NSUF 18-1168: TEM in situ 4-point bend fracture testing of irradiated ODS alloys
The objective of this work is to evaluate fracture properties of irradiated oxide dispersion strengthened (ODS) steels using transmission electron microscopic (TEM) in situ 4-point bend testing. Fracture is amongst the most critical properties used to qualify materials for service in irradiation-facing components of nuclear power reactors. However, assessing the influence of irradiation on fracture toughness is an enormous challenge because of existing testing methods and high costs. Conventional fracture test geometries (e.g. compact tension, Charpy) require large material volumes, and often multiple specimens per material/condition. In-reactor neutron irradiations of such specimens are increasingly unfeasible given the limited space in test reactors, and the costs and complications of post-irradiation fracture testing in hot cells. And although strides have been made toward utilizing ion irradiations to emulate neutron irradiated microstructures, the ~few micrometer ion irradiated surface layer precludes the use of conventional (and even most “miniature”) fracture testing configurations. Hence, there is a critical need to develop nano- to micro-scale fracture test methods that can effectively scope the influence of neutron and ion irradiation on fracture properties. Geometries of TEM in situ mechanical test samples are sufficiently small that they resolve the irradiated volume concerns posed by both neutron and ion irradiation.
When mechanical tests are miniaturized, the specimen size effect can pose a challenge. The specimen size effect results in an inflated yield strength when specimen dimensions are so small that mechanical loads must first introduce geometrically necessary dislocations before they can initiate dislocation motion. However, our prior NSUF support has shown that the size effect is negligible in a model Fe-9Cr ODS alloy as long as specimen dimensions exceed 100 nm. This prior work lends credence to our hypothesis that a 4-point bend design can be miniaturized to in situ TEM scale, and can provide a meaningful assessment of fracture properties in the same ODS. We have already conducted such a test on the as-received ODS, from which we estimate a plane stress fracture toughness value which is within the expected range. In the proposed project, we refine a 4-point bend design using finite element modeling (FEM), then test our hypothesis against the same ODS heat irradiated to three conditions: with fast neutrons, protons, or Fe self-ions, all to 3 displacements per atom (dpa) at 500°C. This experiment matrix allows us the tremendous opportunity to evaluate three particle types on the identical alloy heat. During TEM in situ testing, we will record load-displacement data while simultaneously collecting TEM-resolution video; the combination of these data and our FEM will enable us to predict fracture toughness. Scientifically, this work will generate a meaningful method for assessing fracture in irradiated ODS alloys. More broadly, this work will lead to a reliable FEM-based design for TEM in situ microbeams, which can be adapted to other nuclear/irradiated materials.
Additional Info
| Field | Value |
|---|---|
| Abstract | The objective of this work is to evaluate fracture properties of irradiated oxide dispersion strengthened (ODS) steels using transmission electron microscopic (TEM) in situ 4-point bend testing. Fracture is amongst the most critical properties used to qualify materials for service in irradiation-facing components of nuclear power reactors. However, assessing the influence of irradiation on fracture toughness is an enormous challenge because of existing testing methods and high costs. Conventional fracture test geometries (e.g. compact tension, Charpy) require large material volumes, and often multiple specimens per material/condition. In-reactor neutron irradiations of such specimens are increasingly unfeasible given the limited space in test reactors, and the costs and complications of post-irradiation fracture testing in hot cells. And although strides have been made toward utilizing ion irradiations to emulate neutron irradiated microstructures, the ~few micrometer ion irradiated surface layer precludes the use of conventional (and even most “miniature”) fracture testing configurations. Hence, there is a critical need to develop nano- to micro-scale fracture test methods that can effectively scope the influence of neutron and ion irradiation on fracture properties. Geometries of TEM in situ mechanical test samples are sufficiently small that they resolve the irradiated volume concerns posed by both neutron and ion irradiation. When mechanical tests are miniaturized, the specimen size effect can pose a challenge. The specimen size effect results in an inflated yield strength when specimen dimensions are so small that mechanical loads must first introduce geometrically necessary dislocations before they can initiate dislocation motion. However, our prior NSUF support has shown that the size effect is negligible in a model Fe-9Cr ODS alloy as long as specimen dimensions exceed 100 nm. This prior work lends credence to our hypothesis that a 4-point bend design can be miniaturized to in situ TEM scale, and can provide a meaningful assessment of fracture properties in the same ODS. We have already conducted such a test on the as-received ODS, from which we estimate a plane stress fracture toughness value which is within the expected range. In the proposed project, we refine a 4-point bend design using finite element modeling (FEM), then test our hypothesis against the same ODS heat irradiated to three conditions: with fast neutrons, protons, or Fe self-ions, all to 3 displacements per atom (dpa) at 500°C. This experiment matrix allows us the tremendous opportunity to evaluate three particle types on the identical alloy heat. During TEM in situ testing, we will record load-displacement data while simultaneously collecting TEM-resolution video; the combination of these data and our FEM will enable us to predict fracture toughness. Scientifically, this work will generate a meaningful method for assessing fracture in irradiated ODS alloys. More broadly, this work will lead to a reliable FEM-based design for TEM in situ microbeams, which can be adapted to other nuclear/irradiated materials. |
| Award Announced Date | 2018-02-01T14:12:19.197 |
| Awarded Institution | Center for Advanced Energy Studies |
| Facility | Microscopy and Characterization Suite |
| Facility Tech Lead | Yaqiao Wu |
| Irradiation Facility | None |
| PI | Kayla Yano |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 1168 |