Hynek Hadraba

A Novel Composite Material Designed from FeSi Powder and Mn0.8Zn0.2Fe2O4 Ferrite

A design of the novel microcomposite material composed of spherical FeSi particles and Mn0.8Zn0.2Fe2O4 ferrite is reported together with a characterization of basic mechanical and electrical properties. The sol-gel autocombustion method was used for a preparation of Mn0.8Zn0.2Fe2O4 ferrite, which has a spinel-type crystal structure as verified by XRD and TEM analysis. The final microcomposite samples were prepared by a combination of the traditional PM compaction technique supplemented with unconventional microwave sintering process of the prepared green compacts. The composition and distribution of the secondary phase formed by the spinel ferrite were examined by SEM. It is demonstrated that the prepared composite material has a tight arrangement without any significant porosity, which manifests itself through superior mechanical properties (high mechanical hardness, Young modulus, and transverse rupture strength) and specific electric resistivity compared to the related composite materials including resin as the organic binder.

Alumina and Zirconia Based Composites: Part 1 Preparation

Electrophoretic deposition of Al2O3 and ZrO2 powders from isopropanol suspension in the presence of monochloroacetic acid under constant-current conditions was studied. The similar charge and electrophoretic mobility of Al2O3 and ZrO2 in the suspensions was found. Adjusting to properly controlled kinetics of deposition deposits were prepared of pre-defined thicknesses. In view of the negative charge of Al2O3 and ZrO2 particles in the isopropanol suspensions used, the prepared layers were deposited on the anode and thus they were not affected by possible solvent electrolysis, which contributed to their defect-free and low-porosity structure.

Control of Electrophoretic Deposition Kinetics for Preparation of Laminated Alumina/Zirconia Ceramic Composites

Electrophoretic deposition of Al2O3 and ZrO2 powders from isopropanol suspension in the presence of monochloroacetic acid under constant-current conditions was studied. The deposits were prepared under different electrical conditions of deposition. It was found that the deposits prepared at low current densities (500 µA) contained smaller pores and achieved higher green densities than deposits prepared at high current densities (20 mA). The deposits prepared at low current densities were formed just by part of the particles in the suspension because of decreasing of efficiency of electrophoretic deposition process.

Micromechanical Aspects of Transgranular and Intergranular Failure Competition

Quantification of characteristics that govern intergranular fracture initiation and propagation of this fracture micromechanism in competition with cleavage one is main aim of the paper. A NiCr steel of commercial quality and the same steel with an increased content of impurity elements, Sn and Sb, were used. Step cooling ageing was applied in order to induce intergranular embrittlement. Standard and pre-cracked Charpy type specimens were both tested in three-point bending to determine fracture toughness characteristics. In order to characterise the quantitative differences in fracture surfaces roughness a fractal analysis was applied. A boundary level of fractal dimension has been determined to be 1.12: fracture surface roughness with a higher value reflects high level of intergranular embrittlement and thus fracture resistance degradation.

Fracture Behaviour of TiAl Intermetalics

The role of microstructural parameters in fracture behaviour of TiAl intermetalics at room and increased temperatures will be in focus of interest. Based on experimental findings sampled in this field and arising from literature knowledge an overview of typical fracture micromechanisms acting in different microstructures of TiAl intermetalics will be presented. Where possible semi-quantitative evaluation of toughening effects will be presented. Tensile properties, flexural strength and fracture toughness have been evaluated mainly for Ti-40Al-2Cr-2Nb-1B and Ti-46Al-0.7Cr-0.1Si-7Nb-0.2Ni alloys. For fracture toughness determination chevron-notch specimen technique have been applied except for standard pre-cracked samples. Positive effect of Nb on fracture resistance has been found; fracture toughness of the high Nb alloy increased at contemporary increase of flexural strength. Fracture surfaces have been evaluated using scanning electron microscopy showing the key role of mechanical twinning in deformation of both alloys followed. As a governing mechanism of crack nucleation microcrack formation at boundaries between mechanical twins and γ grains has been confirmed.

Alumina and Zirconia Based Layered Composites: Part 2 Fracture Response

Laminated materials are used for special applications where combination of properties of two components is needed. Ceramics is inherently brittle and above all superior properties (wear resistance, temperature durability, stiffness, low density and others) the brittleness is limiting factor for massive implementation in wide range applications. The laminated structure can be capable to overcome this handicap. Electrophoretic deposition (EPD) is the technique able to prepare ceramic laminated structures having strong interface between layers [1]. It is possible to prepare dense and crack free materials with tailored residual stresses controlled by layer thickness and deposition conditions by this technique. Crack propagation through layered composites based on Al2O3 and ZrO2 was studied. Cracks, produced by an indentation technique, propagated in direction to layer interfaces deflected towards the interface in the compressed layers and away from the interface in the layers containing tension stress [2,3]. Changes in the direction of crack propagation for the whole range of angles of incidence (0° - 90°) were described. The biggest change in the crack propagation was observed for the angle of incidence 45° for A/Z systems and was ca. 15°. The change in the crack propagation was independent on the level of residual stresses in the layers. Behaviour of indentation cracks observed in laminates under investigation was compared with the results obtained on standard SEVNB specimens having inclined the fracture plane with respect to the composite lamellar structure. This model condition helps to understand crack propagation in bulk material. All experimental work was supported by fractographical techniques enabling explanation of fracture micromechanism.

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.

The Prediction of Size Effect on J-R Curve for Eurofer97 Steel by Simplified Mechanical Model

The possibilities to derive fracture toughness from small specimens are naturally limited due to constraint requirements which are especially restrictive in toughness testing. The loss of constraint at the crack tip is more likely to occur as specimen size decreases. Application of miniature specimens in fracture toughness testing thus requires a suitable methodology or correction procedure to deal with phenomenon of the constraint loss. Schindler et al. have proposed a simplified mechanical model that can be used to scale-up the key test characteristics from miniature specimen to the larger one. The model is applied to the miniature bending specimens to describe size effect on J-R curve of the Eurofer97 steel. The examined steel exhibits quite high toughness values at upper shelf region of fracture toughness. As a result, experimentally determined J-R curves of three different sizes of pre-cracked bending specimens showed high values of J-integral, which were significantly different each other. Using semi-empirical definition of the exponent of the power law function of J-R curve the performance of the Schindler’s model was quite successful. It was shown that the model is able to handle with size effect of tested pre-cracked three-point-bend specimens.Copyright

Microfibres Containing Laminates Prepared by EPD: Kinetics of Co-Deposition

The aim of this work was preparation of alumina laminates and fibre reinforced alumina containing zirconia micro-fibres inside the inter-layer interface. Electrophoretic deposition was performed from concentrated isopropanolic suspensions stabilised by monochloracetic acid containing mixture of alumina powder and certain amount of zirconia micro-fibres. A method for in-situ monitoring of deposited amount of mixture of particles and fibres mass was applied to control deposition kinetics. Two different approaches for composite deposition were applied: pre-milling of powders mixture prior electrophoretic deposition and milling of suspension containing stabilization aid. Applying the optimised procedure the alumina laminates were prepared consisting of fibres in the interlayer boundary affected dramatically fracture response of these materials.

Mechanical Properties Determination of Unknown and 10Ch2MFA Steel Based on Non-Destructive Instrumented Hardness Test

The aim of the paper is to present developed methodology for evaluation of mechanical properties using nondestructive (NDT) methods. The final methodology will focus on evaluating the mechanical properties of the heterogeneous weld structure. The mechanical properties are determined by conducting NDT instrumented Vickers hardness test. The developed methodology for the basic mechanical properties determining is based on the inverse FEM modeling of instrumented hardness test and the indentation curve and the measured surface imprint are the output of indentation. The outputs from the experimental hardness test (the corresponding values from indentation curve and measured surface) are not always corresponding. These differences affect the accuracy of the developed methodology, because it is achieved of the basic material mechanical properties by comparing the outputs of the experimental instrumented hardness test and modeled hardness test using FEM.