Zdeněk Drozd

Strain-hardening behaviour of AZ31 magnesium alloys

Abstract The in-plane tensile behaviour of rolled sheets of the magnesium alloy AZ31 was investigated in both an H24 state and an aged state. Whereas the very high initial strain-hardening rate decreases monotonically with strain for the H24 samples, the annealed structure exhibits a striking increase in the strain-hardening rate. To explain this phenomenon, the occurrence of sessile dislocations associated with a secondary (pyramidal) slip is proposed. The absence of increasing hardening for the H24 state is supposed to be related to higher yield stresses (predeformations) within the grains of the polycrystalline aggregate.

Plastic Properties of Microcrystalline Mg with Ceramic Nanoparticles

The Mg micropowder was mixed and ball milled with ceramic nanoparticles. The material was compacted and then hot extruded. Samples have been deformed in tensile as well as compression tests at temperatures between 20 and 300 °C at a constant initial strain rate. The flow stress is significantly influenced by temperature. The yield stress and maximum stress decrease with increasing temperature. Stress strain curves obtained at lower temperatures in tensile tests substantially differ from ones estimated in compression. Stress relaxation tests were conducted in order to find the internal stress as well as to identify possible thermally activated process(-es).

Anisotropy of mechanical and thermal properties of AZ31 sheets prepared using the ARB technique

In the accumulative roll bonding (ARB) technique, repeated stacking of material followed by conventional roll-bonding is carried out. For this process the surfaces are cleaned with ethanol and then joined together by rolling. The rolled material is then cut into two halves, again surface treated and roll-bonded. This process may be repeated several times. For the magnesium alloy AZ31 (Mg-3Al-1Zn) rolling at an elevated temperature of 400 °C is necessary for ARB because of the low plasticity of hexagonal magnesium alloys at lower temperatures. Samples for this study were prepared using 1 to 3 ARB passes through the rolling mill. It was found that the ARB substantially refined the grain size of sheets to the micrometer scale. The microstructure and texture of the deformed samples were studied by light and electron microscopy. The mechanical properties of the ARB samples were explored using tensile test-pieces cut from the sheets with the tensile axis taken either parallel or perpendicular to the rolling direction, where a significant anisotropy in both mechanical properties and Youngs modulus was found. Anisotropy is explained on the basis of the specific microstructure and texture formed during the ARB process.

Characterisation of an Al-BN nanocomposite prepared by ball milling and hot extrusion

Aluminium-matrix-nanocomposites were manufactured by ball milling of microscale aluminium powder with BN nanoparticles in air, followed by subsequent consolidation by hot extrusion. The microstructure of the samples was studied using scanning electron microscopy. Vickers microhardness measurements were used to probe the mechanical properties of the samples. The amplitude dependent damping of the nanocomposites was measured at room temperature after thermal treatment of samples, and the linear thermal expansion was measured over a wide temperature range from room temperature up to 670K in the as-extruded state. The experimental results give a comprehensive picture of the behaviour of this nanocomposite system over the range of thermomechanical treatment conditions examined in this study. Based on these experimental data some possible influences of BN nanoparticles on the anelastic, plastic and thermal properties of microcrystalline aluminium are discussed.

Experimental Study on the Relation between Elastic and Thermal Deformation of the AZ31 Magnesium Alloy and Composite

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.

Deformation behaviour of microcrystalline magnesium reinforced by alumina nano-and microparticles

Abstract Mg-based micro- and nanocomposites were prepared by the powder metallurgical route. Specimens were deformed in tension and in compression at temperatures between RT and 300 °C at a constant crosshead speed giving an initial strain rate of 5.5 × 10– 5 s– 1. Both a yield stress and an ultimate stress improvement were observed in the composites in comparison with the unreinforced microcrystalline Mg. A different deformation behaviour in tension and in compression was observed. The changes in the deformation behaviour can be attributed, among others, to twinning, which occurs in the composites at higher stages of deformation. Various contributions to the composite strengthening are discussed.

Amplitude-dependent internal friction in AZ31 alloy sheets submitted to accumulative roll bonding

Fine grained magnesium alloy AZ31 sheets were submitted to the accumulative roll bonding (ARB). After ARB, the microstructure of samples was refined, and the sheets exhibited pronounced texture. The amplitude-dependent internal friction (ADIF) was measured at room temperature. Microstructure changes as the increased dislocation density, grain size refinement, twins, and texture influenced the ADIF. A significant anisotropy of the properties was observed. Experimental results are discussed on the base of physical mechanisms responsible for internal friction.Fine grained magnesium alloy AZ31 sheets were submitted to the accumulative roll bonding (ARB). After ARB, the microstructure of samples was refined, and the sheets exhibited pronounced texture. The amplitude-dependent internal friction (ADIF) was measured at room temperature. Microstructure changes as the increased dislocation density, grain size refinement, twins, and texture influenced the ADIF. A significant anisotropy of the properties was observed. Experimental results are discussed on the base of physical mechanisms responsible for internal friction.

Effect of hexagonal boron nitride and graphite nanoparticles on the mechanical and physical properties of magnesium

Magnesium nanocomposites reinforced with 3 vol.% of hexagonal boron nitride (hBN) and ultrafine-grained magnesium with 3 vol.% of graphite (Gr) nanoparticles were prepared by milling followed by hot extrusion. The influence of the hBN and Gr nanoparticles on the mechanical and physical properties of magnesium were subsequently investigated. The microstructure of the nanocomposites was analysed by electron microscopy. The nanocomposites were deformed in compression over a wide temperature range from room temperature to 300 degrees C and true stress-true strain curves were determined. The linear thermal expansion coefficient was measured over a wide temperature range from room temperature up to 400 degrees C. Pre-deformation in compression was used to estimate the influence of twins on the thermal expansion coefficient. The results are analysed and possible physical mechanisms for the observed mechanical and physical properties are discussed.