Naoko Oono

Charpy impact property related to {100} cleavage fracture in 15CrODS steel

Abstract Based on the former results that delamination along {100} cleavage fracture leads to the improved absorbed energy, {100} cleavage plane was formed in parallel to the rolled plate by recrystallisation and cold roll processing in 15CrODS steel plate. Charpy impact properties were investigated in terms of notch direction against {100} cleavage plane. Notch with normal direction against {100} cleavage plane induced delamination fracture, but provided similar absorbed energy to that of conventional extruded bar. The transverse notch direction against {100} cleavage plane served higher absorbed energy at low temperature. Higher upper shelf energy and lower DBTT were attained in conventional hot extruded bar, which contains any crystallographic planes rotated around <110> axis at the transverse direction. The delamination fracture leading to high absorbed energy is significantly affected by a crystallographic morphology of the {100} cleavage plane perpendicular to transverse plane.

Consideration of the oxide particle–dislocation interaction in 9Cr-ODS steel

Abstract The interaction between oxide particles and dislocations in a 9Cr-ODS ferritic steel is investigated by both static and in situ TEM observation under dynamic straining conditions and room temperature. The measured obstacle strength () of the oxide particles was no greater than 0.80 and the average was 0.63. The dislocation loops around some coarsened particles were also observed. The calculated obstacle strength by a stress formula of the Orowan interaction is nearly equaled to the average experimental value. Not only cross-slip system but also the Orowan interaction should be considered as the main interaction mechanism between oxide particles and dislocation in 9CrODS ferritic steel.

Effect of the dilation caused by helium bubbles on edge dislocation motion in α-iron: molecular dynamics simulation

ABSTRACT Various types of nanometric defects such as voids and helium (He) bubbles produced by high-energy neutron irradiations are known to degrade the mechanical properties of irradiated materials. In this study, we have evaluated the obstacle strength of He bubbles to the mobility of an edge dislocation in α-iron for 2 and 4 nm bubbles with He-to-vacancy (He/V) ratios ranging from 0 to 1 at 300 and 500 K, by molecular dynamics simulation. Results showed that as the He/V ratio increases, the obstacle strength needed for the release of a dislocation from the bubble becomes stronger up to a moderate He/V ratio (0.6 and 0.4 for 2 and 4 nm bubbles, respectively, at both temperatures), and a further increase in the He/V ratio leads to weakening of the obstacle strength. For He/V = 1, the obstacle strengths are 10–30% weaker than those at moderate He/V ratios depending on the bubble size and temperature. The extent of obstacle strength was found to be correlated with the dilation caused by He bubbles depending on the bubble size, He/V ratio, and temperature.

Development of Advanced ODS Ferritic Steels for Fast Reactor Fuel Cladding

Recent progress of the 9CrODS steel development is presented focusing on their microstructure control to improve sufficient high-temperature strength as well as cladding manufacturing capability. The martensitic 9CrODS steel is primarily candidate cladding materials for the Generation IV fast reactor fuel. They are the attractive composite-like materials consisting of the hard residual ferrite and soft tempered martensite, which are able to be easily controlled by α-γ phase transformation. The residual ferrite containing extremely nanosized oxide particles leads to significantly improved creep rupture strength in 9CrODS cladding. The creep strength stability at extended time of 60,000 h at 700 oC is ascribed to the stable nanosized oxide particles. It was also reviewed that 9CrODS steel has well irradiation stability and fuel pin irradiation test was conducted up to 12 at% burnup and 51 dpa at the cladding temperature of 700oC.

Ferrite Grain Coarsening from Hot Rolled Austenite in ODS Steels

The hot rolling at temperature range of 1100 °C to 862 °C and subsequent air-cooling induce a formation of the coarse ferrite grains in the 9CrODS steels. This coarse ferrite is produced by transformation from the severely hot rolled γ-grains to ferrite. Formation process and mechanism of the transformed coarse ferrite are interpreted in terms of a nucleation, growth and coalescence of the same variant ferrite grains under a variant restriction rule.