Atomic scale identification of nano-sized precipitates of Ta/Ti-added RAFM steel and its superior creep strength

Abstract

The influence of 0.015 wt% Ti addition on the formation of MX precipitates and the creep resistances of a reduced activation ferritic/martensitic (RAFM) steel has been studied. 0.1 wt% Ta was also added as same as conventional RAFM steels. Transmission electron micrographs taken from extraction replicas indicated that the area fraction of MX particles in Ta/Ti-added RAFM steel was 2.3 times higher than that in the reference steel. Atom probe tomography was employed to identify the types of MX precipitates and their interactive distribution. By using isoconcentration surfaces with different elemental concentration values, it was newly found that there are three types of MX precipitates, i.e. (Ta,V)-rich MX, Ti-rich MX and W-rich MX in the Ta/Ti-added RAFM steel. They appeared to be distributed independently in the matrix. However, in some cases, smaller sized (Ti or W)-rich MX particles were in contact with large (Ta,V)-rich MX particle. The creep rupture life of the Ta/Ti-added RAFM steel was significantly improved, as compared with the reference steel. The enhanced creep resistance can be rationalized in terms of a high density of dislocations, which were produced by a strong interaction with a higher fraction of the nano-sized MX particles within laths. © 2020 Elsevier Inc.