Papers by Keyword: Nucleation

Abstract: Owing to their several attractive features such as high hardness, high elastic modulus, light weight, high strength to weight ratio, high thermal conductivity, and high temperature strength, composites from Al-Mg₂Si family offers promise towards deployment in several industries such as automobile, aerospace, marine, defence and electronic. The present molecular dynamics (employing LAMMPS) based simulation study is one of the first attempt to investigate the nucleation and grain growth mechanisms of Mg₂Si phase at atomic level in case of novel Al-15Mg₂Si-4.5Si composite, during semi-solid processing. Modified embedded atom method (MEAM) potential has been used to study the atomic interactions in the composite. Reaching the melt state at 1000 K, the temperature of the system is first decreased from 1000 K to 853 K and then the system is held at 853 K for 100 ps. The simulations are performed with three different cooling rates. With lowering of temperature, randomly distributed Mg and Si atoms form atomic clusters at arbitrary locations within the system, which is the nucleation stage for Mg₂Si phase formation. Cluster size, radial distribution function has been used to investigate the structural evolution of Mg-Si clusters. Cooling rate significantly influences the grain size as well as the grain growth kinetics. The information about the thermodynamic state of the system has been revealed by extracting the values of internal energy, enthalpy, specific heat. during the slurry preparation and isothermal holding stages. The growth mechanism of Mg₂Si nucleus has been characterized from the temporal variation of (Mg + Si) atoms taking part in the cluster formation. Power-law variation is observed in the cooling stage whereas a linear variation is observed in the isothermal stage.

Abstract: 3C-SiC islands were grown on atomically flat (111) 4H-SiC terraces and characterized by micro-Raman and FTIR. The islands initially have a triangular shape as defined by three {100} planes and over time evolve into hexagonal shaped islands. The triangular shape reveals the domain orientation of the island and is easily observed with an optical microscope. Examining 347 3C-SiC islands on 17 4H-SiC terraces we found that islands grown on the same terrace have the same domain orientation with 99.6% probability. The orientation of 3C-SiC islands grown on adjacent terraces was found to be close to random. This work confirms an orientation selection rule with high probability, suggesting that 3C-SiC can be grown without anti-phase domains or DPBs when grown on a single atomically flat 4H-SiC terrace, even when there are multiple nucleation sites.

Abstract: Due to high growth temperatures during the physical vapor transport (PVT) it is still almost impossible to gain proper insight into the actual growth conditions. Therefore, computer tomography (CT) is used as an in-situ monitoring during the crystal growth process. With the help of this technique, it is possible to observe the nucleation centers during the initial stage of growth (CT after 0h) of a 4H-SiC single crystal. These growth islands are likely built before the actual growth conditions are reached. Raman investigations of the area around a growth island located directly on the interface between seed and grown crystal is used to support this assumption. In addition, optical analysis after KOH etching were made to reveal the defects around the growth island. The island exhibits a rough doping concentration in comparison to the surrounding grown crystal.

Abstract: Different Ta concentrations together with stochiometric grain refiner (Al-2.2Ti-1B) in Al-Si-Mg based alloys were investigated with the aim to elucidate grain refinement mechanisms. Post-solidification microstructure was characterised using optical microscopy and scanning electron microscopy (SEM), with a special focus on the Ta-rich layer (more likely to be Al₃Ta) on the basal planes (0001) of TiB₂. A significant grain refinement was observed by using the solute Ta together with stochiometric grain refiner (Al-2.2Ti-1B). In order to further elucidate the formation of Ta-rich layer on the basal planes (0001) of TiB₂, the Density Functional Theory (DFT) calculation were also performed to determine the interface energies of different interfaces and sandwich configurations, including Al (111), Al₃Ti (112) and Al₃Ta (112) at the interface of TiB₂ basal plane (0001). It was found that the interface energy for Ti-terminated TiB₂ at the interface throughout all configurations involved in this paper is lower than that for B-terminated TiB₂, indicating that Ti-terminated TiB₂ is more favourable. It was also found that the Al₃Ta configuration yields the same interface energies as the Al₃Ti configuration. Furthermore, the interface energy of the sandwich configuration also shows nearly identical values along the TiB₂ // Al₃Ti and TiB₂ // Al₃Ta interface energy, strongly indicating that the solute Ti can be fully replaced by the solute Ta.

Abstract: Silicide formation by reactive diffusion is of interest in numerous applications especially for contact formation and interconnections in microelectronics. Several reviews have been published on this topic and the aim of this chapter is to provide an update of these reviews by focusing on new experiment results. This chapter presents thus some progress in the understanding of the main mechanisms (diffusion/reaction, nucleation, lateral growth…) for thin and very thin films (i.e. comprised between 4 and 50 nm). Recent experimental results on the mechanisms of formation of silicide are presented and compared to models and/or simulation in order to extract physical parameters that are relevant to reactive diffusion. These mechanisms include nucleation, lateral growth, diffusion/interface controlled growth, and the role of a diffusion barrier. The combination of several techniques including in situ techniques (XRD, XRR, XPS, DSC) and high resolution techniques (APT and TEM) is shown to be essential in order to gain understanding in the solid state reaction in thin films and to better control these reaction for making contacts in microelectronics devices or for other application.

Abstract: The process of a eutectic alloy crystallization is considered when the eutectic alloy is instantly cooled from the liquid state to below the eutectic transformation temperature. The features of such crystallization are considered. The mathematical model of the process is constructed that takes into account the nucleation of new phases particles, their growth and the associated change the concentrations of the melt components. The nuclei of new phases are supposed be spherical. As they grow, they come into contact and become lamellar. The developed approach was applied to the amorphization process of the eutectic alloys. An amorphous state has been reached if the clusters of solid phases can’t grow above nanosize. The model allows researching the necessary amorphization conditions.

Abstract: The nucleation and growth of oxides formed on Fe-Mn (1 wt.%) binary alloy during annealing were investigated. The recrystallized samples were heated to 800°C and kept at this temperature for 60 s in N₂ / H₂ (5 vol.%) with a dew point of-40°C. The heating process was also interrupted at different temperatures, namely 650, 700, 750 and 800°C. The annealed samples were then observed by scanning electron microscopy. Image analysis was performed to obtain the surface density and size of the oxides. The influence of the ferrite grain orientation on the oxides’ nucleation and growth was studied by Electron Back Scattered Diffraction. The annealed Fe-Mn surfaces were covered by oxide crystals whose shape, surface density and size strongly depended on the underlying ferrite grain orientation. The oxides’ nucleation occurred first on Fe {110} grain, their surface density was the largest on Fe {100}, and their size was the largest on Fe {110}.

Abstract: Overall kinetics of lamellar overaging reactions in U-5.5Nb and U-7.5Nb were analyzed by Avrami-Arrhenius analyses of volume fractions measured from an extensive temperature-time (T-t) matrix of specimens. The cellular initiation site (grain boundaries, inclusions) and regimes of lamellar divergency-cum-slowing growth rate were explicitly accounted for. Avrami exponents n from T-t regimes of constant-growth rate were consistent with theory (1<n<3); those from divergent T-t regimes were smaller, n~0.7, which is not surprising given their different growth rate behavior. The apparent activation energies Q were similar for grain-boundary and inclusion-nucleated discontinuous precipitation, indicating that their nucleation site does not alter their overall kinetics. Avrami Analysis of Isothermal Aging Kinetics

Abstract: Gas pores growing inside the columnar grains, instead of at grain boundary regions were observed in a lotus type porous copper. It is suggested that the lack of effective nucleation substrate in the pure copper melt makes the nucleation and subsequent growth of gas pores difficult. Therefore the melt must be sufficiently undercooled to provide enough free energy for the nucleation, and as a result the gas saturated melt will transform into a higher energy state of gas pores inside the columnar grains. Keywords: Porous material; Solidification; Gas Pore; Nucleation.

Abstract: Nucleation mechanisms and the effect of minor elements added with the inoculants are still a subject of extensive research in ductile iron. Oxides, sulfides, silicates and nitrides have been reported to be nuclei for graphite precipitation. Those compounds originate both from the nodulizing treatment and the inoculation process. Previous research works have shown that titanium nitrides or carbonitrides play an active role in graphite nucleation. In order to determine the efficiency and nature of nitrides that can act as nuclei for graphite, and the possible effect of the trace elements added with the inoculant, melts with titanium contents ranging from 0.007% to 0.036% were produced and poured into standard thermal analysis cups, with and without inoculation. Different inoculants rich in titanium, cerium, aluminum or zirconium were used. Two cups were produced with each inoculant, one cooled down to room temperature, and the other quenched in brine immediately after pouring. Nucleation sites were characterized through detector, spectrum, mapping and line scans of a FEG-SEM equipment. Most of the analyzed nuclei exhibited two or three different inclusions: magnesium sulfides or Mg-Ca oxy-sulfides, Mg or Al oxides, and Ti carbo-nitrides or Mg-Si-Al nitrides. The appearance of each type of nitrides is directly related to the titanium content in the base melt. When titanium was added in the inoculant, no titanium nitrides were noticeable. The zirconium added with the inoculant promoted more complex nitrides that appeared in higher amount. Cerium appears occasionally forming sulfides. Aluminum stimulates the formation of complex nitrides. No differences in the nature of the nuclei were observed between the samples quenched and the ones obtained at room temperature, which assures the methodology approach.