Effects of Zr Content on the Microstructure of FeCrAl ODS Steels

Abstract

FeCrAl oxide dispersion strengthened (ODS) steels are an important kind of cladding material for accident-tolerant fuels. Their radiation resistance and mechanical properties are closely related to the grain size and dispersed second phases. In order to tailor the microstructure and provide an experimental basis for the composition design of FeCrAl ODS steels, in this paper FeCrAl ODS steels with different Zr contents were prepared by mechanical alloying and the subsequent hot isostatic pressing (MA-HIP) process. The effects of Zr content on the grain size distribution and the precipitation of dispersed second phases in FeCrAl ODS steels were investigated by electron back-scattered diffraction (EBSD) and transmission electron microscopy (TEM). The results showed that the grain size decreased first and then increased as the Zr content increased, and that the average grain size achieved the minimum value of 2.092 μm when the Zr content was 0.6 wt.%. The Zr content had a negligible effect on the grain orientation of FeCrAl ODS steels, but the dispersed second phase changed from the Al₂Y₄O₉ phase with monoclinic structure to the Y₄Zr₃O₁₂ phase with hexagonal structure as the Zr content increased.

1. Introduction

The performance of nuclear fuel cladding is an important guarantee for the advanced level as well as the safety and reliability of reactors [1]. Zr alloy cladding has been widely used in current light water reactors (LWRs) due to its merits of a small thermal neutron absorption cross section and good water resistance. However, the Fukushima nuclear accident in Japan exposed the potential safety hazards of Zr alloy cladding under accident conditions [2,3]. In order to improve the accident tolerance of nuclear fuel cladding materials, accident-tolerant fuels (ATFs) have attracted extensive attention in the field of international nuclear fuels in recent years.

In addition to the vanadium–chromium–titanium alloys [4,5] and duplex and ferritic stainless steels [6,7,8], which exhibit superior high-temperature service performance, FeCrAl ODS steels are considered as the promising candidates for ATF cladding due to their excellent resistance to high-temperature water vapor oxidation and radiation resistance [9,10]. The microstructure of FeCrAl ODS steels directly affects their radiation resistance and mechanical strength. In the alloying design of FeCrAl ODS steels, the Al element is essential to ensure their excellent resistance to high-temperature water vapor oxidation. The addition of an Al element can produce a dense oxide film on the surface of the materials, which can delay the corrosion process at high temperatures, thus significantly improving the high-temperature corrosion resistance. In 2015, Yamamoto Y et al. [11], in the Oak Ridge Laboratory, proved through oxidation experiments of different FeCrAl alloy models that an Al element content >4 wt.% was the key to ensure that an FeCrAl alloy had excellent resistance to high-temperature water vapor oxidation at 1200 °C. Although the Al element is rather important for the high-temperature oxidation resistance of the FeCrAl alloy, it can lead to the coarsening of Y-Al-O particles in the matrix, which deteriorates the radiation resistance and mechanical properties [12,13,14,15]. Hence, how to control the phase structure and size distribution of precipitated particles in FeCrAl ODS steels is a key technical issue in the development of ATF cladding materials. Qian et al. [16] calculated the energy-minimizing configurations and their corresponding formation energies of such clusters as Y-Zr-O, Y-Al-O, and Y-Ti-O through first-principle calculations, and the results showed that the decrement of free energy of the system caused by the combination of Zr element and the O-Y cluster core was the largest, indicating that the generated Y-Zr-O cluster core was more stable. Their work provided a thermodynamic calculation basis for regulating dispersed oxide particles in FeCrAl ODS steels.

In this study, FeCrAl ODS steels with different Zr contents (0–1.2 wt.%) were prepared by powder metallurgy, and the grains and dispersed oxide nanoparticles in the steels were thoroughly investigated to clarify the effects of Zr content on the microstructure of FeCrAl ODS steels. Our work provides the basis for the alloying design of accident-tolerant FeCrAl ODS steel cladding materials.

2. Experimental Details

2.1. Preparation of FeCrAl ODS Steel Samples

According to the composition design of

Table 1. Composition design of FeCrAl ODS steels containing Zr element (wt.%).

Sample	Fe	Cr	Al	Ti	W	Y2O3	Zr
1	Bal.	13	4.5	0.4	2	0.35	0
2	Bal.	13	4.5	0.4	2	0.35	0.3
3	Bal.	13	4.5	0.4	2	0.35	0.6
4	Bal.	13	4.5	0.4	2	0.35	1.2

, FeCrAl ODS steels were prepared by the powder metallurgy method. First, prealloy powder was obtained by Ar atomization. The prealloy powder consisted of Fe, Ti, Cr, W, and Al, with a granularity of about 200 mesh. Then, the prealloy powder, metallic Zr powder with granularity of about 200 mesh (purity > 99.9%), and Y₂O₃ powder with particle sizes of 30–50 nm (purity > 99.9%) were mixed. The mixed powder was mechanically alloyed with a German Pulverisette 5 planetary high-energy ball mill (FRITSCH Inc., Weimar, German) during which nanoscale Y₂O₃ powder was decomposed and dissolved into the alloy matrix powder. Mechanical alloying is a ball milling process, during which pure element powders or mixture powders placed in the ball mill are subjected to high energy collision and milling. Then, the solid-state reaction takes place and all powders are uniformly mixed. Thus far, the mechanical alloying has been widely used for manufacturing advanced materials due to its convenient operation and lower cost. In this work, the mechanical alloying process was performed by dry ball milling under an Ar gas protective atmosphere. The ratio of ball was 10:1, the speed was 260 r/min, and the milling time was 50 h. After ball milling, the mechanically alloyed powder was packed into a pre-prepared ladle sheath for degassing and seal welding. Finally, the FeCrAl ODS steel samples were obtained through the sintering and densification of the powder by a hot isostatic pressing process under 150MPa at 1100 °C for 2 h.

2.2. Microstructure Analysis Method

The grain size distribution and grain orientation of FeCrAl ODS steel samples were analyzed by electron back-scattered diffraction (EBSD). The preparation process of EBSD samples was as follows. First, the FeCrAl ODS steel was cut into cuboid samples of 3 mm × 5 mm × 10 mm by wire cutting method, and sandpapers of 400#, 800#, 1200#, and 2000# were successively used for grinding the samples. After that, the samples were ultrasonically cleaned to remove surface contamination, and electropolished to remove the internal stress on the sample surface. The electrolyte for electropolishing was 20% perchloric acid + 10% glycerol + 80% ethanol, the voltage parameter was set to 25 V, the current parameter was set to 2 A, and the polishing time was 20 s. For EBSD scanning, the voltage was 20 kV, the step length was 0.25 μm/s, and the scanning area was 50 μm × 50 μm.

The crystal structure and size distribution of oxide nanoparticles dispersed in FeCrAl ODS steel matrix were analyzed by a Tecnai G2 F30 S-TWIN transmission electron microscope (TEM). The samples for TEM analysis were prepared as follows. First, the slices with a thickness of 300 μm were obtained by wire cutting. Then, the slices were mechanically ground to 30 μm. Finally, electrolytic thinning was performed on the slices by using a dual-jet electrolytic thinning machine (Model MTP-1A, Leibo Inc., Changzhou, China). The final thinning by dual-jet electrolysis was carried out with a voltage of 20 V in a 10% perchloric acid alcohol solution, and the temperature was controlled to be −30 °C.

3. Results and Discussion

3.1. Effects of Zr Content on Grain Size and Grain Orientation

The grain size distribution and grain orientation of FeCrAl ODS steels with different Zr contents were analyzed by EBSD. Figure 1 shows the grain orientations of FeCrAl ODS steels with different Zr contents. Different colors of the triangle color card in the lower right corner represented different crystal planes. Herein, the blue color refers to crystal plane (111), the green refers to crystal plane (101), and the red refers to crystal plane (001). As observed, the grain orientations of FeCrAl ODS steels with different Zr contents are random and the grains are equiaxed ones with no preferential orientation, suggesting that Zr content had negligible effect on the grain orientations of HIP-sintered FeCrAl ODS steels.

In order to analyze the effects of Zr content on the grain size distribution of FeCrAl ODS steels, Image-Pro Plus (6.0, MEDIA CYBERNETICS Inc., Rockville, MD, USA) was used for analyzing the grain sizes according to Figure 1. Figure 2 shows the grain size distribution of FeCrAl ODS steels with different Zr contents in terms of area percentage of grains in different size ranges. As observed, the grain size distribution of FeCrAl ODS steel without Zr addition is relatively discrete and shows the characteristic of bimodal distribution, in which the diameters of small-sized grains are concentrated within the range of 2–4 μm, and those of relatively large grains are approximately 6 μm. After the addition of the Zr element, the grain size distribution range of FeCrAl ODS steels is narrowed (mainly within 2–4 μm), and the number of large-sized grains is significantly reduced.

Figure 3 shows the calculated average grain sizes in the as-prepared samples based on Figure 1. As observed, the grain sizes of FeCrAl ODS steel samples with Zr addition are all smaller than that of the sample without Zr addition, suggesting that the addition of the Zr element can refine the grains. With the increase of Zr content, the grain size decreases first and then increases. When the Zr content is 0.6 wt.% (Sample #3), the grain size is the smallest. When the Zr content increases from 0.6 to 1.2 wt.%, the grain size increases, but it is still slightly smaller than that of samples with 0.3 wt.% Zr element. The change trend of the calculated average grain size is consistent with that of the grain size distribution in Figure 2. To be specific, when the Zr content is 0.6 wt.%, the grain size distribution is the most concentrated, and the proportion of grains over 4 μm is the lowest. Hence, the average grain size of samples with 0.6 wt.% Zr element is the smallest.

3.2. Effects of Zr Content on Dispersed Oxide Particles

Figure 4 shows the TEM images of dispersedly distributed nano-sized second phases in the matrix of FeCrAl ODS steels with different Zr contents. As observed, plentiful nanoparticles are distributed in the matrix. In order to more accurately analyze the effects of Zr content on the distribution of dispersed second phases in the matrix of FeCrAl ODS steels, Image-Pro Plus was used to calculate the area distribution and average diameter per unit area of nano-precipitates according to the contrast difference between nanoscale second phases and the matrix in Figure 4. Assume that the nano-sized second phases are tiled with their average diameter thickness in the matrix, and then the number density can be calculated.

Table 2. Statistical results of nano-precipitate distribution in FeCrAl ODS steels with different Zr contents.

Sample	Zr Content (wt.%)	Mean Area (nm2)	Mean Diameter (nm)	Number Density (1023 Particle·m−3)
1#	0	53.55	7.83	0.656
2#	0.3	45.00	7.15	1.870
3#	0.6	30.20	5.58	3.330
4#	1.2	46.00	7.21	1.040

lists the distribution of nano-precipitates in the matrix of FeCrAl ODS steels with different Zr contents.

As shown in Table 2, the nano-sized second phases in the matrix of FeCrAl ODS steels with different Zr contents all exhibit average particle diameters below 10 nm and relatively high number densities. With the increase of Zr content in FeCrAl ODS steels, the average size of precipitated particles dispersed in the matrix decreases first and then increases, while the number density increases first and then decreases. When the Zr content is 0.6 wt.%, the precipitated particles in the FeCrAl-ODS steel matrix exhibit the minimum average size and the maximum number density, indicating an optimized dispersion-enhanced microstructure.

Figure 5 shows the high-resolution TEM images of precipitated particles in FeCrAl ODS steels with different Zr contents, as well as the corresponding diffraction spot analysis results. Based on the TEM diffraction spots, the interplanar spacings of different crystal planes are measured and compared with the possible interplanar spacings on the standard cards (see

Table 3. Interplanar spacing of dispersed nanoscale phases in the matrix of FeCrAl ODS steels.

Sample	#1			#2			#3			#4
Crystal plane	($0\overline{2}$0)	($0\overline{2}1$)	($001$)	(102)	($\overline{1}$30)	($\overline{2}$$3\overline{2}$)	($10\overline{3}$)	($24\overline{2}$)	(141)	($0\overline{4}\overline{1}$)	($4\overline{4}$0)	(401)
Theoretical interplanar crystal spacing (nm)	0.5253	0.1301	1.0534	0.4009	0.3188	0.2612	0.2858	0.1504	0.1804	0.2054	0.2108	0.2054
Actual interplanar crystal spacing (nm)	0.5227	0.4606	1.0720	0.3813	0.3097	0.2549	0.2915	0.1549	0.1821	0.2022	0.2029	0.2025

). To further explore the crystal structure of the second phases, the interplanar angles of different dispersed second phases are calculated and compared with actual measured values (see

Table 4. Included angle of crystal planes of dispersed nanoscale phases in the matrix of FeCrAl ODS steels.

Sample	#1		#2		#3		#4
Crystal plane	($0\overline{2}$0)/(0$\overline{2}1$)	($0\overline{2}$1)(001)	(102)/($\overline{1}$30)	($\overline{1}$30)/($\overline{2}$$3\overline{2}$)	($10\overline{3}$)/(24$\overline{2}$)	(24$\overline{2}$)/(141)	($0\overline{4}\overline{1}$)/(4$\overline{4}$0)	(4$\overline{4}$0)/(401)
Theoretical value (°)	26.51	63.49	84.85	40.45	56.47	31.74	60.85	60.85
Measured value (°)	27.81	65.15	84.12	41.34	57.87	31.91	60.05	60.78

). The above analysis results reveal that the main phase of the dispersed nanoparticles in Sample #1 is Al₂Y₄O₉ of a monoclinic crystal system, while the main phase of the dispersed nanoparticles in Samples #2–#4 is Y₄Zr₃O₁₂ of a hexagonal crystal system.

3.3. Influence Mechanism of Zr Content on the Size Distribution of Nano-Precipitates

From a thermodynamic point of view, the Y-Zr-O cluster is more stable than the Y-Al-O cluster [16], and the addition of the Zr element to FeCrAl ODS steels can generate a Y-Zr-O cluster with a higher number density during the formation of nano-precipitates, thus providing more nucleation sites for nano-oxide particles. At a given O content in the alloy, more nucleation sites of nano-oxide particles can lead to a smaller average size of nano-oxides after nucleation and growth. It should be noted that the Y-Zr-O cluster has a certain saturated concentration during the formation of nano-precipitates in FeCrAl ODS steels. When the Zr content is higher than the saturated concentration of the Y-Zr-O cluster, the remaining Zr would form other larger precipitates. As a result, the average size of precipitated particles increases significantly when the Zr content is 1.2 wt.%. The presence of large-size precipitates in FeCr and FeAl alloys with Zr addition has been demonstrated in previous studies. For instance, Scudino et al. [17,18,19,20] reported the presence of (Fe, Al)₂Zr Laves precipitates with particle sizes of 2–4 μm in both FeCr and FeAl alloys with Zr addition prepared by the smelting method.

4. Conclusions

In summary, FeCrAl ODS steels with different Zr contents were prepared by mechanical alloying and the hot isostatic pressing processes, and the influence rules and mechanism of Zr content on grains and dispersed second phases of FeCrAl ODS steels were studied by EBSD and TEM. Some conclusions can be drawn as follows:

(1)

Zr content had a negligible effect on grain orientation distribution of FeCrAl ODS steels, and the grains of FeCrAl ODS steels with different Zr contents were all equiaxed ones with randomly distributed orientations.

(2)

Zr could refine grains of FeCrAl ODS steels, and its refinement mechanism was the “pinning” effect of nano-sized second phases on grain boundaries during grain growth. As the Zr content increased, the size of nano-sized precipitates increased, which could weaken the pinning ability. Hence, the grain size of FeCrAl ODS steels decreased first and then increased. The minimum average grain size was 2.092 μm when the Zr content was 0.6 wt.%.

(3)

The dispersed nanoparticles in the matrix of FeCrAl ODS steel without Zr addition were mainly the Al₂Y₄O₉ phase of a monoclinic crystal system, and those in the matrix of FeCrAl ODS steels with Zr addition were mainly the Y₄Zr₃O₁₂ phase of a hexagonal crystal system. The dispersed nanoparticles in FeCrAl ODS steels had a large average diameter per unit area.