Microstructure, Inclusions, and Elemental Distribution of a Compacted Graphite Iron Alloyed by Ce and La Rare Earth (RE) Elements

Abstract

This work investigates the microstructure and inclusions of a compacted graphite iron (CGI) alloyed by Ce and La rare earth (RE) elements. In our study, alloying elemental distribution and solute segregation were characterized by methods of secondary ion mass spectrometry (SIMS) and a three-dimensional atom probe (3DAP) with high sensitivity and spatial resolution. RE sulfide, MgS, carbide, and composite inclusions formed during solidification and provided heterogeneous nucleation cores for the nucleation of the graphite. Significant solute clustering in the matrix, coupled with the segregation of solute to grain boundaries, was observed. C, Mn, Cr, and V were soluted in cementite and promoted the precipitation of cementite, while Si was found to be soluted in ferrite. Cu is usually distributed uniformly in ferrite, but some Cu-rich atom clusters were observed to segregate towards the interface between the ferrite and cementite, stabilizing the pearlite. In addition, P, as a segregation element, was enriched along the boundaries continuously. The RE elements participated in the formation of inclusions, consuming harmful elements such as As and P, and also promoted the heterogeneous nucleation of the graphite and segregated, in the form of solute atoms, at its interfaces.

1. Introduction

When researchers added Ce into molten gray cast iron during the process of producing nodular cast iron in 1948 [1,2], a new morphology of graphite was found in the hypoeutectic molten iron that was quite different from flake graphite and spheroidal graphite. The new cast iron was then defined as compacted graphite iron (CGI) due to the tadpole graphite (spheroidal graphite with a small tail) [3]. CGI has good casting process performance, a small tendency of shrinkage cavity and porosity, high mechanical properties, and thermal fatigue properties [4,5,6]. Therefore, the outstanding combination of mechanical properties and low production cost of CGI has made it widely used for producing complicated castings.

The matrix of common CGI consists of pearlite, ferrite, or a mixture of both. In order to modify the microstructure or to improve the mechanical properties of alloy cast iron, it is usually necessary to add several alloying elements to obtain a higher proportion of pearlite. For instance, C is the basic element in CGI, which promotes graphitization, and, like C, Si also plays the same role as a strong graphitizer [7,8,9]. S is usually regarded as an impurity because it has a strong affinity with Mn and Mg, forming stable carbides and, thus, impeding graphitization. Furthermore, S consumes spheroidizing elements in molten iron, resulting in the precipitation of slag inclusions, such as MnS and MgS [10,11]. Mn is a pearlite stabilizer, but excess Mn tends to segregate towards the eutectic cell boundary and generates carbides into an as-cast state. Moreover, the industry always added Mn to control S to avoid the undesirable consequences of elevated S [12,13]. Coincidentally, P is also a harmful element in CGI and needs its content controlled strictly. Due to the limited solubility of its matrix, P is extremely prone to form phosphorus eutectic by pronounced segregation, and the deterioration of the mechanical properties of CGI is noticeably increased [14,15]. In addition, other alloying elements, such as Cu and Ni, increase hardness and strength, while Cr and Mo enhance wear properties [16,17,18,19,20]. A rare earth (RE) element, usually added into molten iron as a vermicular agent, is consumed by forming inclusions, mainly with oxygen and sulfur [21,22]. The remaining RE content in the liquid iron can change the graphite’s shape and strengthen its matrix structure, effectively performing as a microalloying element. In order to ensure the residual amounts of vermicular elements in the molten iron, the actual adding range of the alloying elements is determined according to the temperature of iron adjustment and ladle transfer time. Although the roles of each alloy element in CGI have been reported, a thorough and direct investigation of its element distribution has still not been fully explored because some trace elements, or a small content of the alloying elements, cannot be detected or accurately characterized through traditional analysis methods.

In this paper, the addition of RE is not used as a vermiculizer but as an alloying element to modify CGI. It is commonly acknowledged that the comprehensive properties of CGI depend on the microstructure and morphology of graphite significantly. Some specific factors, such as alloying elements, can influence the graphite’s morphology [4,5]. Previous researchers proposed that graphite was initially precipitated as a sphere and then developed a tail (tadpole graphite) [3]. Large numbers of investigations have been focused on the nucleation and growth of graphite in CGI, while a rare number of studies have been carried out on the distribution of alloying elements among different phases, which is relevant for the variation of its microstructure and corresponding properties. Especially, the solute segregation of RE atoms has not been characterized clearly in CGI alloyed by RE. The present work aims to study further the distribution of various alloying elements among matrix and graphite. In our study, spatially resolved composition measurements were done by means of secondary ion mass spectrometry (SIMS) and a three-dimensional atom probe (3DAP). Graphite and matrix phases were confirmed by scanning electron microscopy (SEM) combined with energy dispersive spectroscopy (EDS). The distinct solubility of alloying elements is explained from the view of different distribution states of the alloying elements. The role of RE, added into CGI as an alloying element, has been illustrated in this paper.

2. Materials and Methods

The melting equipment was a medium-frequency induction furnace, and the melt temperature was controlled in the range of 1480–1490 °C. The molten iron used in our study was inoculated with 75Si-Fe and treated with RE (Ce and La)-Mg-Si-Fe alloy in a ladle. The chemical compositions of the inoculant and vermiculizer are listed in

Table 1. Chemical composition of inoculant and vermiculizer (wt.%).

Element	Si	Al	Ca	Ba	Mg	RE	Fe
Inoculant	68.23	0.93	2.04	4.15	-	-	Bal.
Vermiculizer	45.11	0.74	2.05	-	5.11	4.51	Bal.

. After treatment, the experimental molten iron was poured into a sample cup of 12 kg with the addition of the RE ferrosilicon (20 g, 30.4 wt.% RE, 42.8 wt.% Si, Bal. Fe) alloy for RE alloying. Then, the melting alloys were poured into “Y” molds for casting and solidified into wedge-shaped samples, according to the requirements of GB/T 26655-2011. After cooling for 3 h, test samples were obtained from the shaded areas, as shown in Figure 1.

Table 2. Chemical composition of original and investigated CGI samples (wt.%).

Element	C	Si	Mn	Cu	Cr	P	S	Ce	La	Fe
Investigated CGI	3.37	2.12	0.33	0.51	0.020	0.039	0.020	0.026	0.015	Bal.
Original CGI	3.35	2.04	0.33	0.50	0.025	0.040	0.017	0.008	0.006	Bal.

lists the nominal chemical compositions of investigated CGI, which is very close to that of the original CGI, except that the RE content was added up to 0.041 wt.%.

Metallographic specimens for microstructure observation were ground to 2000 mesh and fine polished. Samples were chemically etched using a solution with 3% nitric acid and alcohol. An optical microscope (OM, Nikon MA100, Tokyo, Japan) and SEM-EDS (Hitachi SU-1500, Tokyo, Japan) were employed to characterize the morphology and composition of the inclusions and graphite. The local distribution of chemical compositions was revealed clearly by SIMS due to its high mass resolution. The primary ion beam, with an energy of 30 keV, was focused on the selected analysis area of 200 μm × 200 μm, and the primary ion current was set to 1pA. The segregation of the alloying elements was confirmed by 3DAP. The 3DAP specimen was cut from the as-cast alloy by using a focused ion beam (FIB, JEOL-JIB-4601F, Tokyo, Japan) to ensure the existence of phase boundaries in the samples. Atom probe analysis was carried out by three-dimensional atomic probe (CAMECA LEAP 3000HR, Gennevilliers, France) with a vacuum pressure of 2.79 × 10^–9 Pa at 20 K, voltage range of 2–3.9 kV, and laser energy of 30 pJ.

3. Results and Discussions

3.1. Microstructure

A common piece of knowledge about the microstructure of conventional CGI is that the matrix is composed of pearlite, ferrite, and vermicular graphite [23]. OM and SEM images of the microstructure of CGI, alloyed by RE, are shown in Figure 2. Compared to the original CGI, the investigated matrix still consists of ferrite and pearlite, with an area percentage exceeding 95%. The kind of graphite is mainly vermicular graphite, which is distributed in the matrix with a random growth direction and a different length and thickness. A small amount of spheroidal graphite, with a size of less than 40 μm, also exists. The vermiculation rate of graphite in the sample can reach more than 80%. After RE alloying, the number of graphite spheres increased slightly, as shown in

Table 3. Comparison of different graphite content (wt.%).

Sample	RE Content/wt.%	Vermicular Graphite/%	Spheroidal Graphite/%
Investigated CGI	0.041	79	21
Original CGI	0.014	88	12

.

3.2. Inclusions

An RE reacts with other alloying elements to form various kinds of inclusions due to its active chemical properties, and the composition, geometry, size, and volume fraction of the inclusions will have a significant influence on their properties [24,25]. The characterization of single inclusions by means of SEM, in combination with EDS, is shown in Figure 3. Four types of inclusions were mainly observed by EDS analysis, including RE sulfide, MgS, carbide, and composite inclusions. In our study, the spheroidal RE sulfide, with a size of about 2–3 μm, strongly partitioned S into its structure, and the partial inclusion was surrounded by As and P (Figure 3a,b). The analysis showed that the inclusion contained a large number of C, Ce, La, and S elements, among which the weight percentages were, respectively, 4.2%, 38.6%, 32.7%, and 24.5%. Excluding the influence of C, the atomic ratio was calculated as follows: (38.6%/140 + 32.7%/139)/(24.5/32) = 0.67. According to the previous study and proportional distribution, RE sulfide is assumed to be made of Ce₂S₃ and La₂S₃ [26,27]. The precipitation of RE sulfide inhibits the formation of blocky MnS due to a more negative standard free energies of formation [27]. Harmful elements can be absorbed by RE and accumulated on the border of RE sulfide to purify grain boundaries or phase boundaries where As and P are supposed to segregate. In addition, RE sulfide exists as a heterogeneous nucleation core and promotes the nucleation rate. The formation of MgS presents as spherical, with a maximum diameter of about 1 μm, as shown in Figure 3c. Mg, as a pretreatment agent, reacts with S to produce MgS, which is insoluble in molten iron. It will float on the top of molten iron but not transfer into slags. Therefore, MgS becomes an impurity and nucleation particle in CGI and presents in the color of dark gray. Blocky carbides containing Ti and V exist in the matrix, with an average size of 3–4 μm (Figure 3d). Although in our study Ti and V were not added into the CGI as alloying elements, raw materials, and especially return scrap, will contain a certain amount of Ti and V that are strongly bonded to C. Mg and RE can both promote the transformation of C from graphite to carbide because supercooling of the ingredient and temperature may occur around the additional small particles containing Mg and RE, which are not completely dissolved or reacted with these additives. The small particles become the potential nucleation cores of blocky carbides [28].

Inclusions with a large contact angle and large size tend to aggregate. Inclusions are prone to grow upwards as a result of element diffusion with an increase in melting time. Similar inclusions form large spherical inclusions, while different inclusions aggregate to form complex compounds with lower energy. In our study, several kinds of composite inclusions were carried out, as shown in Figure 4. The images of MgO surrounded by RE₂S₃ and of RE₂S₃ surrounded by MgO are shown in Figure 4a,b, respectively. MgO and RE₂S₃ grew next to each other in the spherical state and had little difference in shape and size from a single inclusion, such as MgO, MgS, or RE₂S₃. Carbide, accompanied by RE₂S₃, grew upwards into a blocky inclusion (Figure 4c). The size of carbide, with the core of an Al-Mg-Si inclusion, was more than 10 μm and had the shape of an irregular, long strip (Figure 4d). More complicated multiple inclusions, including RE₂S₃, MgO, and carbide, are shown in Figure 4e. The results above fully prove that the originating nuclei are oxides, mainly MgO, on which RE sulfides nucleate and on which Ti carbides nucleate. Although some large-size composite inclusions were retained in the investigated CGI, the formation of composite inclusions with a large size was beneficial to the reduction of energy and the removal of large inclusions through floatation.

The formation mechanism of inclusions is that the inclusion firstly nucleates with heterogeneous points as the core and grows independently. During the growth of inclusions, they attract and get close to each other and eventually merge into large-size inclusions due to the different surface activities of each particle and alloying element. In the process of forming RE inclusions, oxides or sulfides with high melting points are firstly precipitated as nucleation particles. Some RE atoms will be gradually adsorbed to the hardcore surface due to the high activity of RE. In addition, the existence of a chemical potential gradient results in the concentration difference between the nucleation cores and RE atoms around them, which further strengthens the diffusion of the RE elements and facilitates the RE atoms to move towards the nucleation core [29]. RE oxy-sulfides are finally formed through continuous attraction, approaching, and fusion of the RE atoms. Furthermore, the size of inclusions is closely related to the accumulation amount of the RE elements. The more RE elements there are, the more other impurity elements will be adsorbed [30]. Impurity elements tend to concentrate towards grain boundaries, which always leads to the deterioration of the mechanical properties of CGI. The formation of the inclusion adsorbs the harmful elements, changes the alloying element distribution, and, thus, obviously purifies grain boundaries.

3.3. Graphite Nucleation and Growth

It is important to make certain of the distribution of alloying elements in order to understand the nucleation of vermicular graphite during solidification. It has been found that the content of elements such as Mg, Ce, La, and S in the center of graphite is the highest, as shown in Figure 5. According to the EPMA mapping results, heterogeneous nucleation exists in the central region of graphite and graphite flakes grow around the core. A large number of nuclei are formed to precipitate the graphite, and these nuclei are basically MgS, RE₂S₃, or a composite inclusion. According to the previous study, the heterogeneous crystal nucleus is expected not only to be infiltrated by graphite but also to satisfy the relationship of lattice matching, defined as a lattice misfit [31]. Although these inclusion cores offer the nucleation site of graphite, the growth orientation of graphite has not been determined due to the incoherence of the particle cores in graphite [32,33,34]. During the solidification of CGI as a hypereutectic alloy, graphite is the first phase to be solidified, with the nucleation of graphite formed into liquid metals. Then, graphite nodules are formed, surrounded by austenite. As C diffuses from austenite into graphite, austenite transforms into cementite and pearlite at the eutectoid temperature. The addition of a vermicular element can change the surface of the nucleation cores and, thus, alter the growth orientation of graphite. It has been reported that Mg and RE can form stable compounds with surface-active elements (O and S) in a limited range. The graphite grows alternately along the direction of a <100> and c <1000>, and intermediate graphite in a worm-shape between the flake and sphere can be obtained [31,35].

3.4. Distribution of Alloying Elements

In addition to forming inclusions and heterogeneous nucleation, alloying elements are prone to segregate towards interface boundaries. Figure 6 presents the distribution of alloying elemental atoms in the specimens, which were cut from the ferrite/cementite interface.

3.4.1. Distribution of C, Si, and Mn

In our study, the distribution of C could be used to distinguish ferrite (α-Fe) and cementite (Fe₃C). It was found that C and Mn tended to be enriched in cementite, while Si was probably distributed in ferrite. Si increased the proportion of ferrite and reduced the content of cementite through solid solution strengthening in ferrite. Mn had a stronger affinity with C than Fe, which promoted the diffusion and precipitation of C from a solid solution of the matrix.

3.4.2. Distribution of Cr and V

The enrichment of Cr and V was found in the middle of the specimen. The distribution of these two alloying elements was uniform in the cementite phase, as indicated in Figure 6a. The concentration of the Cr and V atoms was 0.7 at.% and 0.4 at.%, respectively, and much larger than that of the matrix, as shown in Figure 6b. Based on the previous study, Cr and V can replace Fe in the form of a solid solution state [36]. Therefore in our study, the addition of alloying elements usually promoted the formation of alloying cementite (Fe, M)₃C (M=Cr, V), by which the stability increased. Because Cr and V can increase the tendency of the cementite formation and delay the precipitation of graphite, they were divided into carbide forming elements.

3.4.3. Distribution of Cu and P

In contrast to Cr and V elements, Cu was prone to be distributed in the ferrite phase. Furthermore, the segregation of Cu atoms was observed to occur at the interface between ferrite and cementite (Figure 6c). The content of Cu in ferrite was more than that in cementite, while large numbers of Cu atoms were found to gather in the local areas, and the atomic concentration (3 at.%) was significantly higher than that in ferrite and cementite. However, the local Cu-rich regions were not distributed along the phase boundaries continuously but formed dispersedly; this is because Cu tended to concentrate at the interface to form isolated Cu atoms or even Cu-rich clusters once exceeding a certain level due to the low solubility of Cu in the matrix. Therefore, the addition of Cu into the molten iron had two main effects: On one hand, Cu-rich in pearlite or Cu-segregation towards the boundaries could refine and stabilize the pearlite microstructure as a stable pearlite element. On the other hand, Cu did not participate in the formation of carbides and was regarded as a graphitization, promoting the element to ensure the casting process performance.

P is a kind of element which is easily concentrated. In our study, the concentration of the P atoms towards the phase boundary was 0.8 at.% with a width of segregation region of about 5 nm, as shown in Figure 6b. The accumulation of the P atoms was found to be continuously distributed at the interface, which is different from the distribution of Cu (Figure 6d). P has low chemical activity and rarely reacts with other elements chemically at the early stage of alloy solidification; thus, a large amount of P remained in the un-solidified liquid. Although the content of P segregation was not high enough to promote the precipitation of phosphorus eutectic, the segregation of the P atoms still reduced the interface binding force between the Fe atoms and increased the interface brittleness.

3.4.4. Distribution of RE Element

In order to study the distribution of RE in more detail, representative two-dimensional ion images gathered by SIMS are summarized in Figure 7. Based on the experimental results, a significant increase in the local concentration of Ce, La, and C is displayed clearly. Many areas containing a high concentration of RE and S were found near the graphite, indicated by the white arrows, and RE inclusions can be confirmed based on the previous study about inclusions in this paper. Besides, an aggregation of Ce and La was detected at the boundaries, including a graphite/pearlite boundary, ferrite/pearlite boundary, and grain boundary. No obvious inclusions were visible in the same region, which indicates that the segregation of RE may exist in the form of atoms towards the grain boundaries. Researchers have confirmed that RE atoms are more likely to occupy the places at the grain/phase boundaries, vacancies, and defects due to the large radius of RE atoms [26,37].

In summary, the addition of RE achieved three goals in CGI alloyed by RE alloying: deoxidization and desulfurization, promoting graphite nucleation, and inhibiting the harmful effects of the interfering elements. Firstly, a small amount of RE in the molten iron quickly interacted with the oxygen and sulfur to form fine sulfides and oxides [22,36]. For another, the formation of RE inclusions provided a large number of heterogeneous nucleation sites for the graphitization of the cast iron, greatly increasing the number of graphite nodules and facilitating the diffusion of the carbon in iron into the graphite nodules. The segregation of RE towards the graphite boundary promoted the growth of vermicular graphite with a small tail. However, access to the RE content worked as an anti-graphitized element because it promoted the formation of carbides [37]. Furthermore, the addition of RE facilitated the formation of RE inclusions by consuming S, P, and As and reduced the negative influence of the impurity elements. In addition to a very small amount of solid solution in the inner grains, the majority of the RE atoms or compounds formed a partial accumulation at the grain or phase boundaries, which reduced the energy of these interfaces and forced the other elements to become stable in the inner grain in the state of a solid solution [22,38]. Moreover, the solute RE atoms enhanced the cohesion of the grain boundary through its segregation. RE, as an interface segregation element, competed with phosphorus for its position on the boundaries and even changed the concentration state of S, P, and As.

4. Conclusions

The understanding of microstructures, including inclusions and graphite, based on the distribution of different alloying elements in the investigated CGI alloyed by RE elements has been deepened by SIMS and 3DAP.

(1)

The addition of RE into CGI formed several kinds of inclusions that were determined as RE(Ce,La)₂S₃, MgS, carbides, and aggregated composite inclusions. These inclusions, containing elements such as S, Ce, La, and Mg, provided heterogeneous nucleation cores for graphite.

(2)

Significant solute clustering in the matrix, coupled with the segregation of the solute to grain boundaries, was observed in CGI alloyed by RE. Cr and V from raw materials promoted cementite precipitation, while a Cu-rich atom cluster was found towards the phase boundary, stabilizing pearlite. P, as a segregation element, was observed to be enriched at the ferrite/ cementite boundaries continuously.

(3)

RE had three main roles in the modified CGI.: RE participated in the formation of relatively regular inclusions containing RE and consumed harmful elements, such as S, As, and P. In addition, the heterogeneous nucleation of the graphite was promoted by RE inclusion nuclei, and the growth of vermicular graphite was deeply altered by the segregation of RE towards the graphite/ferrite or cementite phase boundaries. Moreover, the segregation of RE, in the form of solute atoms towards the grain or phase boundaries, could eliminate the harmful effects of the residual elements, which benefited the strength of the grain boundary.