Papers by Author: Zuzanka Trojanová

Abstract: In the paper the temperature and amplitude dependence of internal friction in the AZ31, AZ63 and AZ91 alloys is reported. A rapid increase in internal friction with increasing temperature was estimated. Internal friction peaks were observed in AZ63 and AZ91 alloys. Position and height of the peaks in the temperature scale depends on the heating rate. Simultaneously, microstructural observations were performed in the selected points of the temperature scale. The amplitude dependence of internal friction in an AZ31 alloy, prepared by rolling, was measured. A significant influence of the rolling texture has been estimated. Main mechanisms of internal friction are connected with precipitation, dislocation motion and twinning.

Abstract: The main reason why new technologies and treatment procedure are being developed is to attain special mechanical properties. However, these developments are nowadays done on a small material volume either using some laboratory simulators, applying sever plastic deformation procedures or chemical composition screening for multicomponent alloys development by laser or electron beam melting. In all these application a small volume of the material assessed is available and standard procedures for crucial mechanical properties determinations are not applicable. Thus small size techniques should be applied. There has been extensively used small punch test technique (SPT) for those cases in recent years. This technique is mainly based on the evaluation using correlation between standard and SPT tests for considered material. In cases when insufficient material volume is available, those correlations cannot be established and thus comparative evaluation only can be carried out. This kind of evaluation is insufficient for the contemporary purposes, when full material potential is to be utilized. Therefore, procedures providing results directly comparable with standard specimens are being developed. Fundamental properties are those determined from tensile tests. The current paper is presenting application of developed miniature tensile test specimen method to materials after SPD processes. Quasi static properties determination is shown here for Magnesium and Titanium alloys for ECAP and Rotary Swaging SPD techniques. The results obtained from testing can be used not only for a direct material properties assessment and comparison, but also as input data for FEM codes, significantly increasing the materials considered application potential assessment.

Abstract: Neutron diffraction method has been applied in the ex-situ investigation of the residual stresses in Mg-5wt.%Al-1 wt.%Sr (AJ51) magnesium alloy reinforced with short Saffil® fibers deformed in compression at room temperature. The residual stresses were measured in the axial and radial directions with respect to the load direction. It is shown that in the initial state the tensile stress is present in the matrix phase. The in-situ acoustic emission measurements were performed with the aim to reveal the main deformation mechanisms operating in the particular stages of the plastic deformation. Ex-situ neutron diffractions experiments showed that the tensile axial residual stress in the matrix increases with increasing plastic deformation while the radial residual stress decreases. In situ acoustic emission measurements indicate that the main deformation mechanisms are twinning and glide of bigger dislocation ensembles in the early stages of the compressive deformation while the fibers breakage was observed in the vicinity of the maximum stress.

Abstract: AS21 magnesium alloy (2.1Al-1Si-balance Mg in wt.%) and the alloy reinforced with short δ-Al₂O₃ fibres (Saffil^®) were deformed in compression at temperatures between 23 and 300 °C. Stress relaxation tests were performed in order to reveal features of the thermally activated dislocation motion. Internal and effective components of the applied stress have been estimated. The activation volume decreases with increasing effective stress. The values of the activation volume and the activation enthalpy indicate that the main thermally activated process in the alloy as well as in the composite is the dislocation motion in non-compact planes.

Abstract: Magnesium alloy EZ10 was deformed in tension at temperatures from room temperature up to 400 °C with an initial strain rate of 2.7x10^-3 s^-1. Deformation tests showed a rapid decrease of the tensile yield strength at temperatures higher than 300 °C. Microstructure of the deformed samples was analysed with light microscope. Fracture mechanisms were estimated using scanning electron microscopy.

Abstract: Aluminium alloys have very useful properties. Because they are light, easily formed, machined and cast, they are widely used in aircraft industry and automotive industry. Their mechanical properties are mostly influenced by former mechanical and thermal treatment. An extruded aluminium alloy EN AW – 2007 was used in the as-received state and the specimens were thermally cycled with increasing the upper temperature step-by-step. After each thermal cycle the stress-amplitude dependence of internal friction was measured at room temperature. The quality factor Q^-1 was measured by a resonant method at a frequency of 20 kHz. Thermal and mechanical treatments influenced the anelastic properties of the material. The thermal treatment affects the internal friction in a positive way, while the mechanical treatment decreases Q^-1. The microstructural changes were observed by optical microscopy.

Abstract: The amplitude dependent damping of two bending beam samples of magnesium alloy AJ91 (9 wt.% Al, 1 wt.% Sr) was measured at room temperature in as cast condition, after quenching from high temperatures into water of room temperature and after various bending cycles to fatigue. Some measurements were performed successively with about 33 Hz and 100 Hz resonant frequency. The measurements show typical dislocation damping in as cast condition, after heat treatment at temperatures lower than 420°C, and cycle numbers lower then 50.000. For higher quenching temperatures the damping increases over the whole measured strain range with increasing quenching temperature and number of cycles to fatigue. After quenching from temperatures higher than 478°C the crack damping becomes dominant. The effects of damping seem to increase with increasing frequency. In one sample damping of individual cracks could be identified in the amplitude dependent damping curves by their characteristic course very similar to the ones postulated in an earlier publication by a simple rheological model [4]. The extending of crack length leads to a shift of the damping to lower strains.

Abstract: The aim of this work is to study the Amplitude Dependent Internal Friction (ADIF) of magnesium alloy AZ31 at room temperature at the frequency 20kHz. The internal friction of AZ31 at room temperature is mostly influenced by mechanical cycling at strain amplitudes in the microplastic deformation region. An excited state of the AZ31 alloy, which can be associated with a higher internal friction and lower dynamic modulus than usual state, was found immediately after mechanical cycling. When the strain amplitude drops, the diffusion of solute atoms restores the Zener atmosphere and the internal friction relaxes exponentially with the second root of time. The measurement methodology and obtained results are presented.

Abstract: The dilatation characteristics of the continuosly cast AZ31 alloy and composite with AZ31 matrix reinforced by SiC nanoparticles were investigated in the temperature range of 20-410 °C. The axis of specimens was either parallel or perpendicular to the casting texture. The linear thermal expansion of the alloy as well as the composite was measured in an argon protective atmosphere using a Netzsch 410 dilatometer. The relative elongation and coefficient of thermal expansion are the main experimental results obtained using dilatometry. The temperature dependence of the elastic modulus can be calculated using analysis of the dilatometry results.

Abstract: The deformation characteristics of the WE54 magnesium alloy reinforced by 13% of SiC particles have been investigated in tension at elevated temperatures. Composite material was prepared by powder metallurgy technique. The strain rate sensitivity parameter m has been estimated by the abrupt strain rate changes (SRC) method. SRC tests and tensile tests with constant strain rate ( ) were performed at temperatures from 350 to 500 °C. Increased ductility has been found at high strain rates. The corresponding m value was 0.3. The activation energy Q has been estimated. Microstructure evolution has been observed by the light microscope and scanning electron microscope.