W.J. Phythian
AEA Technology
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Featured researches published by W.J. Phythian.
Journal of Nuclear Materials | 1995
W.J. Phythian; Roger E. Stoller; A.J.E. Foreman; A.F. Calder; D.J. Bacon
Abstract The use of molecular dynamics simulation and improved many-body potentials make it possible to compare displacement cascade evolution in different materials. However, the extreme variability between individual cascades requires multiple simulations at nominally identical conditions of temperature and energy in order to assure that the comparison is statistically valid. We describe such a comparison of copper and iron in this paper. Over 600 cascades have been investigated, with simulation energies in the range 60 eV to 10 keV and temperatures from 100 to 900 K. The evolution of the cascades is similar in both materials, with the development of a highly disordered core and the emission of focusons and replacement collision sequences during the collisional phase of the cascade. The majority of vacancy-type defects are found in the cascade core when in-cascade recombination is complete, while the interstitial-type defects tend to be distributed around the periphery of this region. The final defect structure has been characterized by the total surviving defect fraction, and the number and size of the point defect clusters produced. Since these parameters have significant implications for the nuclear industry in its assessment of radiation damage, we show how they depend on cascade energy and temperature. To illustrate their importance, we provide an example of how the molecular dynamics results can be used in a rate theory model of ferritic steel embrittlement.
Journal of Nuclear Materials | 1993
W.J. Phythian; C. A. English
Abstract The irradiation-induced changes occurring at the nanometer level in the microstructure of the reactor pressure vessel have been shown to be responsible for a degradation in mechanical properties that occur in large structures with dimensions in the tens of metres. These changes can place severe restrictions on the reactor both at the startup and under continual operation; and in the long term could compromise the safe operation of the plant. The large financial and safety related matters have given the necessary driving force to study the factors controlling the microstructural evolution, resulting in a multi disciplinary approach to the problem. This paper aims to review the current understanding of the subject, giving were possible examples of the approach and techniques used to obtain this. We also highlight areas of current research activity and indicate the type of work still required to provide information on aspects that currently lack a full understanding.
Philosophical Magazine | 1992
A.J.E. Foreman; W.J. Phythian; C. A. English
Abstract The evolution of irradiation damage cascades in a metal has been simulated by molecular dynamics, using a many-body potential. Over 100 cascades have been produced with random knock-on directions and primary knock-on atom (PKA) energies ranging from 60 to 2 keV. The cascade evolution has been followed for times typically up to about 10ps and in some cases up to about 30 ps. The cascades are characterized by the sudden emission of replacement collision sequences and with shape variations due to local channelling events. At the higher energies the core has been shown to be a liquid-like structure with cavitation. The annealing phase leaves loosely clustered vacancies at the cascade centre but collapse to a vacancy loop is not observed. The onset of this thermal spike occurs for cascades with energy of 100 eV or higher; below this the cascade is made up of a series of closely coupled short replacement collision sequences. A feature of the more energetic cascades is the production by a ballistic mech...
Journal of Nuclear Materials | 1995
J.T. Buswell; W.J. Phythian; R.J. McElroy; S. Dumbill; Phn Ray; J. Mace; R.N. Sinclair
A series of model steels and commercial alloys from the IAEA Phase 3 irradiation programme was irradiated to a dose of 14 mdpa (∼ 9 x 10 22 n m -2 (E > MeV)) at 290°C. The steels were examined using small angle neutron scattering and a range of transmission electron microscope techniques. Measured yield stress and hardness changes were interpreted in terms of the irradiation-induced microstructural developments. The irradiation of Cu-containing steels produced Cu-rich precipitates of ∼ 2 nm diameter which were alloyed with Mn and Ni. The Ni content of the precipitates increased markedly for bulk Ni contents > 0.7 wt%. The volume fractions of precipitate increased with increasing bulk Cu content and, also, with increasing bulk Ni content > 0.7 wt%. The precipitate-induced strengthening and hardening varied according to the square root of the volume fraction of precipitate. Yield stress changes in low-Cu steels were consistent with data from the IAEA Phase 2 irradiation programme.
Philosophical Magazine | 1992
A.J.E. Foreman; C. A. English; W.J. Phythian
Abstract The many-body potential of Ackland et al. is currently being used to investigate the production of displacement damage cascades in irradiated copper, using the molecular dynamics computer program MOLDY. The effect of adopting this potential on both the static and the dynamic properties of irradiation-induced defects is evaluated here. It is shown that this potential provides a good representation of the well known properties of point defects. However, for dynamic events involving the close approach of atoms, a short-range ion core repulsive term needs to be incorporated. The modified core term has been optimized to give an excellent fit to the measured threshold displacement energies without significantly affecting the fit to other physical properties.
Journal of Nuclear Materials | 1993
M. L. Jenkins; M.A. Kirk; W.J. Phythian
Abstract We review recent ion-irradiation experiments which have been performed to investigate the collapse of displacement cascades to dislocation loops in a range of metals and alloys. Many of the results including the dependencies of the collapse probabilities on irradiation temperature, and ion dose, energy and mass, can be explained within the framework of a thermal spike/cascade melting model which has been suggested by computer molecular dynamics simulations. Other aspects, such as the dependence of collapse probabilities on the crystal structure and the effects of alloying and impurities, are less well understood.
Journal of Nuclear Materials | 1992
T. Diaz de la Rubia; W.J. Phythian
We review recent molecular dynamics studies of displacement cascades in copper at energies from near threshold to 25 keV where nascent subcascades develop. We discuss the structure of the cascade region and present results on the dynamical evolution and lifetime of the cascades. We show that the defect production efficiency in cascades follows a downward trend with energy, in good quantitative agreement with experiments. The cluster size distributions from the simulations show a strong recoil energy dependence. At low energy, loosely correlated vacancy distributions are generally observed. However, as the energy and hence the lifetime of the cascade increase, large vacancy clusters appear. Evidence is presented for the athermal collapse of a 25 keV cascade to a vacancy dislocation loop. Interstitial production mechanisms are identified. At all energies studied replacement collision sequences are observed. Moreover, vacancy-interstitial separation mechanisms based on ballistic clustering and dislocation loop punching from cascades are presented.
Journal of Nuclear Materials | 1990
C. A. English; W.J. Phythian; A.J.E. Foreman
Abstract The cascade processes are discussed which potentially give rise to a dependence of microstructural evolution on the recoil spectra set up by a flux of fast particles. It is emphasised that the thermal spike phase of cascade development is central to these processes. Recent results from Molecular Dynamics simulations are presented to highlight the potential importance of interstitial clustering in cascades and the impact of cascades on pre-existing features of the microstructure. A qualitative discussion is presented of the likely importance on microstructural evolution at low and high doses of the different cascade processes.
Journal of Nuclear Materials | 1993
J.T. Buswell; P.J.E. Bischler; S.T. Fenton; A.E. Ward; W.J. Phythian
Abstract The microstructures of Magnox submerged-arc welds have been characterised to investigate the effects of surveillance and accelerated irradiation at temperatures in the range 190–290°C. The radiation hardening and embrittlement is influenced by the precipitation of Cu from solid solution. Mn has been found in the Cu-rich precipitates, together with an indication of P. The precipitates have structure coherent with the ferrite matrix and maintain a constant mean diameter during extended irradiation. Evidence has been obtained indicating that dislocation loops contribute to a matrix damage component in these welds.
Philosophical Magazine | 1995
S. J. Wooding; D.J. Bacon; W.J. Phythian
Abstract The damage produced by displacement cascades in the h.c.p. metal α-titanium at 100K has been investigated by molecular dynamics using a many-body interatomic potential. This has been the first simulation of cascades in the h.c.p. structure. 32 cascades ranging in energy from 0.3 to 5 keV were generated in order to study the effects of the crystal structure and cascade energy. Computer-generated plots have been used to visualize the nature and arrangement of the point defects produced in the cascade events. There is no discernible effect of the primary knock-on atom direction, and individual replacement sequences make only a minor contribution to the final damage state. Views of the displaced atoms looking along the basal planes show very clearly regions with a liquid-like disorder, but they are small, even at 5 ke V, and do not lead to vacancy clustering when crystalline order is restored. The efficiency of production of Frenkel pairs declines with increasing cascade energy in a similar fashion t...