Effects of Partial Phase Transformation on Characteristics of 9Cr Nanostructured Ferritic Alloy
Thak S. Byun,
David T. Hoelzer.
The core structures of future nuclear systems require tolerance to extreme irradiation, and some critical components, for example,
the fuel cladding in Sodium-cooled Fast Reactors (SFRs), have to maintain mechanical integrity to very high doses of 200 – 400 dpa
at high temperatures up to 700 °C. The high Cr nanostructured ferritic alloys (NFAs) are under intense research worldwide as a candidate
core material. Although the NFAs have some admirable characteristics for high-temperature applications, their crack sensitivity is very
high at high temperatures. The fracture toughness of high strength NFAs is unacceptably low above 300 °C. The objective of this study is
to develop processes and microstructures with improved high temperature fracture toughness and ductility. To optimize the afterextrusion
heat treatment condition, both the computational simulation technique on phase equilibrium and the basic microstructural and
mechanical characterization have been carried out. 9 Cr-NFA was produced by the mechanical alloying of pre-alloyed Fe-9Cr base metallic
powder and yttria particles, and subsequent extrusion. The post-extrusion heat-treatments of various conditions were applied to the asextruded
NFA. The tensile and fracture toughness tests were conducted for as-extruded and heat-treated samples at up to 700°C. Fracture
toughness of the NFA has increased by more than 40% at every testing temperature after heat-treatment in the inter-critical temperature
range. The increment of fracture toughness of the NFA after post-extrusion heat-treatment is attributed to the increased strength at below
500oC, and an increased ductility at 700oC.
Keywords: Fracture toughness, nanostructured ferritic alloy, nuclear, ODS, strength.
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