STEM and HRTEM study on matrix microstructure and oxide particles in 11Cr ferritic/martensitic ODS steel

Abstract

Fe–Cr oxide dispersion strengthened (ODS) steel is one of the candidate structural materials for high burn-up fuel cladding of sodium-cooled fast reactor and fusion reactor blanket due to its superior high temperature strength and excellent resistances to corrosion, creep and irradiation. The matrix structure and oxides in 11Cr ferritic/martensitic (F/M) ODS steel (Fe–11Cr–0.13C–1.3W–0.4Ni–0.3Ti–0.35Y2O3) have been characterized by scanning transmission electron microscopy (STEM) and high resolution transmission electron microscopy (HRTEM). The matrix of 11Cr F/M ODS steel is composed of tempered martensite (major) and residual ferrite (minor). The dispersion morphology of oxides of residual ferrite is much better, relative to that of tempered martensite, because the oxides in the former have much smaller mean diameter and inter-particle spacing and, however, significantly higher number density and volume fraction. For tempered martensite and residual ferrite, phase identifications on 41 and 69 nanoparticles with peak number fraction and, consequently, most significant contributions to the macroscopic properties, were accomplished, respectively. The proportions of Y–Ti–O, Y–Cr–O, TiO2 and Y2O3 in tempered martensite are 58.5%, 12.2%, 19.5% and 9.8%, respectively. The proportions of Y–Ti–O, Y–Cr–O, TiO2 and Y2O3 in residual ferrite are 65.3%, 8.7%, 15.9% and 10.1%, respectively. There is no considerable difference in phases and their corresponding number fractions of nanoparticles between tempered martensite and residual ferrite. Coherency and crystallographic orientation relationship between the lattice of oxides and that of the residual ferrite matrix are ubiquitous and, moreover, such coherency and crystallographic correlation between the lattice of oxides and that of the tempered martensite matrix were only occasionally detected. The mechanisms of the formation and polymorphic transition of various kinds of oxides, the structure origins of the much better dispersion morphology of the nanoparticles of residual ferrite, relative to that of tempered martensite, and, moreover, the strengthening mechanisms in the 11Cr F/M ODS steel were discussed.