NSUF 23-4703: Understanding the Origin of Irradiation-Induced Yield Drop Phenomena in Grade 91
The objective of this project is to understand the origin of irradiation-induced yield drop phenomena in Grade 91 martensitic steel. Grade 91 is a 9Cr-1Mo steel which is a leading candidate for advanced reactor structural components due to its high temperature mechanical properties, creep strength, and irradiation tolerance. Through our ongoing NSUF project, we have observed yield drops develop after neutron irradiation to ~1 and 3.7 displacements per atom (dpa) at ~390ºC. Yield drops are conventionally ascribed to a Cottrell atmosphere of solute atoms along dislocation lines, which effectively pin the dislocations during alloy fabrication. Upon plastic yielding, the applied stress decreases once dislocations become unpinned from the Cottrell atmosphere, and the dislocations can continue to glide without additional stress. However, in our Grade 91 specimens, we see little evidence of solute segregation to dislocation lines. More notably, we observe extensive dislocation recovery during irradiation, and the dislocations that remain in the microstructure appear pinned on irradiation-induced dislocation loops. Then upon further irradiation, loops nucleate heterogeneously along these pinned dislocations, forming a subgrain structure. Thus, we hypothesize that the yield drop phenomenon in Grade 91 is not Cottrell-driven but is rather due to the heterogeneous pinning of lines on loops and subgrain formation.
We will resolve our hypothesis using a quasi-in situ approach that will enable us to ascertain the strain evolution of deformation microchemistry and microstructure in neutron irradiated Grade 91. Work will focus on ~1 and 3.7 dpa specimens of Grade 91 fabricated by powder metallurgy with hot isostatic pressing (PM-HIP), available in the NSUF library through our ongoing program 15-8242. We will extract TEM lamellae from positions around nanoindents corresponding to varying stress levels, then conduct high resolution scanning TEM (HR-STEM) to characterize deformation microstructures, precession electron diffraction (PED) to understand subgrain evolution, and energy dispersive x-ray spectroscopy (EDX) to map the microchemistry. Combining these techniques offers a significant advantage of being more straightforward, lower cost, and with fewer artifacts than in situ TEM straining experiments yet are still able to provide temporal insight into deformation behaviors. The scientific outcome will be an understanding of how irradiation alters the deformation mechanisms of Grade 91, specifically the genesis of the yield drop phenomenon. This work is timely because it has just been made feasible in NSUF through the recent installation of the ThermoFisher Spectra 300 STEM with PED and high-efficiency Super-X EDX at CAES.
Additional Info
| Field | Value |
|---|---|
| Abstract | The objective of this project is to understand the origin of irradiation-induced yield drop phenomena in Grade 91 martensitic steel. Grade 91 is a 9Cr-1Mo steel which is a leading candidate for advanced reactor structural components due to its high temperature mechanical properties, creep strength, and irradiation tolerance. Through our ongoing NSUF project, we have observed yield drops develop after neutron irradiation to ~1 and 3.7 displacements per atom (dpa) at ~390ºC. Yield drops are conventionally ascribed to a Cottrell atmosphere of solute atoms along dislocation lines, which effectively pin the dislocations during alloy fabrication. Upon plastic yielding, the applied stress decreases once dislocations become unpinned from the Cottrell atmosphere, and the dislocations can continue to glide without additional stress. However, in our Grade 91 specimens, we see little evidence of solute segregation to dislocation lines. More notably, we observe extensive dislocation recovery during irradiation, and the dislocations that remain in the microstructure appear pinned on irradiation-induced dislocation loops. Then upon further irradiation, loops nucleate heterogeneously along these pinned dislocations, forming a subgrain structure. Thus, we hypothesize that the yield drop phenomenon in Grade 91 is not Cottrell-driven but is rather due to the heterogeneous pinning of lines on loops and subgrain formation. We will resolve our hypothesis using a quasi-in situ approach that will enable us to ascertain the strain evolution of deformation microchemistry and microstructure in neutron irradiated Grade 91. Work will focus on ~1 and 3.7 dpa specimens of Grade 91 fabricated by powder metallurgy with hot isostatic pressing (PM-HIP), available in the NSUF library through our ongoing program 15-8242. We will extract TEM lamellae from positions around nanoindents corresponding to varying stress levels, then conduct high resolution scanning TEM (HR-STEM) to characterize deformation microstructures, precession electron diffraction (PED) to understand subgrain evolution, and energy dispersive x-ray spectroscopy (EDX) to map the microchemistry. Combining these techniques offers a significant advantage of being more straightforward, lower cost, and with fewer artifacts than in situ TEM straining experiments yet are still able to provide temporal insight into deformation behaviors. The scientific outcome will be an understanding of how irradiation alters the deformation mechanisms of Grade 91, specifically the genesis of the yield drop phenomenon. This work is timely because it has just been made feasible in NSUF through the recent installation of the ThermoFisher Spectra 300 STEM with PED and high-efficiency Super-X EDX at CAES. |
| Award Announced Date | 2023-06-01T09:05:44.297 |
| Awarded Institution | None |
| Facility | None |
| Facility Tech Lead | Alina Zackrone, Yaqiao Wu |
| Irradiation Facility | None |
| PI | Donna Guillen |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | None |