Effects of Temperature, Stress, and Grain Size on the High-Temperature Creep Mechanism of FeCrAl Alloys

Abstract

FeCrAl exhibits excellent resistance to high temperatures, corrosion, and irradiation, making it a prime candidate material for accident-tolerant fuel (ATF) cladding. This study investigates the high-temperature creep behavior of FeCrAl alloys with grain sizes of 12.0 μm and 9.9 μm under temperatures ranging from 450 °C to 650 °C and applied stresses between 75 and 200 MPa. The texture, grain morphology, grain orientation, and dislocation density of FeCrAl were characterized by electron backscatter diffraction (EBSD). The results indicate that temperature, applied stress, and grain size are the primary factors governing high-temperature creep behavior. The material texture showed no significant difference before and after creep. Large grains tend to engulf smaller ones during the creep process at lower temperatures and stresses, reducing the proportion of low-angle grain boundaries (LAGBs). In contrast, at higher temperatures or under higher stress, dislocations proliferate within grains, leading to a significant increase in the number of LAGBs. As the applied stress increases, the dominant creep mechanism tends to convert from grain boundary sliding to dislocation motion. Moreover, higher temperatures or smaller grain sizes lower the critical stress required to activate dislocation motion and significantly increase dislocation density, severely degrading the creep resistance.

1. Introduction

FeCrAl, as a high-temperature corrosion-resistant alloy [1], has been identified as an important candidate material for ATF cladding in nuclear reactors [2,3]. Compared to the currently used zircaloy cladding in pressurized water reactors, FeCrAl cladding demonstrates superior high-temperature mechanical properties [4], corrosion resistance [5,6], and neutron irradiation tolerance [7]. FeCrAl cladding can withstand temperatures up to 1227 °C, whereas zircaloy cladding is prone to failure when temperatures exceed 927 °C [8]. Thompson and colleagues at the Oak Ridge National Laboratory (ORNL) measured the elastic modulus and Poisson’s ratio of several FeCrAl alloys suitable for nuclear fuel cladding using resonant ultrasound spectroscopy [9]. The results show that, within the temperature range from room temperature to 850 °C, the elastic modulus of these alloys is at least twice that of zircaloys. Experimental studies conducted by Massey et al. [8] under the loss-of-coolant accident conditions indicated that the swelling of FeCrAl claddings due to creep is negligible, and the cross-sectional area remains unchanged before failure. FeCrAl cladding exhibits a lower creep rate under normal operation conditions, significantly delaying mechanical contact with the reactor core compared to zircaloy cladding, thereby enhancing the safety of nuclear reactors. Ma et al. [5] found that the corrosion resistance of FeCrAl alloys surpasses that of zircaloys. Zhao et al. [7] demonstrated that the gap closure rate of FeCrAl alloys under neutron irradiation is much lower than that of zircaloys due to the lack of irradiation growth, effectively extending the accident response time. In situ burst tests conducted by Gussev et al. revealed that first- and second-generation FeCrAl cladding exhibit mechanical behaviors comparable to or better than those of zircaloy cladding [10].

High-temperature creep performance is one of the key factors in evaluating the suitability of materials for nuclear reactor applications. Researchers have conducted a series of uniaxial tensile creep tests to investigate the creep behavior and mechanisms of FeCrAl alloys under elevated temperatures. Using techniques such as scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM), the microstructural features and their evolution have been observed and analyzed. These studies contribute to the fundamental understanding of the creep behavior and mechanisms in FeCrAl alloys. Arzt et al. [11] investigated the creep properties of oxide-dispersion-strengthened (ODS)-FeCrAl alloys. It was pointed out that, with increasing stress, the creep mechanism transits from grain boundary (GB) diffusion to GB sliding; dislocation creep becomes the dominant mechanism under intermediate and high stress levels. Kamikawa et al. [12] studied the relationship between the creep mechanism and the strain rate of ODS FeCrAl alloys at a high temperature of 1000 °C. Creep data suggests that as the strain rate decreases from 10⁻² s⁻¹ to 10⁻⁵ s⁻¹, the dominant mechanism changes from dislocation creep to GB sliding, and then to cooperative GB sliding as the strain rate drops below 10⁻⁷ s⁻¹. Sun et al. [13] conducted in situ micropillar compression tests on FeCrAl nanolaminates with an average grain size of 82 nm. It was found that GB diffusion is the dominating mechanism at a temperature of 350 °C and strain rates range from 10⁻⁴ s⁻¹ to 10⁻¹ s⁻¹. Jaumier et al. [14] reported pronounced creep anisotropy in thin-walled FeCrAl tubes at 14 wt.% Cr, while only slight anisotropy was observed at 9 wt.% Cr. Yano et al. [15] demonstrated that Zr addition enhances the tensile strength and elongation of ODS FeCrAl thin-walled tubes at temperatures above 1000 °C. Joshi et al. [16] confirmed that the dislocation climb is the dominant mechanism in nuclear-grade FeCrAl thin-walled tubes under high-temperature creep conditions. While researchers have characterized the creep properties of FeCrAl alloys and nuclear-grade FeCrAl cladding under various conditions, elucidating the transition of creep mechanisms with applied stress and studies on the effects of temperature and grain size on these mechanisms remain insufficient.

Under normal operating conditions in nuclear reactors, the average temperature of fuel cladding typically ranges from 400 °C to 450 °C. However, during a loss-of-coolant accident, the rapid depletion of coolant can cause the cladding temperature to rise sharply to 600 °C or above. Therefore, the high-temperature creep tests in this study were conducted within the temperature range of 450 °C to 650 °C. Because FeCrAl alloys have a higher neutron absorption cross-section, the cladding thickness must be reduced to 0.3 mm to achieve the same fuel cycle as zircaloys [17]. Nevertheless, considering the mechanical properties and current manufacturing technology of cladding tubes, the typical cladding thickness is around 0.4 mm. To better approximate the target design dimensions of FeCrAl cladding, the alloy plates used in this study’s creep tests were fabricated to a thickness of 0.47 mm.

This study investigates the high-temperature creep behavior of Fe13Cr4Al2Mo (wt.%) alloys with average grain sizes in the range of 9.0–12.0 μm, at temperatures of 450–650 °C, and under applied stresses of 75–200 MPa. To elucidate the relationship between microstructural changes in the alloys before and after creep and the underlying deformation mechanisms, EBSD techniques were employed to characterize the evolution of texture, grain morphology, grain orientation, and dislocation density under different loading conditions. Furthermore, the dominant creep mechanisms of FeCrAl alloys under various conditions were systematically analyzed. These findings will contribute to advancing the research and development of FeCrAl alloys as ATF cladding materials internationally, with considerable academic and engineering value.

2. Materials and Methods

The high-temperature creep test samples were prepared from FeCrAl alloy plates with a composition of Fe13Cr4Al2Mo (wt.%). Dog bone test specimens with a gauge length of 100 mm and a cross-sectional area of 15 mm × 0.47 mm were machined by electrical discharge machining. The tests were conducted using an RD Series computer-controlled electromechanical testing system (RD-100, Changchun Technology Innovation Test Instruments Co., Ltd., Changchun, China). The furnace used for testing was a Changchun Kexin resistance heating furnace, with a temperature range of 300–900 °C. The temperature was measured using K-type and S-type thermocouples, with a resolution of 0.1 °C. The elongation of the samples was measured with a creep extensometer, featuring a measurement range of 0–10 mm and a resolution of up to 0.001 mm.

Before microstructural observation, sample preparation for scanning was carried out. Small specimens with dimensions of 20 mm × 6 mm × 0.47 mm were cut from FeCrAl thin plates using an electrical discharge wire cutting machine (DK7745E, Taizhou Sipaite CNC Machine Tool Manufacturing Co., Ltd., Taizhou, China). The sampling locations on the post-creep specimens are shown in Figure 1. The samples for microstructural examination underwent mechanical grinding with SiC sandpaper and polishing with diamond paste, before they were submitted for Ar ion polishing to mitigate surface stress. The specimens were mounted in conductive resin and sequentially ground using 200#, 400#, 800#, 1200#, 2500#, and 4000# grit sandpaper to remove the scratches left by previous steps, continuing until no deep scratches remained on the surface. After grinding, the specimen’s surface underwent successive polishing stages: coarse polishing with 3 μm suspension, followed by fine polishing with 1 μm suspension, until a scratch-free surface was achieved. The specimens were then ultrasonically cleaned in anhydrous ethanol for 10 min to remove any residual polishing agent and subsequently dried. After preparation, the specimens were wrapped in lint-free paper and stored under vacuum.

A Hitachi SU3500 tungsten-filament scanning electron microscope (SEM, Hitachi (China), Ltd., Beijing, China) was used to characterize the microstructural features of the samples before and after creep. EBSD was employed to characterize grain size and texture, with a step size of 0.7 μm. The magnification was set to 300 times, and the scan area was 400 μm × 300 μm. The raw EBSD data were visualized and analyzed using Aztec Crystal software (Aztec Crystal 2.1.2, Oxford Instruments Technology (Shanghai) Co., Ltd., Shanghai, China), including grain size, GB characteristics, grain orientation, orientation distribution function (ODF), inverse pole figure (IPF), and geometrically necessary dislocation (GND) density.

3. Results and Discussion

3.1. Preliminary Determination of Creep Mechanism

The stress exponent n, a key parameter in the creep constitutive equation, characterizes the dependence of the steady-state creep rate of crystalline materials on applied stress across varying grain sizes, temperatures, and stress levels. Research on the creep characteristics of metal and alloy materials indicates that distinct values of the stress exponent n correspond to significant transitions in the dominant creep mechanism, which includes diffusion creep (n = 1) [18,19,20], GB sliding (n = 2) [21], and dislocation motion (n ≥ 3) [22]. Based on extensive experimental data on the steady-state creep rate $\dot{\epsilon }$ versus applied stress σ at a constant temperature and grain size, the stress exponent n was derived from the following equation:

$$n={\displaystyle \frac{\mathsf{\partial }log\dot{\epsilon }}{\mathsf{\partial }log\sigma }}$$

(1)

It should also be emphasized that the identification of creep mechanisms based on the stress exponent n is derived primarily from studies on conventional metallic systems. Therefore, the creep mechanisms governing FeCrAl alloys under various conditions require further validation and confirmation via detailed microstructural characterization.

The creep data of FeCrAl alloys at different stresses and temperatures are summarized in

Table 1. Creep data for tests at different stresses and temperatures.

T/°C	σ/MPa	Duration (h)	Creep Rate $\dot{\mathsf{\epsilon }}$ (s−1)	Stress Exponent n
450	150	189.5	3.1 × 10−11	3.4
	175	104.2	5.0 × 10−11
	200	50.3	8.3 × 10−11
475	125	198.0	1.5 × 10−10	3.4
	150	137.2	2.5 × 10−10
	175	84.3	4.7 × 10−10
500	125	141.8	9.1 × 10−10	3.3
	150	107.6	1.8 × 10−9
	175	57.3	3.0 × 10−9
	200	32.7	4.2 × 10−9
525	100	80.2	6.0 × 10−9	3.5
	125	60.7	1.4 × 10−8
	150	35.8	2.4 × 10−8
550	100	47.2	3.8 × 10−8	3.3
	125	33.5	1.1 × 10−7
	150	27.5	1.4 × 10−7
600	75	28.2	2.2 × 10−7	2.1
	100	20.4	3.2 × 10−7
	125	11.7	5.2 × 10−7
	150	6.3	9.4 × 10−7

. According to Equation (1), by plotting the variation in creep rate $\dot{\epsilon }$ with applied stress σ on a double logarithmic scale, the slope obtained from linear fitting corresponds to the stress exponent n at different temperatures, as shown in Figure 2. Under various conditions of temperature and stress, n ranges from 2.0 to 3.5, suggesting that the dominant creep mechanisms are primarily associated with GB sliding and/or dislocation motion.

3.2. Stress Effect

Figure 3 presents the grain size distribution, morphology, and orientation on the rolled surface of the FeCrAl alloys prior to creep testing. The specimen exhibits an equiaxed grain structure, with high-angle grain boundaries (HAGBs) substantially outnumbering low-angle grain boundaries (LAGBs). The grain size ranges from 2.5 μm to 40.9 μm, with an average of 12.0 μm.

Figure 4 shows the grain size and crystal orientation distributions of the 12 μm FeCrAl alloys at 600 °C under applied stresses of 100 MPa and 150 MPa. Grain coarsening occurred during creep, and higher applied stress led to more pronounced grain growth. In addition, at lower applied stress, creep deformation reduced the proportion of LAGBs. Conversely, under elevated stress, a significant increase in LAGB density was observed, indicative of stress-enhanced dislocation multiplication. Under lower stress, dislocations initially pinned at grain boundaries can escape from tangles through glide and climb mechanisms. During this process, dislocations may be absorbed by grain boundaries or become annihilated via interactions between oppositely oriented edge dislocations. Additionally, at lower deformation rates and higher temperatures, large grains tend to engulf smaller ones to reduce the overall GB energy, driving microstructural evolution from high- to low-energy states. This dual mechanism also explains the observed reduction in LAGB density. However, under high stress, the specimens exhibit higher deformation rates. The rapid accumulation of large strains within a short period causes dislocation multiplication to exceed the annihilation rate. As a result, LAGBs are gradually formed by the entanglement and pile-up of dislocations.

Figure 5 shows the GND distribution of the 12.0 μm FeCrAl alloys at 600 °C under different stress conditions. As applied stress increases, the GND density initially decreases slightly and then increases significantly. This is because applied stress facilitates overcoming the energy barrier for pinned dislocations, enabling them to be absorbed at grain boundaries or annihilated mutually with dislocations of the opposite sign. At low stress, the dislocation multiplication rate is relatively low, leading to a net decrease in dislocation density. In contrast, high applied stress induces significant strain, resulting in substantially elevated GND density. At 100 MPa, the GND density is comparable to pre-creep levels, while at 150 MPa, it increases up to approximately four times the initial value. Concurrently, the proportion of LAGBs increases to more than twice that before creep at higher stress (Figure 4), confirming accelerated dislocation proliferation. The same phenomenon was also detected in earlier experimental results of Wang et al. [23] on the Fe-18Ni-12Cr alloy. These results demonstrate that dislocation glide and climb progressively dominate the creep process under high-stress conditions. Conversely, under low stress, both dislocation density and configuration exhibit minimal change, exerting negligible influence on creep behavior. Consequently, GB sliding is more likely to dominate the process of creep deformation in FeCrAl alloys at low stress, with possible contributions from atomic diffusion. This is evidenced by a stress exponent of n ≈ 2.0 at 600 °C across the tested stress range (Figure 2), confirming GB sliding as the controlling mechanism.

In the present study, the real-time tracking of the evolution of the grain boundary microstructure during creep testing was not feasible. As evidenced in Figure 4, continuous grain growth occurs under applied stress with substantial increases in the average grain size, while the matrix maintains contiguity. This microstructural evolution necessitates GB sliding to accommodate intergranular deformation during creep. Based on the above analysis, it is concluded that the dominant creep mechanism transits from GB sliding to dislocation motion as stress increases. Through a systematic investigation of the relationship between creep mechanisms and strain rate in FeCrAl alloys and with supported observations by SEM and TEM, Kamikawa et al. reached similar conclusions [12]. The same phenomenon was also found in earlier atomistic simulations conducted by our research team regarding the transition in the high-temperature creep mechanisms of FeCrAl alloys [24,25].

3.3. Temperature Effect

The effects of temperature and grain size on dislocation motion and GB sliding deformation were also investigated. Figure 6 presents the GND distribution of the 12.0 μm FeCrAl alloys at different temperatures and stresses of 75 MPa and 125 MPa. Under the same stress conditions, the GND density increased significantly with temperature. It was found that there was a negligible change in GND density at 525 °C and 125 MPa, while at 600 °C, it rose to over twice the pre-creep value. Using atomistic simulations, Ye et al. [26] investigated the temperature and strain rate effects on the tensile properties and deformation mechanisms of single-crystal FeCrAl alloys. It is pointed out that elevated temperatures promote dislocation proliferation and motion, while low temperatures are demonstrated to effectively suppress dislocation generation. Critically, thermal activation at elevated temperatures facilitates dislocation glide and climb, preventing dislocation pileups and subsequent crystal deformation. This suggests that the critical stress for initiating dislocation motion decreases as temperature increases.

Figure 7 presents the grain orientation distributions of the 12.0 μm FeCrAl alloys under different stress and temperature conditions. Compared to the pre-creep specimen shown in Figure 3, a proportion of LAGBs decreased under lower temperature (525 °C) and higher stress (125 MPa) and increased significantly under higher temperature (650 °C) and lower stress (75 MPa). This may be because higher temperatures facilitate the velocity and mobility of dislocations, leading to faster glide and climb processes. For the convenience of comparison and analysis, the detailed changes in grain size for FeCrAl alloys under different loading conditions are provided in

Table 2. The grain size (GS) of the 12 μm FeCrAl alloys before and after creep under different stress and temperature conditions.

T/°C	σ/MPa	Minimum GS/μm	Maximum GS/μm	Average GS/μm
Before creep		2.5	40.9	12.0
525	125	2.5	39.6	12.1
600	100	2.5	46.5	12.1
600	150	2.5	45.9	13.0
650	75	2.5	57.2	14.4

. Grain coarsening becomes pronounced at elevated temperatures (650 °C), with the average grain size increasing from 12.0 μm to 14.4 μm, while minimal grain growth occurs at lower temperatures. This may be attributed to the accelerated atomic diffusion at GBs under high temperatures, which facilitates the relative motion of atoms along the GBs, thereby accommodating continuous deformation between adjacent grains. In contrast, GB sliding is limited at low temperatures, and grain deformation becomes less prominent over time. Therefore, it is inferred that as temperature increases, the critical stress required for dislocation motion decreases, and the overall sliding process of GBs is accelerated, leading to a significant increase in the amount of GB sliding.

3.4. Grain Size Effect

Figure 8 shows the grain size and orientation distribution of the 9.9 μm FeCrAl alloys before and after creep at 600 °C and 75 MPa. The proportion of LAGBs decreased slightly compared to the initial value. While most grains grew, only a few decreased in size, resulting in a slight increase in the average grain size. At the same temperature, for the alloy with an initial grain size of 12 μm, the average grain size increased by 0.1 μm after creep at 100 MPa (Table 2). In contrast, the alloy with a finer grain size of 9.9 μm underwent creep at a lower stress of 75 MPa and exhibited a larger grain size increase of 0.3 μm. As mentioned earlier, plastic deformation between grains is accompanied by GB sliding to maintain the continuity of the material. These observations indicate that with a decreasing grain size, the process of GB sliding accelerated, thereby enhancing its contribution to the overall deformation.

Figure 9 shows the GND distribution of the 9.9 μm FeCrAl alloys at 600 °C and 75 MPa. From Figure 5 and Figure 9, it can be observed that at the same temperature, the alloy with a 12 μm grain size exhibited no significant increase in GND density under low stress. When the applied stress exceeded 125 MPa, the GND density increased markedly. In contrast, for the alloy with a grain size of 9.9 μm subjected to a low applied stress of 75 MPa, the GND density increased to at least three times its value before creep deformation. From above, it is confirmed that the smaller the GS, the lower the critical stress at which dislocation motion begins to affect creep. The experimental results of Sun [27], Lapera [28], and Spader et al. [29] also indicate that increasing grain size can greatly enhance the creep resistance of FeCrAl and nickel-based high-temperature alloys.

It is well established that alloying elements such as Nb, Ta, and Zr promote the formation of secondary phase in FeCrAl alloys, whereas Mo primarily remains in solid solution within the matrix [30,31,32,33]. Given the specific composition of our alloy (Fe-13Cr-4Al-2Mo, wt.%) containing only minor Mo additions without Nb or Ta, significant secondary phase precipitation should not occur. Consequently, this study focuses on the effects of temperature, applied stress, and grain size on GB sliding and dislocation-mediated creep mechanisms, excluding the influence of secondary phase particles on creep behavior.

3.5. Texture Effect

Figure 10 shows the IPF and ODF maps of the 9.9 μm FeCrAl alloys at 600 °C under different stress conditions. There is no significant difference in the texture orientations across different applied stresses. The texture type was identified as a γ texture with <110>//TD and an α texture with <111>//ND. Based on calculations of the ODF at φ₂ = 45°, the initial texture orientation {221}<110> evolved to {111}<110> at 100 MPa, then reverted to {221}<110> at 150 MPa. Moreover, the maximum texture intensity remained nearly unchanged. FeCrAl alloys exhibit a body-centered cubic structure, the inherent symmetry of which ensures microstructural isotropy and does not favor grains of preferred orientation during deformation. These observations are consistent with the previous creep experiments on accident-resistant FeCrAl cladding [27], which indicate that no significant change in the texture was observed before and during creep. Thus, we can infer that creep behavior is primarily governed by grain size, temperature, and applied stress, with texture playing a negligible role.

4. Conclusions

In this research, the deformation features and the underlying mechanism for the creep process of FeCrAl alloys were investigated through EBSD observation. The dependencies of the mechanism on temperature, stress, and grain size are discussed in terms of the microstructural changes in texture, grain morphology (size and shape), grain orientation, and dislocation density. The results are summarized as follows.

(1)

Temperature, stress, and grain size are primary factors influencing creep behavior, whereas the effect of texture is relatively weak. Large grains tend to engulf smaller grains at lower temperatures and stresses, reducing the proportion of LAGBs after creep. Under high-temperature or high-stress conditions, a substantial multiplication of dislocations within the crystal increases the number of LAGBs.

(2)

As stress increases, the dominant mechanism tends to transition from GB sliding to a dislocation motion within the present research scope. The number and density of dislocations do not change significantly, and GB sliding dominates the deformation process at low stress. Under high-stress conditions, the dislocation density is elevated through strain accumulation, enabling the dislocation motion to dominate the creep process.

(3)

Temperature and grain size have an impact on the mechanisms for dislocation motion under high stress. The higher the temperature or the smaller the grain size, the lower the critical stress required to initiate dislocation motion, thus leading to a marked increase in dislocation density and weakening the alloy’s high-temperature creep resistance.