Kinetic Approach for Growth and Coalescence of Nano-Size Oxide Particles in 9Cr-ODS Steel Using High-Energy Synchrotron Radiation X-rays in SPring-8

Abstract

Nanometer size oxide particles in 9Cr-ODS steel are dispersed finely and densely in a matrix by the hot-solidification process. The size and density distribution of dispersed oxide particles is recognized as one of the main issues for ensuring good microstructural stability and high temperature strength in a high temperature (<700 C) and neutron irradiation (250 dpa) environment. However, the behavior of oxide particles in the hot-solidification process has not been determined yet. This study evaluated the correlation between nano-size oxide particles and the heat treatment temperature and time in order to characterize the mechanism of formation and the behavior during growth and coalescence of these particles in 9Cr-ODS steel raw powder. XRD and SAXS measurements were made using high-energy synchrotron radiation X-rays in SPring-8. This is the first report of the oxide complex particles (Y 2 Ti 2 O 7 and Y 2 TiO 5 ) being formed from 800 to 960 C, and they were observed to grow and coalesce on increasing both heat-treatment temperature and time.

Thermodynamic and kinetic modeling of oxide precipitation in nanostructured ferritic alloys

“…The nucleation and growth processes are therefore complete after this initial heat treatment and further evolution of the particle size distribution involves only coarsening and dissolution of existing particles. While little quantitative data are available regarding the kinetics of precipitation in MA957 or other NFAs, these results are qualitatively consistent with the very rapid appearance of the oxide NFs during the initial hot extrusion consolidation treatment of the mechanically alloyed powders [2,11,13,66]. It also should be noted that while still very rapid, oxide NF precipitation is slower at lower consolidation temperatures, manifested in quasi C-curve type behavior [2].…”

“…Most high-resolution TEM studies show that the nano-oxides are consistent with Y 2 Ti 2 O 7 [8][9][10]. Combined small angle neutron and X-ray scattering [10][11][12] and X-ray diffraction measurements [13,14] are also broadly consistent with this conclusion. However, the Y 2 Ti 2 O 7 may not be fully stoichiometric; and a multitude of other observed oxide phases include, but are not limited to, Y 2 TiO 5 , Y 2 O 3 , CrTiO 3 and TiO 2 , as well as a variety of carbo-nitrides.…”

“…Fig. 3 compares the precipitate volume fraction in a large number of NFAs with varying Ti and Y contents as measured by SANS [2,7,[11][12][13]48,[51][52][53][54] with the equilibrium volume fractions of oxide phases predicted by the thermodynamic model for the same alloys, using the given nominal alloy compositions. For cases where it was explicitly stated in the experimental methods that only the Y and Y-Ti oxides are included in the reported volume fractions [2,11,12], the predicted values include only these phases as well.…”

“…NFAs typically contain (in wt.%) [12][13][14][15][16] Cr and up to 0.5 Mo or 3 W [10] with small additions Ti, Y, and O. The latter solutes precipitate during processing to form a high density of Ti-Y-O nano-oxides.…”

Effect of Ti and Cr on dispersion, structure and composition of oxide nano-particles in model ODS alloys

“…Oxide dispersion strengthened (ODS) steels have been identified as a promising candidate structural material for Generation IV and fusion reactors because of their superior high-temperature mechanical properties and improved radiation resistance compared to conventional ferritic/martensitic steels [1][2][3]. These properties are achieved by introducing stable oxide dispersoids into the ferritic matrix which act as pinning sites for dislocations, sinks for radiation-induced point defects and limit grain growth [4][5][6][7][8].…”