Papers by Keyword: Nucleation

Abstract: We report on growth of 3C-SiC by sublimation process in vacuum with the aim to ultimately select conditions for single polytype growth of bulk crystals. The 3C polytype occurrence, growth mechanism and structure evolution have been in the focus of the study. To gain understanding of the initial formation of the cubic polytype, growth was performed on various substrates, such as 6H- and 4H-SiC (on-axis and vicinal), as well as freestanding 3C-SiC wafers. The growth configuration used allowed a high growth rate, e.g. up to 200 (m/h, respectively very thick layers. The grown material was studied by means of optical microscopy, AFM and HRTEM. 6H-SiC (0001) Si-face substrates may be a good choice if the 3C nucleation is well controlled, which can be achieved by selecting the initial temperature ramp up and substrate orientation. These growth conditions limit the number of nucleation centers and decrease the defective boundaries.

Abstract: Down-scaling is a major principle of modern technology. As a consequence, the stability of many technical devices is controlled by solid state reactions that proceed on the range of a few nanometres only. On such a short length scale even basic aspects of reaction physics as fundamental as e.g. the Ficks laws of diffusion, need to be reconsidered. Only very few dedicated techniques are suitable to study atomic transport and reactions with sufficient accuracy. Among them, the atom probe tomography is exceptional, as it allows the detection and localization of individual atoms with an accuracy of a lattice constant. An almost complete reconstruction of the 3D atomic arrangement of different atomic species gets possible. This article provides an overview on recent atom probe studies of reactive diffusion. After an introduction into the principles of the analysis method, physical mechanisms of solid state reactions are discussed in view of recent experiments at metallic thin film interfaces. How does nucleation of an interfacial product take place In which way do grain boundaries influence the reaction As a technical example, the stability of Cu/NiFe GMR sensor layers is discussed.

Abstract: Self-assembled niobium oxide microcones produced by potentiostatic anodization with varied NaF content (between 100 and 250 mg) in an HF electrolyte are shown to nucleate mineral when immersed in supersaturated solutions emulating mineral content in saliva and blood. The most extensive mineral coverage in 100 mL of 2.5 wt. % HF electrolyte occurs when NaF content is about 100 mg with substantial mineral formation occurring within 24 hours. Higher salt content apparently alters the conditions favoring mineral nucleation by generating smaller nucleation centers that ultimately diminish the extent of mineral coverage. Additionally, nucleation kinetics and morphological contrasts between mineral formed from saliva and blood is briefly discussed in terms of the relative degree of supersaturation with respect to hydroxyapatite. Finally, we show that the integrity of the microcone shape is not critical for mineral nucleation, an observation that builds on our prior hypothesis by promoting the importance of self-assembly and crystal formation. Based on these results, we demonstrate the influence of NaF and stress the role of the self-organization process in producing effective mineral nucleation sites.

Abstract: Solidification behaviour of 357 Al-alloy under intensive forced convection in the rheo-die-casting (RDC) process, was investigated experimentally to understand the effects of the intensity of forced convection and shearing time on the nucleation and growth behaviour. It was found that under intensive forced convection, heterogeneous nucleation occurred continuously throughout the entire volume of the solidifying melt. All the nuclei could survive due to the uniform temperature and composition fields created by the forced convection. This has been named as ‘effective and continuous nucleation’. It is also found that the nuclei grow spherically with an extremely fast growth rate. This makes the primary solidification essentially a slow coarsening process, in which Ostwald ripening takes place by dissolution of the smaller particles. In addition, it was found that intensive forced convection suppresses partially the formation of the primary phase, promote nucleation of the primary particles, and hinders the particle growth.

Abstract: A simple model for recovery of a subgrain structure is used to distinguish and explain the respective influence of the deformed microstructure characteristics, annealing temperature and concurrent precipitation kinetics on the nucleation kinetics of recrystallisation. Simulation results demonstrate how the balance between recovery and precipitation kinetics controls nucleation.

Abstract: This study presents in situ observations of recrystallization texture formation in Al-3mass%Mg using SEM concurrent with electron back scattering pattern (EBSP) with hot stage. In the present discussion, the emphasis is on the characteristics of the preferred growth or the shrinkage of Cube and other oriented grains. The in-situ observations of recrystallization demonstrate clearly that the nucleation, growth and shrinkage of recrystallized grains occur simultaneously in each orientation in each region. The overall development of recrystallization texture depends on the balance of nucleation/growth and shrinkage/disappearance of each orientation during recrystallization. The preferential growth is determined by the grain boundary mobility between recrystallized grains or the clusters composed of several similar oriented grains, i.e. Cube clusters or S clusters, and neighboring deformed matrix, and the competitive growth with the surrounding grains. On the other hand, the isolated oriented grain and the strain-stored grains tend to shrink and disappear during recrystallization and grain growth.

Abstract: By 3 dimensional X-ray diffraction (3DXRD) using high energy X-rays from synchrotron sources it is possible to study in-situ the nucleation and growth during recrystallization. In this paper it is described and discussed how 3DXRD can supplement EBSP measurements of nucleation and growth. Three types of studies are considered: i) orientation relationships between nuclei and parent deformed matrix, ii) recrystallization kinetics of individual bulk grains and iii) filming of growing grains in deformed single crystals.

Abstract: A kinetic model has been developed to simulate the precipitate size distribution and the resulting yield strength during ageing of 7xxx alloys. The η phase is the only one considered. The kinetic model is mean field: precipitates of different sizes see each other through the average solid solution. Precipitates are assumed to be homogeneous in concentration and are allowed to change chemistry. Local equilibrium is assumed at the matrix-precipitate interface; the equilibrium concentrations are corrected by the curvature effect. Values of the equilibrium concentrations at the matrix-precipitate interface are solved by an iterative method: the resulting flux for each element must be compatible with equilibrium conditions and with the changing stoechiometry of the considered precipitate while maximizing the energy gained. The yield strength is derived from the precipitate size distribution through a mixture law combining the effect of each individual precipitate. The model can take into account non-isothermal treatments and can therefore simulate complicated multi-stage ageing treatment as well as a FSW weld. Results of the model are discussed and compared measurements.

Abstract: The role of Cu in precipitation behavior of 6000 series aluminum alloys has been investigated by isothermal calorimetry and transmission electron microscopy. The newly developed analytical techniques have been used to evaluate the precipitation kinetics in alloys with or without an initial pre-aging history. It has been found that Cu addition results in increasing the rate of precipitation when artificial aging is applied immediately after solutionizing and quenching. However, Cu has no significant effect on the kinetics of precipitation in alloys with the pre-aging history, while it produces finer microstructures in the pre-aged and then artificially aged alloys. These observations have been explained by the effect of Cu on increasing the rate of precipitate nucleation in 6000 series alloys. It has also been suggested that the effect of Cu on nucleation arises from the role of Cu on the cluster formation during and/or immediately after quenching.

Abstract: Hypereutectic ductile iron was cast in green sand moulds with four plates with thickness of 1.5, 2, 3 and 4 mm in each mould. Temperatures were measured in the 3 and 4 mm plate. The temperature curves showed that eutectic solidification was divided into two stages: primary and secondary eutectic solidification. The first stage, which was relatively short, had none or very little recalescence. Further under cooling, followed by reheating during recalescence, was necessary to initiate the second part of the eutectic solidification. Both the secondary under cooling and recalescence was larger in the 3 mm plates than in the 4 mm plates. All 1.5 mm plates contained carbides but the other plates solidified without carbides. Metallographic examination showed two populations of graphite nodules. A small group of nodules was larger than rest of the nodules. Color etching revealing the segregation of Si showed a higher Si content in the ferrite around the larger nodules compared to the ferrite around the rest of the nodules. This indicates that solidification took place along the following path: The solidification starts with nucleation and growth of primary graphite nodules. This probably starts during the filling of the mould. The primary nodules act as nuclei for austenite. As austenite easily nucleates on graphite the temperature will be the same for the 3 and 4 mm plate for the first part of the eutectic solidification. This first part of the solidification ends when concentration of carbon around austenite dendrites is too large and new nodules have to nucleate and grow. The larger under cooling for the 3 mm plates compared to the 4 mm indicates that the nucleation of new nodules is governed by kinetics even in very well inoculated melts.