Intergranular Fatigue Cracking Research Articles

High-temperature high cycle fatigue performance of laser powder bed fusion fabricated Hastelloy X: Study into the microstructure and oxidation effects

High-temperature fatigue properties are critical for laser powder bed fusion (LPBF) fabricated Nickel-based superalloy Hastelloy-X (HX) used in aero engines. In this study, superior high-temperature high-cycle fatigue properties of LPBF HX were achieved. The detailed investigations show that the grain boundary oxidation promoted by the different deformation modes between neighboring grains, despite the reduced oxygen-related damage in the coarser LPBF HX microstructures, are the potential causes for the inter-granular fatigue crack initiation and the subsequent crack coalescence. In the meantime, the crack propagation could be hindered by the refined carbides and annealing twins with Σ3 misorientation (60°/ 〈111〉) in LPBF HX. Furthermore, the recrystallized grains formed ahead of the crack tip due to the severe deformation within the plastic zone and high testing temperature have different crystallographic orientations, which leads to the crack propagation path changes and increases the fatigue crack propagation resistance.

The Role of Extrusions and Intrusions in the Initiation and Intergranular Growth of Fatigue Cracks

The role of extrusions and intrusions in the initiation and growth of intergranular fatigue cracks is studied by performing strain‐controlled fatigue experiments on polycrystalline copper and by analyzing the development of intergranular crack initiation and resulting fracture surfaces. Fatigue cracks initiate either in the persistent slipbands within a grain or on the grain boundaries. The grain boundary initiation mechanism is due to the production of extrusions and intrusions on the grain boundaries that decrease the grain boundary cohesion forming submicroscopic crack nuclei. The alternating extrusions and intrusions are found also on the grain boundary facets of the fracture surface of a growing fatigue crack. Up to three slip systems are identified on the facets of cracked grain boundaries. Their form and height are found using scanning electron microscope observations and focused ion beam sectioning. A mechanism similar to intergranular grain boundary crack initiation is considered to explain the formation of grain boundary facets. A novel mechanism of intergranular fatigue crack growth is proposed based on the damaging effect of the extrusions and intrusions produced in the cyclic plastic zone of the growing fatigue crack.

Correlative, ML-based and non-destructive 3D-analysis of intergranular fatigue cracking in SAC305-Bi solder balls

Reliable connections of electrical components embody a crucial topic in the microelectronics and power semiconductor industry. This study utilises 3D non-destructive X-ray tomography and specifically developed machine learning (ML-) algorithms to statistically investigate crack initiation and propagation in SAC305-Bi solder balls upon thermal cycling on board (TCoB). We quantitatively segment fatigue cracks and flux pores from 3D X-ray tomography data utilising a multi-level ML-workflow incorporating a 3D U-Net model. The data reveals that intergranular fatigue cracking is the predominant failure mechanism during TCoB and that dynamic recrystallisation precedes crack initiation. Moreover, we find that fatigue cracks are initiated at surface notches, flux pores and printed circuit board-metallisation intrusions. The work provides important insights regarding the underlying microstructural and mechanical mechanisms for recrystallisation and cracking, uniting the aspects of big-data analysis with ML-algorithms and in-depth understanding about the underlying materials science.

Mechanism of intergranular fatigue crack growth in copper polycrystal

Polycrystalline copper specimens (notched and smooth) were subjected to constant strain amplitude loading and the fatigue fracture surface was inspected and analyzed. The majority of facets on the fracture surfaces indicated intercrystalline fracture. Facets with well-developed relief were inspected. The relief was formed by persistent slip markings consisting of extrusions and intrusions similar to that on the specimen surface in the cyclic plastic zone of the growing crack. The mechanism of grain boundary cracking in the cyclic plastic zone of the growing crack similar to that recently suggested for intergranular crack initiation was drawn up. The crack growth mechanism based on the interconnection of cracked grain boundaries and the tear of the islands between them was proposed. The effect of grain size, orientation, and crack growth rate on the grain boundary cracking was discussed. Preliminary results on other FCC materials indicate a more general validity of the proposed intergranular fatigue crack growth mechanism.

Extrusions and intrusions in the cyclic plastic zone of the growing fatigue crack in polycrystalline copper

Surface relief in the tip and along the path of a growing fatigue crack in polycrystalline copper has been observed and analyzed. A crack started from a semicircular notch in a prismatic specimen cycled with constant strain amplitude. In the notch tip and along the crack path on both sides of the specimen the surface relief of the grains was characterized by a high density of the PSMs belonging to two, up to three slip systems. Extrusions and intrusions of these PSMs intersect forming specific surface relief. Similar surface relief, corresponding to the grain boundary surface, has arisen on the fracture surface. It means that in the persistent slip bands inside the crystal, the material is pushed towards the neighboring grains or removed from the grain boundary (analogically to extrusions and intrusions on the surface). This mass transfer locally increases stresses on the grain boundaries creates discontinuities and weakens grain boundaries. Such novel mechanism of intergranular fatigue crack growth is discussed. The suggestion of an intergranular crack growth mechanism agrees with the predominantly intergranular crack path of the growing fatigue crack.

Effect of texture evolution and defect on the crack initiation of a Ni‐based superalloy electron beam welded joint

AbstractElectron beam welding (EBW), a high‐quality and precise joining process, is widely used in aerospace manufacturing. In this study, through in situ scanning electron microscope (SEM) fatigue experiments and X‐ray computed tomography (CT) scanning technology, we revealed the fatigue crack initiation and propagation mechanism in GH4169 superalloy EBW joints under different post‐welding heat treatment conditions. The welded pores have a decisive effect on the fatigue properties of direct aging treatment welded joints. Fatigue cracks initiate from pore defects on the surface or in the subsurface. Due to the heterogeneous microstructure in weld seam, fatigue cracks tend to grow along the primary dendrite axis and secondary dendrite arms. Besides, in homogenization and aging treatment specimen, intergranular fatigue cracks tend to nucleate in the slip zone of large grains and twin boundaries. Although homogenization treatment can eliminate segregation, it also results in significant grain growth in both weld seam and base material.

On improved fatigue properties of aluminum alloy 5086 weld joints

In this work, we aimed to investigate the fatigue properties of AA5086-H321 and its weld joints. The AA5086-H321 plates were welded in a butt configuration using the friction stir welding. Load-controlled axial sinusoidal cyclic tests were conducted in the ambient conditions, at a frequency 20 Hz, and at different load ratios i.e., Rσ=-1, 0.1, and 0.5. The weld joints, in general, are known to have reduced properties. However, in this study, the base alloy and its weld joint had similar hardness and ultimate tensile strength, but with marginally reduced elongation (∼4.4%). The endurance limits of the weld joints were either higher (∼28% and ∼11% at R=-1 and 0.1, respectively) or same (at R=0.5) as compared to that of the base alloy. The difference in fatigue lives of base alloy and its weld joint kept diminishing with increasing R-ratio. Microscopic examination of fracture surfaces in weld joints revealed three types of crack initiations: voids, inclusions, and tool-marks, and two types of crack propagation mechanisms: intergranular and transgranular. However, in base alloys, only defect-free surface initiation with transgranular crack growth was observed. Mean stress, as expected, had a detrimental effect on fatigue properties, with no definitive control on the type of crack initiation or its propagation mechanism. An increase in the fatigue lives of weld joints was attributed to the crack growth retardation during the intergranular fatigue crack propagation due to frequent defection of crack path, which was absent in the base alloys. Most of the intergranular facets were observed near the surfaces (top or bottom) of the weld nugget zone and disappeared when the crack front reaches its interface boundary with the heat affected zone. All these aspects of fatigue damage in microstructurally heterogeneous weld joints and the plausible explanations for the observed mechanical fatigue properties are presented in this study.

Microstructure-based intergranular fatigue crack nucleation model: Dislocation transmission versus grain boundary cracking

Intergranular fatigue crack nucleation has been reported to occur commonly at high angle grain boundaries (HAGBs) but seldom at low angle GBs (LAGBs) during persistent slip band (PSB)-GB interactions. However, the mechanistic understanding of the role of GB misorientation angles in affecting the GB fatigue cracking remains limited due to the lack of quantitative descriptions. Here a theoretical framework based on the competition between dislocation transmission across GB and GB cracking is established for investigating the GB fatigue crack nucleation. We show that, HAGBs usually provide higher resistances to dislocation transmissions compared with LAGBs, causing a more significant dislocation pile-up and stress concentration, which facilitates the GB crack nucleation. But the quantitative analysis indicates that even at HAGBs with poor plastic compatibility, the critical stress of GB cracking is much larger than the critical transmission stress. This demonstrates the crucial role of GB fatigue damage accumulation, which is associated with PSB extrusion growth, in promoting GB crack nucleation. In this study, the critical stress of dislocation transmission is derived in terms of the energy balance, and the nucleation criterion of GB crack is formulated using the fracture theory, both of which display the dependence on GB misorientation angles and grain sizes. It is found that the GB fatigue cracks preferentially nucleate at HAGBs with poor plastic compatibility and large grain sizes. The competition between dislocation transmission and GB cracking at varying fatigue cycles are also discussed. This theoretical framework offers quantitative analyses for the GB crack nucleation during PSB–GB interactions, clarifies the role of microstructures in affecting the critical cycles of GB cracking, and might offer important guidelines for designing fatigue-resistant metallic materials.

Hydrogen-assisted intergranular fatigue crack initiation in metals: Role of grain boundaries and triple junctions

In this work, small-scale, low-cycle fatigue experiments on hydrogen charged nickel specimens are performed that highlight particular grain boundaries (GBs) and triple junctions as potential intergranular crack initiation sites. To understand the micromechanics and underlying physics, a dislocation density-based crystal plasticity model coupled with slip-rate based hydrogen transport model is developed. A fatigue indicator parameter (FIP) is also developed that models the crack initiation process by considering the contributions of accumulated plastic slip, GB normal stress, and local hydrogen concentration. Depending on the diffusivity, hydrogen binding energy, and misorientation, GBs are categorized as ‘special’ or ‘random’, and their role on hydrogen distribution is analysed using a model microstructure. Special GBs are ones with low diffusivity and high hydrogen binding energy whereas the random GBs have high diffusivity but low hydrogen binding energy. Complying with the experimental observations, the evolution of FIP with load cycles suggests certain triple junction configurations in the microstructure involving in the crack initiation process. For the case of uniform initial hydrogen concentration, special GBs are found to retain more hydrogen with load cycles primarily due to their low hydrogen diffusivity whereas the random GBs diffuse hydrogen out quickly to the bulk. The high hydrogen concentration and favourable stress state in the form of high hydrostatic stress spots generally found at triple junction of special/random GBs fulfil the necessary condition leading to an intergranular crack initiation.

Mechanisms of intergranular fatigue crack initiation in polycrystalline materials

Fatigue failure in polycrystalline materials occurs by initiation and propagation of fatigue cracks. Fatigue cracks can initiate transgranularly or intergranularly. While considerable progress has been achieved in the explanation of the mechanism of transgranular fatigue crack initiation intergranular initiation is still the subject of controversy. A recent study of the crack initiation in superaustenitic Sanicro 25 steel [1] revealed the important role of persistent slip markings (PSMs) in the grains adjoining the grain boundary. A novel mechanism of intergranular fatigue crack initiation has been proposed. Provided the cyclic slip cannot proceed from one grain to the neighbour one, extrusions and intrusions arise not only on the surface but also on the grain boundary. Growing intrusions represent void-like defects and growing intrusions push neighbour grain apart. A systematic study of intergranular cracking has been performed on polycrystalline copper cycled with high and low strain amplitudes. SEM observations of the early stages of fatigue crack initiation on the grain boundaries, EBSD study of grains orientation, FIB cutting of the grain boundaries and simultaneous documentation of the effect of extrusions and intrusions on the grain boundary cohesion are reported simultaneously with the conditions favourable for the initiation of the fatigue cracks.

High cycle fatigue behavior of high-Mn austenitic steel with deformation twins and twin boundary migration at 77 K

The high-Mn austenitic steel exhibits better fatigue performance at 77 K than at RT. Especially when the stress amplitude is in the range of yield strength (YS) to 1.08YS, the fatigue life decreases slightly at 77 K, while decreases significantly at RT. Lowering the stacking fault energy and increasing the stress amplitude at 77 K promotes twinning, which is responsible for improving the tolerance of fatigue damage and cracking delay. Surprisingly, with the stress amplitude increases to 1.04YS and 1.08YS, migration of twin boundaries and detwinning are observed, respectively, which can enhance the resistance against intergranular fatigue cracking.

Hydrogen-accelerated fatigue crack growth of equiatomic Fe–Cr–Ni–Mn–Co high-entropy alloy evaluated by compact tension testing

We investigated the effect of hydrogen on the resistance to mechanically long fatigue crack growth in an equiatomic Fe–Cr–Ni–Mn–Co high-entropy alloy using compact tension (CT) tests at a frequency of 1 Hz and room temperature (20 °C). The CT test specimens were hydrogen charged using 100 MPa of hydrogen gas prior to testing. Fatigue crack growth progressed three times faster after exposure to hydrogen gas. The major change caused by hydrogen charging was the occurrence of intergranular crack growth. Striation features were observed in the intergranular fracture region, indicating that intergranular crack growth occurred via a plasticity-driven cycle-by-cycle mechanism. More specifically, we propose that the dislocation emission from the grain boundaries at a crack tip controls the intergranular fatigue crack growth.

Intergranular fatigue crack initiation in polycrystalline copper

Polycrystalline copper specimens were fatigued in strain-controlled regime with high and low strain amplitude. Stress amplitude, plastic strain amplitude and hysteresis loops were recorded. The loop shape parameter reflecting the onset and evolution of the localization of cyclic plastic deformation was evaluated. Mechanism of intergranular fatigue crack initiation was studied on the electrolytically polished specimen surfaces using scanning electron microscopy coupled with focused ion beam. Intergranular cracks had initiated already at 4% of the fatigue life. Corrugated surface of grain boundary cracks close to the surface of the grains was closely related to the occurrence of the extrusions and intrusions produced on the grain boundaries. The observations give further support to the novel mechanism of grain boundary crack initiation based on the emergence of extrusions and intrusions on the grain boundary surface. Factors affecting the initiation of grain boundary cracks were identified.

Fatigue crack initiation behaviors around defects induced by welding thermal cycle in superalloy IN617B

In this study, fatigue crack initiation behaviors around defects induced by welding thermal cycle in IN617B were investigated by in-situ fatigue tests combined with DIC (Digital Image Correlation) and EBSD (Electron Back Scattering Diffraction) characterization. Two mechanisms of fatigue crack initiation around defects were clarified and the distance of crack initiation position from defects was proposed to determine which mechanism was dominant. Intergranular fatigue crack initiation caused by pile-up of dislocations was dominant in the far-field region from defects, while intragranular fatigue crack initiation caused by cracking at the interface between dislocation bands and matrix was dominant in the near-field region from defects. Welding defects altered the fatigue crack initiation mechanism by changing the deformation mode of the near-field region, thereby accelerating the fatigue failure process of IN617B alloy.

Low cycle fatigue and stress relaxation behaviours of powder metallurgy Ni-based superalloy FGH4098

In this study, the effects of microstructure, temperature and dwell time in a loading cycle on low cycle fatigue (LCF) and stress relaxation behaviours of a powder metallurgy Ni-based superalloy (i.e. FGH4098) for aeroengine turbine disc application were studied along with detailed microscopic characterisation of fracture features and deformation substructures. The results indicate that the LCF failure mode of FGH4098 transits from transgranular to intergranular with the increase of temperature and dwell time, and the propensity of intergranular fatigue cracking is higher in fine-grained FGH4098. FGH4098 predominantly shows cyclic hardening behaviour which is mainly related to dislocation interactions. With the increase of temperature and/or dwell time, the cyclic hardening behaviour diminishes to some extent due to the increasing shearing of secondary γ′ precipitates by stacking fault and partial dislocation pairs. Similarly, stress relaxation in FGH4098 is also associated with the shearing of γ′ precipitates and becomes more prominent at high temperature with long dwell period. Micro-twinning may also contribute to stress relaxation at 750 °C.

Near-Threshold Fatigue Crack Growth Behavior of ECAPed Ultrafine-Grained Copper

Fatigue crack growth resistance of ultrafine grained Cu processed by equal channel angular pressing (ECAP) was investigated. Particular emphasis was devoted to the effects of microstructure evolution on fatigue crack growth in the near-threshold regime. The ultrafine grained Cu exhibits a lower fatigue threshold than coarse-grained Cu at stress ratios of 0.1 and 0.7. Fatigue induced coarsening of the UFG structure near the fatigue crack and intergranular fatigue crack growth are observed.

Towards the prediction of intergranular fatigue crack initiation in metals due to hydrogen

Hydrogen deteriorates the fatigue behaviour of metals by attacking their microstructure in different ways. For better clarity about hydrogen-based degradation of metals, it is vital to identify the microstructural configurations that promote fatigue crack initiation (FCI) in the presence of hydrogen. Intergranular regions in polycrystalline metals are more prone to hydrogen attack due to the prevailing atomic structure, elastic anisotropy, and plastic inhomogeneities causing increased hydrogen accumulation in this region. In this work, FCI is studied in a model material nickel in the uncharged and hydrogen charged state during in-situ strain controlled low cycle fatigue (LCF) testing under a scanning electron microscope (SEM). Crack initiation sites are characterized by investigating the elastic modulus in the loading direction as well as the maximum Schmid factor of the crack neighbouring grains extracted through the electron backscattered diffraction (EBSD) data. The crack frequency for the uncharged and hydrogen charged specimens is then analyzed using the difference in the elastic modulus (ΔE), the difference in the maximum Schmid factor (Δm), and ΔE/Δm ratio between the crack neighbouring grains. The comparison shows that for the hydrogen charged specimens, intergranular FCI sites show high values of ΔE/Δm compared to the uncharged specimens. These findings provide a predictive model for hydrogen linked FCI in metals. In addition, the synergistic role of the Hydrogen Enhanced Local Plasticity (HELP) mediated Hydrogen Enhanced Decohesion (HEDE) mechanism responsible for FCI is also demonstrated.

Effect of machine scatter on the rotating bending fatigue life of materials

AbstractRotating bending fatigue test results vary from specimen to specimen even if materials are tested in a standard laboratory set up because rotating bending fatigue life or fatigue strength depends upon various factors due to its dynamic action. One method of testing may be machine scatter (offset and angular misalignment). Angular misalignments are unavoidable in dynamic machines due to continuous operation leading to transverse vibrations in specimens. The magnitude of transverse vibrations was measured manually by dial gauge arrangement. Experiments were carried out until fracture either with smooth running conditions or with the effects of offset and angular misalignment. The enhanced effect of misalignments on fatigue life was done through the histogram normal distribution technique. There was a clear distinction between the appearances of the fracture surfaces between the specimens subjected to smooth running conditions and specimen vibration, which reveals general fatigue fracture phenomena and intergranular fatigue cracks, larger decohesion of graphite nodules and isolated cleavage facets. The manual technique used without any resistance strain gauges, as they were not efficiently sustainable under repetitive loads, helped in understanding the dynamic machine scatter on fatigue life in an economical way where there were lesser concentrations in these areas.

Influence of gaseous hydrogen on plastic strain in vicinity of fatigue crack tip in Armco pure iron

A multi-scale characterisation of the crack tip plasticity has been investigated in a fatigue crack propagation under gaseous hydrogen at gas pressure of 35 MPa in a commercially pure iron, Armco iron. The dislocation structure beneath a fracture surface was observed by a Transmission Electron Microscopy(TEM), and the cyclic and monotonic plastic zones were evaluated by an Out-of-Plane Displacement (OPD) measurement. By the TEM observation in a non-accelerated regime (ΔK = 11 MPa×m1/2), a dislocation cell structure was observed even in the brittle intergranular fracture in hydrogen. This result indicates a certain amount of plastic strain is introduced into the grains in front of an intergranular crack in hydrogen, and this may explain the mechanism of hydrogen-induced intergranular fatigue crack propagation. On the other hand, in an accelerated regime (ΔK = 18 and 20 MPa×m1/2), a distribution of scattered dislocation tangles without any cell or vein structure was observed in hydrogen. Besides, the inhibition of the cyclic plasticity near the crack path in hydrogen was confirmed by the OPD measurement. These results are clear evidences of hydrogen-induced localization of cyclic plasticity in the vicinity of a crack tip, which suggests a mechanism model of hydrogen-enhanced fatigue crack growth based on the plasticity localization.

Characteristics of hydrogen-assisted intergranular fatigue crack growth in interstitial-free steel: role of plastic strain localization

The behavior of intergranular fatigue crack growth in an interstitial-free (IF) steel in a hydrogen environment was investigated at different frequencies. Focusing on the plastic strain localization, we observed details of the striation-like feature on the intergranular fracture surface, slip behavior around microvoids, and crystallographic orientation gradient underneath the fracture surface. It was determined that the intergranular fatigue crack growth mechanism in the IF steel is microvoid formation at the crack tip and subsequent coalescence with the crack. Moreover, it was found that the grain boundaries, acting as propagation paths, suffer from pre-damage arising from plastic strain localization near the grain boundaries even before the main crack propagates to a certain location. Therefore, fatigue cracks in a hydrogen environment easily propagate to the grain boundaries. The frequency dependence of fatigue crack growth in the hydrogen environment is significantly smaller than that in a low carbon steel, probably because of the frequency dependence of the pre-damage evolution behavior.