M.W. Guinan

A constitutive model for metals applicable at high‐strain rate

A model, applicable at high‐strain rate, is presented for the shear modulus and yield strength as functions of equivalent plastic strain, pressure, and internal energy (temperature). The parameters needed to implement the model have been determined for 14 metals. Using this model, hydrodynamic computer simulations have been successful in reproducing measured stress and free‐surface‐velocity–vs–time data for a number of shock‐wave experiments.

Impact of cylinders on a rigid boundary

An experimental procedure originally proposed by G. I. Taylor to determine the dynamic yield point of metals is studied, using high‐speed computer simulations. A simple method is outlined for determining the yield strength of materials that can be described by elastic‐plastic theory. Results for several metals are presented.

Progress in the development of a molecular dynamics code for high-energy cascade studies

Abstract We discuss recent progress in the development of a new molecular dynamics program for studies of high-energy displacement cascades. The new code, termed MOLDY-CASK, implements a vectorized algorithm to calculate the forces between atoms. Timing runs show a large increase in efficiency when compared to other, not fully vectorized MD codes. The code also implements several types of isotropic many-body interatomic potentials as well as three-body potentials for semiconductors. We present preliminary results of a 25 keV cascade in Cu where the calculation has been carried out in a computational cell containing 500000 atoms.

Pressure derivatives of the elastic constants of α-iron to 10 kbs

Abstract The pressure derivatives of the elastic constants of α -iron have been determined over the pressure range from 0 to 10kb, by a differential ultrasonic pulse-echo method involving pulse superposition. Measurements were made on single-crystal specimens of orientations (100), (110), and (111), thus providing several cross checks. The values found for the elastic constants and their pressure derivatives at 300°K are: B S =1664 kb C=1166kb C′=477kb d B S d P =5⋯29 d C d P =2⋯59 d C′ d P =1⋯07 using the notation B s = (C 11 +2C 12 ) 3 , C = C 44 , C′= (C 11 −C 12 ) 2 . On the basis of a continuum model, a value for the Gruneisen constant has been calculated from the pressure derivatives. This value agrees closely with that obtained from thermal expansion data. Combining the present result with data on temperature dependence, the elastic constants of α-iron are found to be explicit functions of temperature. The present value for the pressure derivative of the bulk modulus, which is in reasonable agreement with previous ultrasonic measurements, is nearly 40 per cent larger than the value calculated from Bridgmans static measurements.

The effect of electronic energy loss on the dynamics of thermal spikes in Cu

We present results of a molecular dynamics simulation study of the effect of electron-ion interactions on the dynamics of the thermal spike in Cu. Interatomic forces are described with a modified embedded atom method potential. We show that the electron-ion interaction acts to reduce the lifetime of the thermal spike and therefore the amount of atomic rearrangement that takes place in energetic displacement cascades in Cu. The results point towards the important effect that inelastic energy losses might have on the dynamics of displacement cascades in the subcascade energy regime where the lifetime of thermal spike is expected to exceed the electron-phonon coupling time.

Shock wave interactions arising from near surface displacement cascades

Abstract Kaminsky has recently observed during 14 MeV neutron irradiation of a variety of materials that substantial (micron-sized) particles are ejected from the surfaces of the bombarded specimens. Similarities between Kaminskys observations and the macroscopic phenomena of spall suggested the possibility that shock waves and transient thermal stresses, generated by energetic primary knock-ons, may be in part responsible. A brief discussion of spall phenomena will be presented as background to calculations of the shock wave interactions. The results indicate that cascade generated stresses may be responsible for the emission of such particles by triggering the release of stored internal energy.

Defect production efficiencies in thermal neutron irradiated copper and molybdenum

We have derived the primary recoil spectra for thermal neutron capture in copper and molybdenum, and have calculated the damage energy cross-sections needed to determine the defect production efficiencies from measured resistivity damage rates. These efficiencies, which are in excellent agreement with fully dynamic computer simulations of collision cascades, are compared to the predictions of displacement functions which are currently in use.

Cascade structure in low temperature fission and fusion neutron-irradiated metals

Abstract With the cryotransfer TEM technique, defects which were formed in fusion- and fission-neutron irradiated metals below 20 K were observed at the early stage of their formation. At a low D-T neutron fluence of 1× 10 20 n/m 2 , TEM visible defects were of interstitial type in Au, Cu, Ag and Cu 3 Au, and no defects were observed in Ni, Fe and Al. This suggests that vacancy defects at isolated displacement damage cascades have sub-microscopic sizes. At a very low D-T neutron fluence of 5 × 10 18 n/m 2 ,few single-isolated clusters of interstitials were observed in Au. In D-T neutron-irradiated Au, Ag and Cu, clusters of interstitials tend to form groups whose size increases with the fluence, while such group formations of point defects were not observed in D-T neutron-irradiated Ni, Fe and Al. Such groups of interstitial point defects may be formed with strain-affected thermal diffusion after the ejection of interstitials from the cascade core. At a medium D-T neutron fluence of 1× 10 21 n/m 2 , TEM visible vacancy clusters were observed even in metals which were irradiated below 20 K. The development of visible vacancy clusters is explained due to the cascade energy overlapping. TEM observations showed that sub-microscopic interstitial clusters are also formed together with visible ones in D-T neutron-irradiated metals. During TEM observations, sub-microscopic clusters of both interstitial and vacancy types grew to visible ones by being assisted with momentum transfer from the TEM electrons. Differences are noticed between observed defects in fission- and fusion-neutron irradiated metals. Point defect processes of damage formation in fission- and fusion-neutron irradiated metals are discussed.

Grazing incidence metal mirrors as the final elements in a laser driver for inertial confinement fusion

Grazing incidence metal mirrors (GIMMS) have been examined to replace dielectric mirrors for the final elements in a laser beam line for an inertial confinement fusion (ICF) reactor. For a laser driver using light with a wavelength from 250 to 500 nm in a 10 ns pulse, irradiated mirrors made of Al, Al alloys, or Mg were found to have calculated laser damage limits of 0.3--2.3 J/cm{sup 2} of beam energy and neutron lifetime fluence limits of over 5 {times} 10{sup 20} neutrons per square centimeter when use at grazing incidence (an angle of incidence of 85 degrees) and operated at room temperature or at 77 K. A final focusing system including mirrors made of Al alloy 7475 at room temperature or at liquid nitrogen (LN) temperatures used with a driver which delivers 5 MJ of beam energy in 32 beams would require 32 mirrors of roughly 10 m{sup 2} each. This paper briefly reviews the methods used in calculating the damage limits for GIMMs and discusses critical issues relevant to the integrity and lifetime of such mirrors in a reactor environment. 9 refs.

Low temperature D-T neutron irradiation and cryotransfer observation of cascade defects of metals

Electron microscope specimens of Au, Ag, Cu, Al and Fe were irradiated below 20 K by D-T neutrons with RTNS-II at a fluence of 1 × 1016 to 1.5 × 1017 n/cm2. After irradiation the foils were dismounted from the cryostat and mounted on a TEM cold holder in liquid nitrogen, and transferred to an electron microscope without warming them. The results presented here describe preliminary observations on only a few foils. Cascade defects composed of small defects in a group were observed in Au, Ag and Cu. The relaxation of cascade defects occurring during the isochronal annealing was examined. No irradiation-induced defect was detected in cryotransferred aluminum at the instant of starting the observation, while observable cascade defects appeared during a subsequent illumination. This process is due to athermal collapsing of cascades induced by the electron illumination. In iron, no defects could be observed at low temperature or after annealing. Generally cascades collapse at low temperature to non-compact metastable structures and change to stable ones during the stage III annealing.