•Mechanisms of high temperature fatigue damage in austenitic steel.•Use of FIB produced lamellae to get HAADAF-STEM, EDS and EBSD images.•Depletion of Cr at grain boundaries due to the formation Cr ...carbides.•Role of preferential oxidation of grain boundaries in fatigue crack initiation.•Intergranular crack paths of surface cracks and transgranular paths in the bulk.
Crack initiation and short crack growth in highly-alloyed austenitic steel Sanicro 25 cyclically strained at a temperature of 700 °C has been studied. Specimens cycled to 10% of the fatigue life and to the fracture were inspected using scanning electron microscopy and electron backscatter diffraction. On the surface, thin lamellae were extracted site-specifically by focused ion beam and then they were further characterized by high spatial resolution scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. The preferential oxidation of grain boundaries was found to play a principal role in the initiation of multiple cracks. The oxide scale was analysed to show the distribution of individual elements during crack initiation and growth. Two types of crack paths were observed: intergranular due to the oxidation and transgranular accompanied by a cyclic plastic zone. Cracks preferentially initiated at the grain boundaries perpendicular to the stress axis and propagated in the bulk of the material. The role of the oxidation in high temperature fatigue crack initiation is discussed as well as the mechanisms of the crack initiation and growth.
•Profiles of persistent slip markings (PSMs) in four fatigued materials using FIB cutting.•Surface lamella production for TEM with undamaged surface.•Documentation of the shape of PSMs as extrusions ...and extrusion-intrusion pairs.•Observation of surface profile and underlying dislocation structure of persistent slip bands.•Observation and documentation of fatigue crack initiation from the tip of intrusions.•Implication of experimental findings to the models of fatigue crack initiation.
The profiles of persistent slip markings produced by uniaxial and biaxial cyclic straining in four different polycrystalline materials with f.c.c. structure were investigated using focused ion beam (FIB) cutting and TEM observation of oriented surface foils. Typical shapes of persistent slip markings are extrusions accompanied by parallel intrusions. In some cases only extrusions were developed and intrusions were produced later in fatigue life. In polycrystalline copper extrusions and intrusions appear on the surface of the grain where persistent slip band characterized by ladder-like dislocation structure egress on the surface. Similar features were observed in fatigued austenitic 316L and Sanicro 25 steels but the extrusion and intrusion shapes were more complicated. Crack-like intrusion shapes produce high stress and strain concentration and primary stage I crack starts to grow from the tip of intrusions. The experimental observations were compared with the predictions of the existing crack initiation models.
Advanced scanning transmission electron microscopy (STEM) was used to study two distinct populations of nanoparticles associated with the extraordinary strengthening of the highly-alloyed austenitic ...stainless steel Sanicro 25 during cyclic loading at 700 °C. Fully coherent and homogeneously dispersed Cu-rich nanoparticles precipitate rapidly as a result of thermal exposure, along with nanometer-sized incoherent NbC carbides that nucleate on dislocations during the cyclic loading at high temperature. The atomic structure of nanoparticles was investigated by probe-corrected high-angle annular dark-field STEM imaging. Compositional analysis of the nanoparticles was conducted using high spatial resolution energy dispersive X-ray spectroscopy combined with electron energy-loss spectroscopy. Experimental observations were validated by image simulations of the Moiré-like contrast exhibited by NbC carbides. The important role of both nanoparticle populations for the overall cyclic response is discussed. As a result of pinning effects and associated obstacles, dislocation motion is significantly retarded preventing formation of substructures with lower stored internal energy. With recovery heavily suppressed, forest dislocation strengthening supported by precipitation and solid solution hardening, leads to the remarkable increase of cyclic strength at elevated temperatures.
Influence of mean stress on fatigue life of the austenitic stainless steel 316 L in air and light water environments (boiling water reactor/hydrogen water chemistry) at 288 °C was determined with a ...series of tests carried out in load-control mode. Fatigue life was found to increase with application of compressive and tensile mean stress in air and light water reactor environments. Secondary hardening was regarded as the main reason for this behavior. A modified Smith-Watson-Topper (SWT) model was considered to account for mean stress and was shown to predict fatigue life accurately in air and water environments. The reduction of fatigue life in water environment, determined with the SWT curves, was about 2.5. Observations of the end-of-life dislocation arrangements by transmission electron microscopy showed that the dislocation microstructure depends essentially on plastic strain amplitude, which in turn is strongly correlated to stress amplitude and mean stress. The microstructures were found consistent with those usually observed after strain-controlled experiments. At rather low plastic strain amplitudes, corduroy structure consisting of small dislocation loops was observed. Acting as significant obstacle to dislocation motion, corduroy structure affects overall dislocation mobility therefore contributing to notable secondary cyclic hardening.
Focused ion beam sections and lamellae for transmission electron microscopy were prepared from fatigued specimens of polycrystalline copper and austenitic Sanicro 25 steel. The profiles of persistent ...slip markings developed on the surface were observed and documented simultaneously with the underlying dislocation structure. In copper fatigued at room temperature and close to liquid nitrogen temperature, persistent slip markings consisting of pronounced extrusions and parallel intrusions appeared at locations where persistent slip bands having the ladder‐like dislocation structure egress on the surface. Stage I cracks initiated from the tip of the intrusions. In Sanicro 25 steel, more planar character of the dislocation structure led to thin extrusions and intrusions and several stage I cracks running parallel to the primary slip plane. Exceptionally, ladder‐like structure and generally alternating dislocation‐rich and dislocation‐poor volumes were observed in the PSBs. Dislocations bands of secondary slip systems in the matrix disappeared on intersections with PSBs. Experimental findings were compared with predictions of recent physically based models of fatigue crack initiation.
Effects of alloying elements (i.e., variable content of Mo and C) on the microstructure and mechanical properties of five types of experimental material based on TiAl alloy containing 7at.% Nb were ...investigated. Mechanical properties (tensile, compression and fatigue) at room and at elevated temperatures were evaluated. Addition of Mo and/or C resulted in different phase compositions and three typical microstructures. To judge the effect of alloying on the microstructure the comparison with reference TiAl alloy with 7at.% Nb without C and Mo addition was used. Evaluation of mechanical properties of individual alloys revealed two most promising variants: (i) alloy with 2at.% Mo containing β phase, having mixed microstructure and exhibiting the highest ductility, (ii) alloy with 0.5at.% C having nearly lamellar microstructure, enhanced amount of α2 phase, without β phase and exhibiting the highest strength. No positive effect of combined Mo and C addition on mechanical properties was identified. Fractographic observations revealed that the major fracture mode in all alloys was trans-lamellar. Inter-lamellar fracture was observed occasionally and mainly in C containing alloys.
Translamellar cleavage found as main fracture micromechanism at room temperature coupled with intergranular fracture at elevated temperatures. Display omitted
•Positive effects of either C or Mo on microstructure and mechanical properties of high Nb TiAl alloys are described.•Combining of C and Mo does not result in improved mechanical properties.•The major fatigue fracture mode in all studied materials is described as translamellar.
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.
Treatment with escalated BEACOPP achieved a superior time to treatment failure over ABVD in patients with disseminated Hodgkin lymphoma. However, recent clinical trials have failed to confirm BEACOPP ...overall survival (OS) superiority over ABVD. In addition, the gain in low-risk patients is still a matter of debate.
We randomly compared ABVD (8 cycles) with BEACOPP (escalated 4 cycles ≥baseline 4 cycles) in low-risk patients with an International Prognostic Score (IPS) of 0–2. The primary end point was event-free survival (EFS). This parallel group, open-label phase 3 trial was registered under #RECF0219 at French National Cancer Institute.
One hundred and fifty patients were randomized in this trial (ABVD 80, BEACOPP 70): 28 years was the median age, 50% were male and IPS was 0–1 for 64%. Complete remission rate was 85% for ABVD and 90% for BEACOPP. Progression or relapses were more frequent in the ABVD patients than in the BEACOPP patients (17 versus 5 patients). With a median follow-up period of 5.5 years, seven patients died: six in the ABVD arm and one in the BEACOPP arm (HL 3 and 0, 2nd cancer 2 and 1, accident 1 and 0). The EFS at 5 years was estimated at 62% for ABVD versus 77%, for BEACOPP hazards ratio (HR) = 0.6, P = 0.07. The progression-free survival (PFS) at 5 years was 75% versus 93% (HR = 0.3, P = 0.007). The OS at 5 years was 92% versus 99% (HR = 0.18, P = 0.06).
Fewer progressions/relapses were observed with BEACOPP, demonstrating the high efficacy of the more intensive regimen, even in low-risk patients. However, additional considerations, balancing treatment-related toxicity and late morbidity due to salvage may help with decision-making with regard to treatment with ABVD or BEACOPP.
Issues of using Longer Heavier Vehicles on Roads Matuszkova, R; Heczko, M; Cepil, J ...
IOP conference series. Materials Science and Engineering,
03/2018, Letnik:
317, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Many logistics companies aim to save on freight costs. Recently, not only on Czech roads and on motorways, longer and heavier vehicles that exceed dimensions' limits appeared. For these vehicles, it ...is necessary to apply for a special permit, which is, however, much more liberal than the permit for oversized and overweight load transport. This paper informs about checking routes of these vehicles by swept path analysis and finding locations on roads that can generate both safety risks and traffic fluency problems.
Microstructure and dislocation structures in high alloyed heat resistant Sanicro 25 austenitic steel cyclically strained in a wide interval of constant strain amplitudes both at room temperature and ...at temperature of 700°C were studied by means of transmission electron microscopy (TEM). The spatial arrangement of dislocations was determined using the technique of oriented foils. The character and the Burgers vectors of dislocations were determined. It was found that at room temperature strong planarity of dislocation slip prevails. Cyclic plastic deformation is localized into thin bands of high dislocation density which have different structure than ladder-like arrangement. No distinctive wall and channel dislocation configurations were observed but bands have character of alternating dislocation rich and dislocation poor areas. Pronounced plastic strain localization leads to cyclic softening. In high temperature cyclic straining enhanced cross-slip leads to substantial increase of dislocation density. Objects of nanometer size coherent with the matrix were found simultaneously with the evidence indicating pinning of dislocations. The high dislocation density, its homogeneous distribution and interaction of nanoclusters and gliding dislocations are the cause of exceptional cyclic hardening of this material at high temperatures. Due to this hardening, the saturated cyclic stress at 700°C is remarkably high.
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