) method. The amorphous cullet (abbreviated as CMSZ glass) was synthesized by mixing CaO, MgO, and SiO2 powders with ZrO2 nucleating agent, which were melted at 1500°C before the quenching process. The results show that CMSZ glasses annealed below the critical nucleation temperature (Tcn) and sintered at 950°C exhibit obvious bloating phenomena. On the other hand, CMSZ glasses annealed above Tcn demonstrate that zirconia nucleation occurs at the first-stage heat treatment (annealing). The second-stage heat treatment (sintering) is characterized by the rapid crystal growth of CaMgSi2O6 phase around the ZrO2 crystallites, indicating growth of nuclei over the critical size above Tcn. Moreover, specimens fabricated using two-stage heat treatment and annealed above Tcn exhibit appropriate microwave dielectric properties and high mechanical strength.

Solidification behavior of Re- and Ru-containing Ni-based single-crystal superalloys with thermal and metallographic analysis

To clarify the solidification behavior of Re- and Ru-containing Ni-based single-crystal superalloys, four experimental alloys with varied contents of Re and Ru were investigated by differential scanning calorimetry (DSC) and metallographic techniques. To obtain the γ′-solvus temperatures, the stepwise solution and aging heat treatments were used. DSC analysis shows that Re leads to the increase in freezing range and γ′-solvus temperature. On the contrast, Ru only has negligible influence on the freezing range, but leads to the lower γ′-solvus temperature. In comparison with Ru, Re leads to more severe segregation and higher eutectic fractions in as-cast microstructures. Furthermore, the castability and phase stability of Ni-based superalloys were analyzed by the results of DSC and metallographic analysis, such as freezing range, critical nucleation temperature, γ′-solvus temperature and eutectic fractions. It shows that Re leads to the wider freezing range and lower critical nucleation temperature, indicating the worse castability of Re-containing Ni-based single-crystal superalloys. The characteristic phase transformation temperatures of Re-containing single-crystal superalloy are determined by DSC analysis. For as-cast samples, the liquidus, solidus, critical nucleation and eutectic reaction temperature are clearly shown in DSC curves. After a stepwise solution and aging heat treatments, the higher amount of γ′ phase with cubic morphology can be obtained. Finally, the precipitation temperature of γ′ phase is determined by DSC analysis for the heat-treated samples.

Effects of system pressure and heat flux on bubble nucleation and growth

Characteristics of bubble nucleation and growth are critical for its application. It is affected by several factors including viscosity, surface tension and temperature. However, the effect of pressure on bubble nucleation and growth has been underreported, although it processes significant effect on above characteristics. In this work, a micro copper electrode is etched on a slab covered with copper to produce bubble on the surface by current input. The nucleation time of bubble is measured under different heat flux and system pressures. The nucleation and growth processes are recorded with a high speed camera in order to discuss the effects of heat flux and system pressure on bubble characteristics. The experiment results indicate that the micro electrode with higher heat flux produces more thermal energy, which makes the time of bubble nucleation shorter and the speed of bubble growth faster. Higher system pressure causes the increase of the critical nucleation temperature and also baffles the bubble nucleation and growth. Bubble growth includes the stages of rapid growth and dynamic equilibrium, with the speed being from fast to slow. In the former part of rapid growth, heat flux plays a dominant role in bubble growth. While the effect of system pressure on bubble growth becomes significant in the latter part of rapid growth. Both the nucleation time and bubble growth agree well with the theoretical analysis. The obtained results help to accurately control bubble nucleation and growth required in different application.

Phase selection of undercooled solidification of Ni–4.5 wt% B alloy

Abstract

Four-zone solidification microstructure formed by laser melting of copper thin films

Polycrystalline copper film microstructures produced by laser melting and rapid lateral solidification are analyzed using transmission electron microscopy. The microstructure is predominantly composed of directionally solidified grains up to 22 μm long and 1 μm wide lying in the plane of the film. We identify four morphologically solidification zones, corresponding to occlusion, steady lateral growth, defective growth, and nucleation. Electron diffraction analysis indicates clustering of 〈1 0 0〉 orientations around the direction of solidification for the grains in the steady lateral growth zone. Simple estimates of solidification times based on heat flow modeling and a critical nucleation temperature suggest that interface velocities of several hundred m/s may be attained during solidification.

Si (111) substrates as highly effective pseudomasks for selective growth of GaN material and devices by ammonia-molecular-beam epitaxy

The unique property of Si (111) as effective pseudomask substrate for selective growth of GaN by ammonia-molecular-beam epitaxy is reported. The critical nucleation temperature of GaN on Si (111) surface is found to be as low as 700°C, much lower than that on sapphire or AlN surface. As a result, selective growth of GaN is possible by ammonia-molecular-beam epitaxy on Si (111) substrates using a patterned AlN buffer layer. The wide range of growth temperatures (700–900°C) available for selective growth is a critical advantage for control and optimization of the facet characteristics of the selectively grown GaN patterns as required for potential fabrication of site-specific GaN or InGaN quantum dots. The demonstrated ease of selective growth of GaN on silicon has also implications in potential on-chip integration of GaN devices with silicon devices.

Melt-Processed Ni3Al matrix composites reinforced with TiC particles

TiC-reinforced Ni3Al metal matrix composites have been investigated. The composites were prepared by in situ precipitation of either 0.01, 0.05, or 0.1 vol fraction TiC in molten Ni3Al. These precipitates are thermodynamically stable in the Ni3Al at 1000 °C up to 50 hours. Although almost ideal precipitate distribution was achieved at low volume fraction of TiC, at higher volume fraction, there was a tendency toward agglomeration. Room and hightemperature mechanical testing showed significant improvement in modulus, yield strength, and ultimate tensile strength, while still maintaining a 2.5 pct elongation at 0.1 vol. fraction TiC and 500 °C. The density of precipitates (number/m3) is strongly dependent on the fraction of TiC, on temperature, and on the holding time at that temperature. The critical nucleation temperature for these precipitates was determined to be at 1475 °C. Experimentally evaluated growth rate constants indicate a diffusion-controlled coarsening mechanism along with coalescence and multiparticle interaction at higher volume fractions of TiC. This is in accordance with either the Lifshitz Slyozov Encounter Modified (LSEM) model or with the Voorhees and Glicksman (VG) model.

The critical nucleation temperature of stacking fault tetrahedra in gold

The nucleation of stacking fault tetrahedra in quenched high purity gold was traced via measurements of electrical resistivity. A critical nucleation temperature exists above which the nucleation process bccomes increasingly difficult. This temperature is function of the quenched in concentration of vacancies, and reflects the necessity of a small cluster for the nucleation process.

Nucleation and growth of sputtered and evaporated trigonal selenium films

Critical nucleation temperature experiments, scanning electron microscopy and light microscopy were used to study the nucleation and growth of trigonal Se films deposited by evaporation and sputtering on both gold and anodized aluminum substrates. The major nucleation and growth processes were shown to be the condensation of Se 2 molecules followed by their polymerization into Se n chains, where n can be very large. In sputtering, these chains have a higher surface mobility, due in part to their higher residual kinetic energy, and thus they can form large trigonal Se crystallites. The high incident kinetic energies also allow the Se to break through the surface contamination of the substrate. This is important since the Se chain mobility should also be dependent on the surface free energy and on the nucleation site density of the substrate. Very adherent large-grained trigonal Se films were grown by sputtering on Au substrates. It was shown that nucleation sites for the large trigonal Se crystallites were not created by the incident flux but are inherent on the substrate. The high energy incident flux did, however, increase nucleation probability for the initial Se 2 condensation. For sputtering on anodized aluminum substrates the combination of low surface energy and high nucleation site density decreased chain mobility and thus prevented formation of large crystallites.

Ice propagation in systems of biological interest. II. Effect of solutes at rapid cooling rates

In concentrated solutions of gelatin, sucrose, and glycerol, crystal formation occurred at the freezing point at slow cooling rates, but below the freezing point at rapid cooling rates. Increasing either the concentration of solute or the rate of cooling increased the difference between freezing point and temperature of freezing. Addition of hexagonal crystals of silver iodide and calcium carbonate raised the “critical nucleation temperature” and thereby the freezing temperature. It is suggested that large ice crystals are formed when freezing occurs above the critical nucleation temperature. However, when freezing occurs at the critical nucleation temperature small ice crystals are formed, and under suitable conditions some water may not freeze. It is further concluded that the effect of solutes on nucleation and rate of crystal growth must be a major factor controlling the pattern of ice propagation.