NSUF 14-512: Investigation of thermal stability of nanoscale Ni-Mn-Si precipitates in highly irradiated RPV steels
Significant hardening and irradiation embrittlement of RPV steels has traditionally been associated with transition phase (bcc) Cu rich precipitates, alloyed with Mn-Ni-Si. The underlying mechanism is radiation-enhanced diffusion due to excess defects created by displacement damage. More than 20 years ago the PI predicted that Mn-Ni intermetallic phases could form even in Cu free steels, but would be slow to nucleate and grow, thus they would be important only at high fluences and were dubbed late blooming phases (LBP). The existence of LBP was proven about 10 years ago, though recent research has shown that LBP are likely to form at the higher fluences experienced during nuclear plant life extension. Current regulations do not account for LBP, which could cause massive embrittlement even in Cu free steels, as verified in a recent high fluence-high flux (accelerated) test reactor irradiation data, where volume fractions of Mn-Ni-Si phases approach 3%, or ˜ 5 to 10 times more than at 40 year fluences. Though the existence of these precipitates is largely accepted, their formation mechanism is under debate. Some groups propose that these precipitates form through a radiation induced segregation mechanism, while our group proposes that the driving force behind their formation is largely thermodynamic, with radiation increasing the rate at which they form. The use of post irradiation annealing followed by atom probe tomography (APT) measurements of precipitate size, number density, and volume fraction is a way to resolve this issue and develop critical new insight. Additionally, combining APT with microhardness testing provides a way to not only determine the microstructural and mechanical property changes, but link the two together, which is imperative in creating new embrittlement prediction models. Thus, the objective of this proposal is to use APT to investigate the microstructure of irradiated reactor pressure vessel steels following annealing at two different temperatures, 375°C and 425°C, and times, 20 and 30 weeks. Previous studies have been done on these steels annealed for 1 and 10 weeks, but longer annealing times are needed to discriminate kinetic versus thermodynamic effects, e.g. to allow an equilibrium volume precipitate fraction to be reached. These results, which may be obtained over the next 4 months, will be combined with already obtained microhardness data on the same annealing conditions to determine what hardening features are likely present and the relative hardening contribution of each feature. An additional primary short term goal of this research if the precipitate formation is proven to be thermodynamically driven is to map out the low temperature phase boundaries of these phases, which are currently unknown. These results will then feed into the much larger, long term goal of creating an embrittlement prediction model that takes into account LBP and can accurately predict reactor pressure vessel embrittlement at 80 year extended life fluences.
Additional Info
| Field | Value |
|---|---|
| Abstract | Significant hardening and irradiation embrittlement of RPV steels has traditionally been associated with transition phase (bcc) Cu rich precipitates, alloyed with Mn-Ni-Si. The underlying mechanism is radiation-enhanced diffusion due to excess defects created by displacement damage. More than 20 years ago the PI predicted that Mn-Ni intermetallic phases could form even in Cu free steels, but would be slow to nucleate and grow, thus they would be important only at high fluences and were dubbed late blooming phases (LBP). The existence of LBP was proven about 10 years ago, though recent research has shown that LBP are likely to form at the higher fluences experienced during nuclear plant life extension. Current regulations do not account for LBP, which could cause massive embrittlement even in Cu free steels, as verified in a recent high fluence-high flux (accelerated) test reactor irradiation data, where volume fractions of Mn-Ni-Si phases approach 3%, or ˜ 5 to 10 times more than at 40 year fluences. Though the existence of these precipitates is largely accepted, their formation mechanism is under debate. Some groups propose that these precipitates form through a radiation induced segregation mechanism, while our group proposes that the driving force behind their formation is largely thermodynamic, with radiation increasing the rate at which they form. The use of post irradiation annealing followed by atom probe tomography (APT) measurements of precipitate size, number density, and volume fraction is a way to resolve this issue and develop critical new insight. Additionally, combining APT with microhardness testing provides a way to not only determine the microstructural and mechanical property changes, but link the two together, which is imperative in creating new embrittlement prediction models. Thus, the objective of this proposal is to use APT to investigate the microstructure of irradiated reactor pressure vessel steels following annealing at two different temperatures, 375°C and 425°C, and times, 20 and 30 weeks. Previous studies have been done on these steels annealed for 1 and 10 weeks, but longer annealing times are needed to discriminate kinetic versus thermodynamic effects, e.g. to allow an equilibrium volume precipitate fraction to be reached. These results, which may be obtained over the next 4 months, will be combined with already obtained microhardness data on the same annealing conditions to determine what hardening features are likely present and the relative hardening contribution of each feature. An additional primary short term goal of this research if the precipitate formation is proven to be thermodynamically driven is to map out the low temperature phase boundaries of these phases, which are currently unknown. These results will then feed into the much larger, long term goal of creating an embrittlement prediction model that takes into account LBP and can accurately predict reactor pressure vessel embrittlement at 80 year extended life fluences. |
| Award Announced Date | 2014-08-11T00:00:00 |
| Awarded Institution | Center for Advanced Energy Studies |
| Facility | Microscopy and Characterization Suite |
| Facility Tech Lead | Yaqiao Wu |
| Irradiation Facility | None |
| PI | G. Robert Odette |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 512 |