Crystallographic fatigue crack initiation in nickel-based superalloy René 88DT at elevated temperature

The fatigue behavior of a polycrystalline nickel-based superalloy René 88DT was examined in the lifetime regime of 10⁵–10⁹ cycles at 593 °C in air using an ultrasonic fatigue apparatus operating at frequencies close to 20 kHz. Three experimental techniques were combined to obtain new insights into the crack initiation process: serial sectioning, electron backscatter diffraction and quantitative fractographic analysis. Most fatigue failures initiated from internal microstructural sites comprised of large grains. Large crystallographic facets formed at crack initiation sites due to cyclic strain localization on {1 1 1} slip planes in the region close to Σ3 twin boundaries in large grains having high Schmid factors. The micromechanical mechanism of crystallographic fatigue crack initiation was analyzed in terms of both resolved shear stress and elastic incompatibility stresses in regions close to Σ3 twin boundaries. The influence of critical microstructure features on fatigue crack initiation and fatigue life variability is discussed.     

Fatigue testing and properties of hardmetals in the gigacycle range

Hardmetal products are frequently fatigue loaded in service, such as e.g. cutting tools for milling or percussion drills. In the present work, the fatigue behaviour of hardmetals was investigated into the gigacycle range using ultrasonic resonance fatigue testing at 20 kHz in push-pull mode at R = − 1. Liquid cooling was afforded using water with addition of a corrosion inhibitor. Hourglass shaped specimens were prepared, the surface being ground and polished with subsequent stress-relieving anneal to remove the high compressive residual stresses introduced during grinding. S-N curves with fairly low scatter were obtained, which indicates microstructure-controlled and not defect-controlled failure. Low binder content as well as fine WC grains were found to improve the fatigue endurance strength. In no case, however, a horizontal branch of the S-N curve was observed, i.e. there is no fatigue “limit” at least up to 10¹⁰ cycles. The initiation sites were in part difficult to identify; in such cases when the site was clearly visible, decohesion of the binder from large WC grains seems to have caused crack initiation. This further corroborates that microstructural features and not singular defects as e.g. inclusions are the initiation sites, which underlines the high purity of the hardmetal grades used. Based on fracture mechanical consideration a damage diagram was determined allowing to deduce critical defect sizes.     

Twin stability in highly nanotwinned Cu under compression,torsion and tension

Twin stability under four distinct mechanical loading states has been investigated for highly nanotwinned Cu containing parallel nanotwins ～40 nm thick. Observed deformation-induced microstructural changes under tension, compression, tension–tension fatigue and torsion are qualitatively compared in order to assess twin stability as a function of the loading direction and stress. It is observed that the twins are very stable although small microstructural changes vary with deformation mode. Shear bands, deformation-induced grain growth and detwinning are also discussed.     

Effects of stress ratio on fatigue crack propagation properties of submicron-thick free-standing copper films

The dominant mechanics and mechanisms of fatigue crack propagation in ca. 500 nm thick free-standing copper films were evaluated at the submicron level using fatigue crack propagation experiments at three stress ratios, R = 0.1, 0.5 and 0.8. Fatigue cracking initiated at the notch root and propagated stably under cyclic loading. The fatigue crack propagation rate (da/dN) vs. stress intensity factor range (ΔK) relation was dependent on the stress ratio R;da/dN, increases with increasing R. Plots of da/dN vs. the maximum stress intensity factor (K_max) exhibited coincident features in the high-K_max region (K_max ? 4.5 MPa m^1/2) irrespective of R, indicating that K_max is the dominant factor in fatigue crack propagation. In this region, the fatigue crack propagated in tensile fracture mode irrespective of the R value. The region ahead of the fatigue crack tip is plastically stretched by tensile deformation, causing necking deformation in the thickness direction and consequent chisel-point fracture. In contrast, in the low-K_max region (K_max < 4.5 MPa m^1/2), the da/dN vs. K_max function assumes higher values with decreasing R; in this region, the fracture mechanism depends on R. At the higher R value (R = 0.8), the fatigue crack propagates in the tensile fracture mode similar to that in the high-K_max region. On the other hand, at the lower R values (R = 0.1 and 0.5), a characteristic mechanism of fatigue crack propagation appears: within several grains, intrusions/extrusions form ahead of the crack tip along the Σ3 twin boundaries, and the fatigue crack propagates preferentially through the intrusions/extrusions.     

On the prediction of initiation life for fatigue crack emanating from small cold expanded holes

The cold expansion method is an effective method widely used for retarding the initiation of the fatigue crack. In the present work, the finite element analysis is performed to determine the residual stress distribution around the cold expanded hole and the ? ? N method is implemented to predict the fatigue crack initiation lives. The superposition of the mean stress is used to analyze the fatigue crack initiation. It is found that the effect of K_f, which decreases the K_t effect, must be taken into account for the small stop-hole. Estimates of the fatigue life quantities correlate well with the experimental results.     

The fatigue behavior of I-phase containing as-cast Mg–Zn–Y–Zr alloy

The fatigue behavior of as-cast Mg–12%Zn–1.2%Y–0.4%Zr alloy has been investigated. The S–N curve showed that the fatigue strength at 10⁷ cycles was 45 MPa. Scanning electron microscopy observations on the surfaces of the failed and unfailed specimens (after up to 1 × 10⁷ cycles) suggested that the slip bands could act as preferential sites for non-propagating fatigue crack initiation, and the I-phase could effectively retard fatigue crack propagation (FCP). The macro fracture morphology clearly indicated that the overall fracture surface was composed of three regions, i.e. a fatigue crack initiation region (Region 1), a steady crack propagation region (Region 2) and a tearing region (Region 3). High-magnification fractographs showed that only porosities can act as the crack initiation sites for all specimens. Moreover, for specimens with fatigue lifetimes lower than 2 × 10⁵ cycles, the cracks mostly initiated at the subsurface or surface of the specimen. However, when the fatigue lifetime was equal to or higher than 2 × 10⁵ cycles, the fatigue crack initiation sites transferred to the interior of the specimen. The maximum stress intensity factors corresponding to the transition sites between Regions 1, 2 and 3 were 2 and 4.2 MPa m^1/2, respectively. When the maximum stress intensity factor K_max was lower than 4.2 MPa m^1/2, in the steady crack propagation region, due to the retarding effect of I-phase/α-Mg matrix interfaces, the fatigue cracks tended to pass the I-phase/α-Mg matrix eutectic pockets directly and propagated through the grain cells, resulting in the formation of many flat facets on the fracture surface. However, when the maximum stress intensity factor was higher than 4.2 MPa m^1/2, in the sudden failure region, the rigid bonding of I-phase/α-Mg matrix interfaces was destroyed and the cracks preferentially propagated along the interfaces, which resulted in the fracture surface being almost completely composed of cracked I-phase/α-Mg matrix eutectic pockets. Based on microstructural observation and the fracture characteristics of the two regions, it is suggested that with an increase in crack tip driving force, the FCP mode changes from transgranular propagation to intergranular propagation.     

Thermomechanical fatigue behaviour of a third generation γ-TiAl intermetallic alloy

Thermomechanical fatigue (TMF) tests with loading conditions involving a combined thermal and mechanical strain cycle were carried out on a third generation γ-TiAl alloy with fine duplex microstructure. The experiments were conducted at a total strain amplitude of 0.6% in out-of-phase (OP) and in-phase (IP) modes. The temperature range was kept constant at 250 °C and the maximum temperatures applied were 700, 750 and 800 °C. The TMF results revealed that thermomechanical loading conditions must be considered as very crucial for the design of components made of γ-TiAl alloys. In particular, out-of-phase loading conditions reduce fatigue life tremendously as compared to the corresponding isothermal conditions. The very short out-of-phase loading life can be attributed to an accelerated oxidation-induced crack initiation during tension in the low-temperature part of the TMF cycle and is a consequence of the formation of an embrittled zone at the surface during the high-temperature part of the loading cycle.     

Thermomechanical fatigue behaviours of a third generation γ-TiAl based alloy

In order to clarify the behaviours of thermomechanical fatigue (TMF) of a third generation γ-TiAl based alloy, the influence of related microstructural instability during TMF process on stress–strain response, fatigue life and fracture way under in-phase (IP) and out-of-phase (OP) loading mode was investigated. Cyclic softening at high temperature (>700 °C) arises from the dissolution of α₂ lamellae and recrystallization of γ phase. Cyclic hardening at low temperature (<550 °C) is caused by strong interaction between dislocations. As temperature increases, the mean stress and remained plastic strain range increase, leading to severe TMF damage. Owing to the formation of superfine γ grains in IP condition, a superimposed effect of creep and fatigue damage contributes to the TMF failure. OP loading mode brings about the coarsening of primary equiaxed γ grains. Fatigue damage displays the intergranular fracture and transgranular cleavage fracture ways of coarse γ grains.     

Electrochemical short crack effect in environmentally assisted cracking of a steam turbine blade steel

Measurement has been made of the corrosion fatigue short crack growth rate in a 12Cr steam turbine blade steel subjected to low frequency trapezoidal loading in aerated and deaerated 300 ppb¹ Cl⁻ and 300 ppb ${\text{SO}}_4^{2-}$, simulating early condensate chemistry. No difference in growth rate compared to that for long cracks was observed in deaerated solution but significantly enhanced growth rate was obtained in aerated solution for a short crack of length less than 250 μm. Complementary stress corrosion cracking tests were conducted but to ensure crack development at modest applied stresses the environment adopted was aerated 35 ppm Cl⁻, representing a severe system upset. In this case, the growth rate of the short crack was up to 20 times higher than that for a long crack (>6 mm), even though the crack length had reached 1.6 mm. An explanation for both sets of data based on the difference in potential drop between a short and long crack is expounded.     

In situ TEM observations of fast grain-boundary motion in stressed nanocrystalline aluminum films

Free-standing nanocrystalline Al thin films have been strained in situ in a transmission electron microscope at room-temperature. Extensive grain-boundary migration accompanies the in situ loading and has been observed to occur preferentially at crack tips and only in the presence of the applied stress. This grain growth precedes dislocation activity, and measured boundary velocities are greater than can be explained by diffusive processes. The unambiguous observations of stress-assisted grain growth are compatible with recently proposed models for stress-coupled grain-boundary migration. The growth occurs in a faceted manner indicative of preferential boundaries. The fast collapse of small grains with sizes of 30–50 nm demonstrates the unstable nature of a nanocrystalline structure. Clearly observable shape changes testify to the effectiveness of grain-boundary migration as a deformation mechanism, and preferential grain growth at crack tips resulted in efficient crack tip blunting, which is expected to improve the films’ fracture toughness.     

Predicting fatigue crack initiation through image-based micromechanical modeling

The influence of individual grain orientation on early fatigue crack initiation in a four-point bend fatigue test was investigated numerically and experimentally. The 99.99% aluminium test sample was subjected to high cycle fatigue (HCF) and the top surface microstructure within the inner span of the sample was characterized using electron-beam backscattering diffraction (EBSD). Applying a finite-element submodelling approach, the microstructure was digitally reconstructed and refined studies carried out in regions where fatigue damage was observed. The constitutive behaviour of aluminium was described by a crystal plasticity model which considers the evolution of dislocations and accumulation of edge dislocation dipoles. Using an energy-based approach to quantify fatigue damage, the model correctly predicts regions in grains where early fatigue crack initiation was observed. The tendency for fatigue cracks to initiate in these grains appears to be strongly linked to the orientations of the grains relative to the direction of loading – grains less favourably aligned with respect to the loading direction appear more susceptible to fatigue crack initiation. The limitations of this modelling approach are also highlighted and discussed, as some grains predicted to initiate cracks did not show any visible signs of fatigue cracking in the same locations during testing.     

Measurement of deformations in SnAgCu solder interconnects under in situ thermal loading

Optical microscopy was used to discern the different grain orientations and grain boundaries on the polished cross-sections of near-eutectic lead-free board-level SnAgCu (SAC) solder interconnects. Strain distributions with submicron accuracy of the deformations on the cross-sections of the solder interconnects were measured when the package was subjected to thermal loading from room temperature to 100 °C. The results were correlated with the locations of different grains, grain boundaries and larger primary intermetallics. It revealed anisotropic nature of deformations in different grains of the SAC solder, which is similar to the thermomechanical behavior of pure Sn. The strain distribution in a solder interconnect varied significantly in different grain orientations. The primary intermetallics (Ag₃Sn plates) also behaved very differently from the surrounding Sn matrix under the thermal loading. The demonstrated strain localization along the grain boundaries and bigger primary intermetallics provides a clue for the path of fatigue crack growth that leads to a failure because of anisotropic thermomechanical response of SAC solder during thermal cycling.     

In situ observation of short fatigue crack propagation in oxygenated water at elevated temperature and pressure

A new method is demonstrated for in situ measurement of short crack propagation rates during a corrosion fatigue test of an austenitic stainless steel within a windowed autoclave at 250 °C, 50 atmospheres, oxygenated water. Digital image correlation is used to monitor crack growth through measurement of the opening displacements of defects; either focused ion beam (FIB) milled notches from which fatigue cracks initiated, or stress corrosion cracks that subsequently propagated by fatigue.     

Stress fields and geometrically necessary dislocation density distributions near the head of a blocked slip band

We have examined the interaction of a blocked slip band and a grain boundary in deformed titanium using high-resolution electron backscatter diffraction and atomic force microscopy. From these observations, we have deduced the active dislocation types and assessed the dislocation reactions involved within a selected grain. Dislocation sources have been activated on a prism slip plane, producing a planar slip band and a pile-up of dislocations in a near screw alignment at the grain boundary. This pile-up has resulted in activation of plasticity in the neighbouring grain and left the boundary with a number of dislocations in a pile-up. Examination of the elastic stress state ahead of the pile-up reveals a characteristic “one over the square root of distance” dependence for the shear stress resolved on the active slip plane. This observation validates a dislocation mechanics model given by Eshelby, Frank and Nabarro in 1951 and not previously directly tested, despite its importance in underpinning our understanding of grain size strengthening, fracture initiation, short fatigue crack propagation, fatigue crack initiation and many more phenomena. The analysis also provides a method to measure the resistance to slip transfer of an individual grain boundary in a polycrystalline material. For the boundary and slip systems analysed here a Hall–Petch coefficient of K = 0.41 MPa m^½ was determined.     

Quench sensitivity of toughness in an Al alloy: Direct observation and analysis of failure initiation at the precipitate-free zone

An analysis of toughness in 6156 Al–Mg–Si–Cu sheet was performed using enhanced Kahn tear tests on samples quenched at different rates. Crack initiation energies were hardly affected by changing the water quench temperature from 20 °C to 60 °C; however, a significant reduction was evident on air cooling. Crack propagation energy was reduced for both 60 °C water-quenched and air-cooled materials. Observation of failure initiation through synchrotron radiation computed tomography for the 60 °C water-quenched material revealed failure ahead of the crack tip of grain boundaries oriented at 45° to the main loading axis, and crack “tongues” extending into the material ahead of the main crack. Failure was predominantly intergranular. Fractographic assessment revealed predominantly voiding and shear decohesion in the 20 °C water-quenched material. With the aid of the new findings, past models on the influence of precipitate-free zone parameters on toughness were revised.     

Cyclic deformation of ultrafine-grained aluminum

Cyclic deformation of ultrafine-grained (UFG) Al with different grain sizes has been studied. It was found that UFG Al had shorter fatigue life than its coarse-grained counterparts. For UFG Al, the fatigue life decreases with decreasing grain size. Shear bands (SBs) shorten fatigue life. SBs are always inclined at 45° to the loading axis, and extend across the whole specimen. A SB is a thin sheet of tangled dislocations that have different Burgers vectors; its thickness is much less than the grain size. The strain–stress field inside a SB is very high. SBs produce shear steps, but not surface extrusions/intrusions, on the specimen surface. Thick shear bands (TSBs), about 200–300 μm, were found in the 6.36 μm grain size specimens, which also inclined 45° to the loading axis. TSBs consist of dislocation cells. The formation of TSBs does not reduce the fatigue life.     

On the fatigue behavior of γ-based titanium aluminides: role of small cracks

Gamma-TiAl based alloys have recently received attention for potential elevated temperature applications in gas-turbine engines. However, although expected critical crack sizes for some targeted applications (e.g. gas-turbine engine blades) may be less than ∼500 μm, most fatigue-crack growth studies to date have focused on the behavior of large (on the order of a few millimeters) through-thickness cracks. Since successful implementation of damage-tolerant life-prediction methodologies requires that the fatigue properties be understood for crack sizes representative of those seen in service conditions, the present work is focused on characterizing the initiation and growth behavior of small (a∼25–300 μm) fatigue cracks in a γ-TiAl based alloy, of composition Ti–47Al–2Nb–2Cr–0.2B (at.%), with both duplex (average grain size of ∼17 μm) and refined lamellar (average colony size of ∼145 μm) microstructures. Results are compared to the behavior of large (a>5 mm), through-thickness cracks from a previous study. Superior crack initiation resistance is observed in the duplex microstructure, with no cracks nucleating after up to 500 000 cycles at maximum stress levels (R=0.1) in excess of the monotonic yield stress, σ_y. Comparatively, in the lamellar microstructure cracks nucleated readily at applied maximum stresses below the yield stress (85% σ_y) after as few as 500 cycles. In terms of crack growth, measurements for small fatigue cracks in the duplex and lamellar microstructures showed that both microstructures have comparable intrinsic fatigue-crack growth resistance in the presence of small flaws. This observation contrasts previous comparisons of large-crack data, where the lamellar structure showed far superior fatigue-crack growth resistance than the duplex structure. Such “small-crack effects” are examined both in terms of similitude (i.e. crack tip shielding) and continuum (i.e. biased microstructural sampling) limitations of traditional linear elastic fracture mechanics.     

An intrinsic effect of hydrogen on cyclic slip deformation around a {1 1 0} fatigue crack in Fe–3.2 wt.% Si alloy

The effect of gaseous hydrogen on cyclic slip behavior around a fatigue crack tip introduced along the {1 1 0} plane in a Fe–3.2 wt.% Si alloy is precisely investigated by cross-sectional transmission electron microscopy and fractography. The results clearly suggest that the fatigue crack growth rate is promoted by hydrogen, whereas the number of dislocations emitted per load cycle is reduced. In addition, dislocation distribution is localized around the crack, causing quasi-brittle crack morphology. A sustained load test confirms that no subcritical crack growth caused by cleavage or micro-void coalescence exists along the {1 1 0} plane, which indicates that the observed increase in the fatigue crack growth rate is correlated solely to the intrinsic effect of hydrogen on the cyclic slip-off process around the crack tip.     

Investigation of fatigue crack initiation in Ti-6Al-4V during tensile-tensile fatigue

Fatigue crack initiation in Ti-6Al-4V has been investigated in high cycle fatigue (HCF) and low cycle fatigue (LCF) regimes at stress ratio R=0.1 using the replication technique. In all four tested α/β microstructures, the crack was initiated by fracture of equiaxed alpha grain. Fractured alpha grains are seen on the fracture surface as flat facets with features characteristics of cleavage fracture. In the regime of low stress amplitudes and in the absence of reverse loading, cleavage fracture contributes to crack initiation and early stages of crack growth in Ti-6Al-4V. This mechanism is discussed in relation to the anomalous mean stress fatigue behavior exhibited by this alloy.     

Study on dominant mechanism of high-cycle fatigue life in 6061-T6 aluminum alloy through microanalyses of microstructurally small cracks

The mechanism controlling the fatigue life of a precipitation-hardened Al–Mg–Si alloy (6061-T6) at a high-cycle fatigue (HCF) regime of over 10⁷ cycles was investigated in detail. It was found that over 90% of the total fatigue life was occupied by the growth process of a microstructurally small crack at relatively low stress amplitude. The small crack was often found to be arrested and halted for a long period (more than 10⁶ cycles) before it began to grow again, which resulted in a significantly slow growth process. The small crack was then analyzed not only by the conventional fractography but also by the cross-sectional observation of the crack tip region using a focused ion beam and transmission electron microscopy. These observations, supplemented also by a grain orientation analysis using electron backscattered diffraction, explicitly revealed the following points: (i) the small crack growth observed on the specimen surface is primarily related to facet-type cracking that occurs exclusively at the specimen surface; (ii) the growth direction of the small crack has strong anisotropy (i.e. surface-induced growth); (iii) the facet-type cracking is related to the formation of persistent fine slip bands that accompany no structural change of the matrix. On the basis of these results, the micromechanism of small crack growth and its relation to the concept of fatigue limit at the HCF regime is discussed in detail.