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![Proposed mechanisms for the spheroidal graphite growth: a) plate-like (c - direction); b) helicoidal growth; c) spiral growth; d) columnar growth (c - direction) [12].](profile/Doru-Stefanescu-2/publication/260037322/figure/fig1/AS:297054879404034@1447834810650/Proposed-mechanisms-for-the-spheroidal-graphite-growth-a-plate-like-c-direction-b.png)
Proposed mechanisms for the spheroidal graphite growth: a) plate-like (c - direction); b) helicoidal growth; c) spiral growth; d) columnar growth (c - direction) [12].
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The paper addresses mainly the atomic structure of the spheroidal carbon inclusions (Os) from a magnesium treated cast iron. The qualitative phase analysis based on diffraction patterns shows that the CI by-products contain carbon, ferrite and iron oxides. Based on peak angular positions and their half-width at half-intensity, the distances between...
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... atoms will interact permanently with the surrounding iron atoms and will nucleate and grow as nodule inclusions (Fig. 1c, d). This possible growth mechanism shows how the morphology of the carbon inclusions in cast iron can be explained by an atomistic approach. Four of the most widely accepted growth mechanisms, schematically illustrated in Fig. 2 [11], are plate-like growth, helicoidal growth, spiral growth and columnar ...
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... Based on recorded diffraction patterns, they could assume that turbostratic carbon is present directly next to the nucleus. Their conclusion is based on the results of previous macroscopic graphite studies in SGI iron by Pencea et al. [46] and Ohbuchi et al. [47]. ...
... The presence of turbostratic graphite in SG iron was probably first suggested by Pencea et al. 26 They used wide angle X-ray diffraction (WAXD) to test extracted graphite spheroids from Mg-treated cast iron. The extraction method, consisting in dissolving SG iron chips in boiling HCl, appeared to be aggressive enough so that only the core of the graphite inclusions remained undissolved. ...
... sample. The inset shows the corresponding typical SAED pattern.26 ...
Graphite spheroids are one of the most intriguing forms of aggregation of graphite. They can be found as natural graphite in meteorites and volcanic rocks (metamorphic graphite caused by exposure to heat and pressure in the earth's crust), and in man-made materials such as steel and cast iron, but also in pyrolytic graphite, in carbon nanosheets obtained through vapor deposition, and others. While growth of graphite has been studied in depth during the last decades, the understanding of its physics continues to evolve as modern investigating equipment pushes the knowledge horizon. The complexities of the solidification of cast iron and its dependency on undercooling, a function of the cooling rate and of the nucleation potential, are discussed in conjunction with the asymmetric eutectic coupled zone. A sequential chart summarizes the various stages of graphite aggregation, as the graphite begins growing in contact with the nucleus at the beginning of solidification till it reaches its final crystallography at room temperature. It sets the basis for an in-depth analysis of the multi-mechanisms involved in the growth of graphite in cast iron during solidification and the subsequent solid-state transformation, with emphasis on compacted and spheroidal graphite. It discusses the significance of turbostratic graphite that appears mostly, but not only, at the nucleus/graphite interface and in the initial stages of graphitization, as revealed by recent electron microscopy studies.
... Turbostratic graphite formation has been documented in milled graphite [41], in plasma-enhanced chemical vapor deposition of carbon nanosheets [42] and in pyrolytic carbon [43]. To the best of our knowledge, the presence of turbostratic graphite in SG iron was first suggested by Pencea et al. [44] who used Wide Angle X-ray Diffraction (WAXD) experiments to test extracted nodules from Mg treated cast iron. Evidence of turbostratic graphite was claimed. ...
The role of Mg–Si–Al nitrides in the nucleation of low-sulfur spheroidal graphite Fe–C–Si alloys has been previously documented. It was demonstrated that when acting as nuclei, in most cases, these nitrides are not singular crystals but are part of complex inclusions. To further document the complexities of this phenomenon a Fe–C–Si melt with 3.55%C and 3.1%Si was treated with a 2.5Ca1.6Zr1.4Al inoculating alloy and poured into a thermal analysis (TA) cup. Samples extracted from the TA cup were analyzed through quantitative optical microscopy. Then, Field Emission Gun Scanning Electron Microscopy was used to identify the nature of the non-metallic inclusions serving as nuclei. It was demonstrated that the main nucleation sites are Mg–Si–Al nitrides that many times nucleate on sulfides. Also, some Ti carbo-nitrides were observed growing on the corners of these nitrides. Further, Selected Area Electron Diffraction techniques provided information on the crystallinity of the nucleus, as well as on the graphite/nucleus interface and on graphite growth in the immediate vicinity of the nitride. Transmission Kikuchi Diffraction patterns suggest that the nitrides are MgSiN2 polycrystals with orthorhombic lattice. Turbostratic graphite was identified in the proximity of the nucleus. It supports a better understanding of the growth of graphite on the nucleating phase and brings some clarity to the ongoing debate on the mechanisms of graphite growth.
... X-ray diffraction is a standard laboratory technique for materials characterization which has been applied as well to characterize graphite in cast irons [MAT53] or in Fe-C-Si steels [COW81]. Applied to spheroidal graphite, it was concluded that it is turbostratic rather than polycrystalline [PEN11]. ...
This monograph finds its foundation in a simple fact: there is a paradigm with cast irons, which is that these alloys are produced and cast to shape since thousands of years but are amongst the most complicated metallic alloys when considering the formation of their microstructure by solidification and solid-state transformations. In turn, this complexity opens a wide range of possibilities for shaping their microstructure and engineering their service properties.
The first cast irons were mostly Fe-C alloys and as such solidified mainly in the metastable system, leading to hard and brittle parts that were heat treated for graphite precipitation to give malleable cast irons. The introduction of silicon into the melt increased the temperature difference between the stable and metastable systems, thus promoting the formation of graphite instead of cementite during solidification. This gave rise to the silicon cast irons that are the subject of this monograph.
With the advent of metallographic observations, it was realized that cast iron also often contained phosphides related to the origin of iron ores. A good control of the metallic charge allowed to improving the mechanical properties, in particular by ensuring a minimum elongation before rupture under tensile stress. The essential step, however, was the discovery that it is possible to change the shape of graphite by transforming the interconnected lamellae into discrete spheroids. Cast irons thus became a material for safety parts and were no more restricted to construction.
This historical evolution and the research effort during the first part of the 20th century are described in the vast review carried out by Merchant in the 1960s [MER68]. At that time, there was an explosion of research on cast irons with the aim of describing and understanding the formation of graphite during solidification and, to a lesser extent, during heat treatment. As far as solidification is concerned, the review by Lux [LUX70a, LUX70b] of this research effort is an important step that already contained most of the questions and provisional answers that are still referenced in more recent works [STE05]. It is worth mentioning here Zhou's comprehensive literature review on solidification of different types of cast iron [ZHO09, ZHO10, ZHO11].
This monograph is not intended to be an exhaustive review of the literature as those mentioned above, but rather to provide a coherent view of the formation of the microstructure of silicon cast irons. In fact, the authors felt it was very important to present how various aspects of microstructure formation could be related to each other using schemes based on known physical phenomena, and sometimes supported by ad hoc modelling. Consequently, the works that will be referenced first are those that contain information that has proven to be essential for the development of these schemes. Where appropriate, controversies will be mentioned but not discussed, with reference to the works where they are detailed. Instead, emphasis will be on open questions.
The main text containing basic information and descriptions appears on odd-numbered pages, while details and more in-depth descriptions are limited to even-numbered pages. All references are listed at the end of the monograph, which also contains a glossary of acronyms and unusual terms and an index of the parameters used in the equations and the values employed for physical parameters.
For more than 10 years, our work has certainly benefited from Azterlan's impetus and has greatly benefited from the dynamism of the European Cast Iron (ECI) group. The exchanges within this group, as well as the discussions and controversies that have taken place at its annual meetings have been renewed stimuli. We would like to thank the participants, both academics and industrialists, for their continued contribution to this group.
... However, the paper [11] established by using X-ray diffraction analysis that the inclusions of graphite in cast iron do not constitute poly-crystals but they rather have turbostratic structure. In such a structure, there is no strict order of layers characteristic to the crystal structure of graphite. ...
A set of research of nodular graphite inclusions in ductile cast iron was carried out, including micrographic, x-ray and petrographic methods. Petrographic method has more possibilities and allows conducting research not only in the reflected but also in the transmitted light, to determine the structure and composition of graphite inclusions in the cases where other, even the most accurate methods, cannot apply.
Based on the metallographic, petrographic and X-ray research, we introduced a new mechanism of formation of nodular shaped graphite in ductile cast iron. It is due to the simultaneous formation of a gas bubble, supply channel and its base – the mouth – graphite with a cavity inside that form a single system. We identified three morphological varieties of graphite, formed during different periods of changes in the physical–chemical conditions of liquid cast iron: in the moment of interaction of magnesium with carbon monoxide – clear-cut hexagonal; during the period of disproportionation of carbon monoxide in the volume of the gas bubble − aggregates of crystals of various shapes; during the "tempering" of carbon monoxide – film (cryptocrystalline).
It was found that a full filling-up of the total volume of a gas bubble by graphite is carried out in three stages: the first two stages – by primary graphite, which deposits on the inner shell of the bubble, starting from the periphery to the centre, then it forms a nodular shape of graphite and the third and final – by secondary film graphite, distributed from the centre to the periphery.
The obtained results will make it possible to take into account the peculiarities of the formation of nodular shaped graphite inclusions when designing technological processes of obtaining castings for various purposes, to determine the values of modifying additives, and, ultimately, to control the morphology of graphite phase in ductile cast irons.
While turbostratic graphite is documented in many forms of graphite, there is a paucity of information on its contribution to the crystallization of spheroidal graphite (SG) in cast iron. SAD on SG and discussion of deep-etched SEM samples, demonstrates that it is an integral part of the crystallization sequence of SG. It is found in the core region, but also in the outer shell. This imposes a reevaluation of current theories of SG crystallization.
Metallographic and microscopic x-ray spectrum analyses are used to study the special features of the microstructure of perlite-ferrite cast iron with spheroidal graphite. The internal polycrystalline structure of the spheroidal graphite is discussed, and the presence of ferrite precipitates over the boundaries of pyramidal crystals forming spherulites is proved. Data qualifying the nature of nucleus in a spheroidal graphite inclusion are presented.
The first ingredient necessary to build molecular-scale circuits is wire. These wires must be very thin, long, strong mechanical and electrical conductance have a good. The work analyze the austenite grain increase tendency, during the heating of high - carbon steels, at 900°C and 1000°C, which represents the limits of austeniting range in order to be patenting. When the carbon content is between 0.65 - 0.80 %, manganese is limited at 0.6 %, appeared that the steel has a small tendency of increasing the grain sizes (in the 900 - 1000°C range), and the aluminum microalloing is no more needed. Producing of steel using heat treatment an aluminum deoxidizing lead to a high purity steel and a fine grain structure after patenting characterized by a high ductility.
