Effect of surface nanocrystallization on microstructure and thermal stability of reduced activation steel

Abstract

Nanocrystalline grains in the surface of reduced activation ferrite/martensite (RAFM) steel were produced by means of surface mechanical attrition treatment (SMAT). Analysis results of XRD and TEM showed that grains after SMAT were nanocrystalline. Experiment results after annealing at 550°C showed that the nanocrystallines were stable. Abnormal grain growth was observed from the TEM images after tempered for 120 min, and the grain sizes became uniform after tempered for 240 min (about 250 nm). The XRD diffraction peaks of carbides became weaker and boarder indicated that carbides in the surface layer became smaller after SMAT, and smaller MC type carbides were found from HRTEM images after SMAT. The lattice parameters of M23C6 and MC were 1.0631 and 0.4306 nm calculated from the XRD results. The differences of grain sizes obtained by XRD and TEM could be attributed to the different testing mechanism, different measuring depths and the depth-dependent nanocrystalline microstructure, and results obtained by TEM were more accurate to reveal the real grain size. © Copyright.