Richard J. Kurtz

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.

Interactions of dislocations with disconnections in fcc metallic nanolayered materials

Embedded-atom method potentials and atomistic models of coherent (010) interfaces were used to study slip across interfaces in cube-on-cube oriented Cu/Ni nanolayered materials. (111) disconnections form during slip across Cu–Ni interfaces and become significant barriers to continued deformation. A significant barrier exists for the flat coherent interface owing to the large coherency stresses in the Cu/Ni layers that must be overcome by applied stresses but, once these have been overcome, interface transection occurs readily. A disconnection adds an additional barrier because of a residual dislocation with a Burgers vector magnitude equal to the difference between b Cu and b Ni. This barrier depends on the position of the disconnection relative to the glide plane of the transecting glide dislocation and on the disconnection height. Disconnections cause work hardening that prevents shear band formation during deformation and encourages homogeneous shear processes. Disconnection energies are shown to be relatively small and to depend on the disconnection type and size.

Effect of extrinsic grain-boundary defects on grain-boundary sliding resistance

Abstract A computer simulation was performed to examine the role of extrinsic grain-boundary (GB) defects in the sliding of the {11, (101){131} symmetric tilt boundary in aluminium by means of an embedded-atom method potential. The sliding resistance of the equilibrium GB was determined from a computation of the dependence of the boundary energy on in-plane translation of one grain relative to the other. Low-energy faulted structures were found that correspond to unstable intermediate states and the presence of stable partial GB dislocations. Extrinsic dislocations and disconnections were found to lower significantly the sliding stress relative to the equilibrium GB.

Atomistic simulation of helium-defect interaction in alpha-iron

Molecular dynamics (MD) methods are utilized to study the formation of vacancy clusters created by displacement cascades in α‐Fe containing different concentrations of substitutional He atoms. Primary knock-on atom energies, Ep, from 500eV to 20keV are considered at a temperature of 100K, and the results are compared with those performed in pure α‐Fe. There are distinct differences in the number and size of vacancy clusters within displacement cascades with and without substitutional helium atoms. It is found that many large vacancy clusters can be formed within cascade cores in α‐Fe with helium atoms, in contrast to a few small vacancy clusters observed in pure α‐Fe. The number and size of helium/vacancy clusters generally increase with increasing helium concentration and PKA energy. One of the striking results is that the number of self-interstitial atoms (SIAs) and the size of interstitial clusters are much smaller than those in pure α‐Fe.

Shear punch tests performed using a new low compliance test fixture

Based on a recent finite element analysis (FEA) study performed on the shear punch test technique, it was suggested that compliance in a test frame and fixturing which is quite acceptable for uniaxial tensile tests, is much too large for shear punch tests. The FEA study suggested that this relatively large compliance was masking both the true yield point and the shape of the load versus displacement trace obtained in shear punch tests. The knowledge gained from the FEA study was used to design a new shear punch test fixture which more directly measures punch tip displacement. The design of this fixture, the traces obtained from this fixture, and the correlation between uniaxial yield stress and shear yield stress obtained using this fixture are presented here. In general, traces obtained from the new fixture contain much less compliance resulting in a trace shape which is more similar in appearance to a corresponding uniaxial tensile trace. Due to the more direct measurement of displacement, it was possible to measure yield stress at an offset shear strain in a manner analogous to yield stress measurement in a uniaxial tensile test. The correlation between shear yield and uniaxial yield was altered by this new yield measurement technique, but the new correlation was not as greatly improved as was suggested would occur from the FEA study.

The relation between grain-boundary structure and sliding resistance

Abstract During sliding, the grain-boundary (GB) energy depends on the atomic structures produced during relative translation of the two grains. The variation in the GB energy within the two-dimensional boundary unit cell (BUC) constitutes the GB-γ surface. Maxima in the slope of the γ surface determines the sliding resistance, that is the stress required to move the system over the lowest saddle points along a particular path within the BUC. In this paper we present the results of an atomistic study of the γ surfaces for two types of boundaries in a fcc metal. One of the boundaries is a Σg = 11, {131} which is a low-energy boundary and has a simple γ surface with a single stable configuration located at the corners and centre of the BUC. The resistance to sliding was determined by chain-of-states methods along four shear vectors connecting equivalent states within the BUC and is found to be very high in all cases. The asymmetric, Σ = 11, {252}-{414} GB, has a higher GB energy and its γ surf...

Biaxial thermal creep of V–4Cr–4Ti at 700°C and 800°C

Abstract A study of the thermal creep properties of V–4Cr–4Ti were performed using pressurized tube specimens. Creep tubes nominally 4.57 mm OD by 0.25 mm wall thickness were pressurized with high-purity helium gas to mid-wall effective stresses below the uniaxial yield strength. Specimens were heated to 700°C and 800°C in an ultra-high vacuum furnace and periodically removed to measure the change in OD with a high-precision laser profilometer. The secondary creep rate was found to be power-law dependent on the applied stress with a stress exponent of 3.7 at 700°C and 2.7 at 800°C. The average activation energy for creep of V–4Cr–4Ti was 299 kJ/mol, which is quite close to the activation energy for self-diffusion in pure vanadium in this temperature regime. The predominant mechanism of creep deformation for the conditions employed in this study is most likely climb-assisted dislocation motion.

Effects of interatomic potential on He bubble creation by cascades in α -iron

The effects of using different interatomic potentials in molecular dynamics (MD) simulations of the formation of He-vacancy clusters within displacement cascades in α-Fe are investigated using two sets of potentials. Simulations of cascades produced by primary knock-on atoms of energy Ep=1–20keV were performed in α-Fe containing a concentration of substitutional He atoms varying from 1to5at.% at an irradiation temperature of 100K. Although the effects of interatomic potentials on the nucleation of He-vacancy clusters induced by cascades are relatively small, the number and size of He-vacancy clusters produced are significantly different for the different potentials employed in this study. Thus, these differences may influence the microstructural evolution predicted in damage accumulation models that use the results from MD cascade simulations as input. The observed differences in postcascade configurations can be attributed mainly to the differences in the Fe–Fe and Fe–He potentials.

Computer simulation of extrinsic grain-boundary defects in the ∑11, 〈101〉{131} symmetric tilt boundary

Abstract A computer simulation was performed to investigate the structure and properties of extrinsic grain-boundary (GB) defects in the {11, 〈101〉 {131} symmetric tilt boundary. An embedded-atom method potential was employed to represent aluminium. Standard molecular dynamics relaxation techniques were used to compute low-temperature (about 0 K) equilibrium structures. Extrinsic GB dislocations were introduced into the models by application of the exact anisotropic elastic displacement field for a bicrystal interface. Some dislocations were accompanied by geometrically necessary steps (in which case the pair form a disconnection) to avoid formation of GB faults. The structure and properties of the equilibrium GB were compared with GBs containing steps, dislocations and disconnections. A broad range of GB defects was studied in order to characterize their potential effect on GB sliding resistance.