Radiation-induced Disordering Research Articles

Radiation effects in FCC metals and intermetallic compounds: A molecular dynamics computer simulation study

Abstract We review recent results on the mechanisms of atomic mixing, radiation-induced disordering, and defect production and clustering induced by displacement cascades in Cu, Ag, Cu3Au and Ni3Al. We employ molecular dynamics computer simulation (MD) methods with isotropic many body potentials and recoil energies near the subcascade formation regime (up to 30 keV). Atomic mixing is shown to be dominated by diffusion in the locally molten cascade core in both the pure metals and the intermetallic compounds. Disordering of the intermetallics was found to take place in the core of the cascades. However, because of the short lifetime of the displacement cascade, chemical short range order is preserved in the molten zone. Our results reveal the appearance of very large vacancy and interstitial type defect clusters at high recoil energy and cascade energy density. Vacancies agglomerate and collapse into Frank dislocation loops as the result of the quenching of the cascade molten core. Large interstitial clusters are directly produced in cascades and form prismatic dislocation loops. The fraction of defects in clusters for low temperature cascades increases with recoil energy and approaches ∼70% and 60% for interstitials and vacancies, respectively, at recoil energies near the threshold for subcascade formation. In the case of intermetallic (A3B) compounds, the large energy required to produce and transport a superdislocation appears to inhibit the interstitial prismatic loop punching process and interstitials appear as isolated (100) dumbbells formed by pairs of majority-type atoms.

Radiation-induced disordering and defect production in Cu 3Au and Ni 3Al studied by molecular dynamics simulation

We use molecular dynamics computer simulations to investigate the kinetics of disordering induced by energetic displacement cascades in two ordered intermetallics, Cu 3Au and Ni 3Al. We also study the mechanisms of defect production in these systems and compare the results to recent findings in pure metals. We employ embedded atom potentials which reproduce the main features of the phase diagram, including a disordering transition below the melting point ( T m) in Cu 3Au and an ordered compound up to T m in Ni 3Al. We simulate events with 5 keV recoil energy in crystals containing up to 157216 atoms at 10 K. We present results of the influence of the displacement cascade on the order parameters and describe their temporal and spatial evolution. We discuss atomic mixing in the cascade regions in terms of the atomic mobilities in each sublattice. The influence of the ordering energy and the melting temperature in the behavior of the compounds is emphasized.

Microstructure of α-Al base matrix and SiC particulate composites

The microstructures of a high temperature monolithic AlFeVSi alloy and a metal matrix and SiC particulate composite (MMC) based on this alloy matrix were characterized by conventional and high resolution transmission electron microscopy, microdiffraction and X-ray spectroscopy techniques. Silicide particles of average composition Al 3(Fe,V) 3Si were present in both of these materials. These particles were unstable under the electron beam at voltages above 200 kV, and exhibited radiation-induced disordering. SiC particulates present in the MMC structure were predominantly of the hexagonal 6H polytype, but the rhombohedral 87R-VII polytype was also observed. A very thin reaction layer was present between the matrix and SiC particles. No segregation of alloying elements such as iron, vanadium, or silicon at thee matrix-SiC interface was observed. The second phase present at the α-AlSiC interface was probably a disordered silicide.

Radiation-induced disordering and amorphization—An in situ HVEM study of uranium silicide with comparison to natural processes in zirconolite

We present here results of in situ TEM diffraction observations of the response of U3Si and U3Si2 when subjected to 1 MeV electron irradiation or to 1.5 MeV Kr ion irradiation, and observations of damage occuring in natural zirconolite. High energy electron irradiation or energetic heavy ion irradiation were performed in situ at the HVEM-Tandem User Facility at Argonne National Laboratory. In this Facility, a 2 MV Tandem ion accelerator and a 0.6 MV ion implanter have been interfaced to a 1.2 MeV AEI high voltage electron microscope. This allows a wide variety of in situ experiments to be performed with simultaneous ion irradiation and conventional transmission electron microscopy. During the electron irradiation, the electron beam was focused to a diameter of about 2 μ.m at the specimen thin area. The ion beam was approximately 2 mm in diameter and was uniform over the entire specimen. With the specimen mounted in a heating holder, the temperature increase indicated by the furnace thermocouple during the ion irradiation was typically 8 °K.

High efficiency single quantum well graded-index separate-confinement heterostructure lasers fabricated with MeV oxygen ion implantation

Single quantum well AlGaAs/GaAs graded-index separate-confinement heterostructure lasers have been fabricated using MeV oxygen ion implantation plus optimized subsequent thermal annealing. A high differential quantum efficiency of 85% has been obtained in a 360-μm-long and 10-μm-wide stripe geometry device. The results have also demonstrated that excellent electrical isolation (breakdown voltage of over 30 V) and low threshold currents (22 mA) can be obtained with MeV oxygen ion isolation. It is suggested that oxygen ion implantation induced selective carrier compensation and compositional disordering in the quantum well region as well as radiation-induced lattice disordering in AlxGa1−xAs/GaAs may be mostly responsible for the buried layer modification in this fabrication process.

Brillouin scattering and transmission electron microscopy studies of radiation-induced elastic softening, disordering and amorphization of intermetallic compounds

The crystalline-to-amorphous (c-a) phase transformation in the intermetallic compound Zr 3Al during room temperature bombardment with 1.0 MeV Kr + was investigated. Transmission electron microscopy and Brillouin scattering techniques were used to determine the lattice parameter and shear elastic constant as a function of the degree of long-range order. The results show that a large (about 50%) elastic softening and dilatation strain (about 3%) due to disordering precede the onset of amorphization. These results indicate that chemical disordering is an important driving force for the c-a transformation and that the mechanism is an elastic shear instability. It is also shown that the volume dependence of the shear elastic constant associated with radiation-induced disordering and eventual amorphization is virtually identical to that associated with the heating to melting of many solids. The origin and implications of this parallelism between solid state amorphization and melting are discussed.

Elastic Instability During Amorphization of Internetallic Compounds by Charged-Particle Beams

ABSTRACTTransmission electron microscopy (TEN) and Brillouin scattering methods were used to determine the lattice dilatation and shear elastic constant as a function of the degree of long-range order during room temperature irradiation of Zr3Al with 1.0-MeV Kr+. The results indicate that the onset of amorphization in Zr3Al is triggered by an elastic shear instability, and that the instability is directly related to the volume dilatation associated with the destruction of long-range order. It is also shown that the volume dependence of the shear elastic constant associated with radiation-induced disordering and amorphization is virtually identical to that associated with the heating to melting of many metals.

Phase stability under irradiation — a review of theory and experiment

A critical review of the current state of the theories for phase stability under irradiation is presented. Starting from the primary damage structure a formalism is presented which is used to describe radiation enhanced diffusion, precipitate dissolution, metastable precipitate sequences and phase diagram modification under irradiation. In the case of precipitate dissolution it is shown that current theory has been misinterpreted, and an alternative steady state analysis is presented which shows that the particle distribution depends on the pre-existing distribution or on new nucleation. Current theories for phase diagram modification under irradiation are critically reviewed and extended to cover the effect of radiation-induced disordering and amorphization. For the former case a calculated phase diagram (Ti-Ru) is presented as modified by irradiation.

The effects of electron irradiation on the kinetics of order-disorder transformations

Abstract The response of an ordered alloy to high energy electron irradiation has been theoretically investigated in terms of the magnitude and sign of the change in long-range order parameter, S, with time. A theoretical model is developed for the two competing processes of disordering by atomic displacements and ordering by vacancy migration. This predicts that radiation-induced disordering should be favoured at low irradiation temperatures, but that it should be balanced by radiation-enhanced ordering at higher temperatures where the vacancies produced by irradiation are sufficiently mobile to restore some degree of order to the lattice. The response of an ordered material to irradiation also depends on the initial value of order parameter prior to irradiation, and on the displacement rate.