Surface functionalisation of magnesium alloys for use as bio-implants

 Magnesium-based alloys containing appropriate quantities of Strontium can induce optimal bone formation. Surface modification of these alloys with Collagen-I increased mineral deposition on the peri-implant surface over shorter periods of time as compared to the unmodified alloys, indicating the role of Collagen-I and Strontium concentration in bone resorption and remodelling.

Biocompatible magnesium alloys for hard tissue engineering

 Novel Mg-Zr-Sr and Mg-1Zr-2Sr-xDy/yHo alloys have recently been developed for use as biodegradable implant materials. These alloys are recommended to be promising biodegradable implant materials as they have enhanced corrosion resistance and excellent biocompatibility.

Advances in wrought magnesium alloys: Fundamentals of processing, properties and applications

This important book summarises the wealth of recent research on our understanding of process-property relationships in wrought magnesium alloys and the way this understanding can be used to develop a new generation of alloys for high-performance applications. After an introductory overview of current developments in wrought magnesium alloys, part one reviews fundamental aspects of deformation behaviour. These chapters are the building blocks for the optimisation of processing steps covered in part two, which discusses casting, extrusion, rolling and forging technologies. The concluding chapters cover applications of wrought magnesium alloys in automotive and biomedical engineering. With its distinguished editors, and drawing on the work of leading experts in the field, Advances in wrought magnesium alloys is a standard reference for those researching, manufacturing and using these alloys.

Theoretical study on the (1-012) deformation twinning and cracking in coarse-grained magnesium alloys

This paper theoretically and systematically investigates: (1) the effect of local transformed strains within deformation twinning on twin intersection; (2) the fracture mode based on type I co-zone tensile twin intersection in coarse-grained magnesium alloys, as well as the impacts of twin intersection and grain diameter on interfacial crack nucleation along twin boundaries; and (3) the influence of the local stresses arising from the encountered twin bands on crack growth. A novel dislocation-based strain nucleus model and a Green's function method, which are applicable to any material with local transformations in which elastic properties are reasonably approximated as isotropic, are specifically employed to model the concentrated transformed strain and calculate the local stress field resulting from deformation twinning and the stress intensity factors at crack tips in the magnesium alloys, respectively. In addition, an electron backscatter diffraction (EBSD) measurement is provided for crack nucleation originating from Type I co-zone tensile twin intersection. The theoretical modeling indicates: (i) the local strains within barrier twins strongly dictate the growth of incident twins and enhance the twin propagation stress; (ii) larger grains favor brittle fracture. More specifically, the dislocation reactions and pile-ups at the junctions between tensile twins can result in interfacial crack nucleation and growth along the twin boundaries, which is a brittle fracture mode based on lower twinning stress and stress concentration in the coarse-grained magnesium alloys; and (iii) the direction of crack propagation is easily changed by high-density twin bands and twin intersections owing to the local strains.

Magnesium alloy applications in automotive structures

The energy absorbed by magnesium alloys (high-pressure die-cast (HPDC) AM20, AM50, AM60, and extruded AZ31) in a buckling test was significantly greater than the aluminum alloy 6061 T6 and particularly mild steel of a similar weight, but was less than that of the aluminum alloy and steel for the same thickness (Figure 6).26 This indicates that mass savings can be achieved by the substitution with magnesium alloys to achieve similar energy-absorbing characteristics.

The Challenge of inhomogeneous deformation in magnesium and its alloys

It is shown that wrought magnesium alloys display a number of significant types of deformation inhomogeneities. These are influenced by the variation in the ease of basal slip amongst grains, micro-textures, shear banding, twinning and grain boundary sliding. Key features of each of these effects are examined and their engineering consequences and challenges are identified.

A semianalytical Sachs model for the flow stress of a magnesium alloy

A semianalytical Sachs-type equation for the flow stress of magnesium-base alloys is developed using the Schmid law, power law hardening, and a sigmoidal increase in the twinning volume fraction with strain. Average Schmid factors were estimated from electron backscattered diffraction (EBSD) data. With these, the equation provides a reasonable description of the flow curves obtained in compression and tension for samples of Mg-3Al-1Zn cut in different orientations from rolled plate. The model illustrates the general importance of basal slip and twinning in magnesium alloys. The significance of prismatic slip in room temperature tension testing is also highlighted. This is supported with EBSD slip line trace analysis and rationalized in terms of a possible sensitivity of the critical resolved shear stress for prismatic (cross) slip to the stress on the basal plane.

Determination of lower-bound ductility for AZ31 magnesium alloy by use of the bulge specimens

Despite the high demand for industrial applications of magnesium, the forming technology for wrought magnesium alloys is not fully developed due to the limited ductility and high sensitivity to the processing parameters. The processing window for magnesium alloys could be significantly widened if the lower-bound ductility (LBD) for a range of stresses, temperature, and strain rates was known. LBD is the critical strain at the moment of fracture as a function of stress state and temperature. Measurements of LBD are normally performed by testing in a hyperbaric chamber, which is highly specialized, complex, and rare equipment. In this paper an alternative approach to determine LBD is demonstrated using wrought magnesium alloy AZ31 as an example. A series of compression tests of bulge specimens combined with finite element simulation of the tests were performed. The LBD diagram was then deduced by backward calculation.

Grain size in Mg alloys: recrystallization and mechanical consequences

The present paper examines the development of grain size during the recrystallization of magnesium alloys and the influence the grain size has on the mechanical response. In magnesium alloys grain refinement improves the strength-ductility balance. This simultaneous increase in both strength and ductility is ascribed to the impact the grain size has on deformation twinning. The mechanisms by which the grain size is established during hot working are shown to be conventional dynamic recrystallization followed by post-dynamic recrystallization. The role of alloying additionon both of these reactions is briefly considered.

Effect of solidification grain refinement on the development of wrought Mg alloys

A recent trial investigated the effect of solidification grain refinement of billet on the grain refinement and properties of alloy ZM20. It was found that even at levels of 0.4Mn, significant grain refinement could be obtained when 0.7Zr was added. At 0.2Mn grain sizes as low as 60μm were
obtained. Billets of Mg-2Zn-0.2Mn with four different grain sizes, due to different Zr and cooling rates were then cast via vertical direct chill casting and extruded conventionally. Benefits of grain refinement of the billet on extrusion were found to be a slight increase in the size of the operating
window, and a reduction of the grain size in the extrudate. However, the effect of the reduction in extrudate grain size due to refinement of the billet was small compared with the amount of grain refinement obtained due to recrystallisation on extrusion.

Effect of composition on the texture and deformation behaviour of wrought Mg alloys

The microstructure and mechanical response of three extruded magnesium alloys, Mg-3Al-1Zn (AZ31), Mg-1.5Mn (Ml) and Mg-lMn-0.4RE (ME10) are examined. The tensile yield strength of ME10 was nearly half that of AZ31 and Ml. The tensile elongations were 6%, 11% and 19% for Ml, AZ31 and ME10, respectively. This range of properties is large and is attributed to the unique extrusion texture produced in ME10, and the high density of fine particles in Ml.

Microstructure evolution in hot worked and Annealed Magnesium Alloy AZ31

The present work examines the microstructure that evolves during the annealing of hot worked magnesium alloy AZ31. First, the influences of deformation and annealing conditions on the microstructures are assessed. It is found that the annealing behaviour is consistent with what one would expect for a recrystallization type reaction. Whilst both the deformation and annealing conditions influence the time required to reach a stable annealed microstructure, the grain size attained is governed solely by the prior deformation conditions employed. At the highest temperature and strain rate examined, the rate of recrystallization is quite high and the grain size was found to be approximately double when annealed for only 1 s prior to quenching. Finally, semi-empirical equations are developed to predict the kinetics of recrystallization, as well as the evolution of grain size, during annealing.

Processing and mechanical properties of open-cell Mg alloys

Mechanical properties of open-cellular magnesium alloys with three types of
geometric cell-structures, that is, a random round cell-structure (type A). a controlled diamond cell-structure for which the angle between the struts and the load direction is 45 degree (type B) and a controlled square cell-structure for which the angle between the struts and the loading direction is 0 degree (90 degree) (type C), are investigated by compressive tests. Results indicate that type C showed a higher collapse stress than the other two types. The collapse mechanism and the effects of the loading direction on collapse stress for the three types of magnesium alloys arc discussed from the viewpoint of bending, buckling and yielding of the struts. It is suggested that collapse for the open-cellular magnesium aHoys is associated with yielding of struts

Influence of alloying additions on the extrudability of magnesium

The extrusion behaviour of a series of magnesium alloys was investigated and compared to a common aluminium alloy using limit diagrams. The variation in the limits was related to the different flow stress and solidus temperature of each alloy. The findings of this work have enabled predictions of the relative extrudability of new prototype alloys.

Low temperature formability of differently processed AZ31B magnesium alloy sheets

The research found changes in the performance (formability) of magnesium alloy sheets with a history of different processes. A key outcome found that the steel sheet metal processing of rolling and heat treatment caused a detrimental effect. The material's internal deformation was found to be linked to the poor formability.