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This article presents a transgranular fatigue crack growth model based on a restricted slip reversal process where the transgranular crack growth rate is related to the cyclic plastic strain range ahead of the crack tip. Upon applying deformation and fracture kinetics theories, a physically based constitutive law for fatigue crack growth rate is derived. In the absence of any environmental contributions to crack growth, the model takes the form of the Paris equation with a power law exponent of 3 at positiveR values. The model expresses the fatigue crack growth rate explicitly in terms of material properties, such as yield strength, work-hardening coefficient, microstructural quantities such as activation energy, activation volume, and work factor, as well as test constraints such asΔK andR. The absence of a fatigue threshold is predicted for test conditions where environment does not influence the crack growth process and the material microstructure remains stable.

In recent years, Kwon and his co-workers reported that the morphology of grain-boundary martensite (GBM) acted as an important factor in controlling the impact toughness and fracture behavior in experimental medium-carbon alloy steel intercritically treated. Continuous, coarse GBM led to intergranular fracture in the 6Ni steel, compared to transgranular fracture observed in its absence in a 2Si steel. In addition, in a 2Si-3Ni steel, a transgranular-to-intergranular fracture transition occurred as discontinuous, fine GBM was changed to a continuous, coarse one with increasing intercritical treatment temperature. In this study, the morphology of GBM and fracture behavior of a 2Si-6Ni steel intercritically treated have been compared with a 2Si-3Ni steel. In addition, the effect of tempering on fracture behavior has been investigated. The chemical composition of the alloy used in this study is 0.37C, 2.04Si, 5.94Ni, 0.006P, and 0.006S (wt pct). In order to obtain an initial martensite structure, Charpy impact specimens were austenitized in a flowing argon atmosphere at 1200 C for one hour and oil-quenched. To investigate the effect of tempering, the intercritically treated specimens were tempered at 600 C for one hour and water-quenched.

The effect of ordering on susceptibility to hydrogen embrittlement of a Ni-base superalloy (alloy C-276) has been investigated by means of tensile tests in air and with hydrogen-charging in 1N-H2SO4 solution. The annealed specimen has exhibited intergranular fracture by hydrogen-charging, resulting in a marked reduction in tensile elongation and ultimate tensile strength. The mode of fracture was changed by aging at 773 K, and the transgranular fracture has been found to be dominant in the aged specimens. The susceptibility to hydrogen embrittlement, as identified by the test method used in this study, seems to be reduced by short-term aging, though it turns out to be increased again by further aging. The fractured boundaries have been characterized using electron channeling pattern (ECP) analysis of adjacent grains. It is found that the misorientation of grain boundaries plays an important role in fracture, and ∑3 boundaries, twin boundaries in a face-centered cubic (fcc) lattice, are most likely to fracture in the aged specimens. Transmission electron microscopy (TEM) observation has shown that a short-range ordering reaction from a disordered fcc lattice into an ordered Ni2(Cr, Mo) (Pt2Mo type) super-lattice takes place by aging, and hence, superdislocation triplets with APB (antiphase boundary) become predominant when deformed. It is also seen that in the aged specimens, deformation twinning is another mode of deformation, and this leads to the transgranular fracture at twin boundaries by hydrogen-charging. These results suggest that a change in the mode of deformation after aging plays a major role in fracture due to hydrogen embrittlement as a consequence of the heterogeneous interaction between slip dislocations and twin boundaries.

In the 2Si-3Ni steel intercritically treated in the range of 720 ‡C to 790 ‡C, the fracture behavior under the impact testing has been analyzed and the post-tempering effect has also been investigated. The transgranular fracture occurred in the specimens treated below and at 730 ‡C (SN73 specimen) in relatively low intercritical temperature range, but the intergranular fracture occurred in the specimens treated at 750 ‡C and 770 ‡C (SN75 and SN77 specimens) in relatively high intercritical temperature range. In the SN73 specimen, there was little coarse martensite at the prior austenite grain boundaries, whereas there was continuous, coarse martensite at those boundaries in the SN75 and SN77 specimens. The fracture behavior was mainly discussed in terms of the microstructural differences. In addition, no or a little increase in impact toughness, in spite of great decreases in hardness, in the SN75 and SN77 specimens tempered at 600 ‡C is correlated with the easy occurrence of intergranular fracture, which is caused by the carbide aggregates formed in the continuous, coarse martensite at the grain boundaries.

Transmission electron microscopy (TEM) was utilized to characterize the deformation substructure of 304 stainless steel tested for transgranular stress corrosion cracking (TGSCC) in 45 wt pet MgCl2 at 155 °C. The TEM characterization was conducted in thin foils prepared from the fracture surface and from a series of known depths below the fracture surface. The results indicate that the stacking fault energy (SFE) of the material immediately ahead of the crack tip is lowered, with the deformation mode at small distances (a few microns) in front of the growing crack front being entirely coplanar while at larger distances homogeneous. The reduction in the SFE is attributed to absorbed hydrogen formed during the cathodic reaction. Based on this and previous observations of transgranular stress corrosion characteristics of aus-tenitic stainless steels in chloride environments, a “hydrogen-induced cleavage” model is proposed. This model is essentially a modification of a model based on enhanced structural reactivity associated with Lomer-Cottrell locks proposed by Robertson and Tetelmann in 1962.27

Weld metal grain structure and mechanical properties of the Ir-0.3 pct W alloy (DOP-26) doped with 60 ppm Th and 50 ppm Al have been investigated by use of a gas tungsten arc (GTA) welding process. The fusion zone grain structure is strongly influenced by heat input and puddle shape and therefore by the bead width. With increasing bead width from 2.5 to 3.7 mm, the grains in the fusion zone show a sharp change in growth direction near the centerline region and develop a fine columnar structure with grains growing parallel to the welding direction. Mechanical properties of the welds and base metal were characterized by tensile and impact tests from 650 to 1150 °C. The ductility and fracture behavior of DOP-26 welds are sensitive to weld bead width, postweld heat treatment, and weld-test orientation. The ductility of the welded specimens increases with increasing test temperature and decreasing weld bead width. The transverse weld specimen with a wide-bead width (3.7 mm) has the lowest impact ductility, and the longitudinal weld with a narrow-bead width (2.5 mm) has the highest elongation at all the test temperatures. The impact ductility of the transverse weld specimen with the narrow-bead width falls between the limits. All the results are discussed in terms of the fusion zone grain structure and fracture path of the welds.

The tensile properties of pseudobinary Ll3-type intermetallic compounds based on Ni3Al and Ni3Mn were examined on recrystallized specimens. The alloys with manganese composition lower than 15 at. pct were tested from room temperature to 1073 K (1273 K for some alloys). The yield stress appears to be controlled by solid solution hardening (athermal component) at room temperature and by the so-called Kear-Wilsdorf mechanism (thermal component) at elevated temperatures. The activation constant for the latter mechanism,i.e., for the anomalous positive temperature depen-dence of the yield stress, increases with increasing the manganese composition. The tensile elongation shows a maximum at 9 at. pct Mn and at intermediate temperatures (≈700 K). Also, at sufficient higher temperatures (1273 K), an increase of elongation due to dynamic recrystallization was ob-served. The temperature and compositional dependence of the ultimate tensile strength are similar to that of the elongation. The fracture mode was closely correlated with the elongation behavior: the more the transgranular fracture, the higher the ductility.

The fatigue properties of high conductivity Cu-1 pct Cd are reported. In contrast to Cu, the fatigue strength of Cu-Cd is grain-size dependent in high cycle fatigue. Strain aging is responsible for the superior fatigue properties of Cu-Cd, and the effect of dislocation pinning is carried over to the cold work state, giving a work hardened structure which is stable in fatigue loading. A notable feature of the results is that fatigue fracture in Cu-Cd is almost entirely intergranular at long fatigue lives, in contrast to the fracture in tensile loading and in contrast to the transgranular fracture which was observed in Cu. The effect is due to segregation of Cd at the grain boundaries and to the effect of Cd on the cohesive strength of the boundary. Moderate cold work suppresses intergranular fracture.

Near threshold fatigue crack growth behavior of a high strength steel under different tempered conditions was investigated. The important aspect of the study is to compare the crack growth behavior in terms of the closure-free component of the threshold stress intensity range, ΔKth,eff While a systematic variation in the absolute threshold stress intensity range with yield strength was observed, the trend in the intrinsic ΔKth or ΔKth,eff exhibited a contrasting behavior. This has been explained as due to the difference in fracture modes during near threshold crack growth at different temper levels. It is shown that in a high strength and high strain hardening microstructure, yielding along crystallographic slip planes is difficult and hence it exhibited a flat transgranular fracture. In a steel with low strain hardening characteristics and relatively low strength, a tendency to crystallographic planar slip is observed consequently resulting in high ΔKth. Occurrence of a predominantly intergranular fracture is shown to reduce intrinsic ΔKth drastically and increase crack growth rates. Also shown is that crack closure can occur in high strength steels under certain fracture morphologies. A ‘transgranular planar slip’ during the inception of a ‘microstructure sensitive’ crack growth is essential to promote intergranular and faceted fracture. The occurrence of a maximum in the fraction of intergranular fracture during threshold crack growth corresponds to the ΔK value at which the cyclic plastic zone size becomes equal to the prior austenitic grain size.

Electron fractographic and thin foil electron metallographic techniques were used to evaluate the deformation and fracture characteristics of Inconel X-750 at temperatures ranging from 24 to 816 °C. Operative dislocation mechanisms and fracture surface morphologies were related to the overall tensile response of this nickel-base superalloy. At room temperature, failure occurred primarily by an intergranular dimple rupture mechanism associated with microvoid coalescence along grain boundary denuded regions. A fairly high density of dislocations throughout the matrix resulted in relatively high ductility levels even though failure occurred by an intergranular mechanism. Under intermediate temperature conditions (316 to 427 °C), increased transgranular fracture coupled with extensive dislocation activity within the Inconel X-750 matrix caused a slight increase in ductility. At progressively higher temperatures, 538 to 704 °C, all dislocation activity was channeled through narrow slip bands which subsequently initiated localized separation and resulted in a very faceted fracture surface appearance. The absence of a homogeneous dislocation substructure in this temperature regime resulted in a severe degradation in ductility levels. At the highest test temperature (816 °C), a uniform dislocation network throughout the Inconel X-750 matrix coupled with intense dislocation activity in the grain boundary denuded zone resulted in a marked improvement in ductility. Furthermore, the extensive dislocation activity along grain boundary regions ultimately resulted in an intergranular fracture morphology.

The microstructure and tensile properties of two A1-3 wt pct Li-2 wt pct Cu-0.2 wt pct Zr alloys, one Cd-free and one containing 0.2 wt pct Cd, have been investigated. The Cd-free alloy remained unrecrystallized for all solutionizing treatments studied, whereas a special treatment had to be developed to prevent recrystallization during solutionizing of the 0.2 wt pct Cd alloy. In combination with cadmium, zirconium either enters into, or nucleates on, the course Al7Cu2Fe and T2 phases during high temperature annealing. This reduces the volume fraction of small coherent Al3Zr particles in the matrix which normally inhibits recrystallization. Consequently, a low temperature anneal to precipitate Al3Zr is necessary prior to high temperature solutionizing in order to prevent recrystallization in the Cd-containing alloy. Unlike its effect in lower lithium, higher copper content aluminum alloys, cadmium does not significantly affect the nucleation of the strengthening precipitates. If anything, cadmium has a detrimental effect on the age hardening response of this alloy, since it increases the formation of coarse Al-Cu-Li equilibrium phases at grain and subgrain boundaries and thus removes some of the copper and lithium from participating in the formation of the strengthening precipitates T1 and δ′. Subgrain boundary fracture occurred during tensile tests of both alloys in the unrecrystallized condition; however, transgranular fracture occurred in tests of the partially recrystallized 0.2 wt pct Cd alloy. Both types of fractures are believed due to a form of strain localization associated with precipitate free zones and shearable precipitates.

A series of 4130 steels modified with 0.50 pct Mo and 0.75 pct Mo were tempered at temperatures between 300 and 700 °C for one hour. The changes in the carbide dispersion and matrix substructure produced by tempering were measured by transmission electron microscopy. These measurements were correlated with resistance to hydrogen stress cracking produced by cathodic charging of specimens in three-point bending. Scanning electron microscopy showed that specimens tempered between 300 and 500 °C failed by intergranular cracking while those tempered at higher temperatures failed by a transgranular fracture mode. Auger electron spectroscopy showed that the intergranular fracture was associated with hydrogen interaction with P segregation and carbide formation at prior austenite grain boundaries. Transgranular cracking was initiated at inclusion particles from which cracks propagated to produce flat fracture zones extending over several prior austenite grains. The 4130 steels modified with higher Mo content resisted tempering and showed better hydrogen stress cracking resistance than did the unmodified 4130 steel. The transition in fracture mode is attributed to a decohesion mechanism in the low temperature tempered samples and a pressure mechanism in the highly tempered samples.

The effects of boron and carbon additions on the tendency for intergranular fracture in trinickel silicide intermetallics are reported. Melt spinning of Ni77Si23 alloyed with 0.1 at. pet boron results in full bend ductility and complete transgranular fracture compared with brittle intergranular fracture for the unmodified compound. Alloying with 0.1 at. pet carbon also produces full bend ductility but a mixed mode failure (≈30 pct transgranular). For both carbon and boron additions, reducing the Ni concentration of the base compound results in a greater percentage of intergranular fracture. The boron solubility limit depends on the Ni concentration of the base compound. For Ni77Si23, the solubility limit is between 0.1 and 0.2 at. pet boron. For compounds with silicon concentrations of 23.5 and 24.0 at. pct, the solubility limit is less than 0.1 at. pct boron. Boron additions above the solubility limit result in Ni3B precipitates which degrade the bend ductility and increase the percentage of integranular fracture. Alloying with carbon above the solubility limit (

The effect of various grain structures, produced by thermomechanical treatments, on the monotonie tensile properties of the Al-Cu-Li-Mn-Cd alloy 2020 was investigated. Materials having a completely or partially recrystallized structure exhibit elongations between 4 and 8 pct when aged to peak strength. For both cases the low ductility is associated with (a) planar deformation, (b) random texture, (c) the presence of large intermetallic compounds along the recrystallized grain boundaries, and (d) precipitate free zones. The first three enhance crack nucleation at high angle grain boundaries, and subsequent crack propagation occurs along the precipitate free zones. The completely unrecrystallized materials have elongations between 10 and 13 pct in both longitudinal and transverse directions. The high ductility is associated with a sharp texture and a transgranular fracture mode. The maximum ductility is obtained by reducing the unrecrystallized grain size. The results of this study suggest that improved properties of a 2020-type alloy may be obtained by lowering the Fe and Si contents to remove coarse constituent phases, eliminating Cd, and replacing Mn with Zr in order to obtain a highly textured, unrecrystallized structure.

Acoustic emission (AE) has been measured during subcritical crack growth of Fe, Ni, 304 SS, a tool steel, and an Fe−Si single crystal. Tests were conducted using compact tension type samples in a variety of environments. Iron with two levels of S and P grain boundary concentrations was tested in Ca(NO3)2 at +750mV; Ni with P segregation was tested in 1N H2SO4 at +900 mV; two heats of 304 SS were tested in Na2S2O3 at open circuit potentials; Fe and Ni with S-enriched grain boundaries were tested in 1N H2SO4 at cathodic potentials; and RDS tool steel and an [001] oriented Fe−Si single crystal were tested in gaseous hydrogen at a pressure of 1 atm. The number of AE events/mm2 of crack extension increased with an increase in transgranular fracture for all of these test conditions with the exception of the Fe−Si single crystal. In this case, crack extension was 100 pct transgranular, but the number of events/mm2 was negligible. It was concluded that the short, discontinuous crack jumps were not detectable with the AE monitoring equipment. Likewise, for the intergranular subcritical crack growth, it was concluded that crack extension was either anodic dissolution with no accompanying AE or short, discontinuous crack jumps with nondetectable AE. It also was postulated that the transgranular fracture surfaces accompanying the intergranular subcritical crack growth resulted from ligaments which fractured behind the advancing crack front.

The plane strain fracture toughness values,K Ic , the fatigue crack growth rates and the tensile properties of M-50 and 18-4-1 high speed steels have been measured as a function of tempering temperature. The M-50 was a vacuum arc remelted grade (VIM-VAR) and the 18-4-1 an electroslag grade, and both are used in mainshaft gas turbine bearings. At the usual hardness for bearings, Rockwell C 62, 18-4-1 exhibited a slightly higher fracture toughness (21 MPa·m1/2) than M-50 (18 MPa·m1/2). The fatigue crack growth rates were very similar, and in the slow growth region followed the usual power law,dC/dN=(ΔK) m withm=3 to 4. The crack propagation rates were still significant at values as low as ΔK=5 MPa·m1/2. SEM studies of the fracture surfaces showed complex transgranular fracture paths for both steels. The tensile strengths and the elongations of M-50 were somewhat higher than the corresponding values for 18-4-1 but the yield strengths of the two steels were similar. The microstructures of these steels were markedly different, with M-50 exhibiting 2.6 vol pct undissolved carbides and the 18-4-1 showing 15.2 vol pct carbides, but the fatigue and fracture behaviors were similar.

The addition of 0.06 monolayers of antimony to the grain boundaries of iron with 0.3 monolayers of sulfur was found to have no effect on the fracture toughness or subcritical crack growth behavior at cathodic potentials. Tests were conducted using compact tension type samples tested in 1N H2SO4 at cathodic potentials of −0.6V (SCE) to −1.25V (SCE). The absence of any effect of antimony on the fracture toughness was related to iron being in a “minimum” fracture toughness condition such that further segregation of an embrittling element had no effect. Also, the subcritical intergranular crack growth threshold was found to decrease with increasing cathodic potential consistent with results reported by others for transgranular fracture of steels in gaseous hydrogen.

Observations have been made of the fracture surfaces of ordered (Fe22Co78)3V deformed at low strain rates (3.3 and 42 x 10-3 s-1) in tension at temperatures from 20 to 1000 °C, that is, above and below the order/disorder transformation temperature of 950 ± 10 °C. From 20 to 700 °C, transgranular fracture occursvia the nucleation, growth, and coalescence of micro voids; correspondingly, high ductilities are obtained (34 to 44 pct). From 700 to 960 °C, the fracture mode changes from transgranular to intergranular and the ductility falls to a minimum of 4 pct. At 1000 °C a third mode operates, consisting of localized dynamic recrystallization followed by boundary sliding of recrystallized grains; ductility now rises sharply. The underlying mechanisms are considered and are supported through the results of additional experiments performed at distinctly higher strain rates.

The hardness response, tensile behavior, and phase transformations occurring in a quenched and aged metastable β phase Ti-30 at. pct V-l at. pct Si alloy have been inves-tigated. Upon aging at 570°C, as-quenched samples show a broad hardness peak which is associated with the formation of rod-like, hexagonal (Ti,V)xSiy transition phase precipi-tates. The equilibrium silicide is observed upon aging at 570°C in the form of faceted, tetragonal particles. A loss of tensile ductility and a transition to intergranular fracture occurs after extended aging at 570°C and is related to Si segregation to the grain bound-aries. Comparing the behavior of Ti-30V to that of Ti-30V-lSi shows that the presence of Si strongly retards α-phase formation. However, a substantial age hardening re-sponse still occurs upon aging at 450°C, especially after prior cold work (the yield strength increases from 635 to 982 MPa). This hardening response is combined with a retention of a ductile, transgranular fracture even after extended aging at 450°C. Aging first at 570°C followed by aging at 450°C results in an increase in the volume fraction of α-phase formed but a subsequent decrease in ductility and hardness response upon aging at 450°C. These results are discussed in terms of the structure/property relationships which result from the influence of Si on the formation of, a) (Ti.V)xSiy precipitates, b) the equilibrium silicide, and c) the α-phase.

A study of the effect of Cr additions (5, 10, and 12.5 at. pct) on the microstructure of melt-spun Ni3Al-base alloys has been carried out using analytical electron microscopy. The analyses showed that the formation of the β-NiAl phase could not be totally avoided at all three levels of Cr additions studied. However, its volume fraction, morphology, and distribution were affected by the Cr concentration as well as by changes in the rapid solidification conditions. The ordering structure of the matrix γ contains anisotropic antiphase boundaries or cube-plane-oriented thin layers of disordered γ, depending on the local chemistry. A series of microchemistry measurements along the radius of a grain was used to trace the route of Al and Cr segregation, and an unexpected reverse segregation with a partition coefficient greater than 1 was observed for Cr. The bend ductility and tensile properties of the melt-spun ribbons were also studied. It was shown that the Cr additions substantially increased the ribbon strength and modified the mode of failure toward transgranular fracture. The improved tensile properties were partially attributed to the fine matrix γdomains which had mixed interfaces of antiphase boundaries and y thin layers.

The nature of tensile failure in aβ-treated titanium alloy, Ti-6Al-l.6Zr-3.3Mo-0.3Si, is demonstrated to be strongly dependent on cooling rate followingβ treatment. A characterization of microstructure, the fracture surface of tensile tested samples, and the associated slip behavior shows the fracture mode to be determined by the microstructure as well as composition of the a laths. A martensitic matrix with fine grain boundary α particles leads to intergranular failure. At slower cooling rates, α laths form as side plates from grain boundary allotriomorphs, and an fcc interface phase is present at thea lath/retainedβ interfaces. This microstructure results in transgranular fracture caused by ductile failure at the α lath/β interfaces within the grains rather than at grain boundary ⇌. If the composition is equilibrated across the a laths by a long-term anneal followingβ treatment, the slip mode becomes extremely planar leading to slip band related cracking across the grains.

A series of VC + Ni cermets was prepared where the carbon to metal (C / M) ratio of the carbide was varied from 0.86 to 0.75. Transmission electron microscopy and diffraction were used to determine the C/M ratio of the carbide and binder microstructure. At high C/M ratios, the carbide was principally V8C7 or V8C7 + V6C5, while at low C/M ratios the carbide was V6C5 or V6C5 + V4C3. The vanadium content of the binder increases as the C/M ratio of the carbide is decreased. The mechanical behavior of the cermets was evaluated by crack resistance and hardness measurements. In inverse crack resistancevs hardness plots, the VC + Ni cermets exhibit three types of behavior (labeled Type I, II, and III) in order of decreasing fracture toughness for a given hardness. Auger electron spectroscopy and scanning electron microscopy show that the three behavior types are characterized by differences in the dominant fracture mode. The fracture mode is principally transgranular fracture through the binder for Type I, transgranular cleavage of carbides for Type III, and intergranular or mixed fracture mode for Type II. The differences in fracture mode and mechanical behavior are induced by changes in microstructural features, as well as changes in the binder composition and formation of second phases. These results suggest that binder strengthening is a viable means to improving cermet performance only in systems when the cleavage energy of the carbide is sufficiently high to withstand the stresses generated by plastic flow in the binder.

The structure and mechanical properties of ternary Fe-Cr-Co magnet alloys containing 9 to 11 wt pct cobalt have been investigated. Fine scale spinodal decomposition of the iron rich bcc α-phase into (α + α2) structure increased the alloy strength and reduced the ductility. The degree of changes in the mechanical properties depended on the cobalt content and the final aging temperature and time which primarily determines the compositional amplitude. As a result of decomposition, the dislocation movement by slip became more difficult, and the mode of deformation changed from predominantly slip to predominantly twinning. The embrittlement during aging and the fracture behavior of these alloys go through two stages: i) from microvoid nucleation and coalescence type ductile fracture to quasi-cleavage type transgranular fracture (ductile-brittle transition) and ii) from transgranular to intergranular fracture. The cause of the transgranular fracture is attributed to the raised ductile-brittle transition temperature resulting from the increased strength and the tendency for deformation twinning which are likely to make the relief of local stress concentration more difficult. The cause of the intergranular fracture is ascribed to the formation of more or less continuous grain boundary precipitate that forms upon further decomposition at lower temperatures (below ∼540‡C). Both types of embrittlement were found to be reversible upon heat treatment at higher temperatures, either within the (α1 +α2) range or above the miscibility gap.

Metallurgical and mechanical properties of Ir-0.3 pct W alloys have been studied as a function of thorium concentration in the range 0 to 1000 ppm by weight. The solubility limit of thorium in Ir-0.3 pct W is below 30 ppm. Above this limit, the excess thorium reacts with iridium to form second-phase particles. Thorium additions raise the recrystallization temperature and effectively retard grain growth at high temperatures. Tensile tests at 650 °C show that the alloy without thorium additions (undoped alloy) fractured by grain-boundary (GB) separation, while the alloys doped with less than 500 ppm thorium failed mainly by transgranular fracture at 650 °C. Intergranular fracture in the doped alloys is supressed by GB segregation of thorium, which improves the mechanical properties of the boundary. The impact properties of the alloys were correlated with test temperature, grain size, and heat treatment. The impact ductility increases with test temperature and decreases with grain size. For a given grain size, particularly in the fine-grain size range, the thorium-doped alloys are much more ductile and resistant to GB fracture. All of these results can be correlated on the basis of stress concentration on GBs by using a dislocation pileup model.