Microstructural Processes in Irradiated Materials: Ferritic and Ferritic-Martensitic Alloys I
Sponsored by: TMS Structural Materials Division, TMS: Nuclear Materials Committee
Program Organizers: Thak Sang Byun, Pacific Northwest National Laboratory; Chu-Chun Fu, Commissariat à l'énergie atomique et aux énergies alternatives (CEA); Djamel Kaoumi, University of South Carolina; Dane Morgan, University of Wisconsin-Madison; Mahmood Mamivand, University of Wisconsin-Madison; Yasuyoshi Nagai, Tohoku University

Tuesday 8:30 AM
February 28, 2017
Room: Del Mar
Location: Marriott Marquis Hotel

Session Chair: Meimei Li, Argonne National Laboratory; Kevin Field, Oak Ridge National Laboratory

8:30 AM  Invited
Microstructures in Irradiated and Deformed FeCrAl Alloys: Kevin Field1; Samuel Briggs2; Jack Haley3; Maxim Gussev1; Kenneth Littrell1; Philip Edmondson1; Yukinori Yamamoto1; Xunxiang Hu1; Richard Howard1; Zhijie Jiao4; Gary Was4; Kumar Sridharan2; Lance Snead5; Kurt Terrani1; 1Oak Ridge National Laboratory; 2University of Wisconsin; 3University of Oxford; 4University of Michigan; 5Massachusetts Institute of Technology
    A comprehensive database on the microstructures of irradiated and deformed FeCrAl alloys is key to their deployment in nuclear applications. Until recently, no such database existed. Lately, a series of irradiations followed by examination have been completed on over fifteen unique FeCrAl alloys. Irradiations include non-instrumented neutron irradiations up to 14 dpa between 200°C and 550°C, self-ion irradiations at 400°C to 0.5 dpa, in-situ ion irradiations at 320°C up to 2.5 dpa, and instrumented neutron irradiations at 320°C to <1 dpa. Investigations included, but not limited to, electron microcopy, atom probe tomography, tensile testing, and fractography. Observed microstructures include radiation induced precipitation and segregation, dislocation loop populations, and cavities. This work will generalize the observed material behavior and performance by focusing on primary trends within this newly developed comprehensive database and provide detailed strategies for deployment of a nuclear grade FeCrAl alloy.

9:00 AM  
Ballistic Mixing Effect on α' Precipitation in Irradiated Fe-Cr Alloys: Jia-Hong Ke1; Mukesh Bachhav2; Elaina Anderson2; Emmanuelle A. Marquis2; G. Robert Odette3; Dane Morgan1; 1University of Wisconsin-Madison; 2University of Michigan, Ann Arbor; 3University of California, Santa Barbara
    Precipitation of the Cr-rich α' phase in irradiated Fe-Cr ferritic/martensitic alloys is known to be radiation-enhanced and responsible for hardening and embrittlement. Recent studies showed that precipitation microstructures in Fe-Cr model alloys differ dramatically among different types of radiation sources. This unexpected discrepancy suggests a flux- or particle-dependent characteristic, but it remains unclear how irradiation fluxes or displacement cascades affect the stability of α’ precipitates. In the present work, we utilize a Cahn-Hilliard-type phase-field model with an explicit nucleation algorithm and ballistic mixing to study the evolution of precipitate microstructures with different levels of mixing caused by displacement cascades. The simulation result shows that ballistic mixing is able to produce dissolution and cause phase instability of α’ precipitates, which provides insights into the striking difference found between ion and neutron radiation. We also use the model to explore when ballistic mixing effects will play a critical role in precipitate stability.

9:20 AM  
Kinetics of Cr Precipitation in Iron under Irradiation: Frederic Soisson1; Estelle Meslin1; Olivier Tissot1; Jean Henry1; Chu-Chun Fu1; Brigitte Descamps2; Cristelle Pareige3; 1CEA Saclay; 2CSNSM; 3GPM
    At low temperatures, Fe-Cr alloys undergo a coherent phase separation between Fe- and Cr-rich phases. This α-α’ decomposition can be considerably accelerated by irradiation, leading to possible hardening and embrittlement of ferritic steels used in nuclear reactors. The phase separation in Fe-Cr alloys under irradiation is modeled by a multiscale approach coupling ab initio calculations (that provide an accurate description of point defect properties), atomistic Monte Carlo simulations (for the modeling of the α-α’ decomposition), and cluster dynamics (for the evolution of point defect sinks). The simulations are compared to recent analysis of ion and electron irradiated Fe-Cr alloys by atom probe tomography. The conditions for the acceleration of precipitation and the ballistic dissolution of precipitates are studied.

9:40 AM  
Atomistic Modeling of Hardening in Thermally-aged Fe-Cr Binary Alloys: Tomoaki Suzudo1; Yasuyoshi Nagai2; Alfredo Caro3; 1Japan Atomic Energy Agency; 2Tohoku University; 3Los Alamos National Laboratory
    It is widely known that Fe-Cr binary alloys with Cr concentration more than ~20% undergo spinodal decomposition when they are thermally aged. This microstructural evolution causes hardening and loss of ductility of the material. Origin of the hardening can be ascribed to dislocations interacting obstacles, i.e. Cr-rich phases. Since the dislocation motion in such heterogeneous media is extremely complicated, the quantitative prediction of the hardening remains as an unsolved problem. The present study is about the first attempt to tackle this problem by exploiting atomistic modeling techniques. We apply Monte Carlo simulation to creating spinodally-decomposed microstructure and molecular dynamics to simulating edge dislocations moving through this microstructure by imposing shearing deformation. We then measure the critical stress as a measure of hardness for many cases over the progress in spinodal decomposition, and succeed in reproducing an experimentally-discovered proportionality between the phase separation parameter (or the variation parameter) and the hardening.

10:00 AM  
Influence of Secondary Phase Formation on Microstructure Evolution in Self-Ion Irradiated HT9 up to 650 dpa: Elizabeth Getto1; Kai Sun1; Gerrit Vancoevering1; Zhijie Jiao1; Gary Was1; 1University of Michigan
    Determining the microstructure evolution of ferritic-martensitic alloys is important for predicting the safety and structural integrity of fast reactors. Microstructure evolution was characterized in self-ion irradiated ferritic-martensitic alloy HT9 at 460°C up to 650 displacements per atom (dpa). Irradiations were performed with 5 MeV Fe++ ions on samples pre-implanted with 10 atom parts per million He and irradiated using a rastered beam with a 3 MV Pelletron accelerator at the Michigan Ion Beam Laboratory. The precipitate behavior was analyzed using scanning transmission electron microscopy (STEM) in bright field (BF) with additional high resolution STEM microscopy to determine coherency of phases formed. Voids were analyzed using high angle annular dark field (HAADF) and dislocation loops and network were analyzed using STEM BF. Two secondary phases, M2X and G phase, were observed. The influence of these phases on the void and dislocation microstructure was explained using a cluster dynamics model.

10:20 AM Break

10:35 AM  
Ion Irradiation Induced Segregation and Precipitation in F/M Steel HT9: Ce Zheng1; Maria Auger2; Djamel Kaoumi1; 1North Carolina State University; 2University of Oxford
    In-situ transmission electron microscopy (TEM) was used to investigate the microstructural evolution of HT9 under Kr++ ion irradiation at elevated temperatures (420-470°C), in combination with Ex-situ Fe++ ion irradiations performed on bulk HT9 at similar irradiation conditions. ChemiSTEM and atom probe tomography (APT) techniques were then used to characterize radiation-induced segregation and precipitation effects in both in-situ/ex-situ irradiated HT9 samples. Radiation-induced solute segregation to defect sinks (such as grain boundaries and carbides interfaces) and Ni-Mn-Si rich G-phase precipitation were observed in ion-irradiated HT9, in accordance with previous observations done on the same material under neutron irradiation. The microstructure developed in-situ is compared with the ex-situ observations done on the bulk irradiated samples. This study serves to generate baseline data on ion irradiation effects on HT9 in an effort to learn how this alloy respond to irradiation and how the phases in this alloy evolve with dose and temperature.

10:55 AM  
Microstructural Studies of Irradiated and Deformed FeCr Model Alloys: Mercedes Hernández-Mayoral1; Elvira Oñorbe1; Marta Serrano1; 1CIEMAT
    The microstructure induced by ion irradiation in FeCr model alloys is being studied. The materials were irradiated with Fe ions at 300ºC up to a dose of 0.5 dpa. To understand the mechanical response of the material, a thorough characterization of the deformed microstructure has been undertaken. The irradiated and unirradiated material has been tensile tested at RT and 300ºC until rupture in order to know and select the suitable parameters to perform the interrupted tests. The same procedure was followed for the small punch testing and deformation. The dislocation structures are being examined for the materials after tensile or small punch deformation, taking special attention to the dislocation-irradiation induced defect interactions in order to understand how the radiation induced microstructure may affect the mechanical response of the FeCr irradiated materials.

11:15 AM  
Emulation of Reactor-irradiated Microstructural Features with Dual Ion-irradiation in T91 Steel: Stephen Taller1; Zhijie Jiao1; Kevin Field2; Gary Was1; 1University of Michigan; 2Oak Ridge National Laboratory
    High fidelity emulation of neutron irradiation microstructures using ion beams requires capturing the effects of transmutation gas, such as helium. Irradiations using 5 MeV Fe2+ ions to induce damage with co-injected He2+ to simulate gas transmutation buildup were performed at the Michigan Ion Beam Laboratory on a ferritic-martensitic alloy T91, irradiated to 16.6 and 35 dpa at 432°C with helium co-injected in ratios of 0.02 and 0.22 appm He/dpa. The same heat was irradiated in the BOR-60 fast reactor to 16.6 dpa at average temperatures of 386°C and 412°C. Microstructural features, such as cavities, precipitates and dislocation loops were characterized using TEM to understand the development of the irradiated microstructure of reactor and ion irradiated T91. Dislocation loops, cavities assisted by helium, and nickel-silicon clusters were found to agree with those irradiated in BOR-60 in average size and density within a factor of 2.

11:35 AM  
He Implantation of Fe-Y2Ti2O7 Bilayers: Furthering NFA Understating: Tiberiu Stan1; Yuan Wu1; Robert Odette1; Yongquiang Wang2; Richard Cox3; 1University of California Santa Barbara; 2Los Alamos National Laboratory; 3Pacific Northwest National Laboratory
    Nanostructured Ferritic Alloys (NFAs) contain 2-3 nm pyrochlore Y2Ti2O7 nano-oxides (NOs) embedded in a bcc Fe-Cr ferrite matrix. To compliment characterization of the NOs themselves, bilayers were fabricated by Fe deposition onto {100}, {110} and {111} Y2Ti2O7 single crystal surfaces to create mesoscopic interfaces. Following implantation, helium partitioned between the Fe matrix, Y2Ti2O7, and the associated interface. The helium fates, in either forming bubbles or dissolving in the oxide, were characterized by TEM, SIMS and mass spectroscopy. Both the atomic interfacial structures and the implications of this work to measuring, modeling and managing helium in irradiation tolerant NFAs are discussed.