MAX Phases for the Nuclear Industry: Possibilities and Pitfalls

By now it is well-established that the layered, machinable, hexagonal carbides and nitrides, with the general formula, M_n+1AX_n, (MAX) where n = 1 to 3, M is an early transition metal, A is an A-group (mostly groups 13 and 14) element and X is either C and/or N - sometimes referred to as polycrystalline nanolaminates because every basal plane is a potential deformation or delamination plane - combine some of the best attributes of metals and ceramics. Like metals, they are electrically and thermally conductive, most readily machinable (manual hack saw will suffice) not susceptible to thermal shock, plastic at high temperatures, and exceptionally damage tolerant. Like ceramics, some of them are elastically quite rigid, lightweight, and maintain their strengths to high temperatures. The ternaries Ti₃SiC₂ and Ti₂AlC are also creep, fatigue and oxidation resistant. The impetus for this work was our conjecture - proven herein - that the presence of the pure A-layers in the MAX phases would result in extraordinary damage tolerance to neutron. After irradiation to 0.14 dpa at 121 deg. C and 735 deg. C, black spots are observed in both ternaries. After irradiation to higher doses, at 735 deg. C, basal dislocation loops, with a Burgers vector of b = 1/2 [0001] are observed in TAC with loop diameters of the order of the order of 25 nm. In TAC larger dislocation loops (75±34 nm) are observed after 3.4 dpa at 735 deg. C, in addition to stacking faults. In sharp contrast to the MAX phases, the TiC impurity particles in the former, form extensive dislocation loops at all conditions. Voids are observed at grain boundaries and within stacking faults after 3.4 dpa irradiation, with extensive void formation in the TiC regions at 1085 deg. C. Most remarkably, defect free, denuded zones, of the order of 1 μm, were observed after irradiation to 3.4 dpa at 735 deg. C. Small grains, 3-5 μm in diameter were totally damage free after irradiation at 1085 deg. C to this dose. When these results are compared to those of TiC, there is little doubt that the A-layers in the MAX phases, provide enhanced neutron irradiation tolerance. Based on these, and other results that will be touched upon, it is reasonable to conclude that Ti₃SiC₂ is quite a promising candidate for high (> 500 deg. C) temperature nuclear applications. At lower temperatures anisotropic swelling results in microcracking. (authors)