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Article

Effect of Rare-Earth Ce Addition on Thermal Stability and Corrosion Resistance of Inconel 718 Superalloy

by
Muhammad Shakeel
1,
Tahir Ahmad
1,*,
Muhammad Kamran
1,
Muhammad Abubaker Khan
2 and
Jamieson Brechtl
3
1
Institute of Metallurgy & Materials Engineering, University of the Punjab, Lahore 54590, Pakistan
2
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
3
Oak Ridge National Laboratory, Buildings and Transportation Science Division, Oak Ridge, TN 37830, USA
*
Author to whom correspondence should be addressed.
Metals 2025, 15(10), 1090; https://doi.org/10.3390/met15101090
Submission received: 23 August 2025 / Revised: 19 September 2025 / Accepted: 23 September 2025 / Published: 29 September 2025
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Abstract

Inconel 718, which is a high-performance superalloy, is known for its strength under elevated temperatures and corrosive conditions. However, boosting the durability of this alloy in challenging industrial environments is required. Accordingly, this study investigates the effect of rare-earth Cerium (Ce) additions (0.1–0.4 weight percent (wt.%)) on the microstructure, thermal stability, and corrosion resistance of Inconel 718 superalloy synthesized via powder metallurgy. The alloys were comprehensively characterized following sintering and annealing treatment. The results demonstrate that an addition of 0.2 wt.% Ce is optimal, leading to a refined microstructure and significantly improved high-temperature thermal stability. The 0.2 wt.% Ce alloy demonstrated superior thermal stability, with a lower rate of mass gain observed at temperatures up to 1200 °C. This improvement is attributed to Ce promoting a more stable and protective surface oxide layer. Most notably, the annealed (1000 °C/2 h) 0.2 wt.% Ce alloy exhibited a dramatic enhancement in corrosion resistance, as evidenced by a corrosion current density (Icorr) of 3.690 × 10−6 A/cm2, which is over an order of magnitude lower than the baseline Inconel 718. In summary, this work establishes that a minor 0.2 wt.% Ce modification is an effective strategy for substantially improving the durability of Inconel 718 for demanding applications.

1. Introduction

Superalloys exhibit a great combination of various properties including excellent mechanical strength, resistance to creep, corrosion, and oxidation resistance at high temperatures. They can retain most of their strength even after long exposure to temperatures above 650 °C [1,2]. Nickel based superalloys are important structural materials for many engineering applications [3]. Among the different types of superalloys, Ni-based Inconel 718 is still today’s most widely used and produced alloy in the world. In addition to presenting a remarkable set of properties, this material has the capacity to maintain its high strength while employed in cryogenic to high-temperature applications [4,5]. The structural stability of Inconel 718, which is a fundamentally important property, is observed in the alloy even almost up to its melting point. Its high-temperature strength is mainly achieved through its complex chemical composition and precipitation hardening [6,7], making it suitable for a wide range of applications such as components for liquid-fueled rockets, castings and sheet metal parts for aircraft, and land-based gas turbine engines [8]. Furthermore, various components consisting of Inconel 718 can be manufactured through different routes including casting, powder metallurgy, or thermo-mechanical processing of either cast or sintered preforms to form the wrought materials [9,10].
The recent trend in superalloys is moving towards alloying additions of rare-earth (RE) elements, and research in this field has made important progress over the last few decades [11,12]. The incorporation of REs into alloys are thought to improve their high-temperature oxidation resistance and mechanical properties [13,14]. As the desirable properties of superalloys need to be further improved, especially for high-temperature conditions, the problem is expected to be solved by RE alloying modification [15]. For instance, REs can significantly refine the grain of superalloys by segregating at grain boundaries, which hinders the growth of grain cells. At the same time, they can effectively optimize the precipitation phase of these types of alloys. Furthermore, with only a minute addition, REs can enhance the tensile properties, rupture life, and oxidation resistance of superalloys [16]. Among various rare-earth elements, Y, La, and Ce have seen widespread adoption, while their oxides can also be used to minimize the oxidation rate of superalloys [16,17,18]. Following the incorporation of various REs, Inconel 718 benefits from a range of strengthening mechanisms, including solution strengthening, precipitation hardening, and dispersion of second-phase particles [19]. Luis et al. [20] worked on various RE additives in Inconel 718 and reported a small enhancement in the tensile and hardness values. The enhanced mechanical properties observed in the alloy, in as-aged conditions, were attributed to strengthening by the precipitation of γ′ and γ″ phases. However, the results of impact testing showed that no significant change occurred during the aging process. Chuanyong et al. [21] reported the effects of Ce addition on Ni-Co-based superalloys, where it was found that an increase in the Ce content resulted in an increase in the volume fraction of the (Ni, Co)3Ce phase. It was also found that the rupture life of these alloys improved significantly with optimal Ce addition. Lirong et al. [22] reported the effect of adding Ce to steel, which was found to improve the oxidation resistance of the material. Furthermore, Ce additions resulted in enhanced surface stability by reducing the harmful impact of elements, such as S and P, on grain boundaries. But its effect on the mechanical properties and microstructure of Ni-based cast superalloys is yet to be judged.
In light of various reported works, the role of Ce in superalloys is complex and currently not well understood. Therefore, it is not fully known how Ce will impact the microstructural or mechanical properties of superalloys. It is, therefore, necessary and important to investigate the effect of Ce on the various properties of nickel-based superalloys. In this paper, the effect of Ce content (wt.%) on the microstructure, mechanical properties, and corrosion behavior of IN 718 is investigated as an attempt to find the optimum percentage of Ce in these newly developed alloys.

2. Experimental Procedure

Inconel 718 nickel-based superalloy (IN 718) samples, with five different Ce compositions, were fabricated using 99.9% pure elemental powders (Shanghai Macklin Biochemical, Shanghai, China). The alloy designation and chemical composition are listed in Table 1, while chemical analysis of the developed IN 718 superalloys with rare-earth metal Ce addition is shown in Table S1. XRF analysis showed the presence of each element in the alloy which confirmed the good dispersion and homogeneities of the developed IN 718 superalloys with the rare earth Ce. All of the constituent powders were sieved through a 400 mesh size to achieve a similar particle size range. The sieved constituents were weighed to an accuracy of 0.001 gm by using an electric weight balance with respect to the calculated amounts with the mass of each element. All of the weighed constituents were mixed in a ball mill, and to achieve a proper homogenized mixture the mill was used for three hours at 100 rpm.
All of the developed compositions were compacted at 50 MPa via an auto-pellet machine. The green compacts were sintered at 1000 °C for one hour in a three-zone vacuum tube furnace (Nabertherm RS 50/500/A3, Nabertherm, Lilienthal, Germany) under an argon atmosphere. Another batch of the same composition was sintered and then annealed, under an argon atmosphere, at 1000 °C for two hours by using the same tube furnace.
All of the sintered and annealed specimens were hot mounted and then polished. For the optical microscopy (OM) characterization, an inverted Leco LX-31 metallurgical microscope (LECO Corporation, St. Joseph, MI, USA) was used. The microstructure and compositional characterization were performed using an FEI Inspect S50 USA scanning electron microscope (SEM) (FEI Company, Hillsboro, OR, USA) with energy dispersive spectroscopy (EDX). Different phases of the developed alloys were identified using a Bruker D8 X-ray Diffraction (XRD) machine (Bruker AXS, Karlsruhe, Germany) with a CuKα X-ray source. Differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of the alloys were performed using a Linseis STA PT1600 (Linseis Messgeräte, Selb, Germany) at 1200 °C under normal atmosphere. The hardness of these developed alloys was assessed using a Leco LM700AT microhardness tester (LECO Corporation, St. Joseph, MI, USA). The room temperature corrosion behavior of wire cut (10 × 10 mm), sintered, and annealed samples in 3.5% NaCl solution was measured using a Gamry Potentiostat 1000E (Gamry Instruments, Warminster, PA, USA) with a graphite counter electrode and silver–silver chloride reference electrode for a scan rate of 5 mVs−1.

3. Results and Discussion

OM images of the sintered and annealed IN 718 superalloys are shown in Figure 1 and Figure 2, which shows irregular and rounded morphology phases within the austenitic matrix phase. Figure 1a and Figure 2a show the microstructure of baseline IN 718 after sintering and annealing, respectively, observed with mostly fine equiaxed grains in annealed condition. Figure 1a,b and Figure 2a,b show a matrix gamma phase which contains finely dispersed gamma prime particles that strengthen it. The other secondary phases also include different types of metallic carbides [23], which is also confirmed by the results of the EDX analysis. In these types of superalloys, carbides are typically located within the grain and at the grain boundaries, and are responsible for matrix strengthening and grain size control. Luis et al. [20] reported that these types of microstructures, which give a high resolution of the matrix, produce precipitated phases. For instance, these structures were found to cause segregation of Cr and C to the grain boundaries, which led to the formation of the M23C6 phase there. These carbides are important as they help to inhibit grain displacement during creep.
SEM images of the sintered and annealed alloys are shown in Figure 3 and Figure 4, respectively. These figures cover only IN-718 and IN-0.2 wt% Ce alloys, while SEM images of the remaining alloys are shown in Figures S1 and S2. Apparently, two types of phases (γ, γ′) can be distinguished, i.e., round-shaped and irregular-shaped, in Figure 3a,b. As observed in Figure 4a,b, among all experimental compositions in sintered as well as in annealed conditions, two distinct features can be seen, namely, minimal porosity (best sinterability) and the finest second phases. These findings indicate that sintering of the 0.2 wt.% Ce in the IN 718 sample, under the given conditions, led to adequate grain refinement and a uniform second-phase distribution with optimum sinterability. Annealing also reduced the porosity of the sample, as shown in Figure 4a,b. This refinement and homogenization of the microstructure, coupled with reduced porosity, is critical for improving performance. A finer grain structure enhances strength by increasing the number of grain boundaries that impede dislocation motion, while the uniform distribution of secondary phases prevents the formation of localized stress concentrations. These grains actually refer to the γ matrix grains, in which the Ce addition helps to further to refine them by segregating at the GBs, leading to the restriction of grain growth during sintering and annealing. This refinement not only improves mechanical stability but also enhances corrosion resistance by reducing paths for localized attack. Furthermore, the reduction in porosity is directly responsible for the superior corrosion resistance observed later, as pores can act as initiation sites for corrosive attack.
The EDX analysis, as shown in Figure 5, confirms the presence of different second phases, metallic carbides, and a Ce dispersed intermetallic phase in the matrix. Figure 5a,a1 show the presence of a γ matrix phase, whereas other secondary phases, including γ′ and γ″, can be observed in Figure 5b,b1, respectively. The presence of Nb and Ti in the pattern shown in Figure 5b1 further corroborated that these secondary phases were present in the developed IN 718 superalloys. Different metallic carbides can be observed in Figure 5c,c1, while the Ce-based intermetallic phase within the matrix is shown in Figure 5d,d1. Figure 5c,c1 also indicate that Mo and Cr carbide phases are present in the sample, as indicated by the results of the EDX point analysis. Similar to Figure 5b,b1, the EDX point analysis results from Figure 5d,d1 reveal the presence of Nb in the sample, which also confirmed the formation of the γ″ phase in the sample. These figures also show the dispersion of Ce in the matrix of the alloy.
The results of XRD analysis for the sintered and annealed alloys are given in Figure 6 and Figure 7, respectively. Table 2 displays the 2θ angles for the various observed phases. The results shown in the figures reveal the existence of different diffusional phases, which implies that in addition to minimizing porosity, the given sintering time and temperature allowed diffusion in the pellets. In general, the comparison of the diffraction patterns of the sintered and annealed alloys reveals that in terms of composition, there were no significant differences except for the presence of Ce-Ni phases. The formation of Ce-Ni phases indicates the diffusion of Ce in the Ni-rich phase. Strong diffraction peaks corresponding to γ″ Ni3Nb, γ′ Ni3(Al, Ti), and γ (Ni-Cr-Fe) phases are observed. In addition to the detection of all major phases, peaks of very low intensity corresponding to minor microstructural constituents are also observed, with some diffraction spectra coinciding (or overlapping) with each other. Ce addition promotes the formation of intergranular phases, such as CeNi3 and Ce2Ni7, that can introduce additional diffraction peaks into XRD patterns. Moreover, the addition of Ce can also causes a minor shift in peak positions due to minimal changes in the lattice parameters. Also, broadening in some peaks may occur due to the presence of mechanical strain or crystalline defects (by Ce addition) as the larger atomic radius of Ce, when solubilized in the nickel matrix phase, causes lattice distortion in the alloy [24].
A comparison of the Vickers hardness values, for the sintered and annealed alloys, is provided in Figure 8. The obtained results indicate that the Ce-modified sintered alloys are harder than the baseline IN 718. At the same time, the Ce-modified annealed alloys possess a greater hardness value as compared to their sintered counterparts. It is noticeable that after annealing, as compared to sintered alloys, the hardness value of all developed alloys has increased. Overall lower hardness is observed in all Ce-modified alloys (lowest in case of 0.2 wt.% Ce) when compared to the annealed baseline IN 718, while after annealing the baseline IN 718 alloy reaches values nearly equal to its reported hardness values [25]. Considering the hardness values of these alloys, one can find that IN-0.2 wt.% Ce possesses the lowest hardness value. Hence, it can be anticipated that the IN-0.2 wt.% Ce sample might have the highest ductility and toughness among all experimental compositions. Juan et al. [26] reported in their work that the microhardness of the Al-Cu-Fe-Ce alloy, with different Ce additions, gives first an increasing and later a decreasing trend.
The results of the DTA analysis, for the sintered and annealed alloys, are shown in Figure 9 and Figure 10, respectively. The different exothermic and endothermic peaks of the Ce-modified sintered and annealed samples corresponded to different reactions and precipitations during solidification. For instance, the peak observed at around 270 °C corresponded to the precipitation of the γ′ phase. Similarly, two other peaks were observed at 600 °C and 700 °C, which indicated the formation of the γ″ phases. The peak seen around 915 °C corresponded to the formation of the Ni3Ce compound, while for the annealed condition the formation of Ce2Ni7 was observed at around 1030 °C. Masoumi et al. [27] reported similar DTA results in which various endothermic peaks below 1000 °C could be observed. Similarly, other peaks were also identified at 1160 °C and, taking into account the shape of the peak, all witnessed thermal effects reflect the characteristics for melting.
The results of the TGA characterization, for the same developed alloys, can be observed in Figure 11 and Figure 12, respectively. The results revealed that when exposed to elevated temperatures in an oxidative environment, the alloys formed various protective oxide layers that resulted in an increase in mass. Furthermore, it was determined that the growth of these stable oxides led to a continuous weight gain and that the oxidation became more intense at 700 °C for both the sintered and annealed alloys. In the case of the sintered alloys, no difference in thermal stability was observed as the baseline and modified alloys exhibited almost the same weight gain (Figure 11), while in the case of the annealed alloys (Figure 12), a significant change was observed as Ce enhanced the formation of stable oxide layers (e.g., Ce oxide (CeO2)) that provide effective protection against oxidation. During high-temperature exposure, 0.2 wt.% Ce addition (in the annealed condition) resulted in better resistance and higher thermal stability, with lower observed rates of mass change up to a maximum temperature of 1200 °C. The enhanced thermal stability of the IN-0.2Ce sample, as evidenced by its lower mass gain during TGA (Figure 12), was attributed to the ‘reactive element effect’ of Ce. At elevated temperatures, it was thought that Ce promoted the formation of a more adherent and stable oxide layer that was likely composed of a dense CeO2 or a mixed (Ni, Cr, Ce)O scale. This layer, which acted as a more effective diffusion barrier against oxygen ingress as compared to the layer formed on the baseline alloy, was responsible for slowing the rate of oxidation and improving the material’s high-temperature performance. Migas et al. [28] reported the results of TGA of Co-Al-W alloys and found that in the as-cast state, Ce segregated to interdendritic regions where it combined with other elements to form intermetallic phases. Interdendritic precipitate melting was observed at 1160 °C, and Ce-modified alloys were found to be less prone to oxide spallation. Similarly, Alena et al. [29] reported that a very small amount of Ce addition led to the thermal stabilization of the alloy at elevated temperatures of about 400 °C. In this particular investigation, weight gain was observed in both cases of sintered and alloys with a noticeable difference that was primarily attributed to oxidation processes.
Nickel-based superalloys are attributable to the formation of protectively passive films; higher passive current density enhances difficulties for passive film formation, which results in poorer corrosion resistance [30]. Electrochemical measurements of the developed compositions are shown in Figure 13, Figure 14, Figure 15 and Figure 16. Both techniques of electrochemical measurements, including PD polarization and EIS, were performed using a three-electrode cell setup comprising Ag/AgCl as the reference electrode, graphite as the counter electrode, and the IN 718 developed alloys, each with an area of 10 mm2, as the working electrode [31,32]. Figure 13 and Figure 14 illustrate the potentiodynamic curves of sintered and annealed specimens in 3.5% NaCl solution. The corrosion potential (Ecorr) indicates corrosion tendency only, while the corrosion current density (Icorr) reflects the corrosion resistance directly. A higher value of Icorr corresponds to a faster corrosion rate [33]. As can be seen in this investigation, the percentage of rare-earth elements affected the material performance. To evaluate the corrosion behavior of the specimens, the electrochemical parameters Vf, Ecorr, Icorr, and corrosion rate were determined from polarization plots. The electrochemical characteristics of both types of alloys, as obtained from the open circuit potential and potentiodynamic polarization tests, are given in Table 3 and Table 4.
In the as-sintered alloys, the range of Icorr was from 12.30 × 10−6 to 103 × 10−6, among which the lowest corrosion current density was exhibited by 0.2 wt.% Ce. The highest Icorr value for the baseline IN 718, which was directly proportional to the corrosion rate, confirmed that the 0.2 wt.% Ce-containing alloys possessed the highest corrosion resistance. On the other hand, the baseline IN 718 showed the least corrosion resistance. Compared with the annealed alloys, the range of the Icorr was from 3.690 × 10−6 to 65 × 10−6, which was reduced compared to as-sintered alloys—indicating that the annealing treatment increased the overall corrosion resistance of the IN 718 superalloy. In comparison with the wt.% of Ce, the annealed samples exhibited the same trend as the as-sintered samples. Furthermore, the base material (IN 718 superalloy) possessed the least corrosion resistance while IN 718 with 0.2 wt.% Ce exhibited the highest corrosion resistance. It was also observed that as compared to the as-sintered specimens, the Icorr of the annealed specimens was lower, which indicated that the corrosion resistance of the annealed specimens was significantly higher than that of the as-sintered specimens in 3.5% NaCl. In summary, the IN 718 superalloy with 0.2 wt.% Ce had the best corrosion resistance in both cases.
The EIS Nyquist curves for the sintered and annealed compositions are shown in Figure 15 and Figure 16, respectively. The equivalent circuit model and the impedance parameters obtained after fitting Nyquist curves are shown in Figure 17 and Table 5 and Table 6. Each element of the EEC model represents the electrochemical variants where ‘Rs’ is the solution resistance between the reference electrode (Ag/AgCl) and working electrode, ‘Rct’ is the charge transfer resistance, and ‘Yo’ and ‘n’ represent the magnitude and micro-fluctuation on the surface, respectively, and collectively characterized as capacitance ‘Q’. When compared to PD polarization curves, EIS always uses a small excitation amplitude, which causes a slight perturbation to the corrosion potential. This technique offers a unique approach to directly investigate the passive film properties and estimate the corrosion resistance behavior of different materials [34,35]. For this purpose, EIS measurements were performed on both sintered and annealed specimens in 3.5% NaCl at room temperature. As can be observed in Figure 15 and Figure 16, the Nyquist plots exhibited incomplete semicircular arcs due to charge transfer processes at the electrolyte/electrode interface [36]. With respect to the capacitance loop, a larger diameter corresponds to better corrosion resistance [33]. Analysis of both figures indicates that the diameter of the semicircular arc of IN 718 with 0.2 wt.% Ce, in both the as-sintered and annealed conditions, was the largest. The potentiodynamic polarization and EIS results are therefore in strong agreement, reinforcing the conclusion that the IN-0.2Ce annealed sample offers superior corrosion protection. In addition, the largest semicircular arc diameter in its Nyquist plot corresponds to the lowest observed corrosion current density (Icorr = 3.690 × 10−6 A/cm2), which signifies a high charge transfer resistance at the electrode/electrolyte interface. Thus, a more stable and protective passive film formed on the surface of the 0.2 wt.% Ce alloy that effectively stifled the corrosion process. This result implies that the corrosion resistance of both the sintered and annealed IN 718 with 0.2 wt.% was highest among all other compositions, which was in good agreement with the potentiodynamic polarization results. The corrosion mechanism reported here relies on the occurrence and stability of the passive oxide film. In the unaltered IN 718 alloy, chloride ions adhere to active sites of the passive layer, resulting in film degradation and pitting corrosion. However, Ce addition dramatically alters this behavior. RE elements like Ce exhibit a pronounced affinity for oxygen and preferentially accumulate near active corrosion sites, facilitating the development of stable oxides such as CeO2 and Ce-Cr mixed oxides. These phases occupy flaws and micro-voids in the passive film, improving its density and adhesion to the substrate. Consequently, chloride ion penetration is inhibited, pit instigation is delayed, and systemic corrosion resistance is enhanced. This impact is most significant in the IN-0.2Ce alloy, which exhibits the greatest charge transfer resistance in the EIS results, signifying the existence of a highly protective passive film. These findings align with prior research regarding the advantageous effect of Ce on enhancing the corrosion resistance of Ni- and Fe-based alloys. It is further elaborated that the confirmation of corrosion products, including CeO2 and Ce-Cr mixed oxides, in modified Ce alloys is based on the described mechanisms in various reported literature and the electrochemical trends observed in the current investigation. Since the characterization, i.e., SEM/EDS and XRD, of corrosion products is not performed, the presence of these phases cannot be experimentally confirmed in the present study. Therefore, to verify the composition and structure of the passive films formed after electrochemical testing, future work will focus on direct analysis of the corroded alloy surface [37,38].

4. Conclusions

  • This investigation determined that an addition of 0.2 wt.% Ce is the optimal composition for enhancing the properties of the Inconel 718 superalloy when processed via powder metallurgy and subsequent annealing.
  • The addition of Ce led to the formation of homogeneously distributed Ce-Ni intermetallic phases (Ni3Ce and Ce2Ni7) within the alloy matrix.
  • The 0.2 wt.% Ce alloy demonstrated superior thermal stability, with a lower rate of mass gain observed at temperatures up to 1200 °C. This improvement was attributed to Ce promoting a more stable and protective surface oxide layer.
  • Electrochemical analysis results conclusively showed the enhanced corrosion resistance of the optimized alloy. The IN-0.2Ce sample exhibited the lowest corrosion current density (Icorr = 3.690 × 10−6 A/cm2), which corresponded to the best performance in a 3.5% NaCl solution.
  • These findings present a practical method for improving the durability of Inconel 718 in demanding industrial environments. It is recommended that future research focuses on evaluating the creep and fatigue resistance of the IN-0.2Ce alloy to fully validate its potential for high-performance applications.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/met15101090/s1, Figure S1: SEM micrographs of sintered alloy; (a) IN-0.1 wt.% Ce, (b) IN-0.3 wt.% Ce, (c) IN-0.4 wt.% Ce; Figure S2: SEM micrographs of annealed alloy; (a) IN-0.1 wt.% Ce, (b) IN-0.3 wt.% Ce, (c) IN-0.4 wt.% Ce; Table S1: Chemical composition (wt.%) of the developed alloys.

Author Contributions

M.S.: Conceptualization, investigation, formal analysis, writing—review. T.A.: Supervision, conceptualization, writing—review and editing. M.K.: Supervision, conceptualization, writing—review and editing. M.A.K.: Conceptualization, investigation, analysis, writing—review and editing. J.B.: Writing—review and editing, validation, formal analysis, data curation. All authors have read and agreed to the published version of the manuscript.

Funding

This research is supported by University of the Punjab Lahore research grant.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to [privacy].

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. OM images of the sintered alloy microstructures (a) IN 718 and (b) IN-0.2 wt.% Ce, showing the distribution of γ′, and different metallic carbides, i.e., Nb-, Mo-, and Cr-rich carbides in the matrix phase.
Figure 1. OM images of the sintered alloy microstructures (a) IN 718 and (b) IN-0.2 wt.% Ce, showing the distribution of γ′, and different metallic carbides, i.e., Nb-, Mo-, and Cr-rich carbides in the matrix phase.
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Figure 2. Optical microstructures of annealed alloys (a) IN 718, (b) IN-0.2 wt.% Ce, showing the distribution of γ′, and different metallic carbides, i.e., Nb-, Mo-, and Cr-rich carbides in the matrix phase.
Figure 2. Optical microstructures of annealed alloys (a) IN 718, (b) IN-0.2 wt.% Ce, showing the distribution of γ′, and different metallic carbides, i.e., Nb-, Mo-, and Cr-rich carbides in the matrix phase.
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Figure 3. SEM micrographs of the sintered alloys (a) IN 718 and (b) IN-0.2 wt.% Ce, with the distribution of γ′ into the matrix phase along with some of the metallic carbides, i.e., Nb- and Cr-rich carbides.
Figure 3. SEM micrographs of the sintered alloys (a) IN 718 and (b) IN-0.2 wt.% Ce, with the distribution of γ′ into the matrix phase along with some of the metallic carbides, i.e., Nb- and Cr-rich carbides.
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Figure 4. SEM micrographs of annealed alloy: (a) IN 718, (b) IN-0.2 wt.% Ce.
Figure 4. SEM micrographs of annealed alloy: (a) IN 718, (b) IN-0.2 wt.% Ce.
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Figure 5. (a,a1) IN-0.3 wt.% Ce, EDX analysis of experimental alloys; (b,b1) IN-0.4 wt.% Ce, formation of γ′ and γ″ phases; (c,c1) IN-0.4 wt.% Ce, formation of carbide phases; (d,d1) IN-0.2 wt.% Ce, presence of Ce in the alloy matrix.
Figure 5. (a,a1) IN-0.3 wt.% Ce, EDX analysis of experimental alloys; (b,b1) IN-0.4 wt.% Ce, formation of γ′ and γ″ phases; (c,c1) IN-0.4 wt.% Ce, formation of carbide phases; (d,d1) IN-0.2 wt.% Ce, presence of Ce in the alloy matrix.
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Figure 6. XRD diffraction patterns of the sintered alloys.
Figure 6. XRD diffraction patterns of the sintered alloys.
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Figure 7. XRD diffraction patterns of the annealed alloys.
Figure 7. XRD diffraction patterns of the annealed alloys.
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Figure 8. Comparison of the Vickers microhardness of the sintered and annealed alloys.
Figure 8. Comparison of the Vickers microhardness of the sintered and annealed alloys.
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Figure 9. DTA results for the sintered alloys.
Figure 9. DTA results for the sintered alloys.
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Figure 10. DTA results for the annealed alloys.
Figure 10. DTA results for the annealed alloys.
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Figure 11. Thermogravimetric (TG) analysis of sintered alloys.
Figure 11. Thermogravimetric (TG) analysis of sintered alloys.
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Figure 12. The results of TGA characterization of the annealed alloys.
Figure 12. The results of TGA characterization of the annealed alloys.
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Figure 13. Potentiodynamic polarization curves of the sintered alloys.
Figure 13. Potentiodynamic polarization curves of the sintered alloys.
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Figure 14. Potentiodynamic polarization curves of the annealed alloys.
Figure 14. Potentiodynamic polarization curves of the annealed alloys.
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Figure 15. EIS Nyquist curves of the sintered alloys.
Figure 15. EIS Nyquist curves of the sintered alloys.
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Figure 16. EIS Nyquist curves of the annealed alloys.
Figure 16. EIS Nyquist curves of the annealed alloys.
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Figure 17. Equivalent Electrical Circuit (EEC) models were used to simulate the impedance spectrum of the samples.
Figure 17. Equivalent Electrical Circuit (EEC) models were used to simulate the impedance spectrum of the samples.
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Table 1. Chemical composition (wt.%) and designation of the developed alloys.
Table 1. Chemical composition (wt.%) and designation of the developed alloys.
AlloyElements
NiCrMoNbTiAlCuCCeFe
IN 71852.51935.10.90.50.150.08-18.77
IN-0.1Ce52.51935.10.90.50.150.080.118.67
IN-0.2Ce52.51935.10.90.50.150.080.218.57
IN-0.3Ce52.51935.10.90.50.150.080.318.47
IN-0.4Ce52.51935.10.90.50.150.080.418.37
Table 2. Phases determined from the XRD analysis of the sintered and annealed alloys.
Table 2. Phases determined from the XRD analysis of the sintered and annealed alloys.
Phases2θ
γ44.3, 51.3, 75.5, 92.0
γ′44.3, 51.3, 75.5, 92.0
γ″44.5, 51.5, 75.6, 92.1
MC35.0, 75.2
CeNi334.2, 36.2, 45.1
Ce2Ni736.2
Table 3. Electrochemical characteristics of the sintered specimens obtained from OCP and the potentiodynamic polarization.
Table 3. Electrochemical characteristics of the sintered specimens obtained from OCP and the potentiodynamic polarization.
SamplesVf
(mV)		Ecorr
(mV)		Icorr
(A/cm2)		Corrosion Rate
(mpy)
IN 718−178.0−198.0103 × 10−642.49
IN-0.1Ce255.0243.022.60 × 10−69.298
IN-0.2Ce−340.0−358.012.30 × 10−65.056
IN-0.3Ce−315.0−329.041.80 × 10−617.25
IN-0.4Ce−291.0−306.012.40 × 10−65.098
Table 4. Electrochemical characteristics of the annealed specimens obtained from OCP and the potentiodynamic polarization.
Table 4. Electrochemical characteristics of the annealed specimens obtained from OCP and the potentiodynamic polarization.
SamplesVf
(mV)		Ecorr
(mV)		Icorr
(A/cm2)		Corrosion Rate
(mpy)
IN 718−220.0−234.065 × 10−626.74
IN-0.1Ce−175.0−189.014.10 × 10−65.785
IN-0.2Ce−170.0−186.03.690 × 10−61.521
IN-0.3Ce−245.0−256.012.50 × 10−65.152
IN-0.4Ce−390.0−408.028.70 × 10−611.85
Table 5. Impedance parameters of sintered specimens obtained from the Nyquist plot fitting.
Table 5. Impedance parameters of sintered specimens obtained from the Nyquist plot fitting.
SamplesRs
(Ω.cm2)		Rct
(Ω.cm2)		Yo
(µS.secn/cm2)		N
IN 718298.331.028.647 × 10−40.3446
IN-0.1Ce23.11626.44.594 × 10−50.7402
IN-0.2Ce3.6912,5502.978 × 10−61
IN-0.3Ce168.8144.37.64 × 10−50.5988
IN-0.4Ce7.386822.51.294 × 10−50.7733
Table 6. Impedance parameters of annealed specimens obtained from the Nyquist plot fitting.
Table 6. Impedance parameters of annealed specimens obtained from the Nyquist plot fitting.
SamplesRs
(Ω.cm2)		Rct
(Ω.cm2)		Yo
(µS.secn/cm2)		N
IN 71864.8521.77.744 × 10−40.3508
IN-0.1Ce6.343543.83.149 × 10−40.6222
IN-0.2Ce4.81247958.27 × 10−50.8524
IN-0.3Ce5.17437511.136 × 10−40.7014
IN-0.4Ce5.263670.22.124 × 10−40.6976
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Shakeel, M.; Ahmad, T.; Kamran, M.; Khan, M.A.; Brechtl, J. Effect of Rare-Earth Ce Addition on Thermal Stability and Corrosion Resistance of Inconel 718 Superalloy. Metals 2025, 15, 1090. https://doi.org/10.3390/met15101090

AMA Style

Shakeel M, Ahmad T, Kamran M, Khan MA, Brechtl J. Effect of Rare-Earth Ce Addition on Thermal Stability and Corrosion Resistance of Inconel 718 Superalloy. Metals. 2025; 15(10):1090. https://doi.org/10.3390/met15101090

Chicago/Turabian Style

Shakeel, Muhammad, Tahir Ahmad, Muhammad Kamran, Muhammad Abubaker Khan, and Jamieson Brechtl. 2025. "Effect of Rare-Earth Ce Addition on Thermal Stability and Corrosion Resistance of Inconel 718 Superalloy" Metals 15, no. 10: 1090. https://doi.org/10.3390/met15101090

APA Style

Shakeel, M., Ahmad, T., Kamran, M., Khan, M. A., & Brechtl, J. (2025). Effect of Rare-Earth Ce Addition on Thermal Stability and Corrosion Resistance of Inconel 718 Superalloy. Metals, 15(10), 1090. https://doi.org/10.3390/met15101090

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