TEM investigation on the microstructural evolution of Hastelloy N induced by Ar⁺ ion irradiation. 
Hastelloy N alloy has been selected as the primary structure material for molten salt reactor.  In this article, Hastelloy N alloy samples were irradiated to different doses at room temperature using 300 keV Ar(+) ions.  The microstructural evolution was investigated by transmission electron microscopy (TEM) and energy-dispersive spectroscopy (EDS).  Black dot defects emerged in sample irradiated at low dose (0.4 displacement per atom (dpa)), and they grew up with irradiation doses (0.4-2 dpa).  A high density of small dislocation loops (nano meters in size) were observed in the sample irradiated to 4 dpa.  When the ion dose increased to 12 dpa, complicated structures with defects (including dislocation lines, larger loops and smaller black dots) were observed.  Dislocation networks were detected from high-angle annular dark field (HAADF) images.  Larger dislocation loops (size: 30-80 nm) were visible in the sample irradiated to 40 dpa.  Irradiation with dose of 120 dpa led to the formation of face-centered cubic nanocrystallites with preferred orientations.