Void Formation In Metals Research Articles

Materials selection for nuclear applications in view of divacancy energies by comprehensive first-principles calculations

We investigated formation and binding energies of di-vacancy in 51 type pure metals with bcc, fcc, and hcp structures to understand the clustering and dissolution of vacancies in metals by means of a comprehensive first-principles study. First, mono- and di-vacancy formation energies as the first (1nn) and second (2nn) nearest neighbor were compared using three typical functionals. Comparing with other Groups, the formation energies of mono- and di-vacancy reach the maximum in Group VI metals whose d-band is half filled. The interaction between two vacancies is attractive in most metals and the energy difference between the 1nn and 2nn configurations is obvious. We predicted the relationships of di-vacancy formation energy with melting temperature and cohesive energy and discussed vacancy clustering and void formation in metals by the binding energy and dissolution energy of vacancies and their clusters. The present work provides the fundamental data of di-vacancy in metals and a theoretical guideline for nuclear materials selection and design.

Phase field modeling of void nucleation and growth in irradiated metals

Motivated by the need to develop a spatially resolved theory of irradiation-induced microstructure evolution in metals, we present a phase field model for void formation in metals with vacancy concentrations exceeding the thermal equilibrium values. This model, which is phenomenological in nature, is cast in the form of coupled Cahn–Hilliard and Allen–Cahn type equations governing the dynamics of the vacancy concentration field and the void microstructure in the matrix, respectively. The model allows for a unified treatment of void nucleation and growth under the condition of random generation of vacancies, which is similar to vacancy generation by collision cascade in irradiated materials. The basic features of the model are illustrated using two-dimensional solutions for the cases of void growth and shrinkage in supersaturated and undersaturated vacancy fields, void–void interactions, as well as the spontaneous nucleation and growth of a large population of voids.

High dose neutron irradiation damage in alpha alumina

Bulk samples of single crystalline and polycrystalline alpha alumina have been neutron-irradiated in the Experimental Breeder Reactor-II (EBR-II) to doses of 1026 n/m2 at temperatures of 925 K and 1100 K. The samples were found to swell macroscopically between 3% and 6%, depending on the temperature of irradiation and the form of the material. The damaged microstructures were investigated via transmission electron microscopy in order to understand the origin of the macroscopic swelling. In both single crystals and polycrystals the damage consists of a high density of dislocations containing predominately b = 1/3<101> dislocation loops on the (0001) planes coexistent with a high density of voids, which are aligned along the c-axis in this rhombohedral material. The established theory of void formation in metals is utilized to explain the formation of voids in alumina. The polycrystalline samples were extensively microcracked, and this is thought to be due to anisotropic swelling of the grains which in turn leads to stresses and fracturing at the grain boundaries.

II. Effect of oxygen and helium on void formation in metals

Abstract A model is presented that determines the effect of oxygen and helium on the energies of the common vacancy-cluster configurations, namely the void, dislocation loop and stacking-fault tetrahedron. Representative calculations are given for the case of irradiated copper. The presence of oxygen tends to enhance the stability of the void during nucleation compared to the other vacancy-cluster morphologies by decreasing the void surface energy through a chemisorption process. Helium also tends to stabilize void formation, because of the high binding energy of helium to a vacancy (or cluster of vacancies) compared to a dislocation. Gas concentrations as low as 0·01 a.p.p.m. He and 5 a.p.p.m. O are predicted to stabilize void formation in copper at certain temperatures. The predictions of the model are in good agreement with the available experimental results.

Inhomogeneous microstructural growth by irradiation

In the present paper we discuss the development of heterogeneous microstructure for uniform irradiation conditions. It is shown that microstructural inhomogeneities on a scale of 0.1 μm can develop purely from kinematic considerations because of the basic structure of the rate equations used to describe such evolution. Two aspects of the growth of such inhomogeneities are discussed. Firstly, it is shown that a local variation in the sink densities of the various microstructural defects will tend to enhance the inhomogeneity rather than remove it. Secondly, such inhomogeneities will lead to point defect fluxes that result in a spatial growth of the inhomogeneous region, which will be stopped only when the microstructural density around this region becomes large. The results have important implications in the formation of denuded zones and void formation in metals.

Void formation and denudation in ion-irradiated nickel

Abstract As a continuing investigation of the effect of carbon on void formation in metals, 4 MeV Ni++ ions were used to bombard three grades of nickel to 25 dpa at 594°C after helium implantation to 15 appm at room temperature. All irradiated specimens showed dense void formation irrespective of the carbon content. The present study thus reveals that the carbon effect disappears at high damage doses because too many vacancies are produced in comparison with the number of carbon atoms present in the matrix and also because the trapping of vacancies by the carbon atoms is not assisted by recombinations. Meanwhile, the dual-irradiated Ni specimens displayed unusual behavior of void formation and denudation along twin and grain boundaries. The one-side appearance of void denudation along many grain boundaries and the formation of unusually large voids at the edge of the denuded zone are particularly conspicuous and may have repercussions on the use of the denudation phenomenon to partially solve the swelling...

On the lower and upper irradiation temperature limits for void formation in metals

On the lower and upper irradiation temperature limits for void formation in metals

Dislocation Vibration as a Possible Cure for High Temperature Void Formation in Metals under Fast Neutron Irradiation

EXTENSIVE void formation occurs in metals subjected to fast neutron irradiation in the temperature range of 0.3 Tm to 0.5 Tm where Tm is the melting temperature of the metal1–4. This phenomenon is a serious problem in reactor technology. In conditions of fast neutron irradiation large numbers of lattice vacancies and interstitial atoms are produced in equal amounts. Dislocation lines and voids (pre-existing ones or newly nucleated ones) act as sinks to these point defects. If a void is just as efficient a sink for an interstitial atom as for a lattice vacancy and if a dislocation line also is just as efficient a sink for each of these types of defect there can be no tendency for a void to grow2–4. Equal numbers of vacancies and interstitials will be destroyed in a void as well as at dislocation lines. But dislocation lines are probably a more efficient sink for interstitials than for vacancies. Thus more interstitials are destroyed at dislocation lines than are vacancies. The excess vacancies are destroyed at voids, causing them to grow. This very plausible explanation has been presented in the literature to account for growth of voids (see refs. 2–4 and the references cited there).