When metals are subjected to displacive irradiation, especially at elevated temperatures, an intricate and coordinated coevolution of microstructure and microchemistry commences that is dependent pri­marily on the alloy starting state, the dpa rate, and the temperature, and secondarily dependent on vari­ables such as He/dpa rate and applied or internally generated stresses.

In general, the starting microstructure and micro­chemistry of the alloy determine only the path taken to the radiation-defined quasi-equilibrium state, and not the final state itself. If an alloy experiences enough displacements, it effectively forgets its start­ing state and arrives at a destination determined only by irradiation temperature and dpa rate. This quasi­equilibrium or dynamic-equilibrium state consists of microstructural components existing at relatively fixed densities and size distributions, but individual dislocations, loops, precipitates, or cavities at any one moment may be growing, shrinking, or even disap­pearing by shrinkage or annihilation.

The displacement process produces two types of crystalline point defects, vacant crystalline posi­tions (vacancies) and displaced atoms in interstitial crystalline positions (interstitials). These two defect types are both mobile, but move with different dif — fusional modes and at vastly different velocities, with interstitials diffusing much faster than vacan­cies. Therefore it is obvious that all diffusion-driven processes will be strongly affected by radiation. Both defect types have the ability to recombine with the opposite type (annihilation) or to form agglomerations of various types and geometries. These agglomerations and their subsequent evolution alter both the microstructure and elemental distribu­tion of the alloy.

It is important to note that interstitial agglomera­tions are constrained to be two-dimensional, while vacancies can agglomerate in both two-dimensional and three-dimensional forms. This dimensional dis­parity is the root cause of the void swelling phenom­enon covered in a later section.

The developing ensemble of various defect agglomerations with increasing dose induces signifi­cant time-dependent and dose-dependent changes in physical and mechanical properties, as well as resulting in significant dimensional distortion. Most importantly, under high displacement rates stainless steels and other alloys are driven far from equilibrium conditions as defined in phase diagrams, affecting not only phase stability but also all physical, mechanical, and distortion processes that involve phase changes in their initiation or evolution.

During irradiation, the phase evolution can be significantly altered, both in its kinetics and in the identity and balance of phases that form.46,47 Phases can be altered in their composition from that found in the absence of irradiation, and new phases can form that are not found on the equilibrium phase diagram of a given class of steels. In 300 series stainless steels these new or altered phases have been classified as radiation-induced phases, radiation-modified phases, and radiation-enhanced phases.48-51 These classifica­tions are equally applicable to phases formed in other classes of steel.

Radiation-induced alterations of microstructure and microchemistry occur because new driving forces arise that do not occur in purely thermal environ­ments. The first of these new driving forces is the presence of very large supersaturations of point defects, especially at relatively low irradiation tem­peratures (250-550 °C). Not only are vacancies present in uncharacteristically high levels, thereby accelerating normal vacancy-related diffusional pro­cesses, but interstitials are also abundant. Solutes that can bind with either type of point defect tend to flow down any microstructurally induced gradient of that defect, providing a new mechanism of solute segre­gation referred to as solute drag.52 This mechanism has been proposed to be particularly important for binding of smaller solute atoms such as P and Si, and sometimes Ni, with interstitials.

A second new driving force is the inverse Kirkendall effect 53 whereby differences in elemental diffusivity via vacancy exchange lead to segregation of the slowest diffusing species at the bottom of sink — induced vacancy gradients. This mechanism is par­ticularly effective in segregating nickel in austenitic Fe-Cr-Ni alloys at all sinks which absorb vacancies, leading to nickel-rich shells or atmospheres on grain boundaries and other preexisting or radiation — produced microstructural sinks. This type of segre­gation arises because the elemental diffusivities of Fe-Cr-Ni alloys are significantly different, with Dcr > DFe > DNi at all nickel levels.54-57

A third new driving force results from the action of the other two driving forces when operating on microstructural sinks that are produced only in irra­diation environments. These are Frank interstitial loops, helium bubbles, and voids that may have devel­oped from helium bubbles. Precipitates are often observed to form and to co-evolve on the surface of such radiation-induced sinks. Examples of typical radiation-induced microstructures in stainless steels are shown in Figures 12-15. These microstructural sinks have been implicated as participating in the evolutionary path taken by the precipitates and thereby influencing the microchemical evolution of the matrix.1,58-60

Figure 12 Frank loops observed in a 316 stainless flux thimble from a PWR power reactor (a) 70dpa, 315 °Cand (b) 33dpa, 290 °C imaged edge-on on one set of the four (111) planes using the dark-field relrod technique. Reproduced from Edwards, D. J.; Garner, F. A.; Bruemmer, S. M.; Efsing, P. G. J. Nucl. Mater. 2009, 384, 249-255. The image in (c) is from Frank loops that are slightly inclined to the beam direction imaged using a relrod in the diffraction pattern.

Minor solute elements such as Si and P have much higher diffusivities than those of Fe, Ni, and Cr and also participate in the segregation process. Addition­ally, these elements increase the diffusivities of the major elements Fe, Ni, and Cr.54

When the solute drag mechanism, operating between interstitials and smaller size Si and P atoms, combines with nickel segregation via the inverse Kirkendall mechanism, phases that are rich in nickel, silicon, or phosphorus often form (g0, G-phase and Ni2P for example), although in 300 series stainless steels these phases do not form thermally. Other phases that are normally stable in the absence of radiation (carbides, intermetallics) can be forced dur­ing irradiation to become enriched in these elements.1

The removal of nickel, silicon, and phosphorus from the matrix by radiation-induced precipitation exerts a large effect on the effective vacancy diffusiv — ity.57,61 On a per atom basis, phosphorus has been

Figure 14 Void swelling (~1%) and М2зСе carbide precipitation produced in annealed 304 stainless steel after irradiation in the reflector region of the sodium-cooled EBR-II fast reactor at 380°C to 21.7dpa at a dpa rate of 0.84 x 10~7dpas~1. Reproduced from Garner, F. A.; Edwards, D. J.; Bruemmer, S. M.; etal. In Proceedings, Fontevraud 5, Contribution of Materials Investigation to the Resolution of Problems Encountered in Pressurized Water Reactors; 2002; paper #22. Dislocations and dislocation loops are present but are not in contrast.

Figure 15 Reverse contrast image showing void and line dislocation microstructure in Fe-10Cr-30Mn model alloy irradiated in FFTF fast reactor to 15 dpa at 520 °С. Average void sizes are ~40 nm. Reproduced from Brager, H. R.; Garner, F. A.; Gelles, D. S.; Hamilton, M. L. J. Nucl. Mater. 1985, 133-134, 907-911. Frank loops have unfaulted to produce a line dislocation network whose segments end either on void surfaces or on upper and lower surfaces of the thin microscopy specimen. The voids are coated with ferrite phase due to Mn depletion from their surfaces via the Inverse Kirkendall effect.

shown to exert an even larger effect on the effective vacancy diffusivity57 and its removal into Ni2P and other precipitates has a strong influence on matrix diffusion. Silicon is the next most effective element on a per atom basis. As the effective vacancy diffusion coefficient falls with decreasing matrix levels of Ni, Si, and P, conditions for void nucleation become more favorable.

The radiation-induced evolution of diffusional properties has been strongly implicated in determin­ing the transient duration before void swelling accel — erates.1 This evolution often does not necessarily proceed by only one path but occurs in several inter­active stages. Some phases such as nickel phosphides and TiC, especially when precipitated on a very fine scale, are thought to be beneficial in resisting the evolution of nickel silicide type phases.59,62,63 It has been shown, however, that continued radiation — induced segregation eventually overwhelms these phases by removing critical elements such as Ni and Si from solution, causing their dissolution and replacement with nickel-rich and silicon-rich phases that coincide with accelerated swelling.63-65

In high-nickel alloys that normally form the y0 and y00 ordered phases, irradiation-induced segregation processes do not significantly change the identity or composition of the phases, but can strongly change their distribution, dissolving the original distribution but plating these phases out on voids, dislocations, and grain boundaries, with the latter often leading to severe grain boundary embrittlement.66,67

The original dislocation microstructure quickly responds to mobile displacement-generated point defects, increasing their mobility and leading to reductions in dislocation density and distribution in the cold-worked steels most frequently used for fuel cladding and structural components.1 These dislocations are quickly replaced by new micro­structural components, often at very high densities, with two-dimensional interstitial Frank loops first dominating the microstructure, then generating new line dislocations via unfaulting and interaction of loops. In well-annealed alloys there are very few pre­existing dislocations but the same radiation-induced loop and dislocation processes occur, eventually reaching the same quasi-equilibrium microstructure reached by cold-worked alloys.

At lower temperatures found in water-cooled test reactors especially, the microstructural features appear to be three-dimensional vacancy clusters or stacking fault tetrahedra and two-dimensional vacancy or interstitial platelets, which are probably also small dislocation loops. These ‘defect clusters’ at temperatures below ^300°C are usually too small to be easily resolved via conventional transmission

electron microscopy and are often characterized as either ‘black dots’ or ‘black spots.’ These dots are generally thought to be very small Frank interstitial loops.

The cluster and dislocation loop evolution is fre­quently concurrent with or followed by the loss or redistribution of preexisting precipitates. Most importantly, new radiation-stabilized precipitates at high density often appear with crystal structure and composition that are not found on an equilibrium phase diagram for austenitic steels.

As a consequence of these various processes the microstructure at higher doses often develops very high densities of crystallographically faceted, vacuum — filled ‘cavities’ called voids, thought to nucleate on helium clusters formed by transmutation, although residual gases in the steel often help nucleate voids at lower concentrations. Voids have frequently been observed in charged particle irradiations where no helium was introduced.

The void phenomenon is not a volume- conservative process and the metal begins to ‘swell’ as the microscopic voids in aggregate contribute to macroscopic changes in dimension, sometimes increasing the metal volume by levels of many tens of percent.

Concurrently, the dislocation microstructure responds to the local stress state, moving mass via a volume-conservative process designated irradiation creep. In general, irradiation creep is not a directly damaging process but it can lead to component failures resulting from distortion that causes local blockage of coolant flow or strong postirradiation withdrawal forces. Both swelling and irradiation creep are interrelated and are interactive processes that can produce significant distortions in component dimensions. Figure 16 shows some pronounced examples of such distortion.68,69

Eventually, the microstructural/microchemical ensemble approaches a quasi-equilibrium condition

	CW 316 SS, thimble tube
	70 dpa, 330 °C
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Figure 17 High densities of nanocavities observed using highly under-focus conditions in a PWR flux thimble tube constructed from cold-worked 316 stainless steel. Reproduced from Edwards, D. J.; Garner, F. A.; Bruemmer, S. M.; Efsing, P. G. J. Nucl. Mater. 2009, 384, 249-255. The irradiation conditions were ~70 dpa and 330 °C, producing ~600 appm He and 2500 appm H. Note the high density of cavities on the grain boundary.

or ‘saturation’ state, usually at less than 10 dpa for mechanical properties but at higher doses for swelling. As a consequence, the mechanical properties tend to stabilize at levels depending primarily on tempera­ture and to a lesser extent on dpa rate. The two major deformation processes, swelling and irradiation creep, do not saturate but reach steady-state defor­mation rates when quasi-equilibrium microstructures are attained. This coupling of saturation microstruc­ture with steady-state behavior has been character­ized as ‘persistence.’70

Interestingly, the saturation states of each property change are almost always independent of the starting thermal-mechanical state of the material.1,70,71 If irradiation continues long enough, the memory of the starting microstructural state and the associated mechanical properties is almost completely lost. The only deformation-induced microstructural component that succeeds in resisting this erasure process is that of preexisting, deformation-induced twin boundaries.

If this quasi-equilibrium is maintained to higher neutron exposure no further change occurs in the steel’s mechanical properties. However, some slowly developing second-order processes are nonsaturable and are often nonlinear. Eventually, these processes force the system to jump toward a new quasi­equilibrium. These new states usually arise from either the microstructural or microchemical evolu­tion, with voids dominating the former and the latter involving continued segregation, continued transmutation, or a combination of these factors.70-72 A number of such late-stage changes in quasi­equilibrium state are discussed later in this paper.