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The main goal of our project is to maintain the level of performance of the suite, extending the pure message passing paradigms with mixed parallelism using MPI and OpenMP (where possible). In particular, we will start working on one of the most relevant algorithms of the suite: an ad-hoc 3D Fast Fourier Transform. We try 2 approaches to ibridize the code: an implicit approach linking native multi-thread library provided by third-part vendors and an explicit one using OpenMP directly into the code. If the experimental hybrid/mixed paradigm with the FFT is successful, we will try to extend it to other relevant and heavy computational cores of the suite, particularly in the iterative diagonalization of Hamiltonian, which is the second algorithm into suite in order of importance.
We implemented two test programs for a detailed performance analysis of the parallel ad-hoc hybrid 3D FTT. Both programs share the same main modules of Quantum ESPRESSO and are able to initialize the FFT grid like a real simulation using a real input parameters. We devoted a large part of the time to design a test program that shares the same structure of the main modules of Quantum ESPRESSO in order to simplify the procedure of integrating the modifications into the main package. This point is very important because Quantum ESPRESSO has fixed data structures that cannot be changed. Both master-only and funneled approach have been developed. We have test the modified algorithm using different combination of MPI and threads number.
After the testing phase, we have transferred all the modifications into the main code of Quantum ESPRESSO and our improvements have been committed into the official repository. We have had the possibility to work on CRAY XT4 (HECToR) and IBM POWER5 (HPCx) clusters hosted by Edinburgh Parallel Computing Centre and, also, IBM BladeCenter LS21 (BCX) in CINECA. Now we are able to run Quantum ESPRESSO using MPI and OpenMP together.
The mechanical properties of FeCr alloys heavily rely on atomic distribution and can be affected by phenomena such as Cr precipitation. While precipitation of FeCr alloys of various Cr concentrations had been studied before, dissolution of already existing Cr precipitates in FeCr alloys, had not; this was the focus of this study. Our means of investigation was MD computer simulation using the parallel code PARCAS and a 2-band EAM potential. We set up a number of configurations of FeCr alloys containing Cr precipitates of various sizes embedded in matrices of either pure Fe or with a 15% random Cr distribution, and examined their behaviour after thermal aging at T ranging between 600-2000K. The T range was selected so that it would include the (α+α’)-α transition in the standard FeCr phase diagram.
High T results provide insight to the mechanisms that govern the dissolution: Cr precipitates dissolve by vacancy exchange, leading to a random distribution of Cr atoms in an Fe matrix, as the short-range order parameter shifts from a positive value (clustering) to zero (random atomic arrangement). Precipitates at lower T seem to be stable, in agreement with previous experimental and simulation studies that challenge the standard phase diagram’s reliability.
Graphite crystals possess a layered structure with hexagonal symmetry, as originally elucidated by Bernal in the 1920s. Crystals of graphite exposed to ionizing radiation undergo significant expansion along their c-axis, and contraction in the basal directions. This is now believed to be due to buckling and folding over of the layers. However, it was originally believed that this behaviour was caused by the aggregation of interstitial atoms into discs of interstitial prismatic dislocation loops, at the expense of exisitng layers, thereby explaining the observed dimensional change. Nevertheless, Frenkel defects remain important, owing to their role in the buckling process, and that they store large amounts of energy in irradiated graphite, known as Wigner energy.
When irradiated graphite is annealed, self-diffusion allows Frenkel pairs to recombine and release Wigner energy (about 14 eV per pair). It was the release of this Wigner energy which led to the catastrophic accident at the Windscale nuclear pile in 1957. Interstitial atoms also cause buckling of graphite layers, which is subsequently released upon annealing, allowing the layers to fold over upon themselves by the accumulation of dislocations. This also releases Wigner energy, and results in dimensional change.
Although this problem has been studied for many decades, a completely satisfactory explanation of what is happening at the atomic scale has not emerged. There are inconsistencies among the various models and experimental observations.
Thus, a central objective of the originally proposed work was to resolve the problem of self-interstitial migration and aggregation in graphite, which thus far has evaded efforts to find a model which appears to be consistent with experiment. A second aim in the proposal was investigate the migration and aggregation of lattice vacancies in graphite. These defects occur in graphite which has been exposed to ionizing radiation, as might happen in a graphite-moderated nuclear reactor.
Ab initio models employing the AIMPRO program package, which is based on density-functional theory, have been developed that describe how self-diffusion occurs at the atomic scale in graphite containing interstitial atoms and lattice vacancies, as would occur in material which has been exposed to ionizing radiation. The models are constructed in supercells possessing four graphite sheets with orthorhombic symmetry, and contain either 128 or 288 atoms when no defect is present.
Migration of interstitial atoms can occur in several ways involving a number of steps, it is found. In AB-hexagonal graphite, a self-interstitial adopts a structure known as a spiro-interstitial, lying between two neighbouring graphite sheets. The full migration path, involves reorientation around the six equivalent sites associated with the pair of nearest alpha atoms, passing through an intermediate, metastable grafted interstitial state, and a split-interstitial state at the beta site. From the beta split-interstitial, the atom can go to an equivalent grafted state, or its partner can move to a grafted state on the opposite side of the sheet. The rate-limiting step is from the spiro state to the grafted state, which has an activation energy Ea = 2.0 eV. An alternative route for self-interstitial diffusion involves relative translations of graphite layers, or shear. During this process the direction of the shear vector changes in the basal plane, and carries the interstitial atom over an energy barrier with Ea = 1.2 eV. A related process without shear vector reorientation is found to have an activation energy Ea = 1.5 eV.
Lattice vacancies are believed that they posses a reconstructed structure and net magnetic moment of about 1-1.5 µB per atom. This is confirmed by the calculations. There is a relatively small barrier of about 0.2-0.3 eV associated with reorientation of the reconstruction bond, also confirming earlier work. However, it is also found that small displacements of the unpaired atom out of the basal plane easily quench the magnetic moment of the defect. There is growing evidence that the migration barrier for lattice vacancies in graphite is much smaller than an earlier accepted figure of 3.1 eV. According to the simulations here, Ea = 1.2 eV, in agreement with recent experiments and other calculations, and in contrast to the conventional view. Note, it is crucial to take into account the effects of magnetism.
In agreement with earlier work, the recombination of Frenkel pairs has a barrier to the final step, and involves a state known as an intimate Frenkel pair. However, the barrier to recombination is significantly smaller than found previously (1.4 eV). In this state the interstitial atom is delicately balanced in a metastable state immediately adjacent to the vacancy. The barrier depends on whether the vacancy is at an alpha site, a beta site, or the graphite sheets are sheared by a dislocation, where alpha and beta sites loose their distinction. Earlier calculations were not able to resolve these differences. In the present work, it is found that the alpha-type intimate Frenkel pair have a barrier to recombination much less than 0.1 eV, while beta ones experience a barrier Ea = 0.7 eV. In the sheared state Ea = 0.4 eV when there is no constraint on relative translations of the layers, and Ea = 0.25 eV when the shear is frozen with the magnitude and direction which makes alpha and beta sites equivalent.
Results have also been obtained for the final stages of aggregation of vacancies into divacancy complexes, of which there are several.
From this work it can now be understood how the broad spectrum of Wigner energy release arises from low temperatures, to the peak at about 200 C, and on to higher temperatures. The processes at each stage are identified, and it is seen how they are related to observed dimensional changes and other phenomena during the annealing of irradiated graphite.