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Acta Metall Sin  2022, Vol. 58 Issue (7): 883-894    DOI: 10.11900/0412.1961.2020.00533
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Microstructure Evolution at Elevated Temperature and Mechanical Properties of MoNb-Modified FeCrAl Stainless Steel
WEN Donghui1, JIANG Beibei2, WANG Qing3(), LI Xiangwei1, ZHANG Peng1, ZHANG Shuyan1()
1.Centre of Excellence for Advanced Materials, Dongguan 523808, China
2.Analysis and Test Center, Guangdong University of Technology, Guangzhou 510006, China
3.School of Materials Science and Engineering, Dalian University of Technology, Dalian 116024, China
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MoNb-modified FeCrAl ferritic stainless steel (C35MN: Fe-13Cr-4.5Al-2Mo-1Nb, mass fraction, %) exhibits excellent comprehensive properties, including oxidation and corrosion resistance, as well as moderate mechanical properties, machinability, and neutron irradiation-resistance, making them potential accident-tolerant fuel (ATF) cladding materials for pressurized water reactors. However, the microstructural evolution and corresponding mechanical properties of C35MN alloys at the loss-of-coolant accident temperature have not been systematically studied. Herein, the microstructural evolution and mechanical properties of C35MN alloys during 400 h aging at 800oC and 1 h annealing at 1000-1200oC were systematically investigated. The alloy ingots were prepared by vacuum induction melting and cast into round bars, followed by 1150oC hot-forging, 800oC hot-rolling, and aging at 800oC for 400 h. The samples annealed at 1000-1200oC for 1 h were preaged at 800oC for 24 h. The C35MN alloy exhibited excellent microstructural stability at 800 and 1000oC, which is attributed to the precipitation of the Laves phase. The alloy showed a good combination of strength and ductility. However, when the annealing temperature increased above 1100oC, a large amount of the Laves phase dissolved into the ferritic matrix, resulting in the coarsening of the matrix grains. Annealing above 1200oC for 1 h, the grain size increased to 310 μm, severely degrading the mechanical property of the C35MN alloy below the requirement of ATF cladding materials. The microstructural stability of the C35MN alloy was influenced by the thermal stability of the Laves phase, which depends on the composition of the phase. The thermal stability of the Laves phase depends on the solid solubility of Laves phase forming elements in the ferritic matrix: the lower the solid solubility, the higher thermal stability of the Laves phase. The mechanical properties of C35MN were significantly affected by the grain size. The alloy exhibited ductile fracture when the grain size was less than 50 μm and brittle cleavage fracture when the grain size was above 130 μm.

Key words:  fuel cladding material      FeCrAl stainless steel      microstructural stability      Laves phase      mechanical property     
Received:  30 December 2020     
ZTFLH:  TG113.1  
Fund: Guangdong Basic and Applied Basic Research Foundation(2019A1515110051);Guangdong Innovative and Entrepreneurial Research Team Program(2016ZT06G025)
Corresponding Authors:  WANG Qing,ZHANG Shuyan     E-mail:;

Cite this article: 

WEN Donghui, JIANG Beibei, WANG Qing, LI Xiangwei, ZHANG Peng, ZHANG Shuyan. Microstructure Evolution at Elevated Temperature and Mechanical Properties of MoNb-Modified FeCrAl Stainless Steel. Acta Metall Sin, 2022, 58(7): 883-894.

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Table 1  Nominal and measured compositions of Fe-13Cr-4.5Al-2Mo-1Nb (C35MN) alloy
Fig.1  OM (a) and SEM (b) images of the 800oC, 24 h aged C35MN alloy
Fig.2  XRD spectra of C35MN alloy with different heat-treatments
Fig.3  SEM images of the C35MN alloy aged at 800oC for 2 h (a), 8 h (b), 50 h (c), 100 h (d), 200 h (e), 300 h (f), and 400 h (g), and the corresponding variation curves of the volume fraction and precipitate size of Laves phase (h)
Fig.4  OM images of the C35MN alloy aged at 800oC for 2 h (a) and 400 h (b)
Fig.5  OM (a, c, e) and SEM (b, d, f) images of the C35MN alloy annealed at 1000oC, 1 h (a, b), 1100oC, 1 h (c, d), and 1200oC, 1 h (e, f) after 800oC, 24 h aging
Fig.6  Variation of the volume fraction of Laves phase and grain size of C35MN alloy with annealed temperature
Fig.7  TEM bright-field images (a-c) and SAED pattern (inset) of the C35MN alloy annealed at 1000oC, 1 h after 800oC, 24 h aging
Fig.8  Engineering tensile stress-strain curves of the 800oC, 24 h aged C35MN alloy at different temperatures (RT—room tmperature) (a), and variation of mechanical properties with temperature (b)
Fig.9  SEM fracture images (a, b) and BSE-SEM images showing the deformed microstructure (c, d) at RT (a-c) and 800oC (d) of the 800oC, 24 h aged C35MN alloy near fracture surface
Fig.10  Engineering tensile stress-strain curves of the annealed and annealed + 800oC, 24 h re-aged C35MN alloy after 800oC, 24 h aging (a), and variation of mechanical property parameters with heat treatment states (b)
Fig.11  Variation of the micro-hardness of C35MN alloy with aging time at 800oC, the hardness of annealed alloys at different temperature also exhibited



CompositionSolid solubility[29]
Table 2  Compositions of Laves phase and the solid solubilities of Mo and Nb elements in ferrite in C35MN alloy
Fig.12  SEM fracture images of the annealed and annealed + 800oC, 24 h re-aging alloys after 800oC, 24 h aging
(a) 1000oC, 1 h (b) 1000oC, 1 h + 800oC, 24 h
(c) 1100oC, 1 h (d) 1100oC, 1 h + 800oC, 24 h
(e) 1200oC, 1 h (f) 1200oC, 1 h + 800oC, 24 h
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