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

Abstract

Nuclear reactor lifetimes may be limited by nano-scale Cu-Mn-Ni-Si precipitates (CRPs and MNSPs) that form under neutron irradiation (NI) of pressure vessel (RPV) steels, resulting in hardening and ductile to brittle transition temperature increases (embrittlement). Physical models of embrittlement must be based on characterization of precipitation as a function of the combination of metallurgical and irradiation variables. Here we focus on rapid and convenient charged particle irradiations (CPI) to both: a) compare to precipitates formed in NI; and, b) use CPI to efficiently explore precipitation in steels with a very wide range of compositions. Atom probe tomography (APT) comparisons show NI and CPI for similar bulk steel solute contents yield nearly the same precipitate compositions, albeit with some differences in their number density, size and volume fraction (f) dose (dpa) dependence. However, the overall precipitate evolutions are very similar. Advanced high Ni (> 3 wt.%) RPV steels, with superior unirradiated properties, were also investigated at high CPI dpa. For typical Mn contents, MNSPs have Ni16Mn6Si7 or Ni3Mn2Si phase type compositions, with f values that are close to the equilibrium phase separated values. However, in steels with very low Mn and high Ni, Ni2-3Si silicide phase type precipitate compositions are observed; and when Ni is low, the precipitate compositions are close to the MnSi phase field. Low Mn significantly reduces, but does not eliminate, precipitation in high Ni steels. A comparison of dispersed barrier model predictions with measured hardening data suggests that the Ni-Si dominated precipitates are weaker dislocation obstacles than the G phase type MNSPs.

Atom probe characterisation of segregation driven Cu and Mn–Ni–Si co-precipitation in neutron irradiated T91 tempered-martensitic steel

“…MNSPs also develop in low Cu steels, although even trace amounts of Cu are known to be potent catalysts for their formation [53]. The first experimental proof of MNSP formation in irradiated RPV steels was reported in 2004 [54]; and, since then, MNSPs have been widely observed [16,47,50,51,[55][56][57][58]. Notably, CRPs and MNSPs are well predicted by thermodynamic and kinetic modelling [45,46,59].…”

“…RPV steel studies have also revealed significant solute segregation to loop and network dislocations. The segregated dislocations are a favoured nucleation sites for heterogeneous nucleation of MNSPs [43,47,50,57,58], as are cascade generated solute cluster complexes. However, apparently random homogeneous nucleation is also frequently observed in RPV steels and is predicted by models at sufficiently high solute contents (supersaturations), particularly Ni.…”

“…Figure 2: The averaged MNSP composition (in at%) of each APT dataset (over >5 million ions) for both 2.14 dpa and 8.82 dpa T91 irradiation conditions displayed on a 277 o C isothermal section of the Mn-Ni-Si ternary system projection of a Fe based phase diagram[41,48,74]. The size of the data point is scaled to the MNSP average APT dataset volume (nm 3 ).…”

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

“…phases [47]. Notably the XNis is ≈ 3. previous observations of 425°C PIA after a high dpa neutron irradiation [16].…”

Characterizing accelerated precipitation in proton irradiated steel

“…4.3 we compare the volume fraction and radius of CIPs in our 300 • C proton-irradiated specimen with literature on RPV steels with similar copper and high nickel content. This literature data set consists of neutron irradiated and Fe ion irradiated steels spanning over 6 orders of magnitude in dose rate, from neutron dose rates ≈ 0.1 ndpa s −1 matching RPV operational conditions [56][57][58], to Fe ion dose rates up to 150 000 ndpa s −1 [43]. The results from our 300 • C proton irradiation at 880 ndpa s −1 dose rate are found to be in good quantitative agreement with the literature data set, bearing in mind the known effects of nickel (Sec.…”