Effect of fast-neutron irradiation on the tensile properties of steels

[eneacasucci]

"Austenitic steels (fcc) irradiated and tested at high temperatures show no increase in either yield".
I understand that irradiation hardening occurs because the movement of dislocations is hindered by the presence of defects. This phenomenon can be observed in both body-centered cubic (bcc) and face-centered cubic (fcc) structures.

Regarding image a), which shows irradiation at high temperatures for fcc, I assume that at elevated temperatures, some annealing and recombination of defects occur. However, I am unclear why the only observed effect is the reduction of maximum strain. What is the mechanism behind this phenomenon?

(ref. Fundamental Aspects of Nuclear Reactor Fuel Elements Donald R. Olander)

 

[berkeman]

Paging @Astronuc

 

[Astronuc]

"Austenitic steels (fcc) irradiated and tested at high temperatures show no increase in either yield".

That's an awkward statement; it depends on what one means by 'high' temperature. LWR cores operate with stainless steel in the range of 300 to 350°C, while liquid-metal-cooled fast reactors operate with stainless steels operating at 400-700°C (coolant inlet temperatures range from 300°C to 400°C or coolant exit temperatures 500 to 600°C.

One might operate stainless steel in the range of 300 to 350°C, but one has to address hypothetical conditions such as a LOCA or some other transient where cooling is reduced before power levels are reduced or shutdown, so one might tested at a hypothetical peak temperature, e.g., 800°C or 900°C.

I understand that irradiation hardening occurs because the movement of dislocations is hindered by the presence of defects. This phenomenon can be observed in both body-centered cubic (bcc) and face-centered cubic (fcc) structures.

Yes. Radiation, e.g., neutron collisions with nuclei cause atomic displacements. In displacement cascades, most atoms return to more or less the same lattice, but many are left out of places. Irradiation hardening produces much the same hardening (strengthening) effect as cold working. As temperature increases, annealing of defects occurs and the material softens - the higher the temperature, the less time to soften the material. At some temperature, one observers 'recrystallization' of the lattice, accompanied by grain growth.

Regarding image a), which shows irradiation at high temperatures for fcc, I assume that at elevated temperatures, some annealing and recombination of defects occur. However, I am unclear why the only observed effect is the reduction of maximum strain. What is the mechanism behind this phenomenon?

One is seeing the effect in a) in the image of irradiation hardening (increase in yield and tensile strength (provided the irradiation temperature is 'low', which could mean < 350°C, and the concommitant effect of a reduction in ductility (irradiation embrittlement).

One would see ductility reduced by cold-working a metal alloy. When cold working, or plastically deforming a metal (alloy), manufacturers will heat 'anneal' the material during steps of working the material in order to reduce the resistance to further deformation and to protect against internal flaws or cracks.

I'll see if I can dig up some plots on irradiating stainless steel at different temperatures and a work hardening curve, which shows YS, UTS and elongation (a measure of ductility) as a function of cold work.