Precipitation and hardening in irradiated low alloy steels with a wide range of Ni and Mn compositions

Abstract

Mn-Ni-Si intermetallic precipitates (MNSPs) that are observed in some Fe-based alloys following thermal aging and irradiation are of considerable scientific and technical interest. For example, large volume fractions (f) of MNSPs form in reactor pressure vessel low alloy steels irradiated to high fluence, resulting in severe hardening induced embrittlement. Nine compositionally-tailored small heats of low Cu RPV-type steels, with an unusually wide range of dissolved Mn (0.06-1.34 at.%) and Ni (0.19-3.50 at.%) contents, were irradiated at ≈ 290°C to ≈ 1.4x10 20 n/cm 2 at an accelerated test reactor flux of ≈ 3.6x10 12 n/cm 2 -s (E > 1 MeV). Atom probe tomography shows Mn-Ni interactions play the dominant role in determining the MNSP f, which correlates well with irradiation hardening. The wide range of alloy compositions results in corresponding variations in precipitates chemistries that are reasonably similar to various phases in the Mn-Ni-Si projection of the Fe based quaternary. Notably, f scales with ≈ Ni 1.6 Mn 0.8 . Thus f is modest even in advanced high 3.5 at.% Ni steels at very low Mn (Mn starvation); in this case Nisilicide phase type compositions are observed.

On the use of charged particles to characterize precipitation in irradiated reactor pressure vessel steels with a wide range of compositions

“…Thus, CPI were used here to characterize precipitation for a matrix of advanced alloys with compositions that extend beyond the typical range the current RPV steels. The focus is on developing so-called super-clean steels, with very high Ni contents (> 3 wt.%), that have superior unirradiated properties [11][12][13][14][15][16]. However, high Ni levels lead to 4 enormous irradiation hardening and embrittlement in RPV steels with typical Mn and Si contents [11].…”

The mechanistic implications of the high temperature, long time thermal stability of nanoscale Mn-Ni-Si precipitates in irradiated reactor pressure vessel steels

“…The underprediction of embrittlement at high fluence is primarily attributed to the delayed formation of large volume fractions (f) of Mn-Ni-Si precipitates (MNSPs), which are not accounted for in current regulatory models [4][5][6][7][8]. The existence of MNSPs has been demonstrated in both surveillance and test reactor irradiations [4,[9][10][11][12][13][14][15][16][17][18][19][20][21][22], but their detailed character and formation mechanisms is one of the most highly debated issues in the field of radiation effects [4,14,[23][24][25][26]. The central question relates to the driving force for MNSP formation.…”

“…Note, even if it is shown that the MNSPs are stable, and the driving force for forming MNSPs is largely thermodynamic, RIS may affect the precipitate compositions somewhat [11]. Indeed, it is important to emphasize that RIS and RED work in tandem, and both can and do play a role in MNSP evolution in RPV and other steels.…”

Phase prediction of Ni-base superalloys via high-throughput experiments and machine learning

“…The multicomponent alloys including Ti-base [1], Nibase [2,3], Fe-base [4], Al-base [5], and high entropy alloys [6], are composed of five or more metallic elements, which makes it increasingly complex to study the relationships among composition, phases, process, and properties. For example, the Ni-base superalloys, used for turbine blades and disks, generally contain over eight alloying elements, the phase precipitation and mechanical properties are found very sensitive to the composition variation [3,7].…”