Atom probe tomography was used to characterise two low-Cu (< 0.04 at. %) model steels after exposure to long-term thermal ageing. Mn-Ni-Si-rich features were observed to form after as little as 20,731 h (similar to 2.4 years) of ageing. The composition of these features were compared to those predicted by thermodynamic models and the similarities and differences are discussed. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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 approximate to 290 degrees C to approximate to 1.4 x 10(20) n/cm(2) at an accelerated test reactor flux of approximate to 3.6 x 10(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 approximate to 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 Ni-silicide phase type compositions are observed. (C) 2019 Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Irradiation embrittlement of steel pressure vessels is an important consideration for the operation of current and future light water nuclear reactors. In this study we employ an ensemble of artificial neural networks in order to provide predictions of the embrittlement using two literature datasets, one based on US surveillance data and the second from the IVAR experiment. We use these networks to examine trends with input variables and to assess various literature models including compositional effects and the role of flux and temperature. Overall, the networks agree with the existing literature models and we comment on their more general use in predicting irradiation embrittlement.

SANS experiments were performed on a high Ni weld surveillance sample from the Ringhals NPP and the Maximum Entropy method was applied to determine the most probable size distribution of irradiation-induced scattering features. The results were shown to be consistent with atom probe observations. The sensitivity of the data analyses with respect to constraints such as the limited experimentally available Q range was explored. The calculated volume fraction and the mean volume-weighted diameter of the precipitates were found to be relatively insensitive to Q(max) (the maximum scattering vector) greater than -0.40 angstrom(-1). However, use of a lower Q(max) results in a shift of the size distribution to larger diameters and a reduced particle number density. Simulations demonstrated that the experimentally observed decrease in the A-ratio at higher Q values is consistent with the presence of vacancies or higher Mn contents in smaller features. Importantly, features which are experimentally unresolvable do not add to the apparent volume fraction of the features which are resolved. Crown Copyright (C) 2018 Published by Elsevier B.V. All rights reserved.

To understand the contribution of long term thermal ageing to Reactor Pressure Vessel (RPV) embrittlement two high Cu steel welds with different Ni contents were thermally aged for times up to 100,000 h at 330 °C and 365 °C. Microstructural characterisation using Atom Probe Tomography was performed. Thermal ageing produced a high number density of nano-scale Cu-enriched precipitates. The precipitate-matrix interfaces were enriched in Ni, Mn and Si. The characterisation of these interfaces using a double cluster search approach is the subject of this work. The interface region around thermally-induced precipitates was found to be wider in steels with higher bulk Ni contents and where precipitates had larger core radii. The effect of ageing temperature on interface width was small when comparing precipitates of equal core radius. The narrower interface width in the lower Ni steels is reflected in the composition of the interface, which has a lower Ni content than in the higher Ni material. The reduction in interfacial energy due to the segregation of Ni, Mn and Si has been calculated and shows enhanced reductions in interfacial energy with increasing precipitate size, but no obvious effect of temperature.

Probably the most important range of materials for consideration as the blanket material for the tokamak design for fusion reactors ITER and DEMO is the high alloy Fe–9Cr oxide dispersion strengthened ferritic steels. Ferritic steels possess exceptional thermal conductivity and low thermal expansion and are resistant to void swelling. Their main drawback is high ductile to brittle transition temperatures, particularly in the oxide dispersion strengthened versions. This paper describes attempts to reduce the DBTT in an un-irradiated ferritic steel by a novel heat treatment procedure. New batches of high alloy Fe–9Cr oxide dispersion strengthened (Eurofer) ferritic steel have been produced by a powder metallurgy route, and relatively homogeneous material has been produced by hot isostatic pressing (HIP). Mini-Charpy test specimens were made from materials which had been subjected to a matrix of heat treatments with varying solution treatment temperature (ST), cooling rate from the ST temperature, and tempering treatment. The initial DBTT was in the range of 150–200°C (423–473K). Downward shifts of up to approximately 200°C (473K) have been observed after solution treatment at 1300°C (1573K) followed by slow cooling. This paper describes the microstructure of this material, and discussion is made of the likely microstructural factors needed to produce these DBTT downward shifts.

Controlled amounts of cold work are shown to cause a minimum in the ductile to brittle transition temperature (DBTT) in a ferritic steel at a critical level of ∼1·5. Mechanical property assessments show that the hardness values exhibit the same trend. A theory is advanced for explanation of these effects, based on work hardening and Cottrell–Bilby locking models. Consideration is given to an alternative Ashby–Embury model, but it is concluded that the former approach is most successful in predicting the observed DBTT shift behaviour. Although independent of fracture surface type, the degree of plastic deformation shows some dependency on the grain boundary character. This leads to the conclusion that the matrix yield strength is the primary factor in determining the DBTT in these steels. Discussion focuses on methods for exploiting the effect to give higher toughness steels utilising knowledge of how to control matrix hardening and cleavage fracture strength.

Probably the most important range of materials to consider for the blanket material in the tokamak design for fusion reactors such as ITER and DEMO is the high alloy Fe9Cr oxide dispersion strengthened (ODS) ferritic steels. These steels possess exceptional thermal conductivity and low thermal expansion while being strongly resistant to void swelling. Their main drawback is the high ductile-to-brittle transition temperature (DBTT), particularly in the ODS versions of the material. This paper describes attempts that are being made to reduce this DBTT in as yet unirradiated materials by a novel heat treatment procedure. The principle behind this approach is that low DBTT in the unirradiated materials will lead to relatively low DBTT even in He-containing material that has been irradiated with fusion blanket-type irradiations. New batches of high alloy Fe9Cr ODS (EUROFER) ferritic steel have been produced by a powder metallurgical route, and relatively homogeneous material has been produced by a hot isostatic pressing procedure. Mini-Charpy test specimens were made from materials that had been subjected to a matrix of heat treatments designed to show up variations in solution treatment (ST) temperature, cooling rate from the ST temperature and tempering treatment. The initial DBTT was in the range 150-200 degrees C. Extremely interesting results have been obtained. DBTT downward shifts of up to 200 degrees C have been observed by using a high 1300 degrees C ST temperature and a low cooling rate. The paper goes on to describe the microstructure of this material, and discusses the possible microstructural factors needed to produce these very high DBTT downward shifts. Low dissolved carbon and higher proportions of low-angle grain boundaries seem to provide the key to the understanding of the alloy behaviour.

Eurofer ODS steel is a potential candidate for fusion reactor application due to its excellent swelling resistance, low thermal expansion coefficient and high temperature properties. One of the main issues is that high fluence neutron irradiation induces a significant increase of ductile-to-brittle transition temperature (DBTT) at temperatures below 400°C which restricts its application. The aim of this study is to explore the methods to lower the initial DBTT of Eurofer ODS steel by heat treatment optimization. Two heats of Eurofer ODS steels with different C contents are heat-treated at different normalizing temperatures, cooling rates and tempering conditions, and are compared with Eurofer 97 and T92 steels heat-treated with similar conditions. The microstructure is characterized by optical microscopy, FEG-TEM and OIM-EBSD techniques. The effect of normalization, cooling rate and temper on grain size, precipitation, grain boundary misorientation and hardness are investigated. The influences of these properties on DBTT are discussed.

It is shown that controlled amounts of cold work can cause a minimum in the DBTT in normalised, ferritic/pearlitic pressure vessel steels at a critical level of about 1.5%. Detailed mechanical property assessments of the material show that the hardness passes though a minimum with the same critical degree of deformation. The type of fracture surface on the lower shelf is independent of the amount of plastic deformation, but there are some changes in grain boundary character that depend on the amount of plastic deformation. This leads to the conclusion that the matrix yield strength is the primary factor in determining the DBTT in these steels. A theory is advanced for explanation of these effects, based on work hardening and upper yield point models. Consideration is given to an alternative Ashby-Embury model but it is concluded that the former approach is most successful in predicting the observed DBTT shift behaviour in plain carbon steels. Discussion will centre on methods for exploiting the effect to give higher toughness steels utilising knowledge of how to control matrix hardening and cleavage fracture strength.

Eurofer ODS is a potential candidate for fusion reactor application due to its excellent swelling resistance and high temperature properties. One of the main issues is that high fluence neutron irradiation induces a significant increase of ductile-to-brittle transition temperature (DBTT) at temperatures below 400 ◦C which restricts its application. The aim of this study is to lower the initial DBTT of Eurofer ODS steel by heat treatment optimization. Two heats of Eurofer ODS steels with di fferent C contents are heat-treated at di fferent normalized temperatures, cooling rates and tempered conditions. Their microstructure is characterized by optical microscopy, FEG-TEM and OIM-EBSD techniques. The effect of normalization, cooling rate and temper on grain size, precipitation, grain boundary chemistry, grain boundary misorientation, hardness, and DBTT are investigated. Monte Carlo models of precipitation are used with thermodynamic software MTDATA to forecast the phase evolution kinetics in Eurofer ODS. The predicted results are compared with experimental data.

Eurofer ODS is a potential candidate for fusion reactor application due to its excellent swelling resistance and high temperature properties. One of the main issues is that high fluence neutron irradiation induces a significant increase of ductile-to-brittle transition temperature (DBTT) at temperatures below 400 C which restricts its application. The aim of this study is to lower the initial DBTT of Eurofer ODS steel by heat treatment optimization. Two heats of Eurofer ODS steels with di erent C contents are heat-treated at di erent normalized temperatures, cooling rates and tempered conditions. Their microstructure is characterized by optical microscopy, FEG-TEM and OIM-EBSD techniques. The e ect of normalization, cooling rate and temper on grain size, precipitation, grain boundary chemistry, grain boundary misorientation, hardness, and DBTT are investigated. Monte Carlo models of precipitation are used with thermody- namic software MTDATA to forecast the phase evolution kinetics in Eurofer ODS. The predicted results are compared with experimental data.