Effect of niobium content on irradiation microstructure and hardening in FeCrAl-based alloys

doi:10.1016/j.jmst.2021.04.020

Abstract

Abstract:

Iron-chromium-aluminum (FeCrAl) alloys with different content of niobium (Nb)—0, 0.4 wt%, 0.8 wt%, and 1.2 wt%—were designed and prepared. All samples were then irradiated with 2.4 MeV Fe²⁺ ion to the dose of 1 and 15 displacements per atom (dpa) at 400 °C. The formations of dislocation loops induced by self-ion irradiation in these alloys were investigated by transmission electron microscopy (TEM). Nano-indentation tests were used to assess the hardness and irradiation hardening of samples. For the samples before irradiation, the (Fe, Cr)₂(Nb, Mo) Laves phases density and the nano-indentation hardness increased with increasing Nb content of the samples. After irradiation to 1 and 15 dpa, both of a/2<111> and a<100> dislocation loops were produced but no voids or αˊ phase were found in all samples. With increasing Nb content of the samples, the size of dislocation loops increased first and then decreased, while the total volume number density decreased and then increased. The fraction of a<100> dislocation loops increased first and then decreased with increasing Nb content, and increased with increasing irradiation dose. Dislocation networks and the amorphization of the Laves phases were observed in the samples with irradiation dose of 15 dpa. Irradiation hardening of Nb free samples was two to four times that of Nb containing samples, and the irradiation hardening increased with increasing Nb content of Nb containing samples. The experimental results indicate that the increase of Nb content in FeCrAl alloys can increase the density of Laves phases, leading to the decrease of Mo content and increase of Cr content in the matrix. The competition between the two types of solutes affects the nucleation and growth of the dislocation loops.

Key words: FeCrAl alloy, Dislocation loops, Radiation damage, Accident tolerant fuel cladding, Laves phase

Xiong Zhou, Hui Wang, Liping Guo, Yiheng Chen, Fang Li, Yunxiang Long, Cheng Chen, Ziyang Xie, Hongtai Luo, Shaobo Mo. Effect of niobium content on irradiation microstructure and hardening in FeCrAl-based alloys[J]. J. Mater. Sci. Technol., 2021, 95: 181-192.

Figures/Tables 17

Table 1 Compositions of model FeCrAl alloys (wt%).

Alloy	Fe	Cr	Al	Mo	Nb	Si	Y
0Nb	80.2	13.0	4.5	2.0	0	0.25	0.05
0.4Nb	79.8	13.0	4.5	2.0	0.4	0.25	0.05
0.8Nb	79.4	13.0	4.5	2.0	0.8	0.25	0.05
1.2Nb	79.0	13.0	4.5	2.0	1.2	0.25	0.05

Fig. 1. SRIM predictions for 2.4 MeV Fe2+ irradiation of Fe-13Cr-4.5Al model alloy at a fluence of 1.45 × 1015 ions cm-2. The red horizontal line corresponds to the value of 100 appm/dpa below which the ion-irradiation induced Fe concentration can be ignored and is suitable for irradiation damage analysis.

Fig. 2. Bright-field images of unirradiated samples of (a) 0Nb, (b) 0.4Nb, (c) 0.8Nb, (d) 1.2Nb. The red arrow indicates the Laves phases in Nb-containing samples.

Fig. 3. Laves-Fe2Nb precipitates in the unirradiated region of the 1.2Nb sample: (a) BF-TEM image and Nano-beam electron diffraction of particle pointed by the red arrow, (b) HADDF image, and (c) EDS line scan compositional profiles across the particle drawn by a red line in (b).

Table 2 Summary of average size/number density of identified features in unirradiated FeCrAl alloys.

Property	Alloy
	0Nb	0.4Nb	0.8Nb	1.2Nb
Nb content (wt%)	0	0.4	0.8	1.2
ρ_dislocation (m ^{- 2})	3.7 ± 1.0 × 10¹³	5.2 ± 0.5 × 10¹³	6.4 ± 3.1 × 10¹³	7.1 ± 2.6 × 10¹³
ρ_{Laves phase} (m ^{- 3})	0	2.33±0.66 × 10¹⁹	3.17±0.51 × 10¹⁹	4.42±0.47 × 10¹⁹
d_{Laves phase} (nm)	0	145.95±63.48	169.82±65.67	156.49±49.96
d_grain (μm)	7.67±10.34	4.44±15.09	4.05±9.40	2.89±1.93

Fig. 4. Bright-field images of the dislocation loops in samples irradiated to 1 dpa at 400 °C with a) 0Nb g = 1-10 near the [110] zone axis, (b) 0.4Nb g = 1-10 near the [001] zone axis, (c) 0.8Nb g = 1-10 near the [110] zone axis, and (d)1.2Nb g = 1-10 near the [110] zone axis. The black arrow indicates the direction of g-vector.

Fig. 5. Bright-field images of the dislocation loops in samples irradiated to 15 dpa at 400 °C with (a) 0Nb g = 1-10 near the [001] zone axis, (b) 0.4Nb g = 1-10 near the [001] zone axis, (c) 0.8Nb g = 1-10 near the [110] zone axis, and (d)1.2Nb g = 1-10 near the [110] zone axis. The black arrow indicates the direction of g-vector.

Fig. 6. Size distribution statistics of dislocation loops in samples irradiated to 1 dpa at 400 °C with (a) 0Nb, (b) 0.4Nb, (c) 0.8Nb, (d) 1.2Nb.

Fig. 7. Size distribution statistics of dislocation loops in samples irradiated to 15 dpa at 400 °C with (a) 0Nb, (b) 0.4Nb, (c) 0.8Nb, (d) 1.2Nb.

Fig. 8. (a) Average sizes of dislocation loops as a function of Nb content at doses of 1 dpa and 15 dpa, and (b) volume number densities as a function of Nb content at doses of 1 dpa and 15 dpa. The data points were slightly shift along Nb content axis for comparison.

Fig. 9. (a) Volume number densities of a/2<111> and a<100> dislocation loops as a function of Nb content at doses of 1 dpa and 15 dpa, and (b) percentages of a<100> loops as a function of Nb content at doses of 1 dpa and 15 dpa.

Fig. 10. Bright-field images and nano-beam electron diffraction of particle pointed by the red arrow in 1.2Nb, (a) at dose of 1 dpa, (b) at dose of 15 dpa.

Fig. 11. Average hardness as a function of indentation depth for different Nb content FeCrAl alloys at (a) unirradiated, (b) 1 dpa, (c) 15 dpa, and (d) the value of H0 as a function of Nb content for before and after irradiation samples.

Table 3 Summary of H0 and ΔH based on the Nix-Gao model in the investigated FeCrAl alloys and irradiation doses.

Alloys	Dose (dpa)	H₀ (GPa)	ΔH (GPa)
0Nb	0	2.91±0.36	-
	1	3.96±0.48	1.05
	15	4.29±0.46	1.39
0.4Nb	0	2.98±0.34	-
	1	3.22±0.27	0.25
	15	3.34±0.55	0.36
0.8Nb	0	3.05±0.29	-
	1	3.50±0.23	0.45
	15	3.73±0.39	0.67
1.2Nb	0	3.37±0.34	-
	1	3.52±0.59	0.15
	15	4.06±0.47	0.69

Fig. 12. Concentration of Mo and Cr in the matrix for different Nb content samples in the unirradiated areas determined with EDS line scanning.

Table 4 Summary of ΔH based on the Nix-Gao model and DBH model and α value in the investigated alloys and irradiation doses.

Alloys	Dose (dpa)	ΔH^measured (GPa)	ΔH^calculated (GPa)	α
0Nb	0	-	-	0.54
	1	1.05	1.05
	15	1.39	1.25
0.4Nb	0	-	-	0.13
	1	0.25	0.26
	15	0.36	0.29
0.8Nb	0	-	-	0.22
	1	0.45	0.44
	15	0.67	0.56
1.2Nb	0	-	-	0.30
	1	0.15	0.64
	15	0.69	0.69

Fig. 13. The hardness increment ΔH from DBH model calculation and nano-indentation test as a function of Nb content.

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