Microstructure of reduced activation ferritic/martensitic steel after transformation super plasticity

• M23C6 disappeared in JLF-1 after TSP treatment. However, the M23C6 that disappeared by TSP treatment was precipitated again after tempering heat treatment at 780 °C for 1 h. This result suggests that M23C6 was dissolved in the matrix during TSP test by the normalization effect at 1000 °C. Therefore, future TSP-type SPFs will require a tempering process after TSP deformation. • MX did not disappear after the TSP in JLF-1. This is presumably because highly stable MX could not be dissolved at the normalizing temperature. The stable MX in the RAFM steel does not inhibit the TSP, implying that the MX formed RAFM steels does not affect creep deformation during TSP, and TSP phenomena could also be induced on steels strengthened by MX, such as Cr-Mo steels. Reduced activation ferritic-martensitic (RAFM) steels are considered promising candidates as structural materials for fusion blankets. However, the poor processability of RAFM steels is one of the critical issues in their application to fusion blankets. To address the issue, a new material processing based on transformation super plasticity (TSP), a type of superplastic forming (SPF), has been investigated for advanced fusion engineering. Our previous studies have demonstrated that the total elongation of a RAFM steel (JLF-1) reached 170 % through tensile tests under constant load and cyclic heat treatment, suggesting that the TSP-type SPF can be used for the macroscopic deformation processing of RAFM steel. For the application of TSP-type SPF, it is essential to study the microstructure before and after TSP. In this study, the effect of cyclic heat treatment on the microstructure before and after deformation in tensile tests using the TSP was investigated. Microstructural TEM observation of M 23 C 6 and MX precipitates showed that only M 23 C 6 disappeared after tensile tests with cyclic and isothermal temperature and precipitated again by the following tempering. The results indicate that the MX alone remaining at the lath boundaries cannot inhibit the TSP creep deformation, and the combination of dissolution of M 23 C 6 at the block boundaries and cyclic phase transformation may enhance the TSP creep deformation. These results will contribute to the development of advanced processing technology for fusion blankets.