Interactions between iron, manganese, and the Al-Si eutectic in hypoeutectic Al-Si alloys

Sand-cast plates were used to determine the effect of iron and manganese concentrations on porosity levels in Al-9 pet Si-0.5 pet Mg alloys. Iron increased porosity levels. Manganese additions increased porosity levels in alloys with 0.1 pet Fe, but reduced porosity in alloys with 0.6 and I pet Fe. Thermal analysis and quenching were undertaken to determine the effect of iron and manganese on the solidification of the Al-Si eutectic. At high iron levels, the presence of large beta-Al5FeSi was found to reduce the number of eutectic nucleation events and increase the eutectic grain size. The preferential formation of alpha-Al15Mn3Si2 upon addition of manganese reversed these effects. It is proposed that this interaction is due to beta-Al5FeSi and the Al-Si eutectic having common nuclei. Porosity levels are proposed to be controlled by the eutectic grain size and the size of the iron-bearing intermetallic particles rather than the specific intermetallic phase that forms.

Light alloy castings for automotive applications: The case of Al vs Mg

The economical and environmental effects of mass reduction through Al and Mg primary alloys substitutions for cast iron and steel in automotive components are discussed using MF. Ashby's penalty functions method The viability of Mg alloy substitutions for existing Al alloy cast components is also considered. The cost analysis shows that direct, equal-volume, Al alloy substitutions for cast iron and steel are the most feasible in terms of the CAFE liability, followed by substitutions involving flat panels of prescribed stiffness. When the creation of CO2 associated to the production of Al and Mg is considered, the potential gasoline savings over the lifespan of the car compensate for the intrinsic environmental burden of Al in all applications, while electrolytic Mg substitutions for cast iron and steel are feasible for equal volume and panels only. Magnesium produced by the Pidgeon thermal process appears to be too primary energy intensive to be competitive in structural applications. Magnesium substitutions for existing Al alloy beams and panels are generally unviable. The current higher recycling efficiency of Al casting alloys confers Al a significant advantage over Mg alloys.

Serrated yielding and the localized shear failure mode in aluminium alloys

A study has been made of serrated yielding in two commercial Al-Zn-Mg alloys in the as-quenched condition. The different serration types produced in the two alloys and the shear failure mechanism observed in both notched-bend and tensile testing are related to the mechanisms of dynamic strain ageing occurring during the test. An estimate of 19.7 kJ/mole for the activation energy for exchange of a solute atom and a vacancy in Al-6.2 wt% Zn, 2.5 wt% Mg has been made. © 1981.

Assessment of biodegradable magnesium alloys for enhanced mechanical and biocompatible properties

Biomaterials have been used for more than a century in the human body to improve body functions and replace damaged tissues. Currently approved and commonly used metallic biomaterials such as, stainless steel, titanium, cobalt chromium and other alloys have been found to have adverse effects leading in some cases, to mechanical failure and rejection of the implant. The physical or chemical nature of the degradation products of some implants initiates an adverse foreign body reaction in the tissue. Some metallic implants remain as permanent fixtures, whereas others such as plates, screws and pins used to secure serious fractures are removed by a second surgical procedure after the tissue has healed sufficiently. However, repeat surgical procedures increase the cost of health care and the possibility of patient morbidity. This study focuses on the development of magnesium based biodegradable alloys/metal matrix composites (MMCs) for orthopedic and cardiovascular applications. The Mg alloys/MMCs possessed good mechanical properties and biocompatible properties. Nine different compositions of Mg alloys/MMCs were manufactured and surface treated. Their degradation behavior, ion leaching, wettability, morphology, cytotoxicity and mechanical properties were determined. Alloying with Zn, Ca, HA and Gd and surface treatment resulted in improved mechanical properties, corrosion resistance, reduced cytotoxicity, lower pH and hydrogen evolution. Anodization resulted in the formation of a distinct oxide layer (thickness 5-10 μm) as compared with that produced on mechanically polished samples (~20-50 nm) under ambient conditions. It is envisaged that the findings of this research will introduce a new class of Mg based biodegradable alloys/MMCs and the emergence of innovative cardiovascular and orthopedic implant devices.^

Low toxicity ionic liquid surface coating for biodegradable Mg-based stents

 Biodegradable Mg stents are a new treatment means for the human coronary artery disease. This study works on investigating a low toxic and corrosion protective surface coating material- phosphate based ionic liquid to control the degradation rate of Mg alloys in the body.

The effect of treatment time on the ionic liquid surface film formation: promising surface coating for Mg alloy AZ31

Mg alloys are attractive materials for medical devices. The main limitation is that they are prone to corrosion. A low toxicity surface coating that enables uniform, controlled corrosion at a desired rate (this usually means it must offer barrier functions for a limited time period) is desirable. Phosphate-based ionic liquids (ILs) are known to induce a coating that can reduce the corrosion rate of Mg alloys, Furthermore, some ILs are known to be biocompatible and therefore, controlling the corrosion behaviour of an Mg alloy and its surface biocompatibility can be achieved through adding an appropriate low toxic IL surface layer to the substrate. In this study, we have evaluated the cytotoxicity of three phosphate-based ILs to primary human coronary artery endothelial cells. Among them, tributyl(methyl)-phosphonium diphenylphosphate (P1,4,4,4dpp) shows the lowest cytotoxicity. Therefore, further work was aimed at developing an appropriate treatment method to produce a homogeneous and passive surface coating based on P1,4,4,4dpp IL, with the focus on investigating the effect of treatment time. The results showed that that the formation of IL coating on AZ31 has proceeded progressively, and treatment time plays an important role. An IL treatment at 100 °C with an extended treatment time of 5 h significantly enhanced corrosion resistance of the AZ31 alloy in simulated body fluid. Additionally, the corrosion morphology was uniform and there was no evidence of "localized pitting corrosion" observed. Such a performance makes this ionic liquid coating as a potential surface coating biodegradable Mg-based implants.

Biomaterials in orthopedic bone plates : a review

This paper aims to review biomaterials used in manufacturing bone plates including advances in recent years and prospect in the future. It has found among all biomaterials, currently titanium and stainless steel alloys are the most common in production of bone plates. Other biomaterials such as Mg alloys, Ta alloys, SMAs, carbon fiber composites and bioceramics are potentially suitable for bone plates because of their advantages in biocompatibility, bioactivity and biodegradability. However, today either they are not used in bone plates or have limited applications in only some flexible small-size implants. This problem is mainly related to their poor mechanical properties. Additionally, production processes play an effective role. Therefore, in the future, further studies should be conducted to solve these problems and make them feasible for heavy-duty bone plates.

Orthopedic bone plates : evolution in structure implementation technique and biomaterial

With many important developments over the last century, nowadays orthopedic bone plate now excels over other types of internal fixators in bone fracture fixation. The developments involve the design, material and implementation techniques of the plates. This paper aims to review the evolution in implementation technique and biomaterial of the orthopedic bone plates. Plates were initially used to fix the underlying bones firmly. Accordingly, Compression plate (CP), Dynamic compression plate (DCP), Limited contact dynamic compression plate (LC-DCP) and Point contact fixator (PC-Fix) were developed. Later, the implementation approach was changed to locking, and the Less Invasive Stabilization System (LISS) plate was introduced as a result. Finally, a combination of both of these approaches has been used by introducing the Locking Compression Plate (LCP). Currently, precontoured LCPs are mainly used for bone fracture fixation. In parallel with structure and implementation techniques, numerous advances have occurred in biomaterials of the plates. Titanium and stainless steel alloys are now the most common biomaterials in production of orthopedic bone plates. However, regarding the biocompatibility, bioactivity and biodegradability characteristics of Mg alloys, Ta alloys, SMAs, carbon fiber composites and bioceramics, these materials are considered as potentially suitable for plates. However, due to poor mechanical properties, they have very limited applications. Therefore, further studies are required in future to solve these problems and make them feasible for heavy-duty bone plates.

Fluidity of mica particle dispersed aluminium alloy

Cast aluminium alloy-mica particle composites were made by dispersing mica particles in a vortex produced by stirring the liquid Al-4 wt% Cu-1.5 wt% Mg alloy and then casting the melt containing the suspended particles into permanent moulds. Spiral fluidity and casting fluidity of the alloy containing mica particles in suspension were determined. Both the spiral fluidity and the casting fluidity of the base alloy were found to decrease with an increase in volume or weight percent of mica particles (of a given size), and with a decrease in particle size (for a given amount of particles). The fluidities of Al-4 wt% Cu-1.5 wt% Mg alloys containing suspended mica particles were found to correlate very well with the surface area of suspended mica particles. The regression equation for spiral fluidity Y (cm) as a function of surface area of mica particles per gram of spiral X (cm2 g–1) at 700° C was found to be Y=42.62–0.42X with a correlation coefficient of 0.9634. The regression equations for casting fluidity Yprime (cm) as a functiono of surface area of mica particles per gram of fluidity test piece Xprime (cm2 g–1) at 710 and 670° C were found to be Yprime=19.71–0.17Xprime and Yprime=13.52–0.105Xprime with correlation coefficients of 0.9194 and 0.9612 respectively. The percentage decrease in casting fluidity of composite melts containing up to 2.5 wt% mica with a drop in temperature is quite similar to the corresponding decrease in the casting fluidity of base alloy melts (without mica). The change in fluidity due to mica dispersions has been discussed in terms of changes in viscosity of the composite melts. However, the fluidities of these composite alloys containing up to 2.5 wt% mica are adequate for making a variety of simple castings including bearings for which these alloys have been developed.

Reactivities of aluminium and aluminium-magnesium alloy powders in polymeric composites

Polymeric compositions containing Al-Mg alloys show higher reactivities, in comparison with similar compositions containing aluminium. This is observed irrespective of the amount of oxidizer, type of oxidizer used, type of polymeric binder, and over a range of the particle sizes of the metal additive. This is evident from the higher calorimetric values obtained for compositions containing the alloy, in comparison to samples containing aluminium. Analysis of the combustion residue shows the increase in calorimetric value to be due to the greater extent of oxidation of the alloy. The interaction between the polymeric binder and the alloy was studied by coating the metal particles with the polymer by a coacervation technique. On ageing in the presence of ammonium perchlorate, cracking of the polymer coating on the alloy was noticed. This was deduced from differential thermal analysis experiments, and confirmed by scanning electron microscopic observations. The increase in stiffness of the coating, leading to cracking, has been traced to the cross-linking of the polymer by magnesium.

Thermodynamics and kinetics analysis of in-situ synthesizing AlN

AIN powders were prepared by in-situ synthesis technique. It is a reaction of binary molten Al-Mg alloys with highly pure nitrogen. It was confirmed through thermodynamics calculation that Mg element in Al-Mg alloys can decrease oxygen content in the reacting system. Thus, nitridation reaction can be performed to form AIN. Moreover, an analysis of kinetics shows that the nitridation reaction of Al-Mg alloys can be accelerated and transferred rapidly with the increment of Mg content.

Atom probe tomography study of the nanoscale heterostructure around an Al20Mn3Cu2 dispersoid in aluminum alloy 2024.

Atom probe tomography (APT) has been used to investigate the surface and sub-surface microstructures of aluminum alloy 2024 (AA2024) in the T3 condition (solution heat treated, cold worked, and naturally aged to a substantially stable condition). This study revealed surface Cu enrichment on the alloy matrix, local chemical structure around a dispersoid Al20Mn3Cu2 particle including a Cu-rich particle and S-phase particle on its external surface. Moreover, there was a significant level of hydrogen within the dispersoid, indicating that it is a hydrogen sink. These observations of the nanoscale structure around the dispersoid particle have considerable implications for understanding both corrosion and hydrogen embrittlement in high-strength aluminum alloys.

Evaluating the effect of yttrium as a solute strengthener in magnesium using in situ neutron diffraction

In situ neutron diffraction of two binary Mg alloys, Mg-0.5 wt.% Y and Mg-2.2 wt.% Y have been carried out in compression. The experimental data has been modelled using the elastoplastic self-consistent methodology in order to determine the critical resolved shear stress for basal slip, second-order 〈c + 〉 pyramidal slip and {101̄2} twinning. It was found that the addition of Y strengthens all three of the deformation modes examined. However, increasing the Y concentration from 0.5% to 2.2% showed no additional hardening in the basal slip and {101̄2} twinning modes, indicating that solute strengthening of these deformation modes is already exhausted by a concentration of 0.5% Y. Second-order pyramidal slip showed additional solute hardening at the higher concentration. © 2014 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.

Finite element analysis of residual stresses in metallic coatings through a compound casting

Al and Mg alloys are widely used in industry as main lightweight alloys. They have excellent properties, such as low density, high ductility, and high specific strength, and so on. Generally speaking, Mg alloys are better than Al alloys. However the corrosion of Mg alloys is much more difficult to control compared Al alloys. Therefore to combine these two lightweight alloys, a composite-like structure is an ideal solution since Al alloys can be used as protective coatings for Mg alloys. Compound casting is a realistic technique to get this coating system. In the current study, we numerically study the compound casting using finite element method (FEM) to make these two alloys, a composite-like structure, satisfy requirements to resist corrosion required from industry, in which the aluminum layer is acting as a protective coating for the magnesium substrate. Several finite element models have been developed by using the birth and death element technique and we focus on compound casting-induced residual stresses in the compounded structure. The numerical results obtained from the proposed finite element models show the distribution profiles of thermal residual stresses. We found the major factors influencing the residual stresses are the temperature to pre-heating the Al substrate and the thickness of Mg deposits. © (2014) Trans Tech Publications, Switzerland.

Microstructure and corrosion of AA2024

AA2024-Tx is one of the most common high-strength aluminium alloys used in the aerospace industry. This article reviews current understanding of the microstructure of sheet AA2024-T3 and chronicles the emergence of new compositions for constituent particles as well as reviews older literature to understand the source of the original compositions. The review goes on to summarise older and more recent studies on corrosion of AA2024-T3, drawing attention to areas of corrosion initiation and propagation. It pays particular attention to modern approaches to corrosion characterisation as obtained through microelectrochemical techniques and physicochemical characterisation, which provide statistical assessment of factors that contribute to corrosion of AA2024. These approaches are also relevant to other alloys.