Fig. 1 Damage dose (red line) and He+ ion implantation profile (black line) calculated by SRIM after irradiated by 350 keV He+ ion to 5 × 1016 ion/cm2 in the LPBF 304L stainless steel

Fig. 2 Microstructural observations of a-c the as-built sample and d-f the solution-annealed sample. a, d EBSD maps taken in the XY plane perpendicular to the build direction. b, e TEM images of the cellular sub-grain boundaries and grain boundaries. e, f TEM images of nano-sized oxide particles

Fig. 3 TEM images of the a as-built sample and b solution-annealed sample of LPBF 304L stainless steel after He+ ion implantation in - 500 nm under-focus condition

Fig. 4 TEM images of the a as-built sample and b solution-annealed sample of LPBF 304L stainless steel after He+ ion implantation and PIA at 600 °C for 1 h in - 500 nm under-focus condition

Fig. 5 Number density and size of helium bubbles in four samples of comparison S1, S2, S3 and S4, the conditions of the samples are specified in Table 2

Fig. 6 TEM images showing the nano-sized oxide inclusions dispersed in the a, b as-built and c, d solution-annealed LPBF 304L samples after He+ ion implantation in - 500 nm under-focus condition. The average diameters of the oxide particles in the as-built and solution-annealed samples were 15 nm and 80 nm, respectively

Fig. 7 TEM images showing the dislocation structures in the a as-built and b solution-annealed 304L samples after He+ ion implantation in - 500 nm under-focus condition. Weak-beam dark-field micrographs are obtained in the zone axis close to [011] and for beam g = (11$\overline{1 }$) condition

Fig. 8 TEM images of the LPBF 304L samples in a, b as-built state and c, d solution-annealed state after He+ ion implantation and PIA at 600 °C for 1 h. TEM images in (a-d) were recorded in - 300 nm, - 130 nm, - 120 nm and - 500 nm under-focus conditions, respectively. In a an oxide inclusion is shown to trap helium bubbles at its interface with the matrix, in b bubbles are gathered nearby the dislocation lines, in c the bubbles are found to accumulate at a grain boundary, in d larger and coarsened helium bubbles are encircled at the grain interior where defect sinks are absent

Fig. 9 Schematics to illustrate the helium bubble microstructure before and after PIA at 600 °C for 1 h: a as-built sample S1, b solution-annealed sample S3, c as-built sample S2 after PIA at 600 °C for 1 h, d solution-annealed sample S4 after PIA at 600 °C for 1 h