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 °C to 700 °C. Transmission electron microscopy and chemical mapping showed the formation of needle-like Cr-rich M ₂ X carbides and spherical Ni/Si/Mn-rich G-phase precipitates at 450 and 500 °C. No M ₂X carbides or G-phase precipitates were observed at temperatures of 600 and 700 °C. On the other hand, pre-existing intergranular M ₂₃C ₆ carbides were found to be stable even at 700 °C. The M ₂X carbides have a semi-coherent interface with an orientation relationship of { 100 } _BCC// { 2 1 ¯ 1 ¯ 0 } _HCP, 〈 011 〉 _BCC// 〈 0001 〉 _HCP, and the M ₂₃C ₆ carbides have a semi-coherent interface with mismatching lattice planes of {110} _BCC//{211} _FCC. The M ₂₃C ₆ carbides exhibited a concentration gradient of Fe/Cr over the interface region of approximately 100 nm. The mean sizes of both M ₂X carbide and G-phase precipitates increased with increasing damage, but the density of M ₂X carbides saturated while the density of G-phase decreased with increasing damage. G-phase precipitates were found to decorate M ₂ X 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 M ₂X carbide formation near the projected range. A spatial distribution difference between M ₂X carbides and G-phase precipitates was observed.