Long before the onset of significant phase evolution or void swelling is observed, the first manifestation of the radiation-induced microstructural/microchemical evolution appears in changes ofthe mechanical prop­erties. As shown in Figure 18 the stress-strain dia­grams of stainless steels begin to change significantly even at very low dpa levels. The strength of the alloy increases, the elongation decreases, and there is a progressive decrease in work-hardening. This behav­ior is dependent somewhat on test temperature but is not very sensitive to neutron spectrum.

Movement of dislocations in metals during deformation following irradiation is impeded by the microstructural components produced by radiation (dislocations, dislocation loops, voids, bubbles, preci­pitates) and therefore the strength of annealed steel

increases. The strength increase usually saturates at relatively low exposure levels (<10dpa) as shown in Figure 19, reflecting a similar saturation of micro­structural densities. Since the concentration of most radiation-induced microstructural components decreases with increasing temperature above ^300 °C, one would expect that the saturation strength would also decrease with increasing temperature, as is shown in Figures 20-23.

Neutron dose (dpa) Figure 19 Strengthening of various annealed 300 series stainless steels versus dpa in various water-cooled reactors at relatively low temperatures (280-330 °C). Reproduced from Pawel, J. P.; Ioka, I.; Rowcliffe, A. F.; Grossbeck, M. L.; Jitsukawa, S. In Effects of Radiation on Materials: 18th International Symposium; ASTM STP 1325; 1999; pp 671-688. At these temperatures strengthening saturates at ~10 dpa.

Irradiation of cold-worked steels also leads to strengthening at lower temperatures but softening can occur at higher temperatures if the saturation strength level at a given temperature is below the starting strength, as seen in Figure 21. Most impor­tantly, both annealed and cold-worked steels con­verge to the same saturation level when irradiated at the same dpa rate and temperature as seen in Figure 22.78

Similar convergence behavior has been observed in the evolution of microhardness.79 Note also that radiation-induced changes in strength are roughly independent of composition within the annealed 300 series stainless steels, especially at lower irradia­tion temperatures, as shown by Figure 19.

Such convergence behavior has been observed many times, but there are exceptions; for example, cold-worked steels converge in their notch tensile strength, but not to the level reached by annealed steels.80 Such behavior is usually observed in steels that twin heavily during deformation and were irradiated at low temperatures that resist recrystallization. Twin boundaries are not easily erased by displacements, so their hardening contribution persists.

Concurrent with an increase in radiation-induced hardening is a loss of ductility,81-83 as shown in Figures 23 and 24.

The concept of saturation or persistence of mechanical properties, especially with respect to tem­perature, applies to the most recent irradiation temperature, as demonstrated by comparing isother­mal and nonisothermal histories. In Figure 25 the mechanical properties of three model alloys are seen to converge during isothermal irradiation without being affected by composition, He/dpa ratio, and mechanical starting state.84 In Figure 26, however, an early detour in temperature led to differences from isothermal behavior, but these differences dis­appeared when the intended isothermal temperature was reestablished.84

Previous saturation states are soon forgotten, usu­ally by ^5 dpa, but only if the hardening components are easily erased and replaced at the new tempera­ture. If hardening arises primarily from dislocation loops and dislocations, this condition is easily met. If the primary hardening arises from a fine density of voids and especially bubbles produced at lower temperatures, then the microstructural memory can­not be easily erased, even at much higher tempera­tures. An example is shown in Figure 27 where a series of Fe-Cr-Ni ternary austenitic alloys were irradiated at 400 and 500 °C in ORR at high He/dpa ratios

(27-58 appmdpa-1) and 395 and 450 °C in EBR-II at very low He/dpa ratios (0.7-1.2 appmdpa-1).85

Note that there are very significant differences in hardening observed between the two experiments and that the differences arose primarily from a very large difference in cavity density, a difference that was too large to be explained in terms of helium content alone. It was later shown that the ORR experiment suffered a very large number (237 over 2 years) of unrecognized negative temperature set­backs of 1-2 h, with decreases varying from 50 to 500 °C.86 Even though the total dpa accumulated during these setbacks was only ~1% of the total dose, the frequent bloom of high densities of small Frank loops at lower temperatures provided a very large periodic increase in nucleation sites for helium bubbles on the new Frank loops that significantly strengthened the matrix. The loops could subse­quently dissolve but the bubbles could not.

In addition to temperature, the most prominent irradiation variable is the dpa rate and it is known that the microstructural densities, especially Frank loops and voids, are known to increase in concentra­tion as the dpa rate increases. Various radiation-stable phases such as %’ are also known to be flux-sensitive, while other phases such as carbides and intermetallics are more time-sensitive.1

Thus, it is not surprising that some sensitivity to dpa rate might be observed in strength properties, as

Test temperature = Irradiation temperature

Figure 23 Neutron-induced changes in tensile properties of annealed 1.4988 stainless steel irradiated in the DFR fast reactor. Reproduced from Ehrlich, K. J. Nucl. Mater. 1985, 133-134, 119-126. Ductility declines as strength increases.

Figure 24 Strengthening and ductility loss observed in two stainless steels irradiated in the HFIR, HFR, and R2 mixed spectrum reactors at 250°C at He/dpa ratios ranging from 10 to 35appmdpa~ Note that both annealed and cold-worked (CW) steels quickly converge to the same elongation levels, while convergence of strength is not developing as quickly. Reproduced from Elen, J. D.; Fenici, P. J. Nucl. Mater. 1992, 191-194, 766-770.

suggested by the behavior shown in Figure 28 where both the transient rate of strength rise and saturation strength appear to increase with increasing dpa rate. Unfortunately, this figure does not represent a single variable comparison, and by itself is not sufficiently convincing evidence of flux sensitivity. The data shown in Figure 29 is much closer to a single variable comparison, indicating that the transient rise may or not be somewhat flux-sensitive, depending on the details of the microstructural evolution of each alloy. The authors of this study used microscopy to confirm the microstructural origins of the observed differ­ences of behavior as a function of dpa rate.

More recently, Chatani and coworkers showed that at relatively low irradiation temperatures char­acteristic of boiling water reactors, the radiation — induced increments in strength of 304 stainless steel increased by the 1/4 power of the increase in dpa rate.87 It was demonstrated that the black-spot micro­structure dominated the strengthening. It was also shown that the concentration of black spots varied with the square root of the flux as expected, and it is known that hardening varies with the square root of the loop density, thereby producing a fourth-root dependence. Thus, in the absence of any significant microchemical or phase stability contributions, it

appears that radiation-induced strengthening is affected by dpa rate but not very strongly.

The loss of ductility proceeds in several stages, first involving convergence of the yield and ultimate strengths as shown in Figures 29 and 30, such that a loss of work-hardening occurs and very little uniform elongation is attained. As the irradiation pro­ceeds, there is a progressive tendency toward flow localization followed by necking. As seen in Figure 31 the failure surface shows this evolution with increas­ing dose.

The flat faces observed at highest exposure in Figure 31 are often referred to as ‘channel fracture’ but they are not cleavage faces. They are the result of intense flow localization, resulting from the first moving dislocations clearing a path of radiation — produced obstacles, especially Frank loops, and thereby softening the alloy along that path. It is not possible to remove the voids by channeling but the distorted
voids provide a microstructural record of the flow localization as shown in Figure 32. Linkage of the elongated voids is thought to contribute to the failure.

Such a failure surface might best be characterized as ‘quasi-embrittlement’, which is a suppression of uniform deformation, differentiating it from true embrittlement, which involves the complete sup­pression of the steel’s ability for plastic deformation. This distinction is made because under some con­ditions quasi-embrittlement can evolve into true embrittlement.

The tendency toward quasi-embrittlement grows with increasing swelling but the alloy is actually softening with increasing swelling rather than hard­ening. As shown in Figure 33 brittle fracture (defined as strength reduction with zero plasticity) of a Fe-18Cr-10Ni-Ti stainless wrapper in BOR-60 at 72 dpa maximum was observed at positions where peak swelling occurs.88 Some decrease of strength is

observed with increasing irradiation temperature, but the primary strength reduction for specimens tested at the irradiation temperature arises from the magnitude of swelling. Testing at temperatures below the irradia­tion temperature (e. g., 20 °C) demonstrates the same dependence on swelling and irradiation temperature, but the strength and plasticity values are higher. As expected, the strengths for tests conducted at 800 °C are uniformly much lower than that observed at lower temperatures, but there is an absence of any relation­ship between strength and swelling at this temperature.

As shown in Figure 34 failure surfaces at high swelling levels exhibit transgranular cup-cone mor­phology where failure proceeded by micropore coa­lescence arising from stress concentration between deforming voids.88 Similar fracture morphology has been observed in studies on other stainless steels.1

Although voids and bubbles initially serve to harden the microstructure,78 large swelling levels allow previously second-order void effects to become dominant.1,88,89 One of these second-order effects is the strong decrease of elastic moduli at high swelling levels. All three elastic moduli decrease initially at ^2% per each percent of void swelling.90-93 At >10% swelling this leads to significant reduction in strength.

As a consequence, the slope of the elastic region (Young’s modulus) of the stress-strain curve decreases, and more importantly, the barrier strengths of all sinks decrease as the shear modulus likewise decreases. Therefore, the yield and ultimate strengths decrease with increasing swelling, even though the elongation strongly decreases. Similar behavior has also been observed in pure copper.94

Fluence (dpa) Figure 28 Differences in strength change exhibited by annealed 316 stainless steel after irradiation at 390°C in the PHENIXand RAPSODIEfast reactors. Dupouy, J. M.; Erler, J.; Huillery, R. In Proceedings International Conference on Radiation Effects in Breeder Reactor Structural Materials, Scottsdale; The Metallurgical Society of AIME: New York, 1977, pp 83-93. Phenix operated at a displacement rate that was ~three times higher than that of RAPSODIE.

The nature of the void-related failure changes from quasi-embrittlement to true embrittlement for tests at or near room temperature, demonstrating another example of a late-term second-order process growing to first-order importance at higher swelling levels.

Hamilton and coworkers observed that above ~10% swelling the previously established saturation strength level of 316 stainless steel suddenly increased very strongly in room temperature tensile tests.95 Similar results were observed in Russian steels 96,97 As shown in Figures 35 and 36 the failure surfaces in such tests had rotated from the expected 45° (relative to the stress axis) to 90° as swelling approached 10%, indi­cating complete brittle failure, as also indicated by the fully transgranular nature of the failure surface. Con­currently, the ductility vanished and the tearing mod­ulus plunged to zero, indicating no resistance to crack propagation. Once a crack has initiated it then propa­gates completely and instantly through the specimen.

Neustroev and coworkers observed such failures in Russian steels that are subject to greater amounts of precipitation and determined that the critical micro­structural condition was not defined solely by the level of swelling, but by the obstacle-to-obstacle distance of the void-precipitate ensemble, indicating that stress concentration between obstacles was one contributing factor.96 However, it was the progressive segregation of nickel to increasing amounts of void surface and the concurrent rejection of chromium from the sur­faces that precipitated the rather abrupt change in failure behavior.1,95 This late-term void-induced micro­chemical evolution induces a martensite instability in the matrix, as evidenced by the failure surface being completely coated with alpha-martensite.95

Figure 29 Strength changes observed in annealed 304 and 316 stainless steels irradiated in EBR-II at 371-426 °C and tested at 385°C. Reproduced from Brager, H. R. Blackburn, L. D.; Greenslade, D. L. J. Nucl. Mater. 1984, 122-123, 332-337. Microscopy showed that the dependence of microstructure on displacement rate was consistent with the macroscopic behavior exhibited by each alloy. In AISI 316, the flux dependence of precipitation canceled the opposite dependence of other microstructural components.

The abrupt jump in strength just before failure observed by Hamilton and coworkers is the result of a stress-induced blossoming of a high density of small, thin, epsilon-martensite platelets, as seen in Figure 37. These platelets are essentially stacking faults that form under stress as a result of the influ­ence of both falling nickel level and low deforma­tion temperature to decrease the stacking fault energy of the matrix.1 When encountered by the advancing crack tip, the epsilon-martensite is con­verted to alpha-martensite in the strain field ahead of the crack, providing a very brittle path for further cracking.

The correlation between void swelling and both quasi-embrittlement and true embrittlement is observed not only in slow tensile tests (Figures 36, 38, and 39) but also in Charpy impact tests as shown

Figure 30 Convergence of ultimate and yield strengths of annealed 304 stainless steel irradiated in EBR-II and tested at 370°C. Reproduced from Holmes, J. J.; Straalsund, J. L. In Proceedings of International Conference: Radiation Effects in Breeder Reactor Structural Materials; 1977; pp 53-63.

in Figure 39. Figures 40-44 present examples of swelling-induced failures in components experien­cing a wide range of physical insults. The example of Porollo et al. in Figure 44 (top) is particularly noteworthy in that it results from significant swelling at 335 °C, a temperature earlier thought not to produce significant amounts of swelling.

If there are no physical insults experienced by the component during irradiation, the continued segregation of nickel to void surfaces and the con­current rejection of chromium can lead to strong changes in composition in the matrix during irradia­tion, pushing the matrix toward ferrite rather than martensite at higher temperatures, especially for steels with nickel content of <10%. In some observations voids encased in austenite shells have been observed to exist in a pure ferrite matrix.98,99 To date, however, no significant component failure has been reported to result from this particular late-term instability.

Finally, there appears to be another late-term phase instability developing at lower irradiation tem­peratures that involves martensite but does not appear to be due to void swelling. Gusev et al. have shown that for irradiation temperatures below -~350 °C a growing tendency for stress-induced mar­tensite formation is occurring in Russian austenitic steels at doses in the range of 25-55 dpa when tested at room temperature.100-102 Surprisingly, this insta­bility results in a restoration of engineering ductility to preirradiation levels. However, the ductility is

Figure 31 Increase in strength, loss of ductility, and change in failure mode observed during tensile testing in annealed 304 safety and control rod thimbles (SRT and CRT) after irradiation at ~370°C in EBR-II. Reproduced from Fish, R. L.; Straalsund, J. L.; Hunter, C. W.; Holmes, J. J. In Effects of Radiation on Substructure and Mechanical Properties of Metals and Alloys; ASTM STP 529; 1973; pp 149-164.

Figure 32 Intense flow localization manifested as shearing of voids below a ‘channeled’ failure surface in a 304 steel tensile specimen at 40 dpa and ~400°C when tested at 370 °C. There is 100-200% strain in the 0.05 mm wide deformation band. Reproduced from Fish, R. L.; Straalsund, J. L.; Hunter, C. W.; Holmes, J. J. In Effects of Radiation on Substructure and Mechanical Properties of Metals and Alloys; ASTM STP 529; 1973; pp 149-164. The swelling was ~5% in this specimen.

regained not because the steel has softened, but because it becomes exceptionally strong and hard­ened during deformation. As a consequence, the steel has lost the ability to neck.

Instead of necking, a moving wave of deformation is initiated at the first attempted necking point. The wave front then travels nearly the full length of the gage section. Initially, there is a local deforma­tion in the order of 40-45%, but as the wave moves forward it leaves a relatively uniform local defor­mation in its wake. Everywhere behind the wave front there is measured 30-35% volume percent of martensite, as shown in Figures 45 and 46. The martensite is not only a product of the wave, but also the cause of the wave. Deformation-induced martensite resists further necking and forces the deformation to be displaced to the adjacent lesser deformed material. The mechanisms that cause the late-term onset of martensite instability have not yet been determined.

A property of important engineering interest is the fracture toughness Jc. While the fracture toughness of various unirradiated stainless steels can be quite

Figure 35 Fractographs of failure surfaces of 20% cold-worked 316 specimens cut from an FFTF duct at high exposure. Reproduced from Hamilton, M. L.; Huang, F. H.; Yang, W. J. S.; Garner, F. A. In Effects of Radiation on Materials: 13th International Symposium (Part II) Influence of Radiation on Material Properties; ASTM STP 956; 1987; pp 245-270. Note change of fracture mode from channel fracture when tested at 205 and 460 °C to brittle fracture when tested at 20 °C.

Swelling (%) Swelling (%) Figure 36 Influence of swelling on fracture properties during tensile testing of an annealed Fe-18Cr-10Ni-Ti steel irradiated in BOR-60 at 400-500 °C. Neustroev, V. S.; Shamardin, V. K. Atomnaya Energiya 1990, 71(4), 345-348, in Russian. Note that softening and rotation of fracture surface by voids is observed at both room and elevated temperatures.

different, it appears that all austenitic steels studied undergo the same general evolution in toughness during irradiation. Mills has shown that three regimes of evolution occur.103,104 The first regime involves a low-dose threshold exposure range (< 1 dpa) where there is essentially no loss of toughness, and the second regime involves an intermediate exposure range (1-10 dpa) where toughness decreases rapidly with exposure, producing an order of magnitude reduction in Jc and two orders of magnitude degra­dation in tearing modulus. Finally, a saturation regime is reached, in which increasing exposure does not produce a further reduction in toughness. This saturation occurs well before any of the void-induced instabilities discussed above can occur. As shown in

Figure 47, the saturation level is remarkably inde­pendent of the original toughness level.

Welds in austenitic alloys were shown by Mills to exhibit lower initial toughness values and lower satu­ration toughness levels as well. The fracture toughness level is sensitive to the test temperature, however, as shown in Figure 48. At high test temperatures, the fracture mode changes from transgranular to inter­granular in nature, reflecting the effect oftest tempera­ture on both matrix strength and also the influence of helium embrittlement at grain boundaries.105 The level ofhelium needed to promote high temperature embrit­tlement is not very high, however, and can easily be reached after moderate neutron exposure in fast reactor-irradiated alloys with the lowest nickel level.