Polycrystalline copper (99.9%) was fatigued at a total strain amplitude of 0.1 and 0.2%, respectively. The tests were performed in situ under vacuum in a Large Chamber-Scanning Electron Microscope. By a repeated combination of in situ fatigue testing and ex situ focused ion beam milling, a deep insight into the mechanism of fatigue crack initiation and early stages of crack initiation at persistent slip bands (PSBs) and their interaction with grain boundaries was obtained. The EBSD-technique showed early slip activation and the exclusive formation of extrusions in favorably oriented grains until a certain extrusion height was reached. At the total strain amplitude of 0.2%, extrusions are formed not only in favorably oriented grains but also in grains with a lower Schmid factor due to high compatibility stresses at the grain boundaries. Extrusion growth through grain boundaries is affected by the orientation of the primary slip systems in the neighboring grains and the additional anisotropy stresses. It is concluded that early stages of crack initiation are the consequence of the formation of extrusions at PSBs in combination with the clustering of vacancies along the PSB boundaries, as it was proposed by the well-known Essmann–Gösele–Mughrabi model.

Titanium–tantalum based alloys can demonstrate a martensitic transformation well above 100 °C, which makes them attractive for shape memory applications at elevated temperatures. In addition, they provide for good workability and contain only reasonably priced constituents. The current study presents results from functional fatigue experiments on a binary Ti–25Ta high-temperature shape memory alloy. This material shows a martensitic transformation at about 350 °C along with a transformation strain of 2 pct at a bias stress of 100 MPa. The success of most of the envisaged applications will, however, hinge on the microstructural stability under thermomechanical loading. Thus, light and electron optical microscopy as well X-ray diffraction were used to uncover the mechanisms that dominate functional degradation in different temperature regimes. It is demonstrated the maximum test temperature is the key parameter that governs functional degradation in the thermomechanical fatigue tests. Specifically, ω-phase formation and local decomposition in Ti-rich and Ta-rich areas dominate when T max does not exceed ≈430 °C. As T max is increased, the detrimental phases start to dissolve and functional fatigue can be suppressed. However, when T max reaches ≈620 °C, structural fatigue sets in, and fatigue life is again deteriorated by oxygen-induced crack formation.

Selective laser melting, a laser-based additive manufacturing process, can manufacture components with good geometrical integrity. Application of the selective laser melting process for serial production is subject to its reliability on mechanical properties, especially on fatigue behavior, when it is required to be applied for dynamic applications. This study focuses on microstructural, quasistatic, high cycle fatigue (HCF), and very high cycle fatigue (VHCF) mechanisms of aluminum alloys manufactured by selective laser melting. Manufacturing of hybrid structures by selective laser melting process is also investigated. Microstructural features were investigated for process-induced effects and the corresponding influence on quasistatic and fatigue properties. The microstructural features can be controlled in the selective laser melting process for required properties. Joining strengths in hybrid structures can be improved by post process heat-treatments. Material constants in different fatigue regions were determined, and higher fatigue strength of hybrid alloys was achieved in HCF as well as VHCF regimes.

Surface treatments such as shot peening, deep rolling, or nitriding are known to be very effective for the protection of a surface against fatigue crack initiation, due to surface hardening and residual compressive stresses introduced below the surface. Thus, crack initiation of cyclically loaded materials occurs predominantly at internal nonmetallic inclusions (NMIs). Two different plasma-nitriding treatments were performed on a quenched and tempered 42CrMo4 cast steel. Ultrasonic fatigue tests were performed up to 109 cycles. Resonant frequency and the nonlinearity parameter were recorded in situ during the fatigue tests. Fractographic analyses were performed by means of scanning electron microscopy in combination with energy-dispersive X-ray spectroscopy. The results showed that nitriding, as expected, led to improvements in both fatigue life and rates of internal crack initiation at NMIs. However, the analysis of in situ parameters revealed that internal crack initiation occurred at stress amplitude levels well below the failure stress amplitude even for repeated loading until the run-out limit of 109 cycles.

Structural instabilities of nanocrystalline and ultrafine-grained (UFG) materials have been recognized as a major challenge during cyclic loading, especially in the low cycle fatigue regime. Although a severe deterioration of the mechanical properties has been reported during cyclic deformation, quantification of the softening portion solely due to grain coarsening was not possible. It will be demonstrated that cyclic high pressure torsion (CHPT) is a versatile method to enable direct measurement of the impact of grain coarsening on cyclic softening, as failure of the sample is prevented. Here, CHPT experiments have been performed on 99.99% UFG nickel. Grain coarsening similar to conventional uniaxial fatigue experiments was observed and could be studied up to large cyclic accumulated macro strains of 50. The correlation of electron back scatter diffraction images with microhardness measurements facilitated quantification of the cyclic softening as a consequence of grain growth for the very first time. Further, structural investigations revealed distinctly enhanced grain coarsening within shear bands. Thus, the cyclic strain seems to be the most important parameter controlling mechanically driven boundary migration during cyclic loading at low homologous temperatures.

Atomistic simulations are carried out to analyze the influence of oxygen environment on nickel and copper surface roughness and notch initiation. The early stages of oxidation of nickel and copper surfaces are first simulated and compared with experimental observations. Various oxygen superstructures observed on metal surfaces are reproduced as well as the nucleation of small NiO embryos. Nickel and copper surface oxidation mechanisms are different and different “oxide” nano layers are formed. None of these superficial nano layers has a major influence on the mechanical behavior of surface slips as they do not change the surface roughness fatigue evolution and micro-notch production. These atomistic results agree with experimental studies which report similar development of persistent slip band surface relief in inert and in air environment. A general model for the estimation of surface slip irreversibility is also provided and the models of environment-assisted surface relief evolution and microcrack initiation are revisited.

The origin of the extraordinary strengthening of the highly alloyed austenitic stainless steel Sanicro 25 during cyclic loading at 700 °C was investigated by the use of advanced scanning transmission electron microscopy (STEM). Along with substantial change of the dislocation structure, nucleation of two distinct populations of nanoparticles was revealed. Fully coherent Cu-rich nanoparticles were observed to be homogeneously dispersed with high number density along with nanometer-sized incoherent NbC carbides precipitating on dislocations during cyclic loading. Probe-corrected high-angle annular dark-field STEM imaging was used to characterize the atomic structure of nanoparticles. Compositional analysis was conducted using both electron energy loss spectroscopy and high spatial resolution energy dispersive X-ray spectroscopy. High-temperature exposure-induced precipitation of spatially dense coherent Cu-rich nanoparticles and strain-induced nucleation of incoherent NbC nanoparticles leads to retardation of dislocation movement. The pinning effects and associated obstacles to the dislocation motion prevent recovery and formation of the localized low-energy cellular structures. As a consequence, the alloy exhibits remarkable cyclic hardening at elevated temperatures.

The near-threshold behavior of long cracks is studied in this paper using precracked flat dogbone specimens of a commercial aluminum alloy in peak-aged and overaged conditions. After introducing the initial crack in compression precracking, the crack was propagated approximately with the constant range of the stress intensity factor at values just above or below the corresponding threshold values. It was found that there were two major mechanisms which kept the crack from continuous extension. First, the crack front was pinned by primary precipitates. This effect was rather pronounced and lead to significant kinking in the crack front and ductile ridges on the fracture surface. The second mechanism was shear-controlled crack extension of very long cracks with plastic zones ahead of the crack tip, very similar to stage-I small cracks. Interaction with primary precipitates deflected the shear-controlled cracks but did not change the crack extension mode.

Complex wrought magnesium-based alloys suffer from poor ductility, strong yield asymmetry, and lower than desired fatigue performance. These unfavourable properties are exacerbated by the heterogeneity of the microstructure and strong texture forming in Mg alloys during conventional thermo-mechanical processing. For the user, severe plastic deformation (SPD) increases flexibility in tailoring the microstructures and selecting the properties to be emphasized in wrought Mg alloys. The effect of SPD by hot multiaxial forging and equal channel angular pressing on the formation of fine grain microstructure and on resultant mechanical properties is discussed. It is demonstrated that SPD is capable of substantial enhancement in ductility and tensile strength which gives rise to concurrent improvement of both low- and high-cycle fatigue properties. The main message of this overview is that the full potential for improving fatigue performance of Mg alloys can be taken advantage of by way of comprehensive understanding the role of the individual effects associated with the SPD-induced microstructures and textures.

Fatigue is an important mechanism for the failure of components in many engineering applications and a significant proportion of the fatigue life is spent in the crack initiation phase. Although a large number of research work addresses fatigue life and fatigue crack growth, the problem of modeling crack initiation remains a major challenge in the scientific and engineering community. In the present work, a micromechanical model is developed and applied to study fatigue crack initiation. In particular, the effect of different hardening mechanisms on fatigue crack initiation is investigated. To accomplish this, a model describing the evolution of the particular dislocation structures observed under cyclic plastic deformation is implemented and applied on randomly generated representative microstructures to investigate fatigue crack initiation. Finally, a method is presented to calculate the S–N curve for the polycrystalline materials. With this work, it is demonstrated how the micromechanical modeling can support the understanding of damage and failure mechanisms occurring during fatigue.

Two austenitic stainless steels of strongly different stacking fault energies (SFEs) and correspondingly different stabilities of the austenite phase were studied with respect to their very high cycle fatigue (VHCF) behavior. The metastable austenitic stainless steel 304L shows a very pronounced transient behavior and a fatigue limit in the VHCF regime. The higher SFE of the 316L steel results in a less pronounced transient cyclic deformation behavior. The plastic shear is more localized, and the formation of deep intrusions leads to microcrack initiation. However, the propagation of such microcracks is impeded by α′-martensite formed very localized within the shear bands. A comprehensive description of the microstructural changes governing the cyclic deformation including the transient resonant behavior was developed and transferred into a mechanism-based model. Simulation results were correlated with the observed deformation evolution and the change of the resonant behavior of specimens during VHCF loading providing a profound understanding of the VHCF-specific deformation behavior.

Fatigue is one of the major failure modes of structural materials. While the effects of strengthening precipitates on the mechanical properties of heat treatable aluminum alloys during forming operations are well-studied, only little is known about the related mechanisms during fatigue. We study the influence of precipitates during low cycle fatigue of an Al–Si–Mg alloy by mechanical testing and microstructure characterisation using (scanning) transmission electron microscopy. Specifically, we have investigated under-aged, peak-aged, and over-aged precipitation states. The experiments reveal considerable influence of the precipitate state on the mechanical properties and the formed dislocation structures. Under-aged AA6016 experiences cyclic hardening accompanied by dynamic precipitation and precipitate growth during cyclic deformation, whereas peak-aged AA6016 shows a saturated cyclic stress behavior and the formation of a ‘prevein’-like dislocation structure aligned along [001]Al directions. Over-aged AA6016 exhibits cyclic softening, which is assumed to be due to frequent Orowan-looping of dislocations around incoherent precipitates.

The ferritic steel 16Mo3 coated with the nickel-base alloy IN625mod by high-velocity oxy-fuel (HVOF) spraying was investigated under uniaxial and biaxial fatigue loading at 200 and 500 °C. Furthermore, bulk HVOF-sprayed specimens of the coating material IN625mod were also investigated under uniaxial isothermal fatigue loading at 200 and 500 °C. Moreover, the thermo-mechanical fatigue behavior of 16Mo3 was studied under in-phase (IP) and out-of-phase (OP) loading between 200 and 500 °C. The fatigue lives of the bulk coating and the compound material are presented. In particular, the thermo-mechanical OP loading leads to a strong reduction of the lifetimes compared to the IP loading. A conservative estimation of the fatigue lives of the thermo-mechanical loading can be given by isothermal tests at 500 °C. The comparison of the uniaxial loading with the biaxial loading cases shows reasonable coincidence by using the distortion energy hypothesis according to von Mises.

Shot-peened CM400 maraging steel was used to study the mechanism of enhanced notch fatigue properties of ultra-high strength materials. After shot peening, the specimen surface became rougher, but the transversal machining traces were reduced. The yield strength was slightly improved while the ultimate tensile strength and hardness maintained constant; as a result, the fatigue limit was promoted by about 1.5 times. The nucleated sites of the fatigue fracture were partly changed from the surface to subsurface/interior of the specimen. To further analyze the influencing factors of fatigue properties, the fatigue damage process may be resolved to two aspects: (a) fatigue damage rate affected by shear deformation and (b) fatigue damage tolerance controlled by the dilatation fracture process. Considering the stress state near the notch tip, the hydrostatic stress and maximum shear stress are considered for better understanding these two aspects. It is observed that the fatigue damage tolerance increased while the fatigue damage rate decreased after shot peening. Therefore, the notch fatigue properties of CM400 maraging steels can effectively be improved.

The present work addresses the competition between dislocation plasticity and stress-induced martensitic transformations in crack affected regions of a pseudoelastic NiTi miniature compact tension specimen. For this purpose X-ray line profile analysis was performed after fracture to identify dislocation densities and remnant martensite volume fractions in regions along the crack path. Special emphasis was placed on characterizing sub fracture surface zones to obtain depth profiles. The stress affected zone in front of the crack-tip is interpreted in terms of a true plastic zone associated with dislocation plasticity and a pseudoelastic zone where stress-induced martensite can form. On unloading, most of the stress-induced martensite transforms back to austenite but a fraction of it is stabilized by dislocations in both, the irreversible martensite and the surrounding austenite phase. The largest volume fraction of the irreversible or remnant martensite along with the highest density of dislocations in this phase was found close to the primary crack-tip. With increasing distance from the primary crack-tip both, the dislocation density and the volume fraction of irreversible martensite decrease to lower values.

In this paper, we report on the microstructural evolution and mechanical properties of a 5052 Al alloy processed by rotationally accelerated shot peening (RASP). A thick deformation layer of ∼2 mm was formed after the RASP process. Nano-sized grains, equiaxed subgrains, and elongated subgrains were observed along the depth of the deformation layer. Dislocation accumulation and dynamic recrystallization were found primarily responsible for the grain refinement process. An obvious microhardness gradient was observed for all of the samples with different RASP processing parameters, and the microhardness in the top surface of 50 m/s-5 min RASP-processed sample is twice that of its coarse-grained (CG) counterpart. The yield strengths of the RASP-processed 5052 Al alloy samples were 1.4–2.6 times that of CG counterparts, while retaining a decent ductility (25–84% that of CG). The superior properties imparted by the gradient structure are expected to expand the application of the 5052 Al alloy as a structural material.

In the present work, specimens of the metastable austenitic stainless steel AISI 347 with different surface morphologies were investigated in stress-controlled fatigue tests in the high cycle fatigue (HCF) regime at ambient temperature. Specific surface morphologies were generated by cryogenic turning with CO2 snow cooling. As a result of the metastable austenite microstructure, phase changes from paramagnetic austenite to ferromagnetic martensite take place in the near-surface regime during cryogenic turning as well as in the whole specimen volume during monotonic and/or cyclic elastic–plastic deformation. The metastability of AISI 347 was characterized according to the M S-temperature determined from the chemical composition and by X-ray diffraction measurements with in situ cooling. Microhardness and strength of both phases were measured. Near-surface microstructure was analyzed by optical and scanning electron microscopy after focused ion beam preparation. Besides a partially martensitic surface layer, a thin nanocrystalline layer, both induced by cryogenic turning, was observed. In case of cyclic loading, the martensitic surface layer leads to a reduction of plastic strain amplitude as well as a retardation of crack initiation and consequently to an increase in fatigue life.