Papers by Author: Pavel Lukáč

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: 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: 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: Stress relaxation tests have been used in order to determine parameters of a possible thermally activated process in AZ31 magnesium alloy. The samples were deformed at a constant initial strain rate of 6.7x10-5 s-1 at various temperatures between room temperature and 300 °C. Stress relaxation, i.e. a decrease in the stress with time, was measured at various stress levels and at various temperatures. An analysis of the stress relaxation curves enabled to estimate the internal stress as a function of the strain and the test temperature. It has been shown that the activation volume is a function of the effective stress independently of the deformation temperature.

Abstract: The deformation behaviour of the ternary magnesium alloy AX41 (4%Al-1%Ca-balance Mg) were investigated in uniaxial tension tests at temperatures between 20 and 300 °C and at an initial strain rate ranging in the order 10-4 s-1. The yield stress of the alloy is very sensitive to the testing temperature. Stress relaxation tests were performed with the aim to reveal physical base of deformation processes.

Abstract: The mechanical properties and deformation behaviour of magnesium alloys are significantly influenced by the testing temperature, texture and grain size. The paper gives an overview on the influence of the grain size on the mechanical properties of Mg and Mg-2Al alloys at various temperatures. The yield stress and the fracture stress increase with decreasing grain size. It was shown that the elongation to fracture increases with inverse square root of grain size. The effect of grain size on the tensile strength and the elongation to fracture is influenced by the testing temperature. The observed grain size dependence of the elongation to fracture is explained by the activity of non-basal slip systems.

Abstract: The stress relaxation phenomena in magnesium alloys, containing Al, Sr and Ca solutes have been investigated. The samples were deformed at a constant initial strain rate at various temperatures between room temperature and 300 °C. Stress relaxation, i.e. a decrease in the stress with time, was measured at various stresses and at various temperatures. The stress relaxation curves were analysed as a function of the internal stress at the beginning of the relaxation and the test temperature.

Abstract: Nondestructive methods may help to detect changes in the internal structure of a material and to explain the behaviour of the material. This paper describes a series of nondestructive tests performed on magnesium composites with a variety of matrices: commercial pure Mg and three magnesium alloys AZ91, ZC63 and ZE41. Short fibres of δ-Al2O3 (Saffil®) were used as the reinforcement. Internal friction measurements and joint dilatation and acoustic emission studies were used to demonstrate how thermal cycling influences the deformation behaviour of Mg based metal matrix composites. The values of the logarithmic decrement are influenced by the upper temperature of the cycle. The acoustic emission activity and the residual strain increase with increasing upper temperature. The results may be explained assuming that internal thermal stresses are generated.