Formation and dissolution of carbides and precipitates in self-ion irradiated HT9 alloy

Microstructural changes of HT9 irradiated with 5 MeV Fe self-ions were studied at damage levels of 300 and 600 displacements-per-atom (dpa) and at temperatures ranging from 450 degrees C to 700 degrees C. Transmission electron mi-croscopy and chemical mapping showed the formation of needle-like Cr-rich M2X carbides and spherical Ni/Si/ Mn-rich G-phase precipitates at 450 and 500 degrees C. No M2X carbides or G-phase precipitates were observed at temperatures of 600 and 700 degrees C. On the other hand, pre-existing intergranular M23C6 carbides were found to be stable even at 700 degrees C. The M2X carbides have a semi-coherent interface with an orientation relationship of {100} BCC//{2110}HCP, (011)BCC//(0001)HCP, and the M23C6 carbides have a semi-coherent interface with mismatching lattice planes of {110}BCC//{211}FCC. The M23C6 carbides exhibited a concentration gradient of Fe/ Cr over the interface region of approximately 100 nm. The mean sizes of both M2X carbide and G-phase pre-cipitates increased with increasing damage, but the density of M2X carbides saturated while the density of G-phase decreased with increasing damage. G-phase precipitates were found to decorate M2X carbides, and such a spatial distribution correlation was believed to increase precipitate stabilities. Ion irradiation introduces defect imbalances due to the injected interstitial effect. This effect reduces the Cr diffusion and suppresses M2X carbide formation near the projected range. A spatial distribution difference between M2X carbides and G-phase pre-cipitates was observed.