Crystallographic analysis of Ga⁺ irradiation-induced phase transformation in austenitic stainless steels

Focused ion beam processing with Ga⁺ ion sources is widely applied for nanoscale fabrication of materials and preparation of specimens for advanced microstructural observation by methods such as transmission electron microscopy and 3D atom probes. It is important to suppress specimen damage and microstructural changes associated with Ga⁺ irradiation, but limited information is available on the mechanism of microstructural changes. In this study, we explored the mechanism of Ga⁺ irradiation-induced phase transformation of an austenitic stainless steel. The irradiation-induced fcc-bcc phase transformation was characterized by microstructural observations of surfaces and cross sections of specimens and analyses of crystallographic orientation relationships. The results of microstructural analysis showed that the formation of the irradiation-induced bcc phase on the specimen surface increased with increasing irradiation dose. At the same irradiation dose, the bcc transformation initiated sooner, and the total amount of transformation was higher, for irradiation of the (111)_fcc planes than the (001)_fcc planes. Microstructural analysis of cross-sections showed that irradiation (8.0 × 10¹⁶ ions/cm²) of the (001)_fcc and (111)_fcc planes caused formation of a transformation phase to depths of 100 nm and 60 nm, respectively; the bcc transformation extended to deeper regions in the (001)_fcc planes than the (111)_fcc planes. In addition, these transformation areas extended over a wider area than the areas of high Ga concentration. The crystallographic orientation relationship before and after the transformation was not a specific orientation relationship, such as Kurdjumov–Sachs or Nishiyama-Wasserman, and its distribution differed depending on the direction of Ga⁺ irradiation. The mechanism of fcc-bcc phase transformation of SUS304 steel by Ga⁺ ion irradiation was explained by chemical and stress effects due to the injection of Ga⁺ ions. Furthermore, the transformation behavior, which depended on the irradiation direction, was discussed in terms of the (111)_fcc orientation angle relative to the irradiation direction.