Effect of particles on the formation of deformation twins in a magnesium-based alloy

The deformation behaviour of the age hardenable alloy Mg–5%Zn after different precipitation treatments has been examined. It has been found that during compressive deformation, fine particles increase the number of twins that form, but reduce the size and total volume fraction of twins. Visco-plastic selfconsistent modelling has been used to show that the presence of precipitates hardens the twin and prismatic slip systems more than the basal slip system. It is proposed that because the {10 ¯12} twin requires basal slip to accommodate the twinning shear, this twin type will always be hardened equal to, or more than, the basal slip system in response to precipitation.

Deformation mechanisms in mg alloys and the challenge of extending room-temperature Plasticity

Magnesium alloys show promise for application in formed components where weight saving is an advantage. In most instances forming is carried out at elevated temperatures. However, there are considerable gains to be had if forming can be carried out under ambient conditions. The present article outlines some of the difficulties that lie in the way of achieving this objective. The underlying metallurgical characteristics of the issues are considered and means for overcoming them are discussed. It is concluded that a combination of microstructure and texture control remains a promising strategy.

Effect of grain size on the performance of extruded magnesium alloy tubes in three-point bending

The performance of extruded AZ31, AZ61 and AM-EX1 tubes was examined in three-point bending. Different extrusion temperatures were used to investigate the effect of grain size on the load-carrying capacity, energy absorption and fracture propensity of the tubes. Results showed that while the peak load increased with a smaller average recrystallised grain size, the retention of large elongated un-recrystallised grains in the microstructure reduced the load. The presence of the large elongated grains also appeared detrimental to the ability of the tube to deform before fracture.

Atomic scale simulation of deformation in magnesium single crystals

The deformation behaviour of magnesium single crystals under plane strain conditions has been examined using molecular dynamics modelling. The simulations were based on an existing atomic potential for magnesium taken from the literature. A strain of 10% was applied at rates of 3x10⁹s^-1 and 3x10⁷s^-1. The simulations predicted the formation of mechanical twins that accommodated extension in the c-axis direction of the hexagonal unit cell. However, the predicted twin is not of the same kind found in magnesium, but is that commonly observed in titanium. It is believed that further analysis of the physical properties predicted by this interatomic potential will shed more light on the atomic processes controlling twinning in Magnesium alloys. It also highlights the need for improvements to the interatomic potential such that more accurate deformation behaviour can be attained.

Biodegradable Mg-Ca and Mg-Ca-Y alloys for regenerative medicine

In this study, Mg-xCa (x = 0.5, 1.0, 2.0, 5.0, 10.0, 15.0 and 20.0 %, wt.%, hereafter) and Mg-1Ca-1Y alloys were investigated as new biodegradable bone implant materials. The compressive strength, ultimate strength and hardness of the Mg-Ca alloys increased, whilst the corrosion rate and biocompatibility decreased, with the increase of the Ca content in the Mg-Ca alloys; higher Ca content caused the Mg-Ca alloy to become brittle. Solutions of simulated body fluid (SBF) and modified minimum essential media (MMEM) with the immersion of Mg-xCa and Mg-1Ca-1Y alloys showed strong alkalisation. The yttrium addition to the Mg-Ca alloys does not improve the corrosion resistance of the Mg-1Ca-1Y alloy as expected compared to the Mg-1Ca alloy. It is suggested that Mg-Ca alloys with Ca additions less than 1.0 wt.% exhibited good biocompatibility and low corrosion rate.

"Rare Earth" texture development in extruded Mg-based alloys

This data collection addresses the problem of low ductility in magnesium alloys, preventing their wider use. It examines a series of dilute alloys in order to determine the effect of composition on the extrusion behaviour and texture, and on the room temperature tensile ductility.

An electrochemical impedance study of ionic liquid film formation and aqueous corrosion of magnesium alloy ZE41

Ionic liquid surface treatments are proposed as a method of controlling corrosion processes on magnesium alloys. An important magnesium alloy, ZE41 (nominally 4% Zn and 1% rare earth), was treated with the ionic liquid trihexyl(tetradecyl)phosphonium diphenylphosphate (P₆₆₆₁₄DPP). Impedance spectra were acquired at intervals during the treatment, indicating the development of a film and allowing a measure of the film formation process to be obtained over time. Mechanically polished and electro-polished surfaces were prepared; these surfaces, treated and untreated, were subsequently exposed to 0.1 M NaCl aqueous solutions. The corrosion behavior of the prepared surfaces were assessed using impedance spectroscopy and optical microscopy. The results indicated a significant role for the method of surface preparation used and, in both cases, the ionic liquid treatment produced a more corrosion-resistant surface.

Solute segregation and texture modification in an extruded magnesium alloy containing gadolinium

The alloy Mg-1.5Gd has been extruded at different temperatures to produce two significantly different textures. At lower extrusion temperatures there was significant solute clustering in the matrix, coupled with segregation of solute to the grain boundaries. At higher temperatures these two phenomena were both less pronounced. It is suggested here that segregation of solute to the grain boundaries plays a significant role in the texture modification effect that rare earth elements have in magnesium alloys.

In vitro studying corrosion behaviour of biocorrodible Mg alloys

The idea of bioabsorbable/biocorrodible stents has gained increasing attention in the last decade. Permanent coronary stents, traditionally made from 316L grade stainless steel, are routinely used for the treatment of blocked arteries. However, these stents can cause complications such as restenosis, thrombosis and the need for the patient to undergo prolonged antiplatelet therapy. Biodegradable metal stents provide an opportunity for the stent to remain in place for a period to ensure restoration of function and then degrade through a carefully controlled bio-corrosion process. Among the number of potentially suitable materials, Magnesium alloys have shown great promise as a stent material due to their non-toxicity [1] and the corrosion rates attainable in biological environments. However, a carefully controlled corrosion process is essential in order to avoid hyper hydrogen generation and the fatal consequences that follow. In addition uniform corrosion is a basic requirement to maintain the mechanical integrity and load bearing characteristics. Work being undertaken in our laboratories focuses on controlling the corrosion behaviour of magnesium in a simulated biological environment in the presence of protein. In the investigation reported here the Mg alloy has been examined using Scanning Electrochemical Microscope (SECM) to visualize the corrosion process and identify the corrosion pattern. Complementary bulk electrochemical techniques (EIS and potentiodynamic polarization) have been used to acquire kinetic and mechanistic information. Early results obtained by SECM have revealed the tendency towards pitting corrosion in the early stages which subsequently develops in to filiform corrosion.