Vacancy loops

Figure 6.6. Vacancy loops in aluminum shock loaded to (a) 13 GPa and (b) 65 GPa at 100 K.

Similar antiphase boundaries form in metals with structures based upon a hexagonal close-packed array of metal atoms, such as magnesium. Condensation of vacancies upon one of the close-packed metal atom planes to form a vacancy loop, followed by subsequent collapse, will result in a hypothetical sequence. . ABABBABAB This arrangement will be unstable because of the juxtaposition... 

Fig. 33. A typical transmission electron micrograph showing the appearance of dislocations (in this case vacancy loops and basal dislocations in M0S2 are visible). 20,000 X. Reprinted with the permission of the Faraday Society (S 7).

Figure 16.6 Transmission electron micrograph showing vacancy loops and helicoids in niobium tested at T = 1508 K, cr = 17.3 MPa 39000 x.

Interstitial clusters can evolve into dislocation loops and vacancy clusters can develop into vacancy loops or cavities. These clusters can contribute to changes in both mechanical properties and dimension. The types of defect clusters formed under radiation depend on the alloy crystal stmcture (e.g., body centered cubic (bcc) versus face centered cubic (fee)), alloy composition, and temperature, as discussed below. 

Atomic displacement within the graphite crystal lattice leads to the formation of interstitial and vacancy loops causing the crystallites to swell in the c-axis and to shrink, at a slower rate, in the a-axis (see Fig. 14.9). 

Various mechanisms have been proposed for the rearrangement of atoms from their correct positions in an ordered lattice to a random distribution of the atoms as irradiation proceeds. These include thermal spikes by Seitz (1949), replacement collisions by Kinchin and Pease (1955), plastic spikes by Seitz and Koehler (1956), collapse of cascades to vacancy loops by Jenkins and Wilkens (1976), and random recombination by vacancies and interstitials by Carpenter and Schulson (1978). Some examples will now be given of experiments that have been undertaken in an attempt to elucidate some of the mechanisms of irradiation-induced disordering. 

Fig. 6. Radiation damage in graphite showing the induced crystal dimensional strains. Impinging fast neutrons displace carbon atoms from their equilibrium lattice positions, producing an interstitial and vacancy. The coalescence of vacancies causes contraction in the a-direction, whereas interstitials may coalesce to form dislocation loops (essentially new graphite planes) causing c-direction expansion.

The dislocation formation mechanism described in the previous section generates dislocation loops. A dislocation loop can also form by the aggregation of vacancies on a plane in a crystal. Vacancy populations are relatively large at high temperatures, and, if a metal, for example, is held at a temperature near to its melting point, considerable... 

Figure 3.14 Formation of dislocation loops (a) the aggregation of vacancies onto a single plane, (b) collapse of the plane to form a dislocation loop, and (c) aggregation of interstitials to form a dislocation loop.

The aggregation of vacancies or interstitials into dislocation loops will depend critically upon the nature of the crystal structure. Thus, ionic crystals such as sodium chloride, NaCl, or moderately ionic crystals such as corundum, AI2O3, or rutile, TiC>2, will show different propensities to form dislocation loops, and the most favorable planes will depend upon chemical bonding considerations. 

The edge dislocations bounding the dislocation loops just described cannot glide, but nevertheless the loop can grow by the continued collection of vacancies or interstitials. This method of movement of an edge dislocation, which allows edge... 

More generally, co is independent of the external gas pressure k is the Boltzmann constant (1.38 x 10 erg deg ) and T is the temperature in Kelvin. Furthermore, the equilibrium between co and a collapsed CS plane fault is maintained by exchange at dislocations bounding the CS planes. Clearly, this equilibrium cannot be maintained except by the nucleation of a dislocation loop and such a process requires a supersaturation of vacancies and CS planes eliminate supersaturation of anion vacancies (Gai 1981, Gai et al 1982). Thus we introduce the concept of supersaturation of oxygen point defects in the reacting catalytic oxides, which contributes to the driving force for the nucleation of CS planes. From thermodynamics. 

Particle irradiation effects in halides and especially in alkali halides have been intensively studied. One reason is that salt mines can be used to store radioactive waste. Alkali halides in thermal equilibrium are Schottky-type disordered materials. Defects in NaCl which form under electron bombardment at low temperature are neutral anion vacancies (Vx) and a corresponding number of anion interstitials (Xf). Even at liquid nitrogen temperature, these primary radiation defects are still somewhat mobile. Thus, they can either recombine (Xf+Vx = Xx) or form clusters. First, clusters will form according to /i-Xf = X j. Also, Xf and Xf j may be trapped at impurities. Later, vacancies will cluster as well. If X is trapped by a vacancy pair [VA Vx] (which is, in other words, an empty site of a lattice molecule, i.e., the smallest possible pore ) we have the smallest possible halogen molecule bubble . Further clustering of these defects may lead to dislocation loops. In contrast, aggregates of only anion vacancies are equivalent to small metal colloid particles. 

However, during the annealing of small dislocation loops (treated in Section 11.4.3), larger variations of the vacancy concentration occur and Eq. 3.68 must be employed. 

The situation is illustrated in Fig. 11.12a. The loop is taken as an effective torus of large radius, RL. with much smaller core radius, R0, and the film thickness is 2d with d Rl. The vacancy concentration maintained in equilibrium with the loop... 

Figure 11.12 (a) Vacancy diffusion fluxes around a dislocation loop (of radius Rl)... 

The concentration, Cy (loop), can be found by realizing that the formation energy of a vacancy at the climbing loop is lower than at the flat surface because the loop shrinks when a vacancy is formed, and this allows the force shrinking the loop (see Section 11.2.3) to perform work. In general, Nyq = exp[—Gy/(kT) according to Eq. 3.65, and therefore... 

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