M n+1AX n Phase Carbides for Nuclear Applications
Novel nuclear systems are essential to improve the safety and efficiency of the nuclear industry. The development of the next generation (Gen-IV) type nuclear reactors aims at realizing these ambitious goals. However, in order to effectively construct these reactors, several practical obstacles have to be overcome. One particular challenge is the compatibility between the reactor coolant and the functional and structural reactor materials. Specifically, the heavy liquid metal (HLM) coolant (Pb, LBE) envisaged in lead fast reactors (LFRs) and in certain accelerator-driven systems (ADS) such as MYRRHA, tend to corrode the candidate steels in the absence of a protective oxide-layer. Therefore, it is necessary to identify more durable, intrinsically stable material alternatives for the highly taxed reactor components. It is in this context, that the potential of MAX phase carbides is investigated. The main objective of this thesis is to synthesise and evaluate Mn+1AXn phase structured-carbides which can be suitable for current and next generation nuclear systems, with a focus on conditions relevant for reactors cooled by Pb or LBE. A powder metallurgical processing route is established allowing to synthesise the envisaged MAX phase structured carbides with a high phase purity. The synthesis method for these nano-laminated ceramics is based on in situ reactive hot-pressing of a fine powder mixture containing transition metal hydrides and the other elemental constituents. New, ternary MAX phase materials can be made in this way, such as Zr2AlC and Zr3AlC2. Besides, the influence of a fourth element on the phase assembly is investigated. The possibility to replace Nb in the Nb4AlC3-structure by another possible M-element is studied with a focus on the (Nb,Zr)4AlC3 solid solution. This quaternary compound is found to be significantly tougher and has an improved high temperature stability compared to Nb4AlC3. For Zr2AlC-based MAX phases, substitution of Nb and Sn on respectively the M and A-sites can improve the MAX phase content and avoid the presence of ZrC in the final material. This example of a double solid solution is used as starting point to analyse the 211 structure in more detail. Considering the mechanical properties, the elastic behaviour, flexural strength and fracture toughness (KIC) of the MAX phase based materials are investigated. A method to improve the fracture toughness of bulk material is presented. Hot, uniaxial compression of MAX phases in a spark plasma sintering equipment results in a strong crystallographic texture. The resistance to crack propagation of the hot worked ceramics clearly improves in the direction parallel to the compression axis, whereas limited or no decrease in KIC is found in the transverse direction. Finally, the stability of the synthesized MAX phases in LBE is studied. 11 MAX phase carbides are exposed to oxygen poor static LBE at 500°C. The low oxygen content reveals the intrinsic stability of most of these compounds in contact with HLM. An intriguing exchange mechanism is observed for the three exposed Zr-rich MAX phase grades. Besides static exposure, the erosion/corrosion stability of some selected MAX phase materials is tested in fast flowing, oxygen poor LBE. No significant erosion damage is detected.