Towards the development of novel cladding materials based on nanolaminated ternary carbides (MAX phases) for different nuclear systems

Tunca Bensu

Promoter

Vleugels Jozef, (KULeuven - MTM), Jozef.vleugels@mtm.kuleuven.be

SCK•CEN Mentor

Lambrinou Konstantza
konstantza.lambrinou@sckcen.be
+32 14 33 31 64

SCK•CEN Co-mentor

Delville Rémi
remi.delville@sckcen.be
+32 14 33 31 65

Expert group

Structural Materials Research

PhD started

2015-10-01

Short project description

MAX phases are compounds, the chemical composition of which is given by the general formula Mn+1AXn, where n = 1, 2, or 3. The letter "M" in the formula is an early transition metal (Ti, Nb, V, Cr, Zr, …), "A" is an A-group element (Si, Al, Sn, …) and "X" is C or N [1]. MAX phases are a fascinating family of layered solids characterized by a unique combination of properties that can be associated with their atomic bonding, nano-laminated crystal structure and the fact that the multiplication and mobility of basal dislocations is possible even at room temperature [1-3]. The nano-laminated nature of MAX phases is reflected in the fact that near-closed-packed M6X layers are interleaved with pure group-A element layers with the X atoms filling the octahedral sites between the group-A element layers [1]. The properties of MAX phases are on one hand close to the properties of their corresponding binary carbides and nitrides, i.e. they are elastically stiff, have good thermal and electrical conductivity, are resistant to chemical attack, and have relatively low thermal expansion coefficients [2,3]. On the other hand, MAX phases can also behave like metals, i.e. they are relatively soft (2–8 GPa) and readily machineable, thermal shock resistant, and damage tolerant; moreover, some are fatigue, creep, and oxidation resistant [2,3]. At room temperature, they can be compressed to stresses as high as 1 GPa and fully recover upon load removal, while dissipating 25% of the mechanical energy [2,3]. At higher temperatures, however, they undergo a brittle-to-plastic transition, and their mechanical behavior depends strongly on the imposed deformation rate [2,3].

The compatibility of MAX phases with heavy liquid metals, such as the envisaged lead-bismuth eutectic (LBE) primary coolant of the MYRRHA system, is another appealing property of these materials, as reported in literature [4] and experimentally verified at SCK•CEN by means of dedicated liquid metal corrosion tests on a broad selection of MAX phases [5]. The superb liquid metal corrosion resistance of MAX phases makes them promising candidate materials for various applications in Gen-IV lead fast reactors (LFRs). At SCK•CEN, exploring the potential of MAX phases for Pb/LBE-cooled fast reactors started with the PhD work of Mr. Thomas Lapauw; this PhD work began in October 2013 in collaboration with the Dept. of Materials Engineering (MTM) of the KU Leuven (promotor: Prof. Jozef Vleugels) and has already produced the first worth-publishing results [6]. The PhD work of Mr. Lapauw is primarily dedicated to the identification of MAX phases that can meet the material property requirements of the MYRRHA pump impeller. For this particular application, the selected structural material should primarily demonstrate an adequate erosion resistance in contact with fast-flowing LBE (predicted flow velocity: 10-20 m/s locally). In order to achieve the high durability of the envisaged application, the geometrically complex pump impeller will be fabricated in the machineable MAX state and will subsequently be heat-treated in a judiciously chosen atmosphere, so as to promote the formation of hard phases (carbides, nitrides) on the component surface. The PhD work of Mr. Lapauw finds support in Task 3.4 of the FP7 MatISSE project, which, apart from all other testing activities, foresees testing of the candidate MAX materials at high LBE flow velocities in the CORELLA facility at KIT, Germany. Moreover, the PhD work of Mr. Lapauw is financially supported by a personal IWT grant/project entitled "Nano-gelamineerde Ternaire Carbiden".

Another promising potential application of MAX phases for Gen-IV LFRs is the fabrication of cladding materials, in view of the promising reported first results on the radiation resistance of this special class of materials [7-11]. One of the key material properties that must be optimized before MAX phases can be used as candidate cladding materials is their fracture toughness, which has not been reported to exceed 15-18 MPa·m1/2 for selected MAX phases with an optimized (i.e. large-grained and textured) microstructure [3,12,13]. Even though such fracture toughness values are already very high for carbides, fracture toughness must be further improved. Since the targeted fracture toughness of these novel clads must be at least comparable with the reported fracture toughness of commercial and already qualified cladding materials, such as zircalloys (beginning-of-life KIC ≈ 75 MPa·m1/2; end-of-life KIC ≈ 25 MPa·m1/2), the envisaged approach is to produce novel clads made of MAX phase-based cermets (i.e. ceramic composites with metallic 'matrix'). Embedding MAX grains in a metallic matrix is expected to provide an additional mechanism for dislocation motion apart from the already-identified mobility of basal dislocations and the formation of kink bands in MAX phases [1].

MAX phases are currently also being considered as cladding materials – either monolithic or as coatings deposited on commercial clads – for Gen-III+ light water reactors (LWRs). In fact, one of the high-priority goals of the post-Fukushima era is the development of accident-tolerant fuels (ATFs), as the combination of uranium-based fuels with zircalloy clads is characterized by some inherent weaknesses of this fuel/clad system under severe accident conditions. These include potential exothermic reactions between zircalloy clad and water coolant that can raise the core temperature to such levels that the cladding, fuel and core structures are at risk of melting, with the associated release of highly-radioactive fission products. The novel ATF cladding materials must ideally tolerate the loss of active cooling in the reactor core for a considerably longer time period during design-basis and beyond design-basis accidents: in fact, they must tolerate temperatures >>1200°C, while fission products must be contained in the fuel rod for a significantly longer time than the approx. 3 hours associated with a conventional zircalloy cladding. This need is clearly identified in the recently-submitted H2020 FALSTAFF proposal, where both the Dept. MTM of the KU Leuven and SCK•CEN are involved with the aim of making and irradiating novel ATF clads for Gen-III+ LWRs. If the FALSTAFF project is approved (decision to be announced in January 2015), then the suitable-for-this-application MAX phases produced by KU Leuven and other project partners will be irradiated in BR2 in a BAMI irradiation device up to a neutron dose of 3 dpa (displacements per atom). The produced neutron irradiation data will be first-of-a-kind for the nuclear community, providing crucial feedback on the stability of MAX phases under neutron irradiation.

References

[1] M. W. Barsoum, MAX Phases – Properties of Machinable Ternary Carbides and Nitrides, 2013 Wiley-VCH Verlag GmBH, Weinheim, Germany

[2] M. W. Barsoum, T. El-Raghy, The MAX Phases: Unique New Carbide and Nitride Materials, American Scientist 89 (2001) 334-343

[3] M. W. Barsoum, M. Radovic, Elastic and Mechanical Properties of the MAX Phases, Annual Review of Materials Research 41 (2001) 195-227

[4] A. Heinzel, G. Müller, A. Weisenburger, Compatibility of Ti3SiC2 with liquid Pb and PbBi containing oxygen, Journal of Nuclear Materials 392 (2009) 255-258

[5] K. Lambrinou, T. Lapauw, E. Charalampopoulou, A. Bakaeva, J. Joris, J. Vleugels, On the corrosion resistance of various MAX phases exposed to oxygen-depleted static liquid LBE at 500°C, soon to be submitted to the Journal of Nuclear Materials

[6] T. Lapauw, K. Vanmeensel, K. Lambrinou, J. Vleugels, Rapid synthesis and elastic properties of fine-grained Ti2SnC produced by spark plasma sintering, under review after re-submission to the Journal of Alloys and Compounds

[7] K. R. Whittle, M. G. Blackford, R. D. Aughterson, S. Moricca, G. R. Lumpkin, D. P. Riley, N. J. Zaluzec, Radiation tolerance of Mn+1AXn phases Ti3AlC2 and Ti3SiC2, Acta Materialia 58 (2010) 4362-4368

[8] M. Le Flem, X. M. Liu, Stability of Ti3SiC2 under charged particle irradiation, Advances in Science and Technology of Mn+1AXn Phases, 2012 Woodhead Publishing, pp. 355-387

[9] J. C. Nappé, C. Maurice, Ph. Grosseau, F. Audubert, L. Thomé, B. Guilhot, M. Beauvy, M. Benabdesselam, Microstructural changes induced by low energy heavy ion irradiation in titanium silicon carbide, Journal of the European Ceramic Society 31 (2011) 1503-1511

[10] Q. Qi, G. J. Cheng, L. Q. Shi, D. J. O’Connor, B. V. King, E. H. Kisi, Damage accumulation and recovery in C+-irradiated Ti3SiC2, Acta Materialia 66 (2014) 317-325

[11] T. Yang, Ch. Wang, C. A. Taylor, X. Huang, Q. Huang, F. Li, L. Shen, X. Zhou, J. Xue, Sh. Yan, Y. Wanget, The structural transitions of Ti3AlC2 induced by ion irradiation, Acta Materialia 65 (2014) 351-359

[12] M. W. Barsoum, T. El-Raghy, Room-temperature ductile carbides, Metallurgical and Materials Transactions A 30A (1999) 363-369

[13] Ch. Hu, Y. Sakka, S. Grasso, T. Nishimura, Sh. Guod, H. Tanaka, Shell-like nanolayered Nb4AlC3 ceramic with high strength and toughness, Scripta Materialia 64 (2011) 765-768

Objective

This PhD thesis has a very strong scientific element but it is at the same time called to address the top-priority industrial needs for novel cladding materials suitable for Gen-III+ LWRs and Gen-IV LFRs. For Gen-III+ LWRs, the aim is to use MAX phases to develop novel cladding materials for ATFs that can demonstrate a superior performance over the commercially-used fuel/clad systems. For Gen-IV LFRs, MAX phases will be used to develop novel liquid metal corrosion-resistant cladding materials that can sustain higher reactor operating temperatures when compared to the currently-envisaged austenitic stainless steels, thus improving the reactor efficiency. The processing, microstructural characterization and property optimization of suitable candidate clads (monoliths and/or cermets) will be done together with the Dept. MTM of the KU Leuven: material processing will follow a powder metallurgical route, microstructural characterization will be done by means of various techniques (light optical microscopy, scanning electron microscopy/energy-dispersive X-ray spectroscopy, electron probe microanalysis, electron backscatter diffraction, X-ray diffraction, transmission electron microscopy) and important mechanical properties (flexural strength, stiffness, fracture toughness, etc.) will be determined via the appropriate mechanical tests. Moreover, the investigation of the effects of neutron irradiation on the selected MAX phase-based materials will also be part of this PhD work, so as to identify the structural stability and mechanical property degradation of neutron-irradiated MAX materials and characterize the irradiation-induced defects. In fact, the post-irradiation examination (PIE) of MAX phases by transmission electron microscopy (TEM) is already foreseen in the framework of H2020 FALSTAFF. Since an important part of this PhD work will be dedicated to the detailed TEM study of the MAX phase-based materials in both the non-irradiated and irradiated states, a close collaboration with EMAT (Electron Microscopy for Material Science) of the University of Antwerp is considered essential.