Superplastic Behaviour of Selected Magnesium Alloys

Superplastic materials exhibit anomalous plasticity, achieving strain until several thousand per cent. The phenomenon of plasticity is limited on special microstructure, temperatures and strain rates. Magnesium and magnesium alloys are known as materials with limited plasticity. This is due to their hexagonal structure of these materials. Finding the superplasticity conditions has a crucial importance for applications of magnesium alloys. In this chapter, we will deal with the superplastic behaviour of AZ91, QE22, AE42 and EZ33 magnesium alloys. Materials were prepared by various techniques: thermomechanical treatments, equal channel angular pressing, hot extrusion, rolling, friction stirring and high-pressure torsion. Strain rate sensitivity and elongation to fracture were estimated at various temperatures. Mechanisms of superplastic flow are discussed. Grain boundary sliding and diffusional processes were depicted as the main mechanisms responsible for high plasticity of these alloys. On the other hand, cavitation at elevated temperatures deteriorates the superplastic properties

HIGH STRAIN RATE BEHAVIOUR OF AN AZ31 + 0.5 Ca MAGNESIUM ALLOY

<em>The paper reports behaviour of magnesium alloy AZ31 (nominal composition 3 % Al - 1 % Zn – balance Mg) with an addition of 0.5 wt. % Ca at high strain rates. Samples were prepared by the squeeze cast technology. Dynamic compression Hopkinson tests were performed at room temperature with impact velocities ranging from 11.2 to 21.9 m.s^-1. A rapid increase of the flow stress and the strain rate sensitivity was observed at high strain rates. Transmission electron microscopy showed extremely high dislocation density and mechanical twins of two types. Adiabatic shear banding is discussed as the reason for the observed behaviour at high strain rates.</em><span> </span

Internal Friction in Magnesium Alloys and Magnesium Alloys- Based Composites

In practice, some problems connected with undesirable mechanical vibrations or interruption of acoustic bridges may be solved using high damping materials. Especially, transport industry needs high damping light materials with proper mechanical properties. Magnesium alloys and magnesium alloys‐based metal matrix composites may be considered as materials exhibiting such behaviour. Damping of mechanical vibrations and their conversion to the heat (internal friction) is conditioned by the movement and redistribution of various defects in the crystal lattice. Generally, internal friction depends on the material microstructure and conversely changes in the material microstructure may be studied using the internal friction measurements. The strain amplitude‐dependent internal friction was investigated at room temperature in commercially available Mg alloys and Mg alloys‐based composites with the aim to identify changes in the microstructure invoked by thermal and mechanical loading. The temperature‐dependent internal friction indicated the following effects: (a) mechanisms connected with dislocations and grain boundaries in the microcrystalline pure Mg, (b) precipitation and phase transformations in alloys and (c) generation as well as relaxations of thermal stresses in composites. The internal friction was measured in the bending mode in two frequency regions: I.: units and tens of Hz and II.: units of kHz

Influence of processing techniques on microstructure and mechanical properties of a biodegradable Mg-3Zn-2Ca alloy

New Mg-3Zn-2Ca magnesium alloy was prepared using different processing techniques: gravity casting as well as squeeze casting in liquid and semisolid states. Materials were further thermally treated; thermal treatment of the gravity cast alloy was additionally combined with the equal channel angular pressing (ECAP). Alloy processed by the squeeze casting in liquid as well as in semisolid state exhibit improved plasticity; the ECAP processing positively influenced both the tensile and compressive characteristics of the alloy. Applied heat treatment influenced the distribution and chemical composition of present intermetallic phases. Influence of particular processing techniques, heat treatment, and intermetallic phase distribution is thoroughly discussed in relation to mechanical behavior of presented alloys.Web of Science911art. no. 88

Strain Hardening in an AZ31 Alloy Submitted to Rotary Swaging

An extruded magnesium AZ31 magnesium alloy was processed by rotary swaging (RSW) and then deformed by tension and compression at room temperature. The work-hardening behaviour of 1&ndash;5 times swaged samples was analysed using Kocks-Mecking plots. Accumulation of dislocations on dislocation obstacles and twin boundaries is the deciding factor for the strain hardening. Profuse twinning in compression seems to be the reason for the higher hardening observed during compression. The main softening mechanism is apparently the cross-slip between the pyramidal planes of the second and first order. A massive twinning observed at the deformation beginning influences the Hall-Petch parameters

Thermal Conductivity of an AZ31 Sheet after Accumulative Roll Bonding

Accumulative roll bonding (ARB) is one of the methods of severe plastic deformation which is relevant for industrial production of sheets. While mechanical properties of several magnesium alloys subjected to the ARB process have been studied, the physical properties have been reported only for some magnesium alloys. These properties are influenced by the texture developed during the ARB process and the temperature load. In the presented contribution, we studied thermal conductivity of an AZ31 magnesium alloy after one and two passes through the rolling mill. Thermal diffusivity was measured with the laser-flash method in the temperature range between 20 and 350 &deg;C. Thermal conductivity depends on the number of rolling passes. The microstructure and texture of sheets are significant factors influencing thermal properties

Strengthening and Thermally Activated Processes in an AX61/Saffil Metal Matrix Composite

AX61 magnesium alloy was reinforced with short Saffil fibres using squeeze cast technology. Samples were cut from the casting in two directions: parallel and perpendicular to the fibre plane. Samples were deformed in compression at various temperatures from room temperature to 300 &deg;C. Various strengthening mechanisms such as load transfer, increased dislocation density, Orowan and Hall&ndash;Petch strengthening were analysed. During deformation, the stress relaxation tests were subsequently performed. The relaxation curves were evaluated with respect to Li and Feltham equations with the aim to find stress components in matrix and parameters of the thermally activated process(es)