Miroslav Šmíd

X-ray Computed Tomographic Investigation of the Porosity and Morphology of Plasma Electrolytic Oxidation Coatings.

Plasma electrolytic oxidation (PEO) is of increasing interest for the formation of ceramic coatings on metals for applications that require diverse coating properties, such as wear and corrosion resistance, low thermal conductivity, and biocompatibility. Porosity in the coatings can have an important impact on the coating performance. However, the quantification of the porosity in coatings can be difficult due to the wide range of pore sizes and the complexity of the coating morphology. In this work, a PEO coating formed on titanium is examined using high resolution X-ray computed tomography (X-ray CT). The observations are validated by comparisons of surface views and cross-sectional views of specific coating features obtained using X-ray CT and scanning electron microscopy. The X-ray CT technique is shown to be capable of resolving pores with volumes of at least 6 μm(3). Furthermore, the shapes of large pores are revealed and a correlation is demonstrated between the locations of the pores, nodules on the coating surface, and depressions in the titanium substrate. The locations and morphologies of the pores, which constitute 5.7% of the coating volume, indicate that they are generated by release of oxygen gas from the molten coating.

Low Cycle Fatigue of Cast γ-TiAl Based Alloys at High Temperature

The low-cycle fatigue properties of cast nearly lamellar TiAl alloys with diverse Nb content at room temperature and 750°C were compared. Monotonic tensile curves, cyclic stress-strain curves (CSSC) and combined fatigue life curves were obtained at both temperatures. After cyclic straining the surface relief and the fracture surfaces were observed using scanning electron microscopy. The increase in Nb content significantly improves tensile properties and CSSCs at both temperatures. The combined fatigue life curves especially at 750 °C are shifted to higher fatigue lives. The effect o Nb content on the tensile and cyclic strength is discussed in relation to the thickness of the lamellae. Persistent slip markings formed along interlamellar interfaces were predominant locations for fatigue cracks. At both temperatures and materials showed similar crack initiation and propagation behavior. The cracks initiated at surface or in sub-surface region creating smooth flat areas corresponding to the persistent slip bands.

Analysis of the Effective and Internal Cyclic Stress Components in the Inconel Superalloy Fatigued at Elevated Temperature

Cyclic multiple step test in strain control have been performed on cylindrical specimens of cast polycrystalline Inconel 738LC and 792-5A superalloys at 800 °C in laboratory atmosphere. Hysteresis loops were analyzed according to the statistical theory of hysteresis loop. The effective and internal stress components were evaluated. The effective stress of γ´ precipitate has significant influence on the stress-strain response both materials. The stress amplitude in IN 792-5A is higher than in IN 738LC at approximately same total strain amplitude due to significantly higher effective stress of γ´ phase. Cyclic hardening/softening curves and cyclic stress-strain curves using short-cut procedure were obtained. Cyclic hardening/softening behavior depends both on temperature and strain amplitude. Low amplitude straining is characterized by the saturation of the stress amplitude. In high amplitude straining slight softening was found. The cyclic stress-strain curves for both materials can be fitted by power law. Cyclic stress-strain response in terms of internal and effective stress components is discussed in relation to microstructural parameters of the materials. The observation of surface relief revealed the presence of persistent slip markings.

Interaction of Creep and High Cycle Fatigue of IN 713LC Superalloy

The study deals with the interaction of creep and high cycle fatigue of cast polycrystalline nickel-based superalloy IN 713LC at high temperatures. Previous works indicated that creep lifetime of superalloy structures was un-affected or even slightly increased in the cases with superimposed vibrations. The reason for this behaviour was not well described up to now. Therefore, set of fatigue tests was conducted at high mean stresses level to observe this phenomenon. The mean stress was kept constant while the stress amplitudes were selected in order to measure wide range of conditions from pure creep to pure fatigue. Fractographic analysis by scanning electron microscopy (SEM) was done with the aim to identify governing damage mechanisms for particular test conditions as a preliminary evaluation of conducted tests.

Effect of Tensile Dwell on Low Cycle Fatigue of Cast Superalloy Inconel 792-5A at 800°C

Effect of tensile dwell on low cycle fatigue of cast Inconel 792-5A is studied in symmetrical strain cycling at 800°C. Cyclic hardening/softening curves, cyclic stress-strain curves (CSSC) and fatigue life curves were obtained in continuous cycling and in cycling with tensile dwells. Dwells have slight effect on hardening/softening curves at high strain amplitudes. CSSC in cycling with dwells is shifted to lower stress amplitudes. No significant effect of dwells on Basquin curves is observed. Density of slip markings in continuous cycling is significantly higher in comparison with cycling with dwells. Samples cycled with dwells are typical of high density of secondary cracks, although sporadic slip markings were also found.

Kinetic study of static recrystallization in an Fe–Al–O ultra-fine-grained nanocomposite

Abstract A nearly abrupt coarsening of grains is observed in a newly developed Fe–Al–O ultra-fine-grained nanocomposite with a significant volume fraction (4%) of alumina nano-precipitates. The microstructure of the alloy was analysed in different states (as-received and annealed) by means of scanning electron microscopy, transmission electron microscopy (TEM) and hardness. The initial grain size 150–200 nm increases up to 50 μm during annealing 1000 °C/8 h and thereafter demonstrates saturation. A linear correlation between volume fraction of coarse grains and hardness was found. It was identified by TEM that alumina nano-precipitates stabilize the dislocation microstructure against recovery very effectively and the grain coarsening is due to fast growth of very few dislocation free grains. Thus, the observed grain coarsening has the attributes of static recrystallization.

A Numerical Analysis of Deformation Processes in Oxide Dispersion-Strengthened Materials - Influence of Dislocation-Particle Interactions

A recently developed 3D discrete dislocation dynamics (DDD) model is employed to study kinetics of dislocation ensembles subjected to high temperature creep in microstructures of metal matrix composites. We particularly focus on a migration of low angle tilt boundaries in a field of rigid impenetrable particles. This type of dislocation boundaries represents a typical microstructural feature mediating plastic deformation during the high temperature loadings. The article compares results of numerical studies that considered distinct dislocation-particle in-teractions in order to describe the response of dislocation structure to the applied stress. The resultssuggest that, regardless the details related to the dislocation-particle interactions, a critical applied stress always exists, below which the boundary migration process ceases [1,2]. The existence of crit-ical threshold is confirmed by creep tests of ODS materials. This critical threshold, contrary to theclassical Orowan stress, is proportional to the dislocation density. The displacements of individual dislocation segments on the micro-scale level reflect the changes in the dislocation-particle interactions quite sensitively. Atthemacro-scale level, the overall strain rate, which averages out velocities of all the individual dislocation segments, is also significantly influenced by the changes in dislocation-particle interaction

Role of defects in fatigue damage mechanisms of cast polycrystalline superalloy MAR-M 247

High-cycle fatigue life of nickel-based superalloy MAR-M 247 was experimentally determined for as-cast material and material processed by hot isostatic pressing (HIP). Fatigue testing was conducted at temperatures 650, 800, 900  ∘ C in laboratory air. HIP was done at the following conditions: 1200  ∘ C/100 MPa/240 min. It has been found that HIP significantly improves the fatigue life. Obtained results indicate that main factors which determine the fatigue strength of material in both conditions are grain size, grain orientation and size and distribution of casting defects. The distribution and size of the casting defects were evaluated by light microscopy on metallographic sections. The data were processed by the extreme value statistics, which enables to estimate the maximum size of a defect likely to occur in a defined volume. Light and scanning electron microscopy were used for fractographic investigation of fracture surfaces and fatigue crack initiation sites. Focused ion beam technique and transmission electron microscopy were applied with the aim to reveal the microstructure in the nearest vicinity of the early cracks. The mechanism of crack initiation, early crack propagation and the role of casting defects were described and discussed.

Description of Small Fatigue Crack Propagation in ODS Steel

The oxide dispersion strengthened (ODS) steels are widely studied as possible candidate material for fission and fusion technology. Due to application of the ODS materials, mainly low cycle fatigue is important. Therefore, small crack initiation and propagation for rather high stress amplitude are in focus. The main aim of the presented work is description of the small fatigue crack propagation for ODS-EUROFER steel. Due to limited quantity of ODS steel special miniaturized cylindrical specimens for fatigue testing were designed. Crack propagation law based on plastic part of J-integral is presented and applied for mentioned material. The resulting fatigue crack propagation rates for ODS-EUROFER steel and EUROFER 97 are compared and discussed.

Surface Relief Evolution in IN792-5A Nickel Superalloy under High Temperature Fatigue Straining with Hold Times

The present paper is focused on scanning electron microscopy investigation of surface relief developed in cyclically strained specimens at high temperature. Symmetrical fatigue tests without hold times and with tensile or compressive hold times were used in strain control regime. The gauge length of specimens was analyzed after cyclic straining in a wide range of total strain amplitudes. Experimental temperature was 800 °C and selected hold time was 10 minutes.Surface relief is developed during the cyclic straining in localized plastic strain areas. Developed surface relief strongly depends on the type of fatigue test and a value of total strain amplitude. Well-developed surface persistent slip markings were observed after cyclic loading without hold times. Another group of observed slip markings with perpendicular orientation to stress axis exhibits irregular and wavy morphology. Similar markings were also observed on specimen surfaces after fatigue tests with tensile hold times as well as compressive hold times. Third group, named imperfect persistent slip markings, was observed after cyclic straining with tensile holds. In some suitably oriented grains, areas with high density of well-developed markings with irregular morphology were found.