microstructural evolution of ferritic-martensitic steels under heavy ion irradiation
Open Access
- Author:
- Topbasi, Cem
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 25, 2014
- Committee Members:
- Arthur Thompson Motta, Dissertation Advisor/Co-Advisor
Clifford Jesse Lissenden Iii, Committee Member
Zi Kui Liu, Committee Member
Clive A Randall, Committee Member
Brian D Wirth, Special Member - Keywords:
- nuclear materials
steels
ion irradiation
iron
defects - Abstract:
- Ferritic-martensitic steels are primary candidate materials for fuel cladding and internal applications in the Sodium Fast Reactor, as well as first-wall and blanket materials in future fusion concepts because of their favorable mechanical properties and resistance to radiation damage. Since microstructure evolution under irradiation is amongst the key issues for these materials in these applications, developing a fundamental understanding of the irradiation-induced microstructure in these alloys is crucial in modeling and designing new alloys with improved properties. The goal of this project was to investigate the evolution of microstructure of two commercial ferritic-martensitic steels, NF616 and HCM12A, under heavy ion irradiation at a broad temperature range. An in situ heavy ion irradiation technique was used to create irradiation damage in the alloy; while it was being examined in a transmission electron microscope. Electron-transparent samples of NF616 and HCM12A were irradiated in situ at the Intermediate Voltage Electron Microscope (IVEM) at Argonne National Laboratory with 1 MeV Kr ions to ~10 dpa at temperatures ranging from 20 to 773 K. The microstructure evolution of NF616 and HCM12A was followed in situ by systematically recording micrographs and diffraction patterns as well as capturing videos during irradiation. In these irradiations, there was a period during which no changes are visible in the microstructure. After a threshold dose (~0.1 dpa between 20 and 573 K, and ~2.5 dpa at 673 K) black dots started to become visible under the ion beam. These black dots appeared suddenly (from one frame to the next) and are thought to be small defect clusters (2-5 nm in diameter), possibly small dislocation loops with Burgers vectors of either ½<111> or <100>. The overall density of these defect clusters increased with dose and saturated around 6 dpa. At saturation, a steady-state is reached in which defects are eliminated and created at the same rates so that the defect density is constant. After saturation, defects constantly appeared and disappeared in a time that is shorter than the time in between frames (normally 34 ms). The average diameter and size distribution of the irradiation-induced defect clusters did not vary with dose during a single irradiation in the temperature range of 50 to 573 K in NF616, and 20 to 673 K in HCM12A. At 673 K, the defects in NF616 grew and coalesced under irradiation which led to larger average defect sizes and low defect density. At high doses extended defect structures in NF616 formed as short segments aligned along 100 directions. At 773 K, the frequency of defect formation per unit area was the lowest amongst all irradiations and all the visible defect clusters that formed eventually faded out gradually (in ~28 seconds) leading to no net defect accumulation in NF616 even at the highest irradiation dose of 10 dpa. Under irradiation, a significant fraction of these defect clusters exhibited sudden one-dimensional jumps (over ~5nm) between 20 and 573 K, that is, some defect clusters move “or jump” along <211> directions which is consistent with the expected Burgers vector direction of (111). Interestingly, at 673 and 773 K, defects in NF616 and HCM12A did not exhibit the sudden jumps and jerks that were frequently observed during lower temperature irradiations. No resolvable loops, voids or precipitates were formed in NF616 and HCM12A. Furthermore, no significant interaction of the irradiation induced defects with the foil surface, pre-existing dislocation network or grain boundaries was observed between 20 and 773 K. A simplified rate theory model was developed to describe the initial defect formation processes. The model is based on the reactions between intra-cascade clusters driven by the one-dimensional movement of sub-visible interstitial clusters in their glide cylinder under irradiation after detrapping from interstitial and substitutional solute atoms by cascade impact. Multiple cascade impacts on previously existing clusters allow them to gather clusters during their glide, leading to the formation of TEM-visible (~2 nm) defects. The low dose defect density approximated by model is in good agreement with the experimental results. In addition, the model rationalizes the threshold dose before which no visible defect clusters were formed.