G. J. Dienes

Radiation Enhanced Diffusion in Solids

A simple theory of radiation enhanced diffusion has been worked out which describes the dependence of this enhancement on flux and temperature under steady‐state conditions. The theoretical study also shows that the measurement of enhanced diffusion as a function of temperature can indicate the mechanism by which defects are removed from the lattice. Alpha‐brass was chosen for the experimental work because it is a kinetically simple system, not complicated by nucleation, in which diffusion is easily followed by measuring the electrical resistivity changes associated with changes in short‐range order. The enhanced diffusion rate during irradiation in the Brookhaven reactor has been measured in this alloy at several temperatures in the range 0 to 190°C. This enhancement is independent of temperature from 0 to 150°C, in excellent agreement with the theoretical predictions for the case where the radiation induced defects finally disappear at internal surfaces. Some implications of radiation enhanced diffusion...

Molecular Dynamic Simulations of Shock Waves in a Three‐Dimensional Solid

Molecular dynamic techniques were used to simulate the motion of a one‐dimensional shock wave in a three‐dimensional solid. A Lennard‐Jones potential was used to simulate a rare‐gas solid. The pressure, volume and temperature were monitored. The pressure and volume were found to agree with dynamic calculations using the Hugoniot relations. A large temperature increase was found, probably the result of kinetic energy transfer by direct collisions with atoms in the shock‐wave front. Reasonable agreement was found between the computer simulation results and static high‐pressure measurements on neon. The general results establish the value of applying molecular dynamic techniques to shock‐wave problems.

A molecular dynamical study of the equation of state of solids at high temperature and pressure

Abstract The molecular dynamics method of computer simulation was used to study the pressure and energy as a function of volume and temperature for f.c.c. neon with 6–12 Lennard-Jones interatomic forces and for b.c.c. iron with Morse interatomic forces. A self-consistent cell model was used to derive analytical formulae which describe the results of the computer experiments. A method for correcting the inherently classical results of the molecular dynamics calculations for the effect of quantum statistics is proposed. The results of the calculations are compared with experimental data and with various approximate methods for calculating the Gruneisen constant.

The self-consistent cell model equation of state for solids

Abstract An anharmonic equation of state for solids using the Self-Consistent Cell Model (SCCM) is given in a form useful for calculating the usual thermodynamic properties. Following Cowley and Shukla, using the Jaswal-Girifalco potential for copper, calculations are compared with other models and with experiment. The results from the analytic expressions using the SCCM are as good or better than those obtained from far more complicated theories of anharmonicity. Using the Lindemann criterion, the pressure at the melting point was obtained as a function of the melting temperature. A melting line was also obtained for iron and the longitudinal velocity and isothermal bulk modulus along the melting line were calculated. The Hugoniot pressures were calculated and compared with experiment. For both the copper and iron the agreement between theory and experiment is remarkably good considering the empirical nature of the potentials, the simplifying approximations of the SCCM calculations, and the large range of densities and pressures that are compared with experiment.

Defects and diffusion mechanisms in Nb3Sn

Abstract The structure and energetics of point defects in A15 Nb 3 Sn were investigated by means of computer simulations based on a pair-potential model of cohesion. The repulsive part of the potential was calculated theoretically using electron-gas methods while the attractive part was derived by matching experimentally determined crystal properties. The properties of vacancies on both the Nb and Sn sublattice, as well as those of simple antisite defects, were investigated. The results show an unusual structure for the vacancy in the Nb sublattice: the vacancy dissociates into a “split” configuration consisting of “partial vacancies” separated by a one-dimensional “stacking fault”. The Sn vacancy was found to be metastable; it decomposes by an activated process into a Nb atom on a Sn site (an antisite defect) adjacent to a “split” Nb vacancy. The defect of lowest energy compatible with the proper three-to-one ratio of sublattice sites is the antisite defect pair. The lowest energy grouping which contains vacancies is found to consist of Nb-sublattice vacancies and Nb-on-Sn-sublattice antisite defects in the ratio of four of the former to one of the latter (quintuple defects). The results also suggest that bulk Sn diffusion is slower than Nb diffusion; this is consistent with the belief that rapid Sn diffusion during Nb 3 Sn layer growth does not occur by bulk but by grain-boundary diffusion. Nb vacancy migration was found to be quasi-one-dimensional with much lower migration energy required for motion along the Nb chains than that for jumps between chains.

Effects of Nuclear Radiations on the Mechanical Properties of Solids

The general nature of radiation effects in solids is reviewed briefly. Current theoretical understanding of the mechanical properties of solids is critically evaluated. The effect of nuclear radiation on the mechanical properties is discussed in detail. It is shown that the changes in the mechanical properties of crystalline substances (mostly metals) can be quite satisfactorily interpreted on the basis of the production of interstitial atoms and vacant lattice sites by fast particle irradiation. Isolated vacancies and interstitials may not be able to account for all the observations, and attention is called to the possible need of postulating the existence of aggregates of these lattice defects. In molecular solids (mostly high polymers) nuclear radiations bring about changes in the substance which are best described as chemical ones. Ions and free radicals are formed leading to subsequent chemical reactions thereby altering the properties of the substance. Drastic changes in the mechanical properties of...

Short−range order in Au−Fe radiation−enhanced diffusion and the effectiveness of 14−MeV neutrons

Solute clustering in a Au−Fe (17% Fe) alloy has been demonstrated by Mossbauer effect measurements of the magnetic ordering temperature. Neutron irradiation at room temperature produces clustering by means of radiation−enhanced diffusion, while annealing at high temperatures is required to produce clustering thermally. The radiation−enhanced diffusion effect was used to compare directly the efficiency of mobile defect production by 14−MeV fusion neutrons and reactor neutrons. The 14−MeV neutrons are more effective by about a factor of 10 than the reactor neutrons (E≳0.1 MeV) in producing mobile lattice defects. Other compositions failed to respond to radiation−induced clustering, suggesting that Au−17−Fe is in a critical composition region regarding the response of the magnetic ordering temperature to variations in short−range order.

Simulations of shock waves in solids

Abstract Molecular dynamic shock wave simulations have been carried out for face centered cubic (f.c.c.) and body centered cubic (b.c.c.) solids using Lennard-Jones and Morse potentials for the interatomic interactions. The Hugoniot conservation relations were accurately obeyed in all of these calculations. The shock wave profiles may vary with the interatomic potential and the crystal structure, effects most clearly shown by the temperature profile near the shock front. The Lennard-Jones solids are intensitive to a change in structure but the Morse solids appear sensitive to crystal structure, at least in comparing b.c.c. with f.c.c. It was shown that the average shock wave temperature can be calculated from a combination of the Hugoniot conservation relations and the Mie-Gruneisen equation of state. The temperature calculated this way is in good agreement with the average shock wave temperature obtained in the computer simulations.