The effects of He on fast fracture, typically charac­terized by shifts in the DBTT measured in CVN impact tests (AT), has long been a subject of
significant controversy. This controversy has been fueled by studies that were interpreted to suggest that even small to moderate amounts of He result in increases in DBTT14,34,240-242 However, it has been shown that at temperatures below about 400 °C embrittlement is primarily due to irradiation harden­ing (Asy), resulting from fine-scale irradiation — induced dislocation obstacles.20,21 The simplest rela­tion is hardening-shift relation, which is given by

AT = C Asy [18]

Here C depends on a number of variables but for irradiated FMS has an average value of «0.4 °C MPa-1 for subsized CVN tests. Thus, it is obvious that He would contribute to embrittlement of FMS to the extent that it contributes to hardening. However, as noted previously, He effects on hardening are minimal up to levels of about 500 appm. Further, most of the data on He effects on embrittlement are confounded by the experimental techniques, like Ni and B doping, or use of atypical fracture test methods. Irradiation embrittlement can also be induced by nonhardening mechanisms associated with changes in the local fracture properties that are controlled by coarse-scale microstructural features, like brittle trigger particles for cleavage, and segregation of ele­ments that weaken GBs.2 ,

The first data that clearly indicated a nonharden­ing role of He were generated in the early STIP experiments, showing a transition from ductile and cleavage fracture modes to extremely brittle IG fracture20,220 and somewhat larger than expected A T Analyses of a large database on irradiation hard­ening and embrittlement, including the STIP data,2 showed that He does not produce significant nonhardening embrittlement at less than about 500appm. However, above this rough threshold the hardening-shift coefficient C (=AT/Asy) increases due to weakening of the GBs associated with He accumulation, to the point where they became the preferred fracture path. The database was used to derive a simple semiempirical model for CVN AT for 300 °C irradiations as

C = 0.4 + 7 x 10-4(XHe — 500)(oC MPa-1) [19]

As shown in Figure 33 the model prediction (dashed curve)242a is remarkably consistent with SPNI and neutron data including more recent results. The STIP data are based on subsized CVN tests (KLST and 1/3 CVN) on different FMS irradiated in STIP — I-III up to about 17dpa at temperatures below 300 °C.19 The solid symbols are small punch test data converted to CVN A T The neutron data were taken from the literature,14,240-244 and these results

Displacement (dpa)

Figure 34 A sketch showing the mechanisms for irradiation-induced hardening (increase of yield stress, Asy) and helium-induced grain boundary weakening effects (decrease in the intergranular fracture stress, s(g) that elevate the brittle to ductile transition temperature.

are also consistent with the analysis of a larger database.20

As schematically illustrated in Figure 34, the synergistic low-temperature hardening-helium embrit­tlement (LTHE) He threshold can be rationalized as follows. Cleavage fracture occurs when the stress concentrated at the tip of a blunting crack, Mcy, exceeds a critical local stress, c*, over a critical vol­ume needed to activate a brittle trigger particle.2 Here, M is a stress concentration factor. Likewise, brittle IG fracture occurs when the crack tip stress exceeds the critical local stress si*g over a sufficient volume needed to crack GBs. The c* is initially higher than sc*; thus, fracture occurs by transgranular cleavage (Figure 34(a)). However, c* decreases with increasing He GB accumulation, and beyond a bulk threshold level, ca. 500appm, c* falls below c* (Figure 34(b)). Thus, the grain boundary becomes the favored crack path. The c* continues to decrease with increasing He accumulation, resulting in an increasing increment of AT, even in the absence of additional hardening. The transition to IG crack paths is marked by a larger fraction of grain boundary facets on the fracture surface. Note that the continued increase in Ac ys with higher He was not recognized at the time that this simple model was developed, thus the new insight and expanded database will be used to refine the model.

Helium that is not clustered into bubbles is likely the most damaging condition, with a monolayer cov­erage producing essentially complete grain boundary decohesion. The actual amount and distribution of helium on GBs has not been established and is a function of the temperature and microstructure as well as bulk XHe. However, even at 400 ° C boundary bubbles are less than 1 nm in diameter. Assuming that grain boundary helium derives from regions in the
adjoining matrix and is located in spherical bubbles with equal numbers of mHe atoms and vacancies, the fractional grain boundary coverage can be estimated asfHe = tHeXHe/[10-4 mHe]; here, tHe is the thickness (mm) of the layers that feed helium to the grain boundary. Thus for example, fHe«0.25, assuming tHe = 0.25 mm and mHe = 5 and XHe = 500appm. Note that this tHe may be too large considering that denuded zones are not evidently observed at GBs in STIP samples. However, the data are not sufficient to reach firm conclusions, and a combination of models and mechanism experiments is needed to determine the partitioning of He to GBs for various microstruc­tures and irradiation conditions.

Other studies245,246 reached similar conclusions regarding the effect of He on grain boundary strength. Indeed, simple and direct evidence is provided by the brittle fracture stresses measured in the tensile tests cited previously, which decreased from «1850 to 1640 MPa with increasing He levels from 1250 to 2500 appm. These helium-degraded c* are well below the cleavage c* « 2000 MPa.

Embrittlement and AT are most properly evalu­ated by fracture toughness tests that are expected to show hardening-He synergisms that are similar to those measured in CVN tests. Figure 35 shows the estimated fracture toughness (Kjq) of various FMS after SPNI based on three-point bend tests on small precracked bars at test temperatures approximately equal to the irradiation temperatures.210,222 Note that

at high dose, Kjq decreases to less than 40 MPa Vm, close to lower shelf fracture toughness of FMS, even at the maximum irradiation temperature of 400 C.