Papers by Author: Kornel Csach

Abstract: Depending on the composition and structure of metallic glasses cells with the dimensions in the range from tenths nanometers to tenths micrometers were observed on the ductile fracture surface. The variation in dimple size was compared with the serrations presented on the loading curve at the nanoindentation of the metallic glasses with different compositions. Higher instantaneous deformation can be connected with simultaneous shearing at more suitable shear band configurations. The cell morphology with the various cell sizes is observed at the failure of the metallic glasses. At the failure of high strength metallic glasses, the cells are formed in short time due to the release of high amount of stored elastic energy. In this case the uniform cell morphology with the cell size of about 20 nm is observed.

Abstract: Plastic deformation after indentation of the metallic glass Cu₄₇Ti₃₅Zr₁₁Ni₆Si₁ at different loading conditions was examined. Discontinuities on the loading curves were observed, the magnitude of which depends on the loading rate. The presence of these discontinuities is influenced by the precise shape of the indentation tip. At lower loading rates and using a cube corner indenter tip the discontinuities on the loading curves are more pronounced. An increase of the loading rate tends to diminish instantaneous plastic deformation as appear by pop-ins. Using a Berkovich type indenter tip the plastic deformation is more steady. It is concluded that the final morphology of the pile-up area strongly depends on the geometry of the indenter tip, whereas no correlation between discontinuities in the loading part of the indentation curve and the formation of shear band patterns was observed.

Abstract: The ultrafine-grained copper was obtained by 12 passes of equal-channel angular pressing method. The uniaxial tensile tests at room temperature and the subambient temperature of 77 K show that the yield stress increases from the value of 128 MPa to the value of 138 MPa, respectively. In addition, the lowering the test temperature tends to the increase of the deformation before the failure. The fractographic analysis shows the transcrystalline ductile failure for all samples. Due to the high plasticity of nanostructured copper no influence of the nanoporosity on the failure process was observed.

Abstract: An amorphous metallic ribbon of Fe₄₀Ni₄₀B₂₀ was used for in-situ observation of the crack propagation and shear band formation during tensile tests. Prior to the tensile tests, two holes (with different positions with respect to the tensile axis) were made by laser ablation as stress concentrators. The nucleation and propagation of shear bands on the ribbon surface during tensile tests were analysed with scanning electron microscopy (SEM). At room temperature inhomogeneous plastic deformation of amorphous alloy occurs via the development of primary and secondary shear bands. The influence of the different loading geometry on the topology of shear bands and crack propagation was studied.

Abstract: In metallic glasses plastic deformation occurs via the creation and the propagation of a softened region in the shear bands. Some of the high strength metallic glasses (as Zr-based metallic alloys) exhibit complex shear band topography and the final failure respects the allocation of the shear bands. We studied the differences in the fracture surfaces of Zr-and Mg-based amorphous alloys. Ductile behaviour of the shear bands in Zr-based amorphous alloy tends to the dimple creation during the failure. On the fracture surfaces the vein pattern morphology manifestations were present. Conchoidal fracture was typical for Mg-based amorphous glass. Two different surface morphologies, plumes and rib marks ornament the fracture surfaces.

Abstract: Several structural states of the CoCrFeNiMnV_x (x=0, 0.25, 0.5, 0.75) high entropy alloys with different content of the intermetallic σ phase are studied in uniaxial compression in temperature range 4.2-300 K. Peculiarities of strain hardening stages and temperature dependences of the yield strength are registered and analyzed considering dislocation sliding as main deformation mechanism in matrix fcc phase. Influence of σ phase particles, deformation twins and stacking faults on the deformation behavior of the CoCrFeNiMnV_x alloy is discussed.

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: Nanoindentation and thermomechanical experiments on three types of metallic glasses with different glass forming ability were carried out. The nanoindentation behaviour at room temperature was associated with the creep at elevated temperatures. Different discontinuity populations and their shape observed on the nanoindentation loading curves were compared with morphology of plastic deformed indent regions. The influence of the differences in thermal stability of studied alloys on the nanoindentation in these alloys were studied as well.

Abstract: We analyzed the failure characteristics of the metallic glass Co₄₃Fe₂₀Ta_5.5B_31.5 (at.%) deformed in bending. The nanoscale fracture surface morphology respects the micromechanisms of the failure of the amorphous structure. The fracture surfaces consist of nanosized dimples (40 nm) arranged in the lines respecting the periodic corrugation zones oriented perpendicular to the crack propagation direction. The corrugation topology exhibits the point nature of the generation site, the concentric form of the stress waves and their interference.

Abstract: Nanoindentation experiments at the loading rates from 0.05 to 100 mN.s^-1 on the amorphous FeNiB alloy were executed. We found that the serrations in the load-displacement (P-h) curve are more pronounced at lower loading rates and gradually disappear upon increasing loading rate. We have estimated the contribution of the inhomogeneous plastic deformation from pop-in events on the P-h curves. The pop-in population was compared with the morphology of indents.