Recovery behavior of neutron-induced damage of AlN irradiated at higher temperatures by thermal annealing

Aluminum nitride (AlN) is a candidate ceramic for electric or MHD insulator materials of liquid Pb cooled system of fusion reactors since it shows high electrical resistivity, high thermal conductivity, high strength and good compatibility with Pb alloys. In this study, recovery behavior of macroscopic length and lattice parameter of neutron-irradiated AlN was observed. Polycrystalline AlN specimens were neutron-irradiated in the Japan Materials Testing Reactor at the following conditions. (1) 4.4 × 10 24 n/m 2 ( E > 0.1 MeV) at 1193–1223 K (920–950 °C), (2) 8.8 × 10 24 n/m 2 at 1273 K (1000 °C) and (3) 2.8 × 10 24 n/m 2 at 573 K (300 °C). In the case of specimen irradiated to 2.8 × 10 24 n/m 2 at 573 K, macroscopic length slightly shrunk up to 1273 K, whereas the other specimens showed slight increase in length up to 1273 K. Rapid recovery of the macroscopic length was observed beyond ∼1273 K in all the specimens. Furthermore, beyond 1673 K, small increase in length was observed for the specimens irradiated to 4.4 × 10 24 and 8.8 × 10 24 n/m 2 , which should be caused by He bubble swelling. The lattice parameter of the specimens showed anisotropic swelling, i.e., expansion of the c -axis was significantly greater than that of the a -axis due to the irradiation in all the specimens. The recovery of the c -axis parameter was found to proceed above ∼1273 K, corresponding to the decrease in macroscopic length. The difference in macroscopic volume change and the unit-cell volume change can be attributed for the formation of microcrack along grain boundaries in the lower fluence specimen and possibly voids formation for the higher fluence specimens.