Ivo Kuběna

Surface Relief and Internal Structure in Fatigued Stainless Sanicro 25 Steel

High-resolution images of persistent slip markings developed on the surface of Sanicro 25 stainless steel during cyclic loading obtained from the FIB-produced surface lamella in TEM simultaneously with the underlying dislocation structure are reported. Extrusions, intrusions, incipient cracks, and the dislocation arrangement corresponding to the bands of intensive cyclic slip are documented and discussed in relation to the models of surface relief formation in cyclic loading.

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.

Strength, elasticity and failure of composites with pyrolyzed matrices based on polymethylsiloxane resins with optimized ratio of D and T components

Two mixtures of T and D siloxane monomeric components labelled as TxDy (molecular ratio x:y equal 3:1 or 4:1) were chosen as matrix precursors for manufacturing Nextel720 reinforced unidirectional composites which, after pyrolysis at 1000 or 1100°C, revealed good endurance in an oxidizing environment up to 1500°C. Vickers hardness of the heat treated (1000–1500°C) samples of pyrolyzed matrices T3D1 and T4D1 are mutually similar (1100–1400 HV0.2) and reach their maximum between 1200–1300°C. Flexural strength of the pyrolyzed composites is 150–170 MPa and 170–250 MPa for T3D1 and T4D1, respectively. After annealing 3 h in air at 1200–1300°C, the strength slightly decreases but similar treatment at 1500°C yields strengths exceeding those of the pyrolyzed material. Shear modulus of the pyrolyzed T4D1 composite is roughly twice that of the T3D1 one (15 GPa vs. 8 GPa) and both increase sharply to 22–25 GPa after annealing at 1500°C, which manifests substantial improvement of the matrix properties. Fracture toughness of the composites, as measured by chevron notch test at RT, 550°C, and 1100°C, yields 4–5 MPa.m−1/2 for T3D1 and 3–4 MPa.m−1/2 for T4D1. For both composite types, the fracture toughness drops by 1 MPa.m−1/2 when measured at 550°C, which can be attributed to suppression of fibre pull-out due to stress state changes caused by the coefficient of thermal expansion (CTE) mismatch. Fracture surfaces generated during flexural tests of the annealed samples reveal decreasing occurrence of pullout towards the highest annealing temperature.

Low Cycle Fatigue Behaviour of ODS Steels for Nuclear Application

This study deals with the low cycle fatigue (LCF) behaviour of two steels strengthened by oxide dispersion. The ODS Eurofer steel shows cyclic softening, slower than in the case of the same steel without oxide particles. The 14Cr ODS ferritic steel is cyclically stable, even a slight cyclic hardening is observed at 650 °C. The reasons for this different behaviour lie probably in the less resistant matrix with a lot of weak subgrain boundaries in ODS Eurofer, because no clear difference in the oxide particles distribution was found.

AFM and FIB Study of Cyclic Strain Localization and Surface Relief Evolution in Fatigued f.c.c. Polycrystals

Atomic force microscopy (AFM) and focused ion beam technique (FIB) were adopted to study the early stages of surface relief evolution in 316L steel and polycrystalline copper fatigued with constant plastic strain amplitudes at different temperatures (316L steel at 93, 173 and 573 K; copper at 83, 173 and 295 K). Qualitative and quantitative data on the morphology and shape of persistent slip markings (PSMs), occurrence of extrusions and intrusions and the kinetics of extrusion growth are reported. They are discussed in relation with recent physically based theories of surface relief formation leading to fatigue crack initiation.

Surface Relief Formation in Relation to the Underlying Dislocation Arrangement

Two fatigued materials with f.c.c. lattice, i.e. pure polycrystalline copper and austenitic Sanicro 25 stainless steel, were subjected to the study of the persistent slip markings (PSMs) developed on the surface of the suitably oriented grains. They were observed using scanning electron microscopy (SEM) and thin surface FIB lamellae were prepared and studied by transmission electron microscopy (TEM). The aim was to correlate the specimen surface profile with the underlying internal dislocation structure. The localization of the intensive cyclic slip into persistent slip bands (PSBs) of the material was observed and associated with the PSMs on the specimen surface. Extrusions, intrusions and the dislocation structure appertaining to them were analysed, documented and discussed in relation to the models of fatigue crack initiation.

The True Shape of Persistent Slip Markings in Fatigued Metals

Persistent slip markings (PSMs) were experimentally studied in 316L steel fatigued to early stages of the fatigue life. High resolution SEM, combined with focused ion beam (FIB) technique and atomic force microscopy (AFM) were used to assess the true shape of PSMs in their early stage of development. General features of PSMs in fatigued metals are extrusions and intrusions. Their characteristic features were determined. They were discussed in relation with the theories of surface relief formation and fatigue crack initiation based on the formation, migration and annihilation of point defects in the bands of intensive cyclic slip - persistent slip bands (PSBs)

Corrosion-Mechanical Behavior of T91 and 14-19Cr ODS Steels Exposed to Flowing Lead-Bismuth

This work describes the behavior of T91 (9Cr ferritic-martensitic steel), MA957 ODS (14%Cr, 0.3Mo, 0.9Ti, 0.25%Y2O3) and PM2000 ODS (19%Cr, 5%Al, 0.25%Y2O3) steels produced by mechanical alloying process, after long-term exposure to liquid lead-bismuth eutectic. Small bend specimens were pre-strained, then exposed to flowing lead-bismuth at 350°C for 100, 500 and 1000 hours. After exposure, the specimens were examined by using SEM equipped with EDX. The resulting changes of surface and built-up oxide layers are discussed. Liquid Metal Embrittlement, LME, crack initiation was not observed in the specimens. The absence of LME in these conditions is discussed.

Slip activity in molybdenum single crystals compressed at 77 K

ABSTRACT High-purity single crystals of molybdenum of five different orientations were compressed at 77 K to plastic strains between 3% and 4% to investigate the initial stage of plastic flow in these metals. Electron back-scatter diffraction was used to determine the sample orientations before and after deformation to quantify the magnitude and direction of lattice rotation. The slip trace analysis on two perpendicular faces of each prismatic specimen, made using the Nomarski interference contrast, reveals that the slip morphology depends strongly on the orientation of the applied load. The sample oriented near the axis and that of the centre-triangle orientation exhibited sharp slip traces on and planes, whereas the samples oriented near the edge of the triangle yielded by macroscopic slip on a series of high-index planes. The orientations of observed slip traces were compared with theoretical predictions of slip activity of the twelve slip systems made by the Schmid law and using the effective yield criterion developed previously using molecular statics simulations of an isolated screw dislocation under stress.

W-B-C Nanostructured Layers - Microstructure and Mechanical Properties

Several W-B-C layers were prepared by magnetron sputtering. The microstructure of thin layers was observed by means of scanning and transmission electron microscopy on cross sections prepared using a focused ion beam. Both undisturbed layers and the volume under indentation prints were inspected. The W-B-C layers are fine nanostructured materials about 2 μm thick and indents with loads up to 1 N do not cause any visible defects (cracks, delamination etc). The results were correlated with mechanical properties characterized by means of nanoindentation experiments in both the static and the dynamic loading regime using a Berkovich indenter. Elastic modulus, indentation hardness and fracture resistance of prepared nanostructured coatings were evaluated and discussed.