Influence of temperature on ductility of nano-ODS steel prepared by low-energy mechanical alloying

Oxide dispersion strengthened (ODS) steels are promising materials for structural applications in the IV Generation of fission, and fusion reactors. The ODS steel prepared by low-energy mechanical alloying (LEMA) followed by hot extrusion (HE) has been subjected to tensile tests at increasing temperature up to 700 °C. The fracture surfaces have been investigated through scanning electron microscopy (SEM) observations and X-ray photoelectron spectroscopy (XPS) analyses in order to understand the ductility changes vs. temperature. Owing to microstructural heterogeneity a mixed mode of fracture (ductile plus brittle) occurs at every test temperature. Both fracture modes depend on the interaction of dislocations with particles and ductility variations with temperature have been discussed by considering the mobility of dislocations and their capability to unlock from particles. Ductile fracture occurs in the zones where nano-precipitates are homogeneously distributed while the grains close to nano-precipitate clusters exhibit cleavage rupture. XPS revealed a relation between brittle fracture and Cr segregation since the Cr-rich zones represent a preferred path for cleavage cracks. The decrease of ductility around 400 °C has been explained on the basis of internal friction measurements showing a re-arrangement of dislocation structures. The results indicate that further improvement of the mechanical properties of ODS steels can be achieved through a more homogeneous distribution of fine particles.