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Extensive SEM work was carried out on deep etched specimens to reveal the evolution of graphite shape in Fe–C–Si alloys of industrial composition during early solidification and at room temperature. The samples had various magnesium and titanium levels designed to produce graphite morphologies ranging from coarse lamellar to interdendritic lamellar...
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... on composition and cooling rate, three main graphite morphologies crystallize from the FeeCeSi melt during solidification: lamellar (LG), compacted or vermicular (CG) and spheroidal (SG), as exemplified in Fig. 1 [1e3]. The internal struc- ture of SG exhibits conical sectors of parallel graphite planes growing radially from the center (Fig. 2-a). The sectors may be partially broken ( Fig. 2-b) in extreme cases causing "exploded" graphite. The annular rings may exhibit zig-zag steps of the (0001) planes, suggesting columnar crystals of graphite with different orientations ...
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... main graphite morphologies crystallize from the FeeCeSi melt during solidification: lamellar (LG), compacted or vermicular (CG) and spheroidal (SG), as exemplified in Fig. 1 [1e3]. The internal struc- ture of SG exhibits conical sectors of parallel graphite planes growing radially from the center (Fig. 2-a). The sectors may be partially broken ( Fig. 2-b) in extreme cases causing "exploded" graphite. The annular rings may exhibit zig-zag steps of the (0001) planes, suggesting columnar crystals of graphite with different orientations ...
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... of the platelets occurs through spiral growth at screw dislocation steps or by 2-D nucleation of the sheet in the c- direction. There is microscopy evidence for both (see for example Fig. 9 in Ref. [19] and Fig. 12 in Ref. ...
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... evolution of graphite shape during solidification can be summarized through the graphs describing the change in aspect ratio as a function of the measured solid fractions, presented in Fig. 12 for the Q-series samples. In all instances the aspect ratio increases as solidification advances and as the fraction solid in- creases. It should be noted that the fraction solid depends on two variables: the cooling rate which increases from the center of the cup to its periphery, and the time of quenching after the beginning of ...
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... as solidification advances and as the fraction solid in- creases. It should be noted that the fraction solid depends on two variables: the cooling rate which increases from the center of the cup to its periphery, and the time of quenching after the beginning of solidification. The general graphite shape descriptors, SG, TPG, CG and LG on Fig. 12, imply that at room temperature the majority of the graphite was of this shape. A sample was marked as having TPG, as soon as tails were observed on graphite spheroids. This was considered to be the beginning of the transition from ...
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... the Mg-free low-S iron, graphite grows in contact with both the austenite dendrites and the liquid (Fig. 13-a). Increasing the Ti level generates a more compact graphite, as demonstrated through Fig. 12-a for Ti levels of 0.18 and 0.32%, promoting the transition from interdendritic LG to SIG. When Mg was added to the iron, solidification started with graphite spheroids (Fig. 12-b,-c and Fig. 13-b). At about 0.4 fraction solid (f S ) some of the graphite spheroids exhibited one or more tails (Fig. 13-c). This is the so- called tadpole ...
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... graphite grows in contact with both the austenite dendrites and the liquid (Fig. 13-a). Increasing the Ti level generates a more compact graphite, as demonstrated through Fig. 12-a for Ti levels of 0.18 and 0.32%, promoting the transition from interdendritic LG to SIG. When Mg was added to the iron, solidification started with graphite spheroids (Fig. 12-b,-c and Fig. 13-b). At about 0.4 fraction solid (f S ) some of the graphite spheroids exhibited one or more tails (Fig. 13-c). This is the so- called tadpole graphite (TPG) [52]. The graphite spheroids are surrounded by austenite, while tadpole graphite was in most cases connected to cementite, suggesting growth in contact with the ...
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... additions produce more compact graphite. This is seen on Fig. 12-a. The average aspect ratio at the end of solidification is 2.77 for the 0.18% Ti, and 2.44 for the 0.32% Ti samples. The sub- structure of Ti modified graphite still exhibits the tiled-roof struc- ture, but the graphite platelets are smaller of the order of several micrometers (Fig. 17). Growth of the aggregate is again in the a- ...
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... Initial CG aggregates are also the result of stacking of the graphite platelets, with significant c-direction dimension, close to that in the a-direction (Fig. 19-b). In addition, in compacted graphite the platelets are assembled into clusters of parallel platelets that have different orientations growing at an angle with respect to one another (Fig. 20). The clusters have a height of several microns, looking like graphite blocks under certain obser- vation conditions. Some graphite aggregates resembling what is commonly known as chunky graphite were also found in the 0.02% Mg samples. They grow radially from a common center and are the product of c-di- rection stacking of Gr platelets ...
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... (Fig. 20). The clusters have a height of several microns, looking like graphite blocks under certain obser- vation conditions. Some graphite aggregates resembling what is commonly known as chunky graphite were also found in the 0.02% Mg samples. They grow radially from a common center and are the product of c-di- rection stacking of Gr platelets (Fig. ...
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... micrographs of deep etched room temperature CG iron samples exhibit the same pattern of growth identified for lamellar and tadpole graphite, i.e. stacking of graphite platelets (Fig. 22). However, the platelets spread both in the a-and c-direction over a wider area than for ...
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... variety of spheroids and imperfect spheroids were found in the early stages of solidification of Mg-treated irons. At the low Mg level of 0.013%, clear radial growth of dendritic appearance was observed (Fig. 23-a). Increase Mg level further moved graphite shape toward that of a spheroid, albeit imperfect ones. Graphite platelets were still observable on some of the spheroids (Fig. 23-b), with some of the platelets appearing to be perpendicular to the radius of the spheroid. The radial sectors become more compact, but still suggest growth from a ...
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... spheroids were found in the early stages of solidification of Mg-treated irons. At the low Mg level of 0.013%, clear radial growth of dendritic appearance was observed (Fig. 23-a). Increase Mg level further moved graphite shape toward that of a spheroid, albeit imperfect ones. Graphite platelets were still observable on some of the spheroids (Fig. 23-b), with some of the platelets appearing to be perpendicular to the radius of the spheroid. The radial sectors become more compact, but still suggest growth from a common center (Fig. ...
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... Mg level further moved graphite shape toward that of a spheroid, albeit imperfect ones. Graphite platelets were still observable on some of the spheroids (Fig. 23-b), with some of the platelets appearing to be perpendicular to the radius of the spheroid. The radial sectors become more compact, but still suggest growth from a common center (Fig. ...
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... magnification of another graphite spheroid (Fig. 24) re- veals the platelet sub-structure. Growth is much disorganized around the nucleus that was removed by deep etching. While conical sectors of platelets growing radially away from the nucleus are seen, not all sectors originate in the nucleus. This is similar with what was found on ion-etched graphite nodules (Fig. 2-a). The large ...
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... another graphite spheroid (Fig. 24) re- veals the platelet sub-structure. Growth is much disorganized around the nucleus that was removed by deep etching. While conical sectors of platelets growing radially away from the nucleus are seen, not all sectors originate in the nucleus. This is similar with what was found on ion-etched graphite nodules (Fig. 2-a). The large number of cavities between the platelets, which are regions where the austenite has been removed by the deep etching, is consistent with growth of foliated dendrites. There is no evidence of growth by rolling or wrapping of the graphene ...
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... radially from a common center. The basic building blocks of the graphite aggregates are hexagonal faceted graphite platelets with nano- meter height in the c-direction and micrometer width in the a- direction. Thickening of the platelets occurs through growth of additional graphene layers nucleated at the ledges of the graphite prism as shown in Fig. 25 for CG iron and in Fig. 16-b for LG ...
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... Mg-free LG irons, the platelets grow as layered crystals exhibiting a tiled-roof configuration (Fig. 26-a) or form foliated dendrites (Fig. 26-b). The tiled roof configuration was also observed through TEM (Fig. 27). Note the similitude between the drawing in Fig. 26-b, which is based on SEM micrographs, and the schematic drawing of foliated dendrites in Fig. 10-a. Branching of the foliated dendrites can occur at screw dislocation defects ...
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... Mg-free LG irons, the platelets grow as layered crystals exhibiting a tiled-roof configuration (Fig. 26-a) or form foliated dendrites (Fig. 26-b). The tiled roof configuration was also observed through TEM (Fig. 27). Note the similitude between the drawing in Fig. 26-b, which is based on SEM micrographs, and the schematic drawing of foliated dendrites in Fig. 10-a. Branching of the foliated dendrites can occur at screw dislocation defects produced on the platelets (Fig. ...
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... Mg-free LG irons, the platelets grow as layered crystals exhibiting a tiled-roof configuration (Fig. 26-a) or form foliated dendrites (Fig. 26-b). The tiled roof configuration was also observed through TEM (Fig. 27). Note the similitude between the drawing in Fig. 26-b, which is based on SEM micrographs, and the schematic drawing of foliated dendrites in Fig. 10-a. Branching of the foliated dendrites can occur at screw dislocation defects produced on the platelets (Fig. ...
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... Mg-free LG irons, the platelets grow as layered crystals exhibiting a tiled-roof configuration (Fig. 26-a) or form foliated dendrites (Fig. 26-b). The tiled roof configuration was also observed through TEM (Fig. 27). Note the similitude between the drawing in Fig. 26-b, which is based on SEM micrographs, and the schematic drawing of foliated dendrites in Fig. 10-a. Branching of the foliated dendrites can occur at screw dislocation defects produced on the platelets (Fig. ...
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... dendrites (Fig. 26-b). The tiled roof configuration was also observed through TEM (Fig. 27). Note the similitude between the drawing in Fig. 26-b, which is based on SEM micrographs, and the schematic drawing of foliated dendrites in Fig. 10-a. Branching of the foliated dendrites can occur at screw dislocation defects produced on the platelets (Fig. ...
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... dendrites were also observed in Mg-modified CG irons (Figs. 22 and 26-c). However the graphite platelets stack along the c-axis building clusters of graphite platelets growing at various angles with respect to one another (Fig. 20). Quasi-cylindrical shapes connected to more or less curved walls (tadpole graphite) and compacted graphite were generated. Sometimes radial stack- ing of the platelets was ...
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... dendrites were also observed in Mg-modified CG irons (Figs. 22 and 26-c). However the graphite platelets stack along the c-axis building clusters of graphite platelets growing at various angles with respect to one another (Fig. 20). Quasi-cylindrical shapes connected to more or less curved walls (tadpole graphite) and compacted graphite were generated. Sometimes radial stack- ing of the platelets was observed. The hexagonal shape of the platelets is less regular, indicating a roughening of the interface produced by the higher constitutional undercooling induced ...
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... the Mg level further increases, chunky graphite and more spheroids crystallized from the melt. The chunky graphite aggre- gate is made of quasi-cylindrical sectors of graphite platelets stacked along the c-axis, growing radially from a common nucleus (Fig. 21). The graphite spheroids exhibit foliated dendrites growing radially from the center with many voids between the plates (Fig. 26-d). The platelets are more irregular than for the lower Mg iron. In general, as the amount of Mg increases, the platelets gradually lose their clear hexagonal shape observed in LG ...
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... chunky graphite and more spheroids crystallized from the melt. The chunky graphite aggre- gate is made of quasi-cylindrical sectors of graphite platelets stacked along the c-axis, growing radially from a common nucleus (Fig. 21). The graphite spheroids exhibit foliated dendrites growing radially from the center with many voids between the plates (Fig. 26-d). The platelets are more irregular than for the lower Mg iron. In general, as the amount of Mg increases, the platelets gradually lose their clear hexagonal shape observed in LG ...
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... is required to break down the plane interface [58]. The large amounts of impurities available in the melt (S, O, N, Mg, etc.) accumulate on the basal faces of the hexagonal graphite platelets and produce high constitutional undercooling. Screw dislocations generated at the leading edge of the graphite platelet may develop into pro- tuberances (Fig. 29). The dislocation will continue to move in the c- direction, and the protuberance will increases in thickness. If the protuberance grows enough to reach a region with lower consti- tutional undercooling, the growth rate anisotropy of the crystal can re-assert itself, lateral growth occurs parallel to the first formed platelet, and a new ...
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... classic mechanisms of two- dimensional nucleation and screw dislocation movement. Indeed, TEM micrographs of lamellar graphite show that a large number of iron containing regions are incorporate in the graphite (Fig. 30). A significant number of dark spots between the graphite platelets, deemed to be iron, were also identified in this work (e.g. Fig. ...
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... the break-down of the hexagonal faces of the graphite because of the high constitutional undercooling may results in branching of the platelet as shown in Fig. ...
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... Mg addition constitutional undercooling is increased. This destabilizes the (0001) faces of the graphite producing a rougher interface. Branching of the platelets followed by growth in various directions different from that of the initial platelet follows (Fig. 20). In addition to the twining faults, rotational stacking faults occur. Stacking of platelets in the c-direction becomes more significant and clusters with blocky appearance are produced. Although the effect of Mg on the graphene layers has not been documented, existing work with other elements such as oxygen [49], allows to postulate ...
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... concluding we need address one more issue. Many SEM pictures of SG iron exhibit a layer growth (cabbage type) on the surface of the graphite. Clearly the graphite grows in the a-direc- tion, by a mechanism that seems to be analogous to that described by Sadocha and Gruzleski [14]. There is also TEM evidence of such growth for SG as shown in Fig. 32 [63] and HREM evidence for amorphous graphite growing in an electronic beam [64]. This growth pattern occurs after encapsulation of the graphite spheroid into an austenitic shell, when growth of the graphite continues through solid diffusion of carbon through the austenite to the growth ...
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... With the increase of FeSi75, the start temperature of austenite formation T AL firstly decreases and then increases in Fig. 4(a). It can be explained that the addition of FeSi75 increases the carbon equivalent, making the eutectic point to shift to right and thus reducing the primary crystallization temperature [27] . However, further increase of the FeSi75 additive increases the silicon content of molten iron and reduces the local carbon solubility, thus the precipitation phase of graphite transforms to be fine spherical shape [28] . ...
Thermal analysis plays a key role in the online inspection of molten iron quality. Different solidification process of molten iron can be reflected by thermal analysis curves, and silicon is one of important elements affecting the solidification of molten iron. In this study, FeSi75 was added in one chamber of the dual-chamber sample cup, and the influences of FeSi75 additive on the characteristic values of thermal analysis curves and vermiculating rate were investigated. The results show that with the increase of FeSi75, the start temperature of austenite formation TAL firstly decreases and then increases, but the start temperature of eutectic growth TSEF, the lowest eutectic temperature TEU, temperature at maximum eutectic reaction rate TEM, and highest eutectic temperature TER keep always an increase. The temperature at final solidification point TES has little change. The FeSi75 additive has different influences on the vermiculating rate of molten iron with different vermiculation, and the vermiculating rate increases for lower vermiculation molten iron while decreases for higher one. According to the thermal analysis curves obtained by a dual-chamber sample cup with 0.30wt.% FeSi75 additive in one chamber, the vermiculating rate of molten iron can be evaluated by comparing the characteristic values of these curves. The time difference ΔtER corresponding to the highest eutectic temperature TER has a closer relationship with the vermiculating rate, and a parabolic regression curve between the time difference ΔtER and vermiculating rate η has been obtained within the range of 65% to 95%, which is suitable for the qualified melt.
... The graphite platelets are the basic building blocks of the graphite aggregates in cast iron. Direct observation of platelets is reported for LG in Ni-C alloys (100-nm-thick in ref. 30 ), in nanofibers obtained through catalytic routes, 31 in CG obtained through interrupted solidification of cast iron, 32 in SG in cast iron, 33 and in SG obtained by heat treatment of medium carbon steel 5,17 (Figure 15a). The platelets are 10-60-nm-thick and a few micrometers long. ...
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.
... Magnesium (Mg), an alloying element, is used in casting to get the best CGI microstructure [36]. Mg has a considerable impact on how the graphite morphology behaves when cooling occur at speeds between 0.5 and 10 K/s. ...
Compacted Graphite Iron (CGI) is a type of cast iron with intermediate properties between grey and ductile cast iron. The vermicular graphite structure in the CGI provides unique properties to manufacture automotive and other engineering components. However, because of its narrow range of metallurgical control, a high volume of CGI production is not attempted in many foundries. The addition of alloying elements and austempering heat treatment process has a significant effect in altering the properties and microstructure of CGI. Hence, the objective of this paper is to review the influence of various alloying elements and austempering processes in manufacturing CGI components. Eventually, the scope for the research on the development of austempered CGI is identified.
... This can be explained roughly from the preferred growth direction of graphite. Each monolayer along the a-axis (perpendicular with [0001] direction) of graphite is a 2-D polymeric graphene sheet bonded by a strong covalent, and all monolayers along the c-axis (parallel with [0001] direction) of graphite are bonded by weak van der Waals force [38]. Therefore, graphite preferentially grows along the a-axis to flake graphite without adding spheroidized elements. ...
To provide the basis for thermal conductivity regulation of vermicular graphite cast iron (VGI), a new theoretical method consisting of shape interpolation, unit cell model and numerical calculation was proposed. Considering the influence of the graphite anisotropy and interfacial contact thermal conductivity (ICTC), the effective thermal conductivity of a series of unit cell models was calculated by numerical calculation based on finite difference. The effects of micro-structure on effective thermal conductivity of VGI were studied by shape interpolation. The experimental results were in good agreement with the calculated ones. The effective thermal conductivity of VGI increases in power function with the decrease in graphite shape parameter, and increases linearly with the increase in graphite volume fraction and thermal conductivity of matrix. When the graphite volume fraction increases by 1%, the thermal conductivity of nodular cast iron increases by about 0.18 W/(m·K), while that of gray cast iron increases by about 3 W/(m·K). The thermal conductivity of cast iron has the same sensitivity to the thermal conductivity of matrix regardless of the graphite shape parameter. The thermal conductivity of matrix increased by 15 W/(m·K) and the thermal conductivity of cast iron increased by about 12 W/(m·K). Moreover, the more the graphite shape deviates from the sphere, the greater the enhancement effect of graphite anisotropy on thermal conductivity than the hindrance effect of interface between graphite and matrix. This work can provide guidance for the development of high thermal conductivity VGI and the study of thermal conductivity of composites containing anisotropic dispersed phase particles with complex shapes.
... The CaO, SrO or BaO oxides may also be contained in complex oxides. The germs of nuclei may also be formed by nitrides of the (MgSiAl)N type [13,14]. The size of the crystallizing germs is 1-3 μm. ...
In many application fields, thin-walled ductile iron castings can compete with castings made from aluminium alloys thanks as their show superior mechanical properties higher stiffness, vibrations damping as well as properties at higher temperatures. As problematic criterion in thin-walled cast-iron castings can be seen the graphitization ability and high sensitivity of the structure and the mechanical properties to the solidification rate.
The tests were curried on plate castings with wall thicknesses of 3, 5, and 8 mm, using inoculants based on FeSi70 with different contents of nucleation-active elements as aluminium, calcium, zirconium and magnesium. The inoculation was made by the in-mould method. In the experiments structures were achieved, differing by the graphite dispersity, structure and mechanical properties.
The experiments have proved particularly a high sensitivity of the structure and the mechanical properties to the cooling rate of the sample castings. The influence of the inoculant type is less important than the influence of solidification rate.
Key words: Thin-wall castings, Ductile iron, Inoculation, Structure, Mechanical properties
... A purely anisotropic elastic behaviour is assumed for graphite. The elastic constants for both phases are taken from [47][48][49]. Except for the Burgers vector , the temperature and the average grain size g , the remaining crystal plasticity parameters are identified from experimental LCF and relaxation tests. Table 2 are mostly within this acceptable range. ...
To predict and better understand the creep-fatigue behaviour of austenitic cast iron D5S under tension and compression dwell at 800∘C, a physics-based crystal plasticity model that describes the complex rate- and temperature-dependent deformation of the material as a function of the dislocation density is implemented. In addition to the tension and compression dwell direction, the effect of three different dwell times (30, 180 and 600 s) on the creep-fatigue properties is investigated. The dislocation density-based crystal plasticity simulations are compared to experimental tests from a prior work. While relaxation tests and low-cycle fatigue (LCF) tests without dwell assist in systematically identifying the material parameters, creep-fatigue (CF) data is used to validate the predictions. The virtual testing is performed on a large-scale representation of the actual test specimen with a polycrystalline structure. To analyse the fatigue damage mechanism, small-scale predictions are also conducted using a micromechanical unit cell approach. Here, a single graphite nodule frequently found in the material is embedded into the austenitic matrix. In the present work, a close agreement is achieved between the predicted CF behaviour and the experimental results. Consistent with the experimental findings, the simulation results show that the addition of compression dwell leads to an uplift of the overall tensile stress level, which significantly reduces the fatigue life of the material. The unit cell studies demonstrate that during this uplift, a strong localisation of stresses and strains arises at the graphite/matrix interface, triggering the nucleation and growth of cavities and/or debonding.
... The microstructure of CI depends on the chemical composition, cooling rate conditions, and subsequent heat treatments [2]. Properly controlling the carbon and silicon contents and the cooling rate, the graphite crystallizes directly from the melt [3,4]. Then, the mechanical properties will strongly depend on the shape, size and distribution of the graphite particles in the metallic matrix. ...
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... Since experimental acquisition of these directions for a 3D model would be prohibitively heavy and virtually infeasible, a novel approach is proposed here to approximate the directions of graphite anisotropy for a microstructural model. The approach is based on microstructural studies presented in the literature [65][66][67], where it has been shown that the nucleation and growth (crystallization) of graphite particles starts from the outer surface and results in either an 'onion'-like structure of graphite in spheroidal (SGI) cast iron, or a stack of graphite plates along the short direction of the lamellas in the lamellar (FGI) cast iron, as schematically shown in Fig. 4(b). The structure of the graphite in compacted graphite cast iron (CGI) is more complex, but in general can be assumed to be intermediate between that of SGI and FGI. ...
This work presents a multi-scale modelling framework for thermo-mechanical behaviour of Compacted Graphite Iron cast iron. A general thermo-elasto-visco-plastic model is developed to describe the matrix (pearlite) behavior under thermo-mechanical cyclic loading, for which the parameters are identified from tests on pearlitic steel. The pearlite model takes into account the temperature dependent rate-dependency and kinematic hardening. The importance of properly accounting for the graphite anisotropy is emphasised, for which a numerical procedure for estimating the local anisotropy directions from the graphite particle geometry and experimental observations is proposed. A high quality conforming finite element mesh is generated on a representative volume element using discrete voxelized microstructural data in combination with signed distance functions from the interfaces. For fully constraint thermal cyclic loading conditions with different holding times, the capabilities of the developed multi-scale model are demonstrated at both scales: the macroscale, where the simulation results are in very good agreement with the experimental data, and the microscale, providing the evolution of local fields.
... The formed steps continue to accumulate carbon atoms to achieve the growth of graphite. [54][55][56] Stefanescu et al. 57) demonstrated that the magnesium can promote the nodularization of graphite in ductile iron. When the concentration of residual magnesium at the growth front is sufficient, the growth of graphite on the a axis is limited and forms graphite nodules. ...
Cast high-nickel austenitic ductile iron (CHNADI) is widely used in high temperature, low temperature and strong corrosive environments due to its good comprehensive mechanical properties. However, its low yield strength limits its application in nuclear power equipment. Herein, the influence of nodularizer and processing temperature on the graphite and mechanical properties of CHNADI was investigated to significantly promote its properties. With the increase of nodularizer dosage, the mechanical properties of tensile strength, yield strength, elongation, and impact toughness first increase and then decrease. When the dosage of nodularizer is 0.9 mass% and the processing temperature is 1480°C, CHNADI exhibits excellent mechanical properties with the hardness of 132 HB, tensile strength of 450 MPa, yield strength of 224 MPa, elongation rate of 46% and impact toughness of 91 J. In this work, a new strategy of preparing CHNADI without heat treatment is proposed to meet the strict requirements of this material, which is significant for the development of CHNADI.
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... Free graphite exists in the metallic matrix as flake (in grey cast iron), intermediate (in compacted or vermicular cast iron), or spheroidal (in nodular or ductile cast iron) [1,[5][6][7][8][9][10]. ...
... This is shown schematically, along with major crystallographic planes and directions, in Fig. 1. It is to be noted that the carbon atoms are sp2 bonded in the basal planes, while weak van der Waals bonds exist between different basal planes (in the prismatic planes) [5]. With this difference in the nature and magnitude of bonding and corresponding differences in interfacial energies, the growth of graphite is expected [5] to be anisotropic. ...
... It is to be noted that the carbon atoms are sp2 bonded in the basal planes, while weak van der Waals bonds exist between different basal planes (in the prismatic planes) [5]. With this difference in the nature and magnitude of bonding and corresponding differences in interfacial energies, the growth of graphite is expected [5] to be anisotropic. The graphite tends to grow such that the total free energy of the system gets reduced. ...
Applications of cast irons, from mundane to more challenging, are decided by the morphology of free graphite in the metallic matrix. The morphology changes from flake to spheroidal, as controlled by magnesium (Mg) addition during metal castings. Though this technology dates back several decades, the exact mechanism remains debatable. This study used a combination of industrial casting trials, analytical microscopy, and molecular dynamics (MD) simulations to address this question. It was experimentally established that the shape change was accompanied by a change in the growth direction, from prism to basal in the graphite, and atomic segregation of Mg at the interface. MD simulations indicated a combination of migration of oxygen (O) atoms and Mg-O interactions, respectively in the prism and basal oriented graphite, resulted in cross-over in interfacial free energy with Mg concentration. The anisotropic growth by high energy interface, controlled by interface chemistry, thus defined the mechanistic origin for the graphite morphology in cast iron.
