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Article

The Effect of Long-Term Aging on the Mechanical Properties of Corrosion-Resistant Nickel-Based Alloys for Their Application in Nuclear Technologies

by
Alfiya F. Gibadullina
1,
Vladislav A. Khotinov
2,
Maxim S. Karabanalov
2 and
Ilya B. Polovov
1,*
1
Department of Rare Metals and Nanomaterials, Ural Federal University, 620062 Ekaterinburg, Russia
2
Department of Heat Treatment and Physics of Metals, Ural Federal University, 620062 Ekaterinburg, Russia
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(11), 6133; https://doi.org/10.3390/app15116133
Submission received: 21 March 2025 / Revised: 22 May 2025 / Accepted: 23 May 2025 / Published: 29 May 2025
(This article belongs to the Special Issue Experimental and Theoretical Studies on the Physical Properties of Materials for Nuclear Engineering)
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Abstract

:
The short-term mechanical properties of commercial corrosion-resistant nickel alloys based on Ni-Cr (Hastelloy® G-35® or UNS N06035), Ni-Mo (Hastelloy® B-3® or UNS N10675), and Ni-Cr-Mo (VDM® Alloy C-4 or UNS N06455, VDM® Alloy 59 or UNS N06059, and KhN62M-VI) systems were analyzed in the as-received state and after long-term (up to 5000 h) aging at 500–700 °C. All alloys exhibited moderate strength and high ductility in the as-received state. Under the influence of high temperatures, these alloys showed a tendency toward the decomposition of Ni-based FCC solid solutions and a change in mechanical properties. It was shown that the difference in chromium and molybdenum content in Ni-Cr-Mo alloys leads to the formation of secondary phases of various composition and morphology, which had varied influence on the short-term mechanical properties of the materials. Grain boundary precipitates had a negligible effect on the strength properties of the investigated alloys, while intragranular precipitates embrittled nickel-based alloys, reducing their possible application at high temperatures.

1. Introduction

Molten salt reactors (MSRs) are fourth-generation nuclear power systems where halide-based salts can be used as both coolants and nuclear fuel. The practical realization of MSRs requires core and structural materials that exhibit high corrosion and radiation resistance, mechanical stability, and manufacturability for long-term operation in a high-temperature molten salt environment [1,2,3,4].
Nickel-based alloys are promising construction materials due to their corrosion resistance in molten chloride- and fluoride-based media [5,6,7,8]. However, the investigation of properties of nickel-based alloys solely in their as-received state is insufficient to predict their service life in MSRs, because prolonged exposure under real operating conditions at elevated temperatures and in aggressive environments leads to structural and phase transformations in these alloys, inevitably altering their corrosion and mechanical properties [9,10,11]. The precipitation of secondary phases along the grain boundaries can significantly influence the corrosion resistance and increase susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) [12,13,14,15]. High capacity for work hardening is reflected by the tensile curves, where an extended uniform deformation stage and low ratio of the yield strength to the tensile strength are demonstrated [9,14]. The processes occurring during the aging of nickel-based alloys (depletion of the FCC solid solution, precipitation of secondary-phase particles, their growth, and ordering) result in dispersion hardening. The extent of dispersion hardening is determined by the density of the particles, their size, and the degree of coherence with the matrix. As a result, dispersion hardening leads to the degradation of tensile properties, increasing yield strength and the ratio of the yield strength to the tensile strength. Thus, these mechanical properties can be used for assessing the alloy’s embrittlement during aging [16,17,18,19].
The investigation of precipitation of secondary phases and the determination of their impact on the mechanical properties of nickel-based alloys during long-term thermal exposure is therefore an important task. This study is focused on the effect of prolonged high-temperature aging (up to 5000 h) of Ni-Cr, Ni-Mo, and Ni-Cr-Mo alloys on their short-term mechanical properties (yield strength, YS; tensile strength, TS; relative elongation, El; the ratio of the yield strength to the tensile strength, YS/TS) and their dependence on the type and morphology of secondary-phase precipitates.
All alloys considered in the present study are nickel-based materials that vary in molybdenum and chromium content, as well as in their ratio, which determines different structural and operational properties. The ternary Ni-Cr-Mo system has been widely recognized as a base for the development of a variety of corrosion-resistant alloys [20,21]. Historically, these materials were classified into three main families, i.e., B, C, and G.
The B-family of nickel alloys with 26–30 wt.% molybdenum as a primary alloying element is represented in this study by Hastelloy® B-3®. This alloy is an improved version of Hastelloy® B and B-2. The B-series alloys were specifically developed for environments containing reducing agents such as hydrochloric acid. Hastelloy® B-3® was also designed for the use in hydrochloric acid environments at various concentrations and temperatures, offering corrosion resistance comparable to that of more expensive materials like tantalum and zirconium [22]. Unlike its predecessors, it also exhibits improved thermal stability, which allows eliminating post-weld annealing. Despite their excellent corrosion resistance, Ni-Mo-based alloys are thermally unstable at elevated temperatures and are subjected to the formation of ordered phases such as Ni₄Mo (D1a) and Ni₃Mo (DO₂₂). These phases significantly reduce the ductility of the material, and this phenomenon is described in detail in a number of works [23,24,25,26,27,28].
The C-family of Ni-Cr-Mo alloys used in this study includes three materials, i.e., the newly developed corrosion-resistant alloy KhN62M-VI, created in collaboration with Russian researchers from UrFU, NIIKhIMMASH, and TsNIIChERMET [29], as well as two corrosion-resistant alloys from VDM Metals—VDM® Alloy C-4 [30,31,32,33], an analog of Hastelloy® C-4, and VDM® Alloy 59, which offers greater phase stability [34,35,36,37], even after prolonged exposure (up to 20,000 h) at 200–427 °C [38]. The selected alloys are characterized by low carbon and silicon content, the absence of tungsten, and reduced iron content. During long-term high-temperature aging, Ni-Cr-Mo alloys can form both ordered phases (Pt₂Mo-type) and topologically close-packed (TCP) phases, such as σ, P, and μ [11]. These phenomena depend on the chromium and molybdenum content in the materials. The indicated phases influence mechanical and corrosion properties of the alloys, often reducing ductility and increasing brittleness, thus limiting their application as structural materials for molten salt reactor applications. For example, both a σ-phase and a P-phase were identified in VDM® Alloy C-4 [39]. The presence of these phases in the face-centered cubic nickel-based matrix has a significant impact on the material’s properties, resulting in strengthening, while simultaneously increasing the risk of embrittlement.
Hastelloy® G-35® alloy represents the fourth generation of the G-family of nickel alloys. It differs from its predecessors by reduced iron content, which significantly affects the phase composition. This alloy is primarily used in wet-process phosphoric acid production [40,41] and fertilizer manufacturing, and it is also found applications in ore processing [20]. Two types of precipitates can be formed in a wide temperature range (550–1000 °C), i.e., a σ-phase with a characteristic lamellar morphology appears along the grain boundaries at higher temperatures; and chromium-rich (α-Cr) precipitates are formed independently at relatively low temperatures [42].
All G- and C-family alloys selected for investigation exhibit a tendency toward ordering phase formation. An ordered Ni2(Cr,Mo) phase (Pt2Mo-type) significantly influences the mechanical properties of Ni-Cr-Mo alloys. This phase is observed in both the binary Ni-Cr system (as Ni2Cr, stable up to 475 °C) and the ternary Ni-Cr-Mo system (as Ni2(Cr,Mo)). The addition of molybdenum extends the temperature stability range of this phase and affects ordering kinetics, depending on the Mo/ratio [43]. Ni2(Cr,Mo) phase has also been identified in VDM® Alloy C-4 after long-term aging at temperatures of up to 600 °C [9,14,28,44,45].
Except for KhN62M-VI, which was specifically developed for operation in molten halides and exhibits high resistance to such environments, all other studied commercial alloys were originally designed for use in the chemical industry at moderate temperatures. However, previous studies have demonstrated that these materials can be used as structural components under conditions that combine high temperatures and aggressive chemical environments [12,13,28].
Phase transformations in Ni-Cr-Mo alloys play a key role in the change of their performance characteristics. The formation of secondary and ordered phases has a complex effect on mechanical and, in some cases, corrosion properties, and therefore, it must be considered when nickel alloys are intended for application in high-temperature and aggressive environments.
However, the relationship between the mechanical properties of prospective construction materials and their microstructure after long-term aging was considered only for the C-4 alloy [9,14]. Therefore, the aim of the present study was investigating the changes in short-term mechanical properties of various industrial Ni-Cr-Mo-based alloys with different chromium (from 1.5 to 33 wt.%) and molybdenum (from 8 to 28 wt.%) contents after long-term aging at temperatures ranging from 500 to 700 °C, as well as establishing the relationship between the mechanical properties of the alloys and the nature of secondary phases that formed.

2. Methods and Materials

2.1. Equipment and Experimental Methods

First, phase modeling of the selected alloys was carried out using Thermo-Calc software and the TCNI10 database to determine the stability regions of ordered and secondary phases that can influence the mechanical properties of the materials. The modeling results were compared with the experimental data on material structures obtained using metallographic analysis with scanning electron microscopy (SEM) in backscattered electron mode (Carl Zeiss Auriga CrossBeam equipped with an Oxford INCA X-Max 80 micro-X-ray spectrometer (Carl Zeiss NTS, Oberkochen, Germany)).
First, the microstructure of the studied materials in the as-received state was examined. Samples were then aged at 500–700 °C for 100 to 5000 h in a muffle furnace in air and their mechanical properties were determined.
Short-term mechanical properties after long-term aging were assessed using an Instron 3382 (Instron, Norwood, MA, USA) tensile testing machine, in accordance with the Russian GOST 1497 standard [46]. Tensile tests were carried out on proportional flat specimens with a thickness of 4 mm and a working length of 65 mm (calculated length of 50 mm) with a tensile speed of 5 mm/min. At least 3 specimens (Figure 1) were used in each aging procedure. The deformation behavior of the samples during aging was analyzed based on changes in the following mechanical properties: yield strength (YS), tensile strength (TS), relative elongation (El), and the ratio of the yield strength to the tensile strength (YS/TS).

2.2. Materials

Five corrosion-resistant nickel-based alloys were selected as the objects of investigation in this study: Hastelloy® G-35®, VDM® Alloy C-4, VDM® Alloy 59, Hastelloy® B-3®, and KhN62M-VI. The position of these alloys on the isothermal section of the ternary Ni-Cr-Mo phase diagram is shown in Figure 2. The chemical composition of the materials according to the manufacturers’ certificates is listed in Table 1.

3. Results

3.1. Modeling

The phase composition diagrams of the selected alloys are presented in Figure 3. They were obtained through thermodynamic modeling using the Thermo-Calc v.2022a software package with the TCNI10 database. These diagrams allow us to compare material structures with experimentally obtained results. According to the calculations, all investigated nickel alloys tend to form topologically close-packed (TCP) phases (P- and σ-phase) as well as the ordered Ni2(Cr,Mo) phase. These results correlate well with the literature data [9,14,31,34,35,39,42,43,45]. There is a possibility of formation of α-Cr particles in the Hastelloy® G-35® alloy, and this was confirmed experimentally [42]. The more complex nature of the phase composition diagram of Hastelloy® B-3® alloy is due to its chemical composition; Ni4Mo and Ni3Mo intermetallic compounds can be formed as a result of high-temperature exposure. The presence of these intermetallic compounds as well as the possible formation of a σ-phase can negatively affect the ductility and corrosion resistance of the alloy.

3.2. Microstructure and Stress–Strain Behavior of Nickel Alloys in the Initial State

3.2.1. Initial Microstructure of Materials

The microstructure of nickel alloys from G- and C-families in the as-received state is characterized by a high degree of homogeneity, representing a metastable nickel-based FCC solid solution (Figure 4). The grains in the samples are large and equiaxial with clearly observed annealing twins. The average grain size is approximately 50 μm. A small number of inclusions containing magnesium and oxygen were detected in all studied materials. The microstructure of Hastelloy® B-3® differs from other alloys exhibiting a small number of unevenly distributed stringer inclusions enriched in nickel and molybdenum. However, Mishra [20] reported that such inclusions had no effect on the corrosion properties.

3.2.2. Deformation Behavior of Alloys in the Initial State

The tensile curves of the studied materials allow to assess the interconnection between strength and ductility in the initial state (Figure 5).
For example, the tensile curve for Hastelloy® G-35® in the initial state exhibits a profile characteristic of FCC metallic materials, i.e., relatively low strength characteristics are combined with high uniform ductility and strain hardening, which can be confirmed by the YS/TS ratio of 0.51. The YS/TS ratio has been proposed as an indicator for estimating the tendency of a material to embrittle under various conditions. The precipitation of secondary phases, depending on their localization within the grain, has a different influence on the parameters of the tensile curve: yield stress and the duration of uniform and localized deformation stages.
C-family alloys exhibit similar tensile behavior; the YS/TS ratio is 0.60 for VDM® Alloy C-4, 0.57 for Alloy 59, and 0.47 for the KhN62M-VI alloy. The YS/TS values for VDM® Alloy C-4 and VDM® Alloy 59 indicate a slightly higher tendency for embrittlement in the initial state compared to the KhN62M-VI alloy.
The tensile curve for Hastelloy® B-3® in the initial state also shows a smooth and monotonous profile, similar to other alloys. However, this alloy is characterized by higher strength and lower total ductility. The YS/TS ratio of 0.55 is comparable to the values obtained for VDM® Alloy 59 and VDM® Alloy C-4.
Therefore, nickel-based alloys in the “as-received” state have high uniform and general plasticity and a low level of strength properties, caused by solid solution hardening.

3.3. Microstructure and Stress–Strain Curves of Nickel Alloys After Long-Term Aging

Nickel-based alloys are highly sensitive to thermal exposure, which affects their structural phase composition and consequently their performance properties. Analysis of tensile curves was performed at two temperature values, at which characteristic changes in the material properties occur. These changes are determined by the formation of excess phases and their morphology.
The temperatures of 500 and 650 °C were chosen as characteristic temperatures for Hastelloy® G-35®, and they are denoted as Regime 1 and Regime 2, respectively.
Higher molybdenum content in C-family nickel-based alloys compared to Hastelloy® G-35® leads to changes in the type and size of precipitates, significantly affecting their tensile behavior. The KhN62M-VI alloy was used to study the tensile curve characteristics at 600 °C (Regime 1) and 700 °C (Regime 2).
The deformation behavior of Hastelloy® B-3® nickel–molybdenum alloy was investigated at 500 °C (Regime 1) and 650 °C (Regime 2), temperatures at which characteristic changes in its mechanical properties occur.

3.3.1. Microstructure and Stress–Strain Curves of Hastelloy® G-35® After Long-Term Aging at 500 °C

Metallographic analysis did not reveal any excess phase formation along grain boundaries in the aged samples (Figure 6). Relatively low sensitivity of Hastelloy® G-35® to deformation during prolonged aging (up to 2000 h) is associated with the interaction of dislocations with Ni2(Cr,Mo) precipitates within the grain that maintain coherency with the matrix. The observed increase in strength after 5000 h of aging is likely due to a growth in the volume fraction of these particles.
Increase in aging duration at 500 °C from 1000 to 5000 h led to a gradual increase in strength properties; YS rose from 380 to 530 MPa, and TS increased from 740 to 890 MPa, indicating a dispersion hardening mechanism. The strengthening trend intensified with prolonged aging, when the YS/TS ratio gradually rose from 0.51 to 0.60. Despite the increase in strength, the alloy retained a high level of ductility (47%). Additionally, long-term aging at 500 °C led to the appearance of a yield plateau phenomenon (Figure 7).

3.3.2. Microstructure and Stress–Strain Curves of Hastelloy® G-35® After Long-Term Aging at 650 °C

Unlike previous regime, aging at 650 °C resulted in different deformation behavior for Hastelloy® G-35®. During prolonged aging at this temperature, the nucleation and growth of chromium-enriched particles along the grain boundaries occurred (Figure 8), and this is consistent with the data presented by Belikov et al. [42].
It is important to note that despite the precipitation of excess phases enriched in chromium and molybdenum along the grain boundaries, the mechanical properties remained largely unaffected (Figure 9, Table 2). However, these grain boundary precipitates can contribute to intergranular corrosion due to the formation of micro-galvanic couples and subsequent leaching upon exposure to a salt melt environment [47].
Regardless of aging duration, the strength properties remained nearly unchanged—TS remained at 710 MPa after aging for up to 5000 h, and YS remained at 370–380 MPa. The YS/TS ratio (0.53 for both 1000 and 5000 h) and the relatively stable ductility range of 54–57% indicate that the observed phase precipitation primarily occurred along the grain boundaries, resulting in a change of strengthening mechanism compared to Regime 1. The tensile curves exhibit a yield drop phenomenon (Figure 10).

3.3.3. Microstructure and Stress–Strain Curves of KhN62M-VI Alloy After Long-Term Aging at 600 °C

One can see that the microstructure of the KhN62M-VI alloy after 1000 h and 5000 h at 600 °C (Regime 1) exposure is similar to the initial state (Figure 11). However, prolonged aging at 600 °C resulted in significant changes not only in the shape but also in the behavior of the stress–strain curves (Figure 12), and it led to substantial material strengthening and gradual embrittlement as the aging duration increased. The rise in strength properties and strain hardening, reflected in the higher yield stress and a steeper slope of the uniform deformation stage, indicates an increasing volume fraction of coherent precipitates. Additionally, the increase in barriers for dislocation movement caused a sharp decline in ductile properties after just 100 h of aging, including a drastic reduction in uniform plasticity, the disappearance of the localized deformation stage, and an increase in the YS/TS ratio. Prolonged aging for up to 2000 h resulted in complete embrittlement of the alloy. However, further aging for up to 5000 h led to a partial recovery of plasticity, likely due to the process of coherent precipitate coarsening.

3.3.4. Microstructure and Stress–Strain Curves of KhN62M-VI Alloy After Long-Term Aging at 700 °C

The behavior of the KhN62M-VI alloy during aging at a higher temperature (Regime 2) differed significantly from Regime 1, as evident from both microstructural changes and stress–strain curve evolution (Figure 13). TCP-phase particles enriched in chromium and molybdenum were formed in the material during prolonged aging (Figure 14, Table 3). At relatively short exposure times, these phases formed thin plates along the grain boundaries and chains along the intergranular boundaries. The density and size of these precipitates grew with increasing aging time, and the particles became larger and had a tendency to coagulate. Grain boundary TCP-phase precipitates did not significantly influence mechanical properties similar to Regime 2 behavior in Hastelloy® G-35®. Wang et al. [48] noted a similar effect—the sigma phase precipitated along the grain boundaries caused a slight reduction in tensile strength due to the formation of cracks at the γ/σ interface but had no obvious effect on ductility. However, potential contact of these particles with molten electrolytes may initiate intergranular corrosion [14].
Analysis of the stress–strain curves indicates that both uniform and localized deformation stages remained for up to 2000 h at 700 °C. Further aging for up to 5000 h led to a gradual reduction in ductility to 44%. The relatively stable and low strain hardening intensity across the entire aging range—YS/TS was equal to 0.49 at 1000 h and to 0.46 at 5000 h—was primarily determined by dislocation interactions within the grains. The effect of grain boundary precipitates became noticeable only after 5000 h, resulting in a reduction in ductile properties (Figure 15). It is worth noting that embrittlement observed after 5000 h may limit the operational reliability of the KhN62M-VI alloy.

3.3.5. Effect of Long-Term Aging on VDM® Alloy 59 and VDM® Alloy C-4 Properties

Analysis of the stress–strain curves of VDM® Alloy 59 and VDM® Alloy C-4 indicates that their deformation behavior during aging was similar to that of KhN62M-VI. A summary plot of the short-term mechanical properties of VDM® Alloy 59 and VDM® Alloy C-4 for selected aging temperatures is presented in Figure 16 and Figure 17. It is important to note that prolonged exposure of VDM® Alloy 59 at 700 °C led to the formation of TCP-phase precipitates at grain boundaries but did not affect strength properties. However, the ductility of the alloys was significantly reduced, particularly after 5000 h.
The correctness of our experimental data is confirmed by the results of Tawancy [28], who observed similar effects in VDM® Alloy C-4 after long-term aging (up to 16,000 h).

3.3.6. Microstructure and Stress–Strain Curves of Hastelloy® B-3® After Long-Term Aging at 500 °C

The microstructure of Hastelloy® B-3® after high-temperature exposure (Figure 18) did not show much difference from the initial state but contained coherent intermetallic phases based on nickel and molybdenum.
A comparative analysis of the stress–strain curves of Hastelloy® B-3® in the as-received state and after aging reveals that exposure at 500 °C for up to 5000 h led to a gradual increase in strength and a more pronounced reduction in ductility (Figure 19). This was primarily due to the shortened (to 21%) uniform deformation stage and an increase in the YS/TS ratio from 0.45 to 0.77.

3.3.7. Microstructure and Stress–Strain Curves of Hastelloy® B-3® After Long-Term Aging at 650 °C

At the aging temperature of 650 °C, precipitates of various types and morphologies were formed in Hastelloy® B-3® alloy, both at grain boundaries and within the grain body. Intragranular precipitates primarily appeared as thin plates (below 1 μm thick) oriented at different angles (Figure 20). This morphology is characteristic of the ordered intermetallic Ni4Mo phase, as confirmed by EDS analysis (Figure 21, Table 4). Some precipitates with a lack of distinct interfaces between their surface and the matrix are coherent or semi-coherent. These results agree well with previous studies of Ni4Mo precipitates, which form through solid solution ordering and may retain partial coherence with the matrix [25,49]. In certain regions, darker zones are observed that can correspond to a Ni3Mo phase.
The plate-like morphology of Ni₄Mo particles is highly unfavorable for maintaining adequate ductility and viscosity during aging. In this case, interphase boundaries act as stress concentrators and crack nucleation sites. As a result of these processes, aging at 650 °C led to complete embrittlement of Hastelloy® B-3® (δ → 0, YS/TS = 1), and these phenomena did not depend on aging duration (Figure 22). Similar results were obtained for the Hastelloy B-2 alloy by Tawancy [27].

4. Discussion

A comprehensive analysis of the microstructure and tensile deformation behavior allowed us to examine the processes of decomposition of Cr- and Mo-enriched supersaturated γ-phases in the studied Ni-based alloys. Additionally, assessment of the contribution of secondary-phase particle type and morphology to the evolution of mechanical properties during long-term aging (for up to 5000 h) was estimated.
The observed property changes can be represented in a structural strengthening and embrittlement diagram (Figure 23), where the increase in yield strength (YS) due to aging is primarily associated with the dispersion-hardening mechanism, and this process is accompanied by embrittlement (expressed through the YS/TS ratio). Based on the impact on mechanical properties, the experimental data on the diagram can be divided into several regions. These regions reflect the influence of intra- and intergranular precipitates.
In the “as-received” state, the investigated corrosion-resistant nickel alloys exhibited relatively low yield strength and the lowest degree of embrittlement, indicating high plasticity and strain-hardening capability.
Analysis of the aged samples showed that material strengthening was directly related to the localization of particle nucleation and the Cr/Mo ratio. The predominance of secondary-phase precipitates at the grain boundaries in Hastelloy® G-35® alloy and partially in the C-group alloys under certain conditions provides a more balanced combination of mechanical properties—moderate strengthening while preserving ductility. In contrast, the formation of intragranular Ni-Mo intermetallic precipitates significantly reduces plasticity and enhances the material’s tendency to embrittlement.
Therefore, Hastelloy® B-3® (Ni-Mo system), during aging at 550–700 °C, undergoes significant strengthening accompanied by complete embrittlement (YS/TS = 1). Such behavior results from the intensive formation of intragranular precipitates and an increase in their volume fraction that leads to a gradual rise in yield strength and degradation of ductile properties. Loss of plasticity becomes more pronounced with increasing aging temperature. The embrittlement of the Hastelloy® B-3® alloy during prolonged aging is unfavorable for applications within the working temperature range of molten salt reactors. Aging at a lower temperature (500 °C) also showed a tendency toward embrittlement, and the YS/TS ratio gradually increased to 0.77.
Alloys of the C-family exhibit the most stable mechanical behavior during long-term aging and have an intermediate position among the studied materials. Their strength properties increase gradually up to 650 °C due to precipitation of ordered Ni2(Cr,Mo) particles but without a drastic drop in ductility, as seen in B-type alloys. Among the studied C-alloys, KhN62M-VI shows the highest dispersion-hardening effect, with the YS/TS ratio increasing up to 0.70 at 550 °C. Within this group, VDM® Alloy C-4 demonstrates intermediate mechanical properties after short-term aging with YS/TS values of 0.63–0.66 at 500–600 °C and 0.54 at 700 °C. The YS/TS ratio for VDM® Alloy 59 remains within the range of 0.60–0.66 across all the investigated temperature regimes. After aging at 700 °C, the strength properties of all studied C-alloys returned to levels comparable to the as-received state due to formation and coarsening of TCP-phase precipitates at the grain boundaries; at the same time, the ductile properties were reduced.
The Hastelloy® G-35® alloy (Ni-Cr system), after aging at 550–650 °C, demonstrates relatively low sensitivity to property changes—the moderate strength increase is not accompanied by significant embrittlement (YS/TS ratio only slightly increases to 0.52–0.53). It is important to note that even prolonged exposure at 500 °C allows the material to retain a certain balance of strength and plasticity, which is confirmed by the rise in the YS/TS ratio to only 0.60.
Therefore, the deformation behavior of the studied alloys under tensile loading is primarily determined by the type of secondary-phase precipitates formed during the decomposition of the supersaturated γ-phase. This process depends on Ni alloy composition, precipitate morphology (spherical or plate-like), and preferred nucleation sites (intragranular and/or intergranular). The impact of these factors shows considerable dependence on the aging temperature range.

5. Conclusions

The short-term mechanical properties of commercial corrosion-resistant alloys based on Ni-Cr (Hastelloy® G-35®), Ni-Mo (Hastelloy® B-3®), and Ni-Cr-Mo (VDM® Alloy C-4, VDM® Alloy 59, KhN62M-VI) systems were analyzed in both the as-received state and after long-term (up to 5000 h) aging at 500–700 °C. The obtained data showed that the content of the alloy’s main components had a pronounced influence on the type of secondary phases released during long-term high-temperature aging, and the nature of these phases predetermined the deformation behavior of Ni–Cr–Mo alloys during tension.
The analysis of the deformation behavior of Hastelloy® G-35® indicates that nickel–chromium-based materials are the most reliable in terms of mechanical property stability for structural applications within the temperature range of 550–700 °C. Hastelloy® G-35®, in similar temperature conditions, exhibits the least sensitivity to aging due to the precipitation of particles identified as σ-phase and α-Cr, which has no significant effect on the material’s ductile properties while slightly increasing its strength.
The high molybdenum content in B-family alloys reduces matrix stability under high-temperature aging, leading to complete embrittlement at 600–700 °C. This is attributed to the plate-like morphology of Ni4Mo precipitates, which are highly harmful to ductile properties, as these excess phases act as internal stress concentrators and crack initiation sites.
C-family alloys have the greatest potential as structural materials for high-temperature applications at 500–700 °C. Their more balanced chemical composition results in the best combination of strength and ductility, making them highly processable. However, it has been demonstrated that intragranular precipitates in the C-type nickel-based alloys slightly embrittle the metal, limiting its applicability within the specified temperature range.
Therefore, it has been demonstrated that grain boundary precipitates have a negligible effect on the strength properties of the investigated alloys, while intragranular precipitates embrittle nickel-based alloys, reducing their possible application at high temperatures.
The obtained results can be useful for the preliminary selection of candidate construction materials for MSRs. The final choice of the construction material requires performing prolonged resource testing in the molten salt medium with a forecast of mechanical properties on the entire service life of the MSR.

Author Contributions

Conceptualization, I.B.P.; Formal analysis, A.F.G. and V.A.K.; Investigation, A.F.G., V.A.K. and M.S.K.; Data curation, V.A.K. and I.B.P.; Writing—original draft, A.F.G.; Writing—review & editing, V.A.K. and I.B.P.; Visualization, A.F.G.; Project administration, I.B.P.; Funding acquisition, I.B.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Ministry of Science and Higher Education of the Russian Federation (Ural Federal University Program of Development within the Priority-2030 Program).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Proportional flat specimen of investigated alloys for tensile tests.
Figure 1. Proportional flat specimen of investigated alloys for tensile tests.
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Figure 2. Position of studied alloys on the isothermal section of Ni-Cr-Mo phase diagram at 650 °C.
Figure 2. Position of studied alloys on the isothermal section of Ni-Cr-Mo phase diagram at 650 °C.
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Figure 3. Calculated phase compositions of Ni-based alloys: (a) Hastelloy® G-35®; (b) Hastelloy® B-3®; (c) VDM® Alloy 59; (d) VDM® Alloy C-4; (e) alloy KhN62M-VI.
Figure 3. Calculated phase compositions of Ni-based alloys: (a) Hastelloy® G-35®; (b) Hastelloy® B-3®; (c) VDM® Alloy 59; (d) VDM® Alloy C-4; (e) alloy KhN62M-VI.
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Figure 4. Microstructure of the studied alloys in the as-received state: (a) VDM® Alloy 59; (b) VDM® Alloy C-4; (c) alloy KhN62M-VI; (d) Hastelloy® B-3®; (e) Hastelloy® G-35®.
Figure 4. Microstructure of the studied alloys in the as-received state: (a) VDM® Alloy 59; (b) VDM® Alloy C-4; (c) alloy KhN62M-VI; (d) Hastelloy® B-3®; (e) Hastelloy® G-35®.
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Figure 5. Typical engineering stress–strain curves of nickel-based alloys in the as-received state.
Figure 5. Typical engineering stress–strain curves of nickel-based alloys in the as-received state.
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Figure 6. Microstructure of Hastelloy® G-35® after long-term aging at 500 °C for 1000 h (a) and 5000 h (b).
Figure 6. Microstructure of Hastelloy® G-35® after long-term aging at 500 °C for 1000 h (a) and 5000 h (b).
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Figure 7. Typical engineering stress–strain curves of Hastelloy® G-35® specimens after long-term aging at 500 °C.
Figure 7. Typical engineering stress–strain curves of Hastelloy® G-35® specimens after long-term aging at 500 °C.
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Figure 8. Microstructure of Hastelloy® G-35® after long-term aging at 650 °C for 1000 h (a) and 5000 h (b).
Figure 8. Microstructure of Hastelloy® G-35® after long-term aging at 650 °C for 1000 h (a) and 5000 h (b).
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Figure 9. Excessive phases formed in Hastelloy® G-35® after 5000 h of aging at 650 °C.
Figure 9. Excessive phases formed in Hastelloy® G-35® after 5000 h of aging at 650 °C.
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Figure 10. Typical engineering stress–strain curves of Hastelloy® G-35® specimens after long-term aging at 650 °C.
Figure 10. Typical engineering stress–strain curves of Hastelloy® G-35® specimens after long-term aging at 650 °C.
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Figure 11. Microstructure of KhN62M-VI alloy after long-term aging at 600 °C for 1000 h (a) and 5000 h (b).
Figure 11. Microstructure of KhN62M-VI alloy after long-term aging at 600 °C for 1000 h (a) and 5000 h (b).
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Figure 12. Typical engineering stress–strain curves of KhN62M-VI specimens after long-term aging at 600 °C.
Figure 12. Typical engineering stress–strain curves of KhN62M-VI specimens after long-term aging at 600 °C.
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Figure 13. Microstructure of KhN62M-VI alloy after long-term aging at 700 °C for 1000 h (a) and 5000 h (b).
Figure 13. Microstructure of KhN62M-VI alloy after long-term aging at 700 °C for 1000 h (a) and 5000 h (b).
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Figure 14. Excessive phases formed in KhN62M-VI after 5000 h of aging at 700 °C for 5000 h.
Figure 14. Excessive phases formed in KhN62M-VI after 5000 h of aging at 700 °C for 5000 h.
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Figure 15. Typical engineering stress–strain curves of KhN62M-VI specimens after long-term aging at 700 °C.
Figure 15. Typical engineering stress–strain curves of KhN62M-VI specimens after long-term aging at 700 °C.
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Figure 16. Tensile properties of VDM® Alloy 59 after long-term aging.
Figure 16. Tensile properties of VDM® Alloy 59 after long-term aging.
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Figure 17. Tensile properties of VDM® Alloy C-4 after long-term aging.
Figure 17. Tensile properties of VDM® Alloy C-4 after long-term aging.
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Figure 18. Microstructure of Hastelloy® B-3® alloy after long-term aging at 500 °C for 500 h (a) and 5000 h (b).
Figure 18. Microstructure of Hastelloy® B-3® alloy after long-term aging at 500 °C for 500 h (a) and 5000 h (b).
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Figure 19. Typical engineering stress–strain curves of Hastelloy® B-3® specimens after long-term aging at 500 °C.
Figure 19. Typical engineering stress–strain curves of Hastelloy® B-3® specimens after long-term aging at 500 °C.
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Figure 20. Microstructure of Hastelloy® B-3® alloy after long-term aging at 650 °C for 500 h (a) and 5000 h (b).
Figure 20. Microstructure of Hastelloy® B-3® alloy after long-term aging at 650 °C for 500 h (a) and 5000 h (b).
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Figure 21. Secondary phases formed in Hastelloy® B-3® after 5000 h of aging at 650 °C.
Figure 21. Secondary phases formed in Hastelloy® B-3® after 5000 h of aging at 650 °C.
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Figure 22. Typical engineering stress–strain curves of Hastelloy® B-3® specimens after long-term aging at 650 °C.
Figure 22. Typical engineering stress–strain curves of Hastelloy® B-3® specimens after long-term aging at 650 °C.
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Figure 23. Correlation between strengthening and embrittlement of selected nickel alloys due to precipitate localization after 5000 h of long-term aging.
Figure 23. Correlation between strengthening and embrittlement of selected nickel alloys due to precipitate localization after 5000 h of long-term aging.
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Table 1. Composition of alloys according to the manufacturers’ certificates.
Table 1. Composition of alloys according to the manufacturers’ certificates.
AlloyContent, wt.%
NiCrMoFeMnTiCuCoAlSiSCP
Hastelloy® G-35®
(UNS N06035) (1)		57.233.18.20.80.220.010.030.070.190.040.0020.0120.003
KhN62M-VI (2)63.223.212.80.470.030.08 0.110.060.0030.0050.004
VDM® Alloy 59 (UNS N06059)60.422.615.40.90.19 0.010.010.230.020.0020.0020.002
VDM® Alloy C-4 (UNS N06455)67.0516.1215.620.840.02 0.010.01 0.010.0020.0020.003
Hastelloy®B-3®
(UNS N10675) (3)		67.11.628.51.410.680.010.10.050.330.040.0010.0020.004
(1) Hastelloy® G-35® also contains about 0.01% W. (2) KhN62M-VI also contains about 0.03% Nb. (3) Hastelloy® B-3® also contains about 0.24% W.
Table 2. Chemical composition of phases in Hastelloy® G-35® aged at 650 °C for 5000 h (at.%).
Table 2. Chemical composition of phases in Hastelloy® G-35® aged at 650 °C for 5000 h (at.%).
SpectrumCrFeNiMo
Spectrum 149.760.6637.6211.96
Spectrum 221.670.4772.595.27
Spectrum 316.730.1877.425.68
Spectrum 445.660.4841.5112.35
Spectrum 550.180.5534.0215.24
Spectrum 612.640.2480.816.32
Table 3. Chemical composition of phases in KhN62M-VI aged at 700 °C for 5000 h (at.%).
Table 3. Chemical composition of phases in KhN62M-VI aged at 700 °C for 5000 h (at.%).
SpectrumCrNiMo
Spectrum 121.7837.5540.67
Spectrum 221.9537.8340.22
Spectrum 370.4018.7510.85
Spectrum 426.3164.908.79
Table 4. Chemical composition of phases in Hastelloy® B-3® aged at 650 °C for 5000 h (at.%).
Table 4. Chemical composition of phases in Hastelloy® B-3® aged at 650 °C for 5000 h (at.%).
SpectrumAlSiCrMnFeNiMo
Spectrum 10.940.943.020.882.8377.5213.85
Spectrum 20.720.132.500.721.7173.2420.96
Spectrum 30.880.882.961.012.7177.5514.00
Spectrum 41.320.692.881.003.0777.8813.16
Spectrum 50.530.401.910.861.6574.0320.63
Spectrum 60.940.943.020.882.8377.5213.85
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Gibadullina, A.F.; Khotinov, V.A.; Karabanalov, M.S.; Polovov, I.B. The Effect of Long-Term Aging on the Mechanical Properties of Corrosion-Resistant Nickel-Based Alloys for Their Application in Nuclear Technologies. Appl. Sci. 2025, 15, 6133. https://doi.org/10.3390/app15116133

AMA Style

Gibadullina AF, Khotinov VA, Karabanalov MS, Polovov IB. The Effect of Long-Term Aging on the Mechanical Properties of Corrosion-Resistant Nickel-Based Alloys for Their Application in Nuclear Technologies. Applied Sciences. 2025; 15(11):6133. https://doi.org/10.3390/app15116133

Chicago/Turabian Style

Gibadullina, Alfiya F., Vladislav A. Khotinov, Maxim S. Karabanalov, and Ilya B. Polovov. 2025. "The Effect of Long-Term Aging on the Mechanical Properties of Corrosion-Resistant Nickel-Based Alloys for Their Application in Nuclear Technologies" Applied Sciences 15, no. 11: 6133. https://doi.org/10.3390/app15116133

APA Style

Gibadullina, A. F., Khotinov, V. A., Karabanalov, M. S., & Polovov, I. B. (2025). The Effect of Long-Term Aging on the Mechanical Properties of Corrosion-Resistant Nickel-Based Alloys for Their Application in Nuclear Technologies. Applied Sciences, 15(11), 6133. https://doi.org/10.3390/app15116133

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