Defect and Diffusion Forum Vol. 368

Abstract: Nanoindentation experiments were executed on amorphous metallic ribbons made of Fe₄₀Ni₄₀B₂₀, Cu₄₇Ti₃₅Zr₁₁Ni₆Si₁ and Zr₆₅Cu_17.5Ni₁₀Al_7.5 that differ in microhardness and glass forming ability. The individual serrated plastic flow events were analyzed in a wide range of the loading rates. In the individual pop-in events of the load-displacement (P-h) curve the contributions of plastic deformation (Δh_pl) were calculated depending on the loading rate and the alloy composition. It is concluded that the contribution of the serrated plastic deformation flow varies with the composition of the alloy. The highest plastic deformation for the individual pop-ins was observed for Zr-based metallic glasses.

Abstract: Technological processes of machining lead to plastically deformation of workpieces. Therefore it is needful to know influence of machining to machined material. The area of plastically deformation caused by friction of tool to machined surface was analysed. Local strain in structure was estimated by measurement of deformation of grains on metallographic cut using stereology. Local plastic deformation in deformation zone around the surface of drilled holes, local plastic deformation in deformation zone near the surface of milled workpiece and local plastic deformation near the surface of turned workpiece were investigated. The working piece was bulk from carbon steel CK45 (1.0503). Local plastic deformation was observed in case of drilling and turning, in case of milling no deformation of surface was present.

Abstract: The operational degradation of surface layers due to a rolling contact process was simulated for Hadfield steel by a special testing rig. A specific limited state based on cumulative depletion of plasticity was achieved.Precise evaluation of the surface layer's mechanical parameters is necessary for a service life prediction. Very low depth of localized dislocation hardening process doesn ́t allow the standard mechanical testing. The comparative instrumented indentation tests using Vickers and cylindrical indenter were used for evaluation of defined stage of surface degradation process. The intensity and reach of deformation hardening are partially limiting for particular methodology. The yield stress of surface layers was estimated according to an analytical model of uniaxial vs. indentation plastic flow ratio.

Abstract: Machined surface of hard steel parts like gears, bearings, camshafts etc. is full of local imperfections which are commonly described in terms of surface integrity. This surface integrity is usually quantified with the level of residual stresses or amplitude of Barkhausen noise signal for instance. The contribution introduces parameter called Sum of Effective Residual Stresses (SERS) to describe the level of surface damage after grinding. Experimental data obtained by X-Ray Diffraction (XRD) and Barkhausen Noise Analysis (BNA) methods of ground gears are shown. The way to use SERS approach for experimental determination of penetration depth of Barkhausen signal in steels is proposed, too.

Abstract: The research focuses on elastic-plastic behaviour of welded joints of materials which are commonly used in both constructional practice and the automotive industry. The examined welded joints are oriented to a common constructional S355 and a modern high-strength steel DOMEX 700MC.The main tool for the assessment of the elastic-plastic response was the methodology based on the instrumented penetration testing with the use of a cylindrical indenter. The indentation tool was applied into narrow zones of welded boundaries. These zones represent critical areas of welded constructions because of a changing structure and different mechanical properties than the basic material possesses. The indentation data were recalculated to the mechanical properties in shear with the help of Hencky ́s hypotheses about material behaviour. The correctness of the used methodology was verified by a tensile test. The aim of the work is to determine optimal methods for defining a yield strength and to find a hardening trend in the zones. The comparison of these steels revealed different changes due to the welding process. A substantial decrease of the yield strength of Domex700MC was observed, as a contrary to a stable, or partially increased, yield strength of the S355 steel. Structural analyses revealed a different material response of the evaluated steels, according to a different hardness in the heat affected zone.

Abstract: Actual shape of the diamond spherical indenter of nominal radius 20 μm was investigated in this study. 3D reconstruction was performed by atomic force microscope and by the method of stereopair using SEM images of the tip taken under several different angles. The results were compared with the shape obtained indirectly by the calibration performed on specimens with known Young’s modulus. It was found that lower effective values of tip radius for the small penetration depths are caused by the irregular geometry of contact between indenter and specimen surface. With increasing penetration depth the radius increased to the theoretical values and it decreased again for high penetration depths. The stress-strain curves were determined using corrected effective indenter radius.

Abstract: This paper shows the results of finite element modelling of contact problems between perfectly rigid body with rough surface and an indenter from elastic-plastic material. In the first phase, spherical indenter from elastic – plastic material is considered, which is pressed into a body with a rough surface. Results show the influence of the amplitude and distances of various undulations at the rough surface to response surface.

Abstract: Mechanical characterization of individual microstructural phases of hardly deformable magnesium alloys is of crucial importance for the development of multi-scale material models. The magnesium alloys are used for preparation of fine tubes with diameter of a few millimeters and tens of millimeters wall thickness. It is hard to control an ordinary drawing process for the preparation of such tubes. However, the tubes can be prepared with a laser dieless drawing process that is, in contrary to conventional drawing, able to draw low formability materials and it is able to produce variable cross-sections of the tube or a wire with high precision. The magnesium alloy tubes are used in various fields as micro-electro-mechanical systems, medicine, electrical, biological and chemical fields. In this paper, preliminary microstructural studies and local mechanical characterization of pure Mg, MgCa0.8 and AZ31 magnesium alloys used for tube extrusion, is provided. The material microstructure is studied by means of scanning electron, atomic force microscopes and nanoindentation. Elastic properties and volume fractions of mechanically distinct phases that are not accessible by standard testing methods are provided in the paper.

Abstract: Zirconium oxide samples were sintered at two different temperatures (1250°C and 1450°C) using SPS method with maximum holding time 10 min. to obtain nonstoichiometric composition. Some of the samples were subsequently annealed for 6 hours at 1100°C to obtain stoichiometric composition. Wear rate and friction coefficient were obtained from tribological tests carried out on the UMT3 equipment (Bruker) using two different static partners (zirconia and alumina ball). Main aim of this work is to study the influence of composition and sintering temperature on the wear behavior, friction coefficient and basic mechanical properties.

Abstract: The iron aluminides seem to be very perspective materials for high temperature structural application. They have many advantages, but unfortunately also some negative properties – e.g. sharp drop in strength above 600°C or limited ductility at room temperature. These disadvantages can be reduced by alloying of binary alloy by other elements.Present work deals with a study of coefficient of thermal expansion (CTE). It was investigated the influence of microstructure and heat-treatment on the values of CTE. Secondary, it was studied the possibilities, how to determine phase transition temperatures from CTE curves. Influence of type of iron aluminides lattice on CTE values was also examined as well as the influence of addition of alloying elements into binary iron aluminides.