Interior Of Sample Research Articles

Elucidating the microstructure evolution and performance response of wire-arc directed energy deposited Al–Zn–Mg–Cu alloy at as-deposited and post-processing heat treatment states

Wire-arc directed energy deposition technology was employed to manufacture ultra-high-strength Al–Zn–Mg–Cu alloy taking advantage of variable polarity cold metal transfer (CMT) process. The grain characteristics and precipitation behavior under as-deposited (AD) and post-processing heat treatment (PHT) states were comparatively analyzed. The relationship between the microstructure evolution and mechanical performances was fully disclosed. The obtained results show that the grain structure of AD sample exhibited a nearly fully-equiaxed feature (average size: ∼45 μm), which was caused by the low heat input and variable polarity of CMT advance mode. The microhardness distribution of AD sample consisted of slight fluctuation and stable areas along the building direction due to the different thermal history and precipitation process. After the PHT process, the average grain size and morphology almost remained unchanged. However, compared with the size and type of precipitation phases under the AD state, high-density nanoscale (∼5 nm) metastable η′ phases were precipitated in the grain interior of the PHT sample, which contributed to the crucial precipitation strengthening effect. The homogeneous microhardness distribution was achieved with a high level (∼180 HV0.2). Meanwhile, tensile properties and elongation significantly increased to 524.68 ± 7.31 MPa and 7.10 ± 2.09 % after the PHT process, respectively.

Effect of Chinese quince proanthocyanidins on the inhibition of heterocyclic amines and quality of fried chicken meatballs and tofu.

Heterocyclic amines (HCAs) have potential carcinogenic and mutagenic activity and are generated in cooked protein-rich foods. Adding proanthocyanidins (PAs) to these foods before frying is an effective way to reduce HCAs. In this study, polymeric PAs (PPA) and ultrasound-assisted acid-catalyzed/catechin nucleophilic depolymerized PAs (UAPA, a type of oligomeric PA) were prepared from Chinese quince fruits (CQF). Different levels of PPA and UAPA (0.05%, 0.1%, and 0.15%) were added to chicken meatballs and tofu; then these foods were fried, and the content of HCAs in them after frying was investigated. The results showed that PPA and, particularly, UAPA significantly inhibited the formation of HCAs in fried meatballs and tofu, and this inhibition was dose-dependent. The inhibition of HCAs by both PPA and UAPA was stronger in the chicken meatballs than in fried tofu. The level of total HCAs was significantly reduced by 57.84% (from 11.93 to 5.03ng/g) after treatment of meatballs with 0.15% UAPA, with inhibition rates of 78.94%, 50.37%, and 17.81% for norharman, harman, and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), respectively. Of note, there was a negative correlation between water, lipid, protein, creatine, and glucose content and HCA content in the crust, interior, and whole (crust-plus-interior) measurements of all fried samples. Interestingly, PPA and UAPA were found more effective in inhibiting HCAs in the exterior crust than in the interior of the fried chicken meatballs. These results provide evidence that further studies on the reduction of the formation of harmful HCAs in fried foods by adding CQF PAs could be valuable to the fried food industry. PRACTICAL APPLICATION: Chinese quince proanthocyanidins treatments significantly inhibited the generation of heterocyclic amines (HCAs) in chicken meatballs and tofu when deep-fried. These results suggest that Chinese quince proanthocyanidins can be used as natural food additive for reducing HCAs in fried foods, laying the foundation for using Chinese quince fruit proanthocyanidins for HCA inhibition in the food industry.

Nanoindentation into a bcc high-entropy HfNbTaTiZr alloy-an atomistic study of the effect of short-range order.

The plastic response of the Senkov HfNbTaTiZr high-entropy alloy is explored by means of simulated nanoindentation tests. Both a random alloy and an alloy with chemical short-range order are investigated and compared to the well understood case of an elementary Ta crystal. Strong differences in the dislocation plasticity between the alloys and the elementary Ta crystal are found. The high-entropy alloys show only little relaxation of the indentation dislocation network after indenter retraction and only negligible dislocation emission into the sample interior. Short-range order-besides making the alloy both stiffer and harder-further increases the size of the plastic zone and the dislocation density there. These features are explained by the slow dislocation migration in these alloys. Also, the short-range-ordered alloy features no twinning plasticity in contrast to the random alloy, while elemental Ta exhibits twinning under high stress but detwins considerably under stress relief. The results are in good qualitative agreement with our current knowledge of plasticity in high-entropy alloys.

Laboratory VIS–NIR reflectance measurements of heated Vesta regolith analogs: Unraveling the spectral properties of the pitted impact deposits on Vesta

AbstractPitted impact deposits on Vesta show higher reflectance and pyroxene absorption band strengths compared to their immediate surroundings and other typical Vestan materials. We investigated whether heating to different temperatures for different durations of Vestan regolith analog materials can reproduce these spectral characteristics using mixtures of HEDs, the carbonaceous chondrite Murchison, and terrestrial analogs. We find no consistent spectral trend due merely to temperature increases, but observed that the interiors of many heated samples show both higher reflectance and pyroxene band I strength than their heated surfaces. With electron probe microanalysis, we additionally observe the formation of hematite, which could account for the higher reflectance. The presence of hematite indicates oxidation occurring in the sample interiors. In combination with heat, this might cause the increase of pyroxene band strengths through migration of iron cations. The effect grows larger with increasing temperature and duration, although temperature appears to play the more dominant role. A higher proportion of Murchison or the terrestrial carbonaceous chondrite analog within our mixtures also appears to facilitate the onset of oxidation. Our observations suggest that both the introduction of exogenic material on Vesta as well as the heating from impacts were necessary to enable the process (possibly oxidation) causing the observed spectral changes.

Investigation of neighboring grain effects on load shedding in titanium alloys under cold dwell fatigue

Load shedding under cold dwell fatigue in titanium alloys has been a threat to the safety of the aero-engine since it is responsible for crack formation. The strain incompatibility between the soft-hard grain pair arises from the significant creep in the soft grain during the dwell period, which is fundamentally related to the load shedding. Such deformational heterogeneity is not only affected by the soft-hard grain itself but also by its neighboring grains, but the effect of the latter is rarely reported. In this paper, the effect of neighboring grains, including location, crystal orientation, texture, etc., on load shedding is quantitatively investigated using a three-dimensional rate-dependent crystal plasticity model by controlling the microstructural morphology of the grains. In particular, the mechanism that the dwell fatigue cracks tend to nucleate from the sample's interior rather than the free surface is revealed. Restriction of rigid body rotation from neighboring grains can further enhance load shedding. The crystal orientation of individual and group neighboring grains sitting at different locations with respect to the soft-hard grain pair is evaluated. The grain directly attached to the hard grain has the strongest influence. The statistics of load shedding are analyzed by conducting hundreds of simulations with different textures. Stronger textures are found to be able to enhance load shedding and thus deteriorate the service lifetime. The change of locations showing the highest stress during the dwell period arising from plastic deformation in neighboring grain may lead to the phenomenon of reverse load shedding. Understanding the neighboring grain effects from both the mechanistic basis and the statistical point of view is important in integrity evaluations and service life assessments of titanium components in aero-engines.

Integration of Iot and NDT for real Time Monitoring System

For the findings of non-destructive testing on buildings have only been provided in specific cases. To facilitate all-encompassing quality control throughout the construction's entire lifespan, it might be ideal to integrate the different findings, track their evolution across duration, and apply them when performing on-the-spot maintenance duties. Under the context of building information modelling (BIM), opportunities are established throughout this achievement, enabling the integration of non-destructive testing findings through digital building models. An inventive augmented reality (AR) visualization technique is shown as an illustration of the combined use of planned and measured results. An augmented reality application is being put into place that overlays the camera picture of a tablet looking at a piece of concrete using images captured by radar and ultrasonic sensors of the sample's interior (actual data) and the three-dimensional design layout of the integrated pieces (target data). The geometric relationship between the inner images and the camera image remains intact whenever the device turns or shifts. Therefore, the device's display provides a view inside the concrete structure's interior. The procedures required to prepare and evaluate the findings of non-destructive testing have been collected, and potential markers for linking the physical and digital realms have been explored. The objective is to assess the state, enable model-based examination, and upkeep operations instantly within the framework by enhancing the preparation of data as well as evaluation outcomes with extra data and displaying the findings in their actual situations.

A smart device of data acquisition with emergency safety features for laboratory furnaces

Laboratory furnaces or ovens are found in scientific, forensic, medicinal, and material processing labs. They provide precise temperature control and homogenous heating for samples in industrial and research applications but cannot measure the sample's interior temperature and most of them do not have the facility to store temperature data. Moreover, a furnace cannot promptly shut off if a sample or the furnace catches fire due to overheating or an electrical failure. To solve these problems, in this work, an intelligent, automated add-on device with wireless monitoring and emergency safety features to use with any commercial furnace or oven is presented. The device can individually measure furnace and sample temperatures at the same time using a two-channel K-type thermocouple unit. The system can store the real-time temperature data generated by the thermocouples, display it on LCD (Liquid Crystal Display) and give the facility to observe this real-time data from anywhere within an area of 800–1000-m radius using the portable wireless monitor device without the need for Wi-Fi or internet. Additionally, the device can quickly detect fire or overheating, immediately cuts the power automatically, activates a loud alarm and dials the user's phone itself using Global System for Mobile Communication (GSM) technology to warn them.

Nano-Patterned Magnetic Edges in CrGeTe3 for Quasi 1-D Spintronic Devices.

The synthesis of two-dimensional van der Waals magnets has paved the way for both technological applications and fundamental research on magnetism confined to ultra-small length scales. Edge magnetic moments in ferromagnets are expected to be less magnetized than in the sample interior because of the reduced amount of neighboring ferromagnetic spins at the sample edge. We recently demonstrated that CrGeTe3 (CGT) flakes thinner than 10 nm are hard ferromagnets; i.e., they exhibit an open hysteresis loop. In contrast, thicker flakes exhibit zero net remnant field in the interior, with hard ferromagnetism present only at the cleaved edges. This experimental observation suggests that a nontrivial interaction exists between the sample edge and the interior. Here, we demonstrate that artificial edges fabricated by focus ion beam etching also display hard ferromagnetism. This enables us to write magnetic nanowires in CGT directly and use this method to characterize the magnetic interaction between the interior and edge. The results indicate that the interior saturation and depolarization fields depend on the lateral dimensions of the sample. Most notably, the interior region between the edges of a sample narrower than 300 nm becomes a hard ferromagnet, suggesting an enhancement of the magnetic exchange induced by the proximity of the edges. Last, we find that the CGT regions amorphized by the gallium beam are nonmagnetic, which introduces a novel method to tune the local magnetic properties of CGT films, potentially enabling integration into spintronic devices.

Nonlinear acoustic characterization of heterogeneous plasticity in bent aluminium samples

Knowledge of the state of plastic deformation in metallic structures is vital to prevent failure. This is why non-destructive acoustic tests based on the measurement of first order elastic constants have been developed and intensively used. However, plastic deformations, which are usually heterogeneous in space, may be invisible to these methods if the variation of the elastic constants is too small. In recent years, digital image correlation techniques, based on measurements carried out at the surface of a sample, have been successfully used in conjunction with finite element modeling to gain information about plastic deformation in the sample interior. Acoustic waves can penetrate deep into a sample and offer the possibility of probing into the bulk of a plastically deformed material. Previously, we have demonstrated that nonlinear acoustic methods are far more sensitive to changes in dislocation density than linear ones. Here, we show that the nonlinear Second Harmonic Generation method (SHG) is sensitive enough to detect different zones of von Mises stress as well as effective plastic strain in centimeter-size aluminium pieces. This is achieved by way of ultrasonic measurements on a sample that has undergone a three-point bending test. Because of the relatively low stress and small deformations, the sample undergoes plastic deformation by dislocation proliferation. Thus, we conclude that the nonlinear parameter measured by SHG is also sensitive to dislocation density. Our experimental results agree with numerical results obtained by Finite Element Method (FEM) modeling. We also support the acoustic results by X-ray Diffraction measurements (XRD). Although intrusive and less accurate, they also agree with the acoustic measurements and plastic deformations in finite element simulations.

Visualization of bulk and edge photocurrent flow in anisotropic Weyl semimetals

Materials that rectify light into current in their bulk are desired for optoelectronic applications. In inversion-breaking Weyl semimetals, bulk photocurrents may arise due to nonlinear optical processes that are enhanced near the Weyl nodes. However, the photoresponse of these materials is commonly studied by scanning photocurrent microscopy (SPCM), which convolves the effects of photocurrent generation and collection. Here, we directly image the photocurrent flow inside the type-II Weyl semimetals WTe2 and TaIrTe4 using high-sensitivity quantum magnetometry with nitrogen-vacancy center spins. We elucidate an unknown mechanism for bulk photocurrent generation termed the anisotropic photothermoelectric effect (APTE), where unequal thermopowers along different crystal axes drive intricate circulations of photocurrent around the photoexcitation. Using simultaneous SPCM and magnetic imaging at the sample's interior and edges, we visualize how the APTE stimulates the long-range photocurrent collected in our Weyl semimetal devices through the Shockley-Ramo theorem. Our results highlight an overlooked, but widely relevant source of current flow and inspire novel photodetectors using homogeneous materials with anisotropy.

Tracking combustion behavior of copper monosulfide, ferrous sulfide, and chalcopyrite tablets by high-speed microscopic videography

A new suspended-combustion-test method involving high-speed digital microscopy and thermal measurements was developed to analyze small tablets of CuS, FeS, and CuFeS2 to elucidate their combustion behavior and phase changes in flash smelting. Moreover, the frequency of splash generation and homogeneous oxidation of the released gaseous sulfur from each sulfide were monitored. Microstructural analysis of quenched samples by X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy revealed that the combustion intensity could be related to the formation of an oxide layer, which blocked the O2 supply to the sample interior. Moreover, the gas formed at various oxygen concentrations agitated the samples. In particular, the generated solid-oxide shell could move, the sulfide was exposed to the surface again, and the combustion reaction proceeded further. Insufficient agitation at lower O2 concentrations resulted in a thick and dense oxide layer, which blocked the O2 supply and caused extinction.

Improving adversarial transferability by temporal and spatial momentum in urban speaker recognition systems

DNN-based speaker recognition systems (SRSs) in smart cities suffer from adversarial attacks, which have caused widespread concern. An attacker can fool the SRSs by adding imperceptible perturbations to benign audio. Recent studies have shown that adversarial attacks could achieve almost 100% attack success rate in the white-box but perform poorly in the black-box. Existing attacks do not effectively use the gradient information of the available white-box models, which is easy to over-fit the target model. To tackle the problem, we propose a temporal and spatial momentum-based iteration gradient sign method (TSMI-FGSM). Specifically, we introduce the sample neighborhood and interior space, and accumulate the gradient information of the randomly sampled points in these two spaces to correct and update direction by tuning strategies during each iteration. The experiment results with 9 SRSs demonstrate that our method significantly enhances the transferability of the adversarial examples compared to state-of-the-art attacks.

A hybrid chemical modification strategy for monocrystalline silicon micro-grinding: Experimental investigation and synergistic mechanism

For high performance manufacturing of micro parts and features, a hybrid chemical modification strategy is proposed to decrease critical energy barrier of mechanical removal of hard and brittle crystal material by refining localized machining condition. The strategy, namely UV-light and IR-laser hybrid chemical modification (UVIR-CM) strategy, includes two steps, an ultraviolet light (UV-light) catalytic advanced oxidation and an infrared laser (IR-laser) assisted selective modification based on Fenton liquid–solid reaction for monocrystalline silicon. The modification effects of UVIR-CM strategy were investigated by surface morphology micro-observation, cross-section transmission electron microscopy (TEM) observation, Raman spectroscopy analysis and nanoindentation test. Experimental results demonstrated that varied degrees of laser texturing appeared on different strategy samples’ IR-laser scanned area. And the IR-laser thermal damage has been successfully inhibited due to the refraction and reflection of energy by bubbles in liquid medium. But for the UVIR-CM strategy, a uniform and amorphous silicate layer is detected in a certain boundary. The UV-light promotes oxidation cycle ability of the chemical solution and ensures sufficient oxide modified layer for subsequent step. Attributing to synergism of photochemical, photothermal and kinetic effects induced by IR-laser, the modified layer displays layered structure with about 600 nm thickness, (2.7 ± 0.60) GPa nanohardness, and (93.7 ± 22.9) GPa indentation modulus. And the layered structure is amorphous layer, nanocrystal and micro-twins layer from the surface to the interior of sample. Consequently, it reveals that the subsequent mechanical removal will become easy due to decreasing energy barrier of monocrystalline silicon in selective area. Meanwhile, its original excellent mechanical properties also are maintained under a certain depth. The results contribute to develop a novel combined micro-machining technology to achieve collaborative manufacturing of structure shape and surface integrity for micro parts and feature.

MICROTOMOGRAFÍA DE RAYOS X PARA CARACTERIZAR VOLUMEN DEL CANAL RADICULAR EXTRAÍDO EN INSTRUMENTACIÓN ENDODÓNTICA

During the last decades, the analytical techniques of X-ray absorption contrast imaging have systematically gained greater relevance, mainly due to the ability to attain non-destructive exploration of the sample interior. The significant improvement in spatial resolution offered by X-ray micro-tomography, as compared to conventional computed tomography, has motivated its insertion in many biomedical fields, among which dentistry stands out. Particularly, for endodontics, microCT appears as a method of remarkable potential interest to study procedures involved in root canal treatments, where one of the main needs is the anatomical characterization of the root canal in the teeth. The present work reports on the adaptation of the microCT equipment of the LIIFAMIRx⃝laboratory at the E. Gaviola Physics Institute, CONICET and UNC, thus allowing to acquire of radiographic images of dental samples of interest, to be later used in the implementation of algorithms, intended to tomographic reconstruction and volume segmentation. As a result, radiographic images of premolar teeth were obtained with good contrast between the different materials present, and three-dimensional representations, whose visualization is comparable with the real samples. Moreover, it was possible to characterize the root canal volume of the tooth both in its natural form and after having undergone the instrumentation process in which the pulp tissue is extracted.

Corrosion Behavior and Mechanism Analysis of Oilwell Cement Under CO2 and H2S Conditions

In order to explore the corrosion effect of CO2 and H2S in oilwell cement, the samples were exposed to a CO2/H2S environment. The permeability, microstructure, chemical composition, and tensile strength of the cement samples were measured by a pulse pore-permeability tester, scanning electron microscope (SEM), X-ray diffraction (XRD), and rock tensile compression tester, respectively. The corrosion behavior and mechanism of the cement samples were analyzed. The experimental results show that the corrosion degree increases with the exposure time, and the permeability increases from 0.0003 md on the seventh day to 0.0646 md on the thirtieth day. It can be concluded from the microscopic morphology that the internal structure of the corroded cement sample is compact, without an obvious loose structure, but there are some particles with different particle sizes. The composition phases of the products after corrosion are mainly calcium carbonate and silica, and the content of calcium carbonate in the corrosion area increases obviously. The tensile strength of the tested cement sample is only 9.8 MPa. The reaction of gaseous CO2 and H2S with calcium-bearing phases in a cement sample is known to cause a lowering of alkalinity, leading to the corrosion of the cement sample. Due to the existence of carbonation, carbon dioxide will gradually diffuse into the interior of the cement sample and react with hydrogen ions, calcium ions, and bicarbonate ions in the pore fluid of the cement sample to form varieties of calcium carbonate, resulting in the shrinkage of the cement sample, surface cracks, and ultimately affecting the compactness and sealing effect of the cement sample.

Achieving better synergy of strength and ductility by adjusting size and volume fraction of coherent κ’–carbides in a lightweight steel

Heterogeneous grain structures with dual-nanoprecipitates have been designed and fabricated in a lightweight steel. The size and the volume fraction of coherent κ’–carbides are increased by aging treatment, while the volume fraction of B2 precipitates and the heterogeneous grain structure are nearly unchanged after aging treatment. The aged samples are found to have a better synergy of strength and ductility, as compared to the unaged samples. The better tensile properties in the aged samples can be owing to the higher hetero-deformation-induced hardening and the higher density of induced geometrically necessary dislocations during tensile deformation. Two distinct precipitation hardening mechanisms are revealed for the dual-nanoprecipitates, i.e., bypassing mechanism for B2 precipitates and shearing mechanism for coherent κ’–carbides. Numerous dislocations can be observed both around and inside coherent κ’–carbides. High density of stacking faults and nanotwins can also be found in the grain interiors of the aged samples, resulting in strong strain hardening by interaction between them and dislocations. The better tensile properties in the aged samples can also be attributed to the stronger shearing precipitation hardening effect by larger size and higher volume fraction of coherent κ’–carbides.

Ignition of thermally-thick blunt body PMMA samples using a heated wire

Ignition limits and ignition delay times of thermally-thick polymethyl methacrylate (PMMA) spheres (4-cm diameter) and cylinders (5.08-cm diameter) are investigated by placing an electrically-heated wire coil beneath the sample in a slow forced flow. The wire serves as both a heater for the solid sample and a gas-phase hot spot to ignite the oxidizer-fuel vapor mixture. In the experiments, the wire's electrical current and power-on time, its distance from the sample surface, the forced flow velocity, oxygen percentage, and pressure are systematically varied. Ignition limits as a function of the oxygen percentage, pressure, and igniter current are determined. Two ignition delay times are determined: igniter-assisted ignition and self-sustained ignition. These critical times are also functions of the degree of heat-up of the sample interior. Solid temperature profiles are measured using embedded thermocouples to determine the sub-surface temperature gradients at the moment of ignition. The heat transfer modes from the igniter to the solid sample are numerically investigated during the pre-pyrolysis heat-up period. At the forward stagnation-point region where ignition occurs, radiation is greater than convection and accounts for approximately 60% of the total heat input. This is consistent with the experimental findings on ignition delay times and heat flux measurements.

Biomineralization of Symbiotic Cultures of Bacteria and Yeast (SCOBY) Cellulose Aerogels

Microbial biomineralization is explored as a strategy for improving the thermal stability of symbiotic cultures of bacteria and yeast (SCOBY) cellulose aerogels. Cellulose pellicles were grown in kombucha tea and subsequently biomineralized with Sporosarcina pasteurii (S. pasteurii). The samples were exposed to one, two, or three cycles of ureolytic biomineralization. The microstructure and material properties of biomineralized samples were compared to experimental controls, which included a non‐biomineralized cellulose aerogel sample and an abiotic cellulose aerogel sample that was exposed to the biomineralization treatment without the addition of S. pasteurii. Results indicate that significant biomineralization (up to 38% CaCO3 by mass) occurred in the biomineralized SCOBY cellulose aerogels. Multiple biomineralization cycles enhance biomineralization of the sample surface and interior. Thermal conductivities of resultant aerogels measured ≈0.043–0.057 Wm−1 K−1 for all samples, indicating that biomineralization did not negatively affect the thermal conductivity of the SCOBY cellulose aerogels. Results further illustrate that biomineralization improved the thermal stability and flame resistance of SCOBY cellulose aerogels compared to experimental controls. Low thermal conductivity, improved thermal stability, and higher flame resistance of biomineralized SCOBY cellulose aerogels together indicate that thermally stable insulation materials can be produced exclusively from biological organisms.

Multi-mode chemical exchange in seafloor alteration revealed by lithium and potassium isotopes

Elemental exchange during seafloor alteration exerts substantial control on the chemical compositions of seawater and oceanic crust, as well as the global elemental cycling. However, detailed processes and mechanisms for element transport and isotopic exchange remain unclear. Here, we report Li and K isotopic compositions across an altered MORB-like pillow basalt from Mariana Trench. We find large and systematic isotopic variations on a centimeter scale, with δ7Li decreasing from +9.07‰ to +6.21‰ and δ41K increasing from −0.12‰ to +0.33‰ from the rim to the core. The heavier-than-mantle Li and K isotopic compositions in the core likely inherit from an enriched mantle source rather than post-magmatic alteration processes. The low K and high Li isotopic compositions at the outmost rims result from adsorption during seawater-rock interactions. The basin-shaped elemental and isotopic variations were subsequently formed by chemical diffusion and spontaneous seawater penetration from the weathered rind to the interior of pillow basalt sample. This study uses a small-scale sampling approach and provides the first evidence for multi-mode chemical exchange in oceanic basalts and improves our understanding of the seafloor alteration processes.

Influence of Residual Stresses on the Crack Initiation and Short Crack Propagation in a Martensitic Spring Steel

The crack initiation and short crack propagation in a martensitic spring steel were investigated by means of in-situ fatigue testing. Shot peened samples as well as untreated samples were exposed to uniaxial alternating stress to analyze the impact of compressive residual stresses. The early fatigue damage started in both sample conditions with the formation of slip bands, which subsequently served as crack initiation sites. Most of the slip bands and, correspondingly, most of the short fatigue cracks initiated at or close to prior austenite grain boundaries. The observed crack density of the emerging network of short cracks increased with the number of cycles and with increasing applied stress amplitudes. Furthermore, the prior austenite grain boundaries acted as obstacles to short crack propagation in both sample conditions. Compressive residual stresses enhanced the fatigue strength, and it is assumed that this beneficial effect was due to a delayed transition from short crack propagation to long crack propagation and a shift of the crack initiation site from the sample surface to the sample interior.