Michel Barsoum

Herein we report on the characterization of defects formed in polycrystalline Ti3SiC2 and Ti2AlC samples exposed to neutron irradiation e up to 0.1 displacements per atom (dpa) at 350 ± 40 C or 695 ± 25 C, and up to 0.4 dpa at 350 ± 40 C. Black spots are observed in both Ti3SiC2 and Ti2AlC after irradiation to both 0.1 and 0.4 dpa at 350 C. After irradiation to 0.1 dpa at 695 C, small basal dislocation loops, with a Burgers vector of b ¼ 1/2 [0001] are observed in both materials. At 9 ± 3 and 10 ± 5 nm, the loop diameters in the Ti3SiC2 and Ti2AlC samples, respectively, were comparable. At 1  1023 loops/m3, the dislocation loop density in Ti2AlC wasz1.5 orders of magnitude greater than in Ti3SiC2, at 3  1021 loops/ m3. After irradiation at 350 C, extensive microcracking was observed in Ti2AlC, but not in Ti3SiC2. The room temperature electrical resistivities increased as a function of neutron dose for all samples tested, and appear to saturate in the case of Ti3SiC2. The MAX phases are unequivocally more neutron radiation tolerant than the impurity phases TiC and Al2O3. Based on these results, Ti3SiC2 appears to be a more promising MAX phase candidate for high temperature nuclear applications than Ti2AlC.

Abstract—Herein we report on the effect of neutron irradiation – of up to 0.1 displacements per atom at 360(20) C or 695(25) C – on polycrystalline samples of Ti3AlC2, Ti2AlC, Ti3SiC2 and Ti2AlN. Rietveld refinement of X-ray diffraction patterns of the irradiated samples showed irradiation enhanced dissociation into TiC of the Ti3AlC2 and Ti3SiC2 phases, most prominently in the former. Ti2AlN also showed an increase in TiN content, as well as Ti4AlN3 after irradiation. In contrast, Ti2AlC was quite stable under these irradiation conditions. Dislocation loops are seen to form in Ti2AlC and Ti3AlC2 after irradiation at 360(20) C. The room temperature electrical resistivity of all samples increased by an order of magnitude after irradiation at 360(20) C, but only by 25% after 695(25) C, providing evidence for the MAX phases’ dynamic recovery at temperatures as low at 695(25) C. Based on these preliminary results, it appears that Ti2AlC and Ti3SiC2 are the more promising materials for high-temperature nuclear applications.

Herein we report on the formation of defects in response to neutron irradiation of polycrystalline Ti3SiC2 and Ti3AlC2 samples exposed to total fluences of ˜6 × 1020 n/m2, 5 × 1021 n/m2 and 1.7 × 1022 n/m2 at irradiation temperatures of 121(12), 735(6) and 1085(68)°C. These fluences correspond to 0.14, 1.6 and 3.4 dpa, respectively. After irradiation to 0.14 dpa at 121 °C and 735 °C, black spots are observed via transmission electron microscopy in both Ti3SiC2 and Ti3AlC2. After irradiation to 1.6 and 3.4 dpa at 735 °C, basal dislocation loops, with a Burgers vector of b = ½ [0001] are observed in Ti3SiC2, with loop diameters of 21(6) and 30(8) nm after 1.6 dpa and 3.4 dpa, respectively. In Ti3AlC2, larger dislocation loops, 75(34) nm in diameter are observed after 3.4 dpa at 735 °C, in addition to stacking faults. Impurity particles of TiC, as well as stacking fault TiC platelets in the MAX phases, are seen to form extensive dislocation loops under all conditions. Cavities were observed at grain boundaries and within stacking faults after 3.4 dpa irradiation, with extensive cavity formation in the TiC regions at 1085 °C. Remarkably, denuded zones on the order of 1 µm are observed in Ti3SiC2 after irradiation to 3.4 dpa at 735 °C. Small grains, 3–5 µm in diameter, are damage free after irradiation at 1085 °C at this dose. The results shown herein confirm once again that the presence of the A-layers in the MAX phases considerably enhance their irradiation tolerance. Based on these results, and up to 3.4 dpa, Ti3SiC2 remains a promising candidate for high temperature nuclear applications as long as the temperature remains >700 °C.

Herein we report, for the first time, on the synthesis and structural characterization of the Zr-based MAX phase, Zr3AlC2, fabricated by reactive hot pressing of ZrH2, Al, and C powders. The crystal structure of Zr3AlC2 was determined by X-ray diffraction and high resolution scanning transmission electron microscopy to be the hexagonal space group P63/mmc. The a and c lattice parameters of Zr3AlC2 are 3.33308(6) Å and 19.9507(3) Å, respectively. The samples include the secondary phases ZrC and Zr-Al intermetallics as confirmed by quantitative electron probe microanalysis. The Vickers hardness (3 kg) of Zr3AlC2 was measured to be 4.4 ±0.4 GPa.