Defect sink

A, B, and C in vicinal (001) twist grain boundary in Au. Static array of screw dislocations in background accommodates the twist deviation of the vicinal boundary shown from the crystal misorientation of the nearby singular twist boundary to which it is vicinal. Excess selfinterstitial defects were produced m the specimen by fast-ion irradiation and were destroyed at the grain-boundary dislocations by climb, causing the boundary to act as a defect sink, (a) Prior to irradiation, (b) Same area as in (a) after irradiation, (c) Diagram showing the extent of the climb. From Komer et al. [24],... 

Some microstructural changes are directly attributable to the reduction in free energy associated with point defect destruction, or to enhanced kinetics. More specifically, freely migrating defects (vacancies or interstitials) and mobile clusters escaping from the initial damage event (i.e. the initial cascade) may interact with point defect sinks such as pre-existing... 

In real cases it may be difficult to reach equilibrium if the diffusion of the cations to and from defect sinks (dislocations, grain boundaries a.o.) is slow. Another point worth mentioning is that the excess second phase may well be a compound of the parent and dopant oxide. For instance, MgO is normally present in excess of the solubility in so-called high-purity alumina (AI2O3) and the second phase in that case is spinel MgAl204. However, this does not alter the oxygen activity dependencies the ternary compound serves as a source/sink for the dopant, with constant activity, and the defect chemical treatment remains essentially the same. 

For example, the formation of nonequilibrium y, y, and G phase have aU been observed in 316 stainless steels [32]. Both RIS and RIP are observed in the 250—300°C range in 300 series austenitic stainless steels. Due to enrichment of nickel and silicon at defect sinks, the G-phase sUicides are present after modest irradiation fluences [48]. In addition, many other types of phases have been observed in irradiated stainless steels, including carbides, Laves, and gamma [32]. The extent of precipitation and type of precipitates are extremely sensitive to the exact temperature, dose, and dose rate, but are also dependent on the specific damage microstructure. 

As discussed in Section 7.2.3, radiation can induce segregation of alloy elements at defect sinks such as grain boundaries [101]. Typically, RIS is a result of inverse Kirkendall (IK) effects in which the evolution of defect concentration field drives the evolution of alloy composition field. ID rate theory modeling [44,101] is widely used to describe the coupled evolution between defect flux and composition flux. These rate theory models considered both vacancy-mediated and interstitial-mediated solute transport, as well as point defect recombination and defect loss to dislocations. At steady state, the solute segregation direction depends on the relative diffiisivity of different species-defect coupled diffusion. In austenitic Fe-Cr-Ni alloys, the vacancy-mediated solute diffusion alone is sufficient in describing the RIS trend and the interstitial-mediated solute diffusion is usually assumed to have a neutral contribution to RIS [44]. However, in Fe-Cr F/M alloys, both interstitial- and vacancy-mediated diffusion should be considered [102]. 

Let us now introduce the physical bases of the irradiation-induced phenomena. Once created, the out-of-equilibrium supersaturation of point defects that the dpa concept represents (see Section 8.2.2) will tend to be reduced through thermally activated phenomena controlled by atomic diffusion toward the defect sinks. The main means of eliminating point defects to be taken into account are ... 

A fraction of the Ti is in the form of fine TiC precipitates and in the presence of phosphorus, in the form of small M2P-type phosphide platelets (Fig. 8.15(b)). These phases can pin the structure dislocations and therefore block restoration of the initial cold working. Once again, they work very favorably on the resistance to swelling because they also act as defect sinks and recombination sites. 

Other phases which are found to precipitate under neutron irradiation in FM steels are described in references [48,75,83] (1) Diamond cubic r] (MsC) carbide, was frequently observed in irradiated FM steels containing more than about 0.3% Ni [47,75,83] (2) bcc x intermetallic phase [47,77,78,84] (see Fig. 9.4(b)) (3) fee G (MyNiigSi , where M = Mn, Cr, or Nb) silicide phase [6,83] (4) a phase and phosphides of M3P and MP types were infrequently reported [82]. The formation of these phases, enriched in minor solutes such as Ni, Si, and P, is thought to be irradiation-induced, i.e., due to RIS of these elements which are known to segregate to point defect sinks (see above). 

A.D. Brailsford, R. Bullough, M.R. Hayns, Point defect sink strengths and void-swelling, J. Nucl. Mater. 60 (1976) 246-256. 

K.G. Field, Y. Yang, T.R. AUen, J.T. Bushy, Defect sink characteristics of specific grain boundary types in 304 stainless steels under high dose neutron environments, Acta Mater. 89 (2015) 438-449. 

In that case, the first requirement is that the scale/substrate interface acts as a source or a sink for the point defects involved in the mass transport within the oxide scale and/or the metallic substrate. This action must be combined with the action of defect sinks/sources within the oxide scale and its substrate to maintain the system in local equilibrium. The second requirement is the absence of any obstacle to the free displacement of that interface and, if required, to the free relative displacements of lattice planes in the oxide scale and the metallic substrate. 

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