Crystallographic analysis of Ga plus 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 micro -structural 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 x 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 concen-tration. 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.

Automated summary

This study investigated the mechanism of Ga+ irradiation-induced phase transformation of an austenitic stainless steel. The results showed that increasing irradiation dose led to an increase in the formation of the irradiation-induced bcc phase on the specimen surface. The orientation of the irradiation and crystallographic relationships also affected the transformation behavior.