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636 results on '"Ukai, S."'

109. Evaluation of Processing Conditions on the γ′ Morphology of an Oxide Dispersion Strengthened Ni‐Based Superalloy.

Author

Aghamiri, S. M. S., Linye, Z., Ukai, S., Oono, N., Shahverdi, H. R., and Soltanieh, M.

Subjects
DISPERSION strengthening, NICKEL alloys, HEAT resistant alloys, SCANNING transmission electron microscopy, HEAT treatment, MECHANICAL alloying
Abstract

Herein, the evolution of the microstructure of a newly developed oxide dispersion strengthened (ODS) Ni‐based superalloy with emphasis on γ′ morphology during different processing conditions is investigated. X‐ray analysis of severely deformed mechanically alloyed (MAed) powders shows that the as‐milled nano‐sized crystallite size increases with annealing associated with the sharp drop of dislocation density. At the same time, by annealing MAed powders γ′ precipitates above 300 °C, and depending on the aging treatment regime, Vickers microhardness changes mainly due to γ′ distribution and dislocation release. The thorough evaluation of γ′ morphology in the microscale shows that in contrast to the spherical shape of γ′, after heat treatment of solutionized MAed ODS superalloy, irregular‐shaped γ′ exist in the MAed ODS superalloy due to severe plastic deformation during mechanical alloying. However, a unique nanostructured lamellar morphology forms in the γ′ precipitates with different lamellae directions. This structure is stable in all subsequent processing steps with γ′ lamellae of (011¯) planes and intervals of 20–30 nm. The scanning transmission electron microscopy (STEM)–high‐angle annular dark‐field (HAADF) composition analysis clarifies that the nanostructured lamellar morphology shows no partitioning of alloying elements, and its origin may be attributed to the formation of antiphase domain boundaries. [ABSTRACT FROM AUTHOR]

Published
2021
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127. Deformation-Induced Dynamic Ferrite Transformation During Hot-Rolling in Oxide Dispersion-Strengthened Ferritic Steel with 9 Wt Pct Cr Content.

Author

Kasai, Shoki, Ukai, S., Yamashiro, T., Zhang, S., Oono, N., Hayashi, S., Ohtsuka, S., and Sakasegawa, H.

Subjects
FERRITIC steel, HOT rolling, DEFORMATIONS (Mechanics), SURFACE temperature, CRYSTALLIZATION
Abstract

9CrODS steel, a candidate fission and fusion structural material, was subjected to hot-rolling with varying parameters of surface temperature and cooling rate just after hot-rolling. The deformation-induced dynamic ferrite transformation was confirmed at the rolling temperature 805 °C above Ar3 (780 °C). This transformation exhibits three characteristic features: transformation for extremely short duration (0.044 second), retaining carbon content equal to the original without long-distance carbon diffusion, and elongated coarse ferrite grains (10 μm). The massive transformation was proposed for the dynamic ferrite transformation from the hot-rolled austenite. The driving force for massive transformation was quantitatively estimated considering dislocations accumulated by hot-rolling. It was also shown that the oxide particles in 9CrODS steel play a critical role for dynamic ferrite transformation by suppressing the dynamic recrystallization at hot-rolling. [ABSTRACT FROM AUTHOR]

Published
2019
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