G.R. Odette

Ferritic/martensitic steels - overview of recent results

Considerable research work has been conducted on the ferritic/martensitic steels since the last International Conference on Fusion Reactor Materials in 1999. Since only a limited amount of that work can be reviewed in this paper, four areas will be emphasized: (1) the international collaboration under the auspices of the International Energy Agency (IEA) to address potential problems with ferri tic/marten si tic steels and to prove their feasibility for fusion, (2) the major uncertainty that remains concerning the effect of transmutation helium on mechanical properties of the steels when irradiated in a fusion neutron environment, (3) development of new reduced-activation steels beyond the F82H and JLF-1 steels studied in the IEA collaboration, and (4) work directed at developing oxide dispersion-strengthened steels for operation above 650degreesC

Primary damage formation in bcc iron

Abstract Primary defect formation in bee iron has been extensively investigated using the methods of molecular dynamics (MD) and Monte Carlo (MC) simulation. This research has employed a modified version of the Finnis-Sinclair interatomic potential. MD was used in the simulation of displacement cascades with energies up to 40 keV and to examine the migration of the interstitial clusters that were observed to form in the cascade simulations. Interstitial cluster binding energies and the stable cluster configurations were determined by structural relaxation and energy minimization using a MC method with simulated annealing. Clusters containing up to 19 interstitials were examined. Taken together with the previous work, these new simulations provide a reasonably complete description of primary defect formation in iron. The results of the displacement cascade simulations have been used to characterize the energy and temperature dependence of primary defect formation in terms of two parameters: (1) the number of surviving point defects and (2) the fraction of the surviving defects that are contained in clusters. The number of surviving point defects is expressed as a fraction of the atomic displacements calculated using the secondary displacement model of Norgett-Robinson-Torrens (NRT). Although the results of the high energy simulations are generally consistent with those obtained at lower energies, two notable exceptions were observed. The first is that extensive subcascade formation at 40 keV leads to a higher defect survival fraction than would be predicted from extrapolation of the results obtained for energies up to 20 keV. The stable defect fraction obtained from the MD simulations is a smoothly decreasing function up to 20 keV. Subcascade formation leads to a slight increase in this ratio at 40 keV, where the value is about the same as at 10 keV. Secondly, the potential for a significant level of in-cascade vacancy clustering was observed. Previous cascade studies employing this potential have reported extensive interstitial clustering, but little evidence of vacancy clustering. Interstitial clusters were found to be strongly bound, with binding energies in excess of 1 eV. The larger clusters exhibited a complex, 3D structure and were composed of 〈111〉 crowdions. These clusters were observed to migrate by collective 〈111〉 translations with an activation energy on the order of 0.1 eV.

Progress and critical issues of reduced activation ferritic/martensitic steel development

The inherent properties of reduced activation ferritic/martensitic (RAFM) steels include reduced swelling and high recycling potential, which make them likely candidates for application in commercial fusion power plants. The International Energy Agency (IEA) agreement has been an effective framework for international co-operation in developing RAFM steels. The progress and critical issues observed in this co-operation are reported. The production of RAFM steels on an industrial scale has been demonstrated. Various methods of fusion welding and solid hot isostatic pressing (HIP) are feasible for joining the steels. Manufacturing of complex shapes with the powder HIP method works well for RAFM steels. Major critical issues addressed concern the effects of simultaneous introduction of helium and displacement damage. The availability of a 14 MeV neutron source is identified as an essential tool to determine this effect. Finally, the potential of oxide dispersion strengthening to increase the operating temperature of RAFM steels is considered as an issue that has to be resolved to enlarge the application temperature window of RAFM steels.

Development of an extensive database of mechanical and physical properties for reduced-activation martensitic steel F82H

Abstract Tensile, fracture toughness, creep and fatigue properties and microstructural studies of the reduced-activation martensitic steel F82H (8Cr–2W–0.04Ta–0.1C) before and after irradiation are reported. The design concept used for the development of this alloy is also introduced. A large number of collaborative test results including those generated under the International Energy Agency (IEA) implementing agreements are collected and are used to evaluate the feasibility of using reduced-activation martensitic steels for fusion reactor structural materials, with F82H as one of the reference alloys. All the specimens used in these tests were prepared from plates obtained from 5-ton heats of F82H supplied to all participating laboratories by JAERI. Many of the results have been entered into relational databases with emphasis on traceability of records on how the specimens were prepared from plates and ingots.

Energetics of formation and migration of self-interstitials and self-interstitial clusters in α-iron

Abstract Energetic primary recoil atoms from fast neutron irradiation generate both isolated point defects and clusters of vacancies and interstitials. Self-interstitial mobility as well as defect cluster stability and mobility play key roles in the subsequent fate of defects and, hence, in the overall microstructural evolution under irradiation. Self-interstitials and two, three and four-member self-interstitial clusters are highly mobile at low temperatures as observed in molecular-dynamics simulations and high mobility probably also extends to larger clusters. In this study, the morphology, energetics and mobility of self-interstitials and small self-interstitial clusters in α-iron are studied by molecular-statics and molecular-dynamics simulations using a Finnis-Sinclair many-body interatomic potential. Self-interstitial migration is found to be a two-step process consisting of a rotation out of the 〈110〉 split-dumbbell configuration into the 〈111〉 split-dumbbell configuration and 〈111〉 translational jumps through the crowdion configuration before returning to the 〈110〉 dumbbell configuration. Self-interstitial clusters of 〈111〉 type split-interstitials assembled on adjacent {110} planes migrate along 〈111〉 directions in an amoeba-like fashion by sequential local dissociation and re-association processes.

Status of R&D activities on materials for fusion power reactors

Current R&D activities on materials for fusion power reactors are mainly focused on plasma facing, structural and tritium breeding materials for plasma facing (first wall, divertor) and breeding blanket components. Most of these activities are being performed in Europe, Japan, the Peoples Republic of China, Russia and the USA. They relate to the development of new high temperature, radiation resistant materials, the development of coatings that will act as erosion, corrosion, permeation and/or electrical/MHD barriers, characterization of candidate materials in terms of mechanical and physical properties, assessment of irradiation effects, compatibility experiments, development of reliable joints, and development and/or validation of design rules. Priorities defined worldwide in the field of materials for fusion power reactors are summarized, as well as the main achievements obtained during the last few years and the near-term perspectives in the different investigation areas.

Critical issues and current status of vanadium alloys for fusion energy applications

Vanadium alloys are widely regarded as possessing desirable mechanical and physical properties for application as structural materials in fusion power systems. The bulk of the recent research on vanadium is focussed on ternaries containing 4–5% Cr and 4–10% Ti. The aim of this paper is to review significant results generated by the international research and development community on this alloy system and to highlight the critical issues that must be resolved before alloy development can proceed to the next stage. Recent progress on understanding the physical metallurgy, fabrication and joining behavior, and compatibility with hydrogen and oxygen containing environments of unirradiated vanadium alloys is discussed. The effect of low-temperature neutron irradiation on mechanical properties and their relationship to the observed microstructure are briefly summarized. Current efforts to characterize the high-temperature mechanical properties, develop constitutive equations describing flow and fracture, and understand and mitigate the effects of non-metallic impurities on properties are presented.

Analytical solutions for helium bubble and critical radius parameters using a hard sphere equation of state

Abstract Considerable theoretical and experimental work has verified the role of helium-stabilized bubbles as the precursor to void formation in fast neutron irradiated stainless steels. The concept of the critical bubble radius or critical helium number for bubble-to-void conversion has received particular attention. A hard sphere equation of state is used to compute these parameters for a variety of irradiation conditions and the results are compared with those computed using the ideal gas law. Simplified analytical solutions are developed which permit the calculation of the bubble radius and the critical bubble parameters without resorting to iterative techniques and yet retain the accuracy of the hard sphere equation of state. The use of these solutions is illustrated using a rate theory model of void swelling which has been calibrated using fast reactor swelling data.

On the effect of dose rate on irradiation hardening of RPV steels

The effect of dose rate (DR), or neutron flux (ϕ), on irradiation hardening (Δσy) and embrittlement of reactor pressure vessel (RPV) steels is a key unresolved issue. We report a rigorous evaluation of DR effects based on a very large Δσy database we developed for RPV steels with a wide range of compositions, including a set of split-melt alloys with controlled and systematic variation in Cu, Ni and Mn content. The steels were irradiated at 290°C in three ϕ-regimes to a wide range of overlapping fluences (ϕt). The contribution of copper-rich precipitates (CRPs) to Δσy increases up to a plateau hardening that is a strong function of the alloy Cu, Ni and Mn content, but is relatively independent of DR. However, the pre-plateau region is shifted to higher ϕt with increasing DR. The shift can be approximately accounted for by defining an effective fluence (ϕte) as ϕte ≈ ϕt(ϕr /ϕ)1/2, where ϕr is a reference flux. The ϕ −1/2 scaling is consistent with a vacancy plus self-interstitial-atom (SIA) recombination rate controlling mechanism. The Δσy data are analysed with a combined model describing: (a) the excess vacancy concentration under irradiation as a function of DR, including the effect of solute vacancy traps on recombination; (b) the corresponding radiation enhanced Cu diffusion (RED) coefficient (D*); (c) the resulting accelerated growth of CRPs; and (d) the contribution of CRPs to Δσy. Recombination is shown to increase with higher alloy Ni and Mn content, consistent with a solute–vacancy trapping mechanism. In spite of high recombination rates, however, RED is extremely efficient, with the D* ranging up to a factor of 60 or more times higher than predicted by simple rate theory models. Various explanations of the high diffusion rates are discussed, including large vacancy–solute binding energies that control the vacancy concentrations and jump frequencies near solutes in a way that can enhance both diffusion and recombination.

Ductile-reinforcement toughening in γ-TiAl intermetallic-matrix composites : effects on fracture toughness and fatigue-crack propagation resistance

Abstract The influence of the type, volume fraction, thickness and orientation of ductile phase reinforcements on the room temperature fatigue and fracture resistance of γ-TiAl intermetallic alloys is investigated. Large improvements in toughness compared to monolithic γ-TiAl are observed in both the TiNb- and Nb-reinforced composites under monotonic loading. Toughness increases with increasing ductile phase content, reinforcement thickness and strength; orientation effects are minimal. Crack-growth behavior is characterized by steep resistance curves primarily due to crack trapping/renucleation and extensive crack bridging by the ductile-phase particles. In contrast, under cyclic loading the influence of ductile phases on fatigue resistance is strongly dependent upon reinforcement orientation. Compared to monolithic γ-TiAl, improvements in fatigue-crack growth resistance are observed in TiNb-reinforced composites only in the face (C-L) orientation; crack-growth rates for the edge (C-R) orientation are actually faster in the composite. In comparison, Nb-particle reinforcements offer less toughening under monotonic loading but enhance the fatigue properties compared to TiNb reinforcements under cyclic loading.