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It has been documented that the hierarchical character of microstructure produced by laser powder bed fusion (L-PBF) is the key to superior mechanical properties. Especially important is a fine cell microstructure possessing heterogeneous distribution of dislocation density and alloying elements. Despite multiple studies that have investigated the effect of such L-PBF structure on the stress-strain response during monotonic loading, just a few investigations were devoted to cyclic behaviour. The present study delivers an insight into the cyclic behaviour of L-PBF processed metastable austenitic stainless steel 304L and its relation to the observed microstructure evo-lution and strain-induced martensitic transformation (SIMT). The combination of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations, and feritscope measurements enabled to follow the onset of strain-induced martensite (SIM) nucleation and underlying dislocation microstructure evolution. The cyclic behaviour consisted of initial cyclic softening regardless of subjected strain amplitude. Afterwards, milder cyclic softening or saturation stage followed until a final failure was characteristic for the tests held at low strain amplitudes (e(a) = 0.5%). The third fatigue life stage, cyclic hardening, was recorded during fatigue tests held at e(a) > 0.5%. The excellent cyclic strength of stainless steel 304L is a direct consequence of cell microstructure containing high dislocation density walls and elemental microsegregation, which effectively inhibit dislocation motion. Cyclic softening was linked with cyclic strain localization into slip bands of decreased dislocation density and heavily altered dislocation cell walls. These bands have been observed for the first time in L-PBF-processed metals. This microstructural feature seems to be a variant of persistent slip bands (PSBs), a typical dislocation arrangement observed in conventionally produced materials subjected to c

Fatigue crack propagation in near-threshold regime was studied in the 304L austenitic stainless steel in two microstructural states: as-received (AR) with finer microstructure and low susceptibility to the transformation-induced plasticity (TRIP) effect, and solution-annealed (SA) with coarser microstructure and higher susceptibility to TRIP. At the load ratio R = 0.1 the threshold was higher in the SA state than in the AR state due to coarser grains and possibly the TRIP effect. In order to clarify the role of crack closure, experiments at R = 0.7 were done. The threshold in the SA state was still higher by 1 MPa center dot m(0.5). This effect was identified as crack tip shielding induced by phase transformation, an example of a non-closure shielding effect. Higher resistance to crack growth in the SA state was attributed to promoted martensitic transformation in non-favorable oriented grain families rather than thicker martensite layers in the crack path area. The conclusions were verified by experiments at R = 0.7 and temperature 150 degrees C > M-s which did not reveal any notable difference in thresholds. However, the threshold values were affected by the load-shedding gradient C = -d Delta K/da, which had to be equalized in both experimental setups inside and outside the furnace.

In this work two approaches to the description of short fatigue crack growth rate under large-scale yielding condition were comprehensively tested: (i) plastic component of the J-integral and (ii) Polak model of crack propagation. The ability to predict residual fatigue life of bodies with short initial cracks was studied for stainless steels Sanicro 25 and 304L. Despite their coarse microstructure and very different cyclic stress-strain response, the employed continuum mechanics models were found to give satisfactory results. Finite element modeling was used to determine the J-integrals and to simulate the evolution of crack front shapes, which corresponded to the real cracks observed on the fracture surfaces of the specimens. Residual fatigue lives estimated by these models were in good agreement with the number of cycles to failure of individual test specimens strained at various total strain amplitudes. Moreover, the crack growth rates of both investigated materials fell onto the same curve that was previously obtained for other steels with different properties. Such a "master curve" was achieved using the plastic part of J-integral and it has the potential of being an advantageous tool to model the fatigue crack propagation under large-scale yielding regime without a need of any additional experimental data.

Lifetime of MAR-M 247 Ni-based superalloy was experimentally studied under high-frequency cyclic loading (120 Hz) with constant tensile mean stress m = 300 MPa at temperature 900 °C in air. The stress amplitude a was changed in order to investigate the superalloy lifetime in a wide range of conditions, from pure creep to pure fatigue. The superposition of the stress amplitude on the mean stress does not influence the specimen lifetime until a threshold value is reached. Scanning electron microscopy was utilized for the fracture surface observation with the aim to identify the governing damage mechanisms for particular test conditions.

B1914 is a nickel-based superalloy with an increased amount of B (boron) and C (carbon) to reach an adequate amount of borides and carbides in the cast structure ensuring improved creep properties. However, the influence of the structure on the material high-cycle fatigue properties is not sufficiently described. The present study brings the experimental results on the high-cycle fatigue properties of a cast polycrystalline nickel-based boron-rich B1914 superalloy, obtained at temperatures of 800, 900 and 950 °C. The cast superalloy was processed by hot isostatic pressing (HIP) to diminish the casting defects. The fatigue tests were performed in symmetrical loading in laboratory air. The fracture surfaces of the specimens were studied by scanning electron microscopy in order to describe the influence of temperature on the fatigue crack initiation and propagation. Change of the primary fatigue crack propagation mechanism from the crystallographic to the non-crystallographic mechanism was observed with increasing temperature. Decrease of a material lifetime and decrease of the fatigue endurance limit due to the increasing testing temperature was observed.

A multi-phase Ti–6Al–4V prepared by electron beam melting and thermal post treatments has been shown to exhibit increased strength and ductility over standard wrought or hot isostatic pressed Ti–6Al–4V. The mechanical improvements are due to a prolonged, continuous work hardening effect not commonly observed in Ti alloys. In situ x-ray diffraction and high resolution digital image correlation are used to examine the strain partitioning between the phases during tensile loading with post-mortem electron microscopy to characterize the deformation behavior in each phase. Specimens heat treated between 850 and 980 °C were tested and the effect of annealing temperature on the micromechanical response is discussed. It is shown that the work hardening is the result of composite load-sharing behavior between three mechanically distinct microstructures: large α lamellae and a martensitic region of fine acicular α' and a third phase not previously reported in this alloy., SCOPUS: ar.j, info:eu-repo/semantics/published

Measurement of the short fatigue crack propagation can come across a lot of difficulties from the experimental point of view and interpretation of the results is also sometimes controversial. For simplicity, usual description of the short cracks is based on linear elastic fracture mechanics (using the concept of stress intensity factor). In this case, significant differences between short cracks and long cracks are usually presented. Most of the discrepancies are simply given by non-validity of the linear elastic fracture mechanics approach. In our case of physically short fatigue cracks the level of applied stress is close to cyclic yield stress of the material and, due to large amount of plasticity, conditions of linear elastic fracture mechanics are not satisfied. As a consequence, non-linear elastic plastic fracture mechanics is used for description of the short crack behavior in this article. Concept based on plastic part of J-integral is proposed for the description of the short crack behavior and data obtained for different strain amplitudes are compared. This concept is validated on experimental data obtained on steel 316L., Měření šíření krátkých únavových trhlin může narazit na mnoho obtíží z experimentálního hlediska a interpretace výsledků je také někdy kontroverzní. Pro jednoduchost je obvyklý popis krátké trhliny založen na lineární elastické lomové mechanice (pomocí faktoru intenzity napětí). V tomto případě jsou obvykle prezentovány významné rozdíly mezi krátkými a dlouhými trhlinami. Většina nesrovnalostí je však zpravidla dána neplatnosti předpokladů lineární elastické lomové mechaniky v případě krátkých trhlin. V našem případě fyzicky krátkých únavových trhlin se použité zatížení blíží cyklciké mezi kluzu materiálu a kvůli výrazné plastické deformaci nejsou splněny podmínky lineární elastické lomové mechaniky. Proto je v tomto příspěvku k popisu chování krátkých trhlin použito elasto-plastické lomové mechaniky. Je navržen koncept pro popis chování krátkých trhlin založený na použití plastické části J-integrálu a jsou porovnána data obdržená pro různé hodnoty amplitudy přetvoření. Tento koncept je ověřen na experimentálních datech získaných na oceli 316L.

Nickel base superalloys exhibit fatigue fracture behavior with features of brittle-like cleavage cracking under high cycle fatigue loading at temperatures up to approximately 800 °C. This specific fracture behavior was already documented in several studies, but a possible mechanism of fatigue crack propagation under this mode has not been made completely clear yet. The aim of this paper is to put more light on the phenomenon by using advanced electron microscopy techniques like electron back-scattered diffraction (EBSD) and focused ion beam (FIB) sectioning. Fractured specimens after high cycle fatigue tests were thoroughly examined with the aim to localize the fatigue crack initiation sites and accompanying features of the fatigue crack propagation. Several specimens were cross-sectioned in order to characterize active slip systems, cyclic plastic deformation localization and fatigue crack propagation. Dislocation structures were studied by transmission electron microscopy (TEM).