Zuzanka Trojanová

Mechanical spectroscopy of commercial AZ91 magnesium alloy

Abstract A damping peak at around 425 K for a frequency of about 1 Hz was found and it was correlated to the characteristic grain boundary peak of magnesium. An increase in the height of the grain boundary peak appears during the temperature cycles, controlled by the decrease of solute atoms in the matrix since the appearance of a precipitation process.

Study of relaxation of residual internal stress in Mg composites by internal friction

Abstract When a metal matrix composite is subjected to temperature changes, thermal stresses arise at the interfaces between the matrix and the reinforcement as a result of the considerable mismatch between the thermal expansion coefficients of the matrix and the reinforcement. Even moderate temperature changes can produce thermal stresses that exceed the matrix yield stress. Consequently, new dislocations are generated at the interfaces causing thermal fatigue (microstructural changes, matrix plastic deformation and irreversible shape changes). The microstructural changes in the matrix can be detected by damping and stress relaxation measurements. The logarithmic decrement and modulus defect were measured after thermal treatment at temperatures between room temperature and temperatures up to 400°C. The strain dependence of the logarithmic decrement can be divided into two regions. The values of the logarithmic decrement are also influenced by heat treatment and foreign atoms in the matrix. The results can be explained assuming that straining and thermomechanical treatment produce changes in the microstructure of the composites. The stress relaxation curves were analysed and the modulus defect, relaxation strength and activation energy for dislocation motion were estimated. It is very difficult to draw conclusions on the mechanisms responsible for the stress relaxation but part of the relaxation strength is due to reversible dislocation movement.

Internal friction in microcrystalline magnesium reinforced by alumina particles

Abstract Using internal friction measurement, changes in the microstructure of microcrystalline magnesium and magnesium with 3 vol.% of Al 2 O 3 microparticles and nanoparticles, due to thermal treatment, have been investigated. Materials have been thermally treated at increasing temperature and then the amplitude dependence of the logarithmic decrement was measured at room temperature. The thermal treatment influences the amplitude-dependent component of the decrement in composite with nanoparticles while in composite with microparticles no influence has been estimated. Thermal stresses induced in the composites due to a difference between thermal expansion coefficient of matrix and ceramic particles may create new dislocations during the cooling from elevated temperatures. Density of new dislocations depends on the particle size. The thermal stresses can achieve yield stress in the matrix and micro-glide of newly created dislocations as well as their annihilation may occur.

Hardening and softening in Zr–Sn polycrystals

Abstract Polycrystalline samples of Zr-0.8Sn and Zr-6.0Sn alloys were deformed in tension at 650 K. The deformation behaviour of polycrystalline is studied using analysis of the work-hardening coefficient ϑ=( d σ d e ) dot e , T vs. stress curves. Three models proposed by Kocks, Estrin and Mecking, and Roberts are used in the analysis. The models are applicable only in a certain stress region. The parameters of the models (mean free path, annihilation length) are compared with some characteristics of the microstructure (grain size, subgrain size, dislocation network).

Changes in the microstructure of QE22 composites estimated by non-destructive methods

Using non-destructive methods changes in the microstructure of QE22 metal matrix composites (MMC) due to thermal cycling have been investigated. Thermal stresses induced in the composites due to a difference between thermal expansion coefficient of matrix and ceramic particles may create new dislocations during the cooling from elevated temperatures. The thermal stresses can achieve the yield stress in the matrix and micro-glide of new created dislocations as well as their annihilation can occur. Thermodynamic processes in the matrix influences these effects.

Influence of rapid solidification on the damping behaviour of some magnesium alloys

Rapid solidification processing of magnesium alloys leads to changes in their microstructure. This paper describes the damping behaviour of Mg, a series of Mg-Ca alloys and QE22 alloy prepared by rapid solidification. We use the logarithmic decrement as the characteristic parameter. The strain dependence of the logarithmic decrement exhibits two regions. At lower strains the logarithmic decrement is strain independent. In the second region the logarithmic decrement depends strongly on strain amplitude. The amplitude independent logarithmic decrement for the specimens prepared by rapid solidification is higher than that for cast specimens. The amplitude independent logarithmic decrement for Mg-Ca alloys prepared by rapid solidification decreases with increasing Ca concentration. Possible mechanisms are discussed.

Tensile and fracture properties of an Mg-RE-Zn alloy at elevated temperatures

Abstract Magnesium alloy EZ10 (Mg-RE-Zn) was deformed in tension at temperatures from 20 up to 520 °C. A rapid decrease of the yield and tensile strength with temperature was observed at temperatures higher than 300 °C. On the other hand, ductility of samples rapidly increased in the same temperature range. Light microscopy and scanning electron microscopy was used to reveal the reason for these behaviours. Intermetallic particles in grain boundaries are responsible for excellent mechanical properties at lower temperatures. Diffusional processes occurring at temperatures higher than 300 °C significantly influenced the deformation mechanism as well as the fracture character.

Physical aspects of plastic deformation in Mg–Al alloys with Sr and Ca

Abstract Magnesium alloys of AJ (Mg – Al – Sr) and AX (Mg – Al – Ca) series were deformed at temperatures between room temperature and 300 °C. Stress relaxation tests were performed in order to reveal features of the thermally activated dislocation motion. The work hardening rate = d/d decreases with increasing stress and temperature. Analysis of the – plots revealed the hardening and softening mechanisms operating during the deformation. Internal and effective components of the applied stress have been estimated. Very high values of the internal stress estimated at lower temperatures decrease rapidly with increasing deformation temperature. The apparent activation volume decreases with increasing effective stress. The values of the activation volume as well as the activation enthalpy indicate that the main thermally activated process is most probably the glide of dislocations in non-compact planes.

Magnesium-based nanocomposites

The Mg micropowder was generated by gas atomisation of a magnesium melt with argon. The ceramic nanoparticles (SiC, ZrO2, Al2O3) and carbon nanoparticles were generated by laser-induced gas phase reaction in a flow reactor. The Mg materials were mixed and ball milled with nanoscaled ceramic powder and then was hot extruded. The mechanical properties of new materials were investigated by tensile test at temperatures between room temperature and 300°C at a constant initial strain rate. The flow stress is significantly influenced by temperature. The nanocomposites exhibit high-damping characteristics. The damping was measured as the logarithmic decrement of the free decay of the vibrating nanocomposite beam. At lower strains, the logarithmic decrement is practically independent of the strain amplitude, whereas at higher strains the decrement increases with increasing strain amplitude. The volume fraction and the kind of reinforcement influence the value of the logarithmic decrement and its temperature dependence.

Damping in Mg-Sc alloys after thermal treatment

Internal friction of Mg–Sc alloys has been studied after sequential thermal treatment. The obtained strain amplitude dependencies of the logarithmic decrement can be divided into two regions. In the region of small strain amplitudes the decrement is dependent on amplitude only slightly while at higher amplitudes the decrement depends strongly on the amplitude. Both regions are separated by critical strain amplitude. The thermal treating of alloys changes the sample microstructure. These changes are reflected by the dependence of the critical amplitude on the thermal treatment temperature. An increase of the critical amplitude with temperature up to ≈160 °C is interpreted on the basis of the theory of vibrating shorter and longer dislocation segments caused by sonic wave. Solving of solute atoms clusters up to ≈160 °C is followed at higher temperatures very probably by a precipitation process. Die Dampfung von Mg–Sc-Legierungen wurde nach sequentiellen isochronen Warmebehandlungen untersucht. Die erhaltenen Abhangkeiten des logarithmischen Dekrements von der Dehnungsamplitude konnen in zwei Bereiche eingeteilt werden. Im Bereich kleiner Dehnungsamplituden ist das logarithmische Dekrement nur sehr wenig von der Dehnungsamplitude abhangig, wahrend sich fur hohe Dehnungsamplituden eine starke Abhangigkeit ergibt. Beide Bereiche werden durch die kritische Dehnungsamplitude getrennt. Die durchgefuhrten Warmebehandlungen andern die Mikrostruktur der Proben. Diese Anderungen werden durch die Abhangigkeit der kritischen Dehnungsamplitude von der Temperatur der Warmebehandlung angezeigt. Der Anstieg der kritischen Dehnungsamplitude mit der Temperatur bis zu ca. 160 °C wird auf der Basis der Theorie der Vibration von kurzeren und langeren Versetzungsstucken diskutiert, die durch Schallwellen angeregt werden. Der Auflosung von Fremdatomen-Clustern bis zu 160 °C folgt bei hoheren Temperaturen wahrscheinlich ein Ausscheidungsprozess.