The origin of "Rare Earth" texture development in extruded Mg-based alloys and its effect on tensile ductility

The extrusion behaviour, texture and tensile ductility of five binary Mg-based alloys have been examined and compared to pure Mg. The five alloying additions examined were Al, Sn, Ca, La and Gd. When these alloys are compared at equivalent grain size, the La- and Gd-containing alloys show the best ductilities. This has been attributed to a weaker extrusion texture. These two alloying additions, La and Gd, were found to also produce a new texture peak with View the MathML source parallel to the extrusion direction. This “rare earth texture” component was found to be suppressed at high extrusion temperatures. It is proposed that the View the MathML source texture component arises from oriented nucleation at shear bands.

Structure and mechanical property variations in Mg-Gd-Y-Zn-Zr alloy depending on its composition and processing conditions

An effect of alloying element content on mechanical properties and precipitate formation in Mg-RE alloys was studied for Mg-8Gd-4Y- 1Zn-0.4Zr (wt%) and Mg-10Gd-5Y-1.8Zn-0.4Zr (wt%). It is shown that small variations in the alloying element concentration can be used to manipulate the alloy microstructure and precipitate formation towards eliminating the asymmetry (tension/compression) and anisotropy of yield stress.

Atom probe tomography of solute distributions in mg-based alloys

Atom probe tomography (APT) has been carried out on three magnesium-based alloys: M1 (Mg-1 wt pct Mn), AZ31(Mg-3 pct Al-1 pct Zn), and ME10 (Mg-1 pct Mn- 0.4 pct misch metal). The aims of this experiment were to measure the composition of the matrix and to investigate solute clustering in the matrix of the three different alloys. For AZ31, the matrix composition was variable but close to the bulk composition. For ME10 and M1, the matrix was depleted in alloying additions, with the remainder residing in precipitates. Most alloying additions were found to exhibit clustering to some extent, with misch metal having the strongest partitioning behavior to clusters. Solute clusters did not appear to affect mechanical twinning. It has been proposed that the clustering behavior of misch metal contributes to its ability to modify the recrystallization texture.

Exploiting low levels of rare earth addition in Mg extrusion alloys

The present work examines the extrusion and mechanical properties of MExlOO alloys, which contain levels of rare earth alloying additions up to 0.4 wt%. It is shown that these alloys can display the high extrudability of alloy Ml with strengths nearing those of AZ31. Most importantly, the grades display high room temperature ductility; values of total tensile elongation as high as 30% have been observed. These benefits derive from a combination of grain refinement and texture weakening.

"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.

Compositional effects on the restoration behaviour in Mg-Zn-Re alloys

Additions of rare earth elements to magnesium alloys are qualitatively reported in the literature to retard recrystallisation. However, their effect in the presence of other (non-rare earth) alloy additions has not been systematically shown nor has the effect been quantified. The microstructural restoration following the hot deformation of Mg-xZn-yRE (x = 2.5 and 5 wt.%, y = 0 and 1 wt.%, and RE = Gd and Y) alloys has been studied using double hit compression testing and microscopy. It was found that, in the absence of rare earth additions, increases in zinc level had a negligible influence on the kinetics of restoration and the microstructure developed both during extrusion and throughout double hit testing. Adding rare earth elements to Mg-Zn alloys was found to retard restoration of the microstructure and maintain finer recrystallised grains. However, in the Mg-Zn-RE alloys, increasing the zinc concentration from 2.5 wt.% to 5 wt.% accelerated the restoration process, most likely due to a depletion of rare earth elements from solid solution and modification of the particles present in the matrix.

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.

Effects of alloying elements on the corrosion behavior and biocompatibility of biodegradable magnesium alloys: a review

Magnesium (Mg) based alloys have been extensively considered for their use as biodegradable implant materials. However, controlling their corrosion rate in the physiological environment of the human body is still a significant challenge. One of the most effective approaches to address this challenge is to carefully select alloying compositions with enhanced corrosion resistance and mechanical properties when designing the Mg alloys. This paper comprehensively reviews research progress on the development of Mg alloys as biodegradable implant materials, highlighting the effects of alloying elements including aluminum (Al), calcium (Ca), lithium (Li), manganese (Mn), zinc (Zn), zirconium (Zr), strontium (Sr) and rare earth elements (REEs) on the corrosion resistance and biocompatibility of Mg alloys, from the viewpoint of the design and utilization of Mg biomaterials. The REEs covered in this review include cerium (Ce), erbium (Er), lanthanum (La), gadolinium (Gd), neodymium (Nd) and yttrium (Y). The effects of alloying elements on the microstructure, corrosion behavior and biocompatibility of Mg alloys have been critically summarized based on specific aspects of the physiological environment, namely the electrochemical effect and the biological behavior. This journal is © the Partner Organisations 2014.

Corrosion behavior of Mg-5Al-xZn alloys in 3.5 wt.% NaCl solution

Five types of Mg-5Al alloys with different weight percentages of Zn ranging from 0 to 4 wt.% were examined using electrochemical techniques and surface analysis. The electrochemical results indicated that the Mg-5Al alloys containing Zn have a lower corrosion and hydrogen evolution rates than the Mg-5Al based specimens with a decrease of value being observed with the decrease in Zn content. Zn addition induced the precipitation of Mg-Al and Mg-Zn phases in the Mg matrix along with grain refinement and increased an interaction of Zn oxide with Mg and Al products serving as a corrosion barrier. © 2014 Elsevier B.V. All rights reserved.

Mechanical properties, in vitro corrosion and biocompatibility of newly developed biodegradable Mg-Zr-Sr-Ho alloys for biomedical applications

Our previous studies have demonstrated that Mg-Zr-Sr alloys can be anticipated as excellent biodegradable implant materials for load-bearing applications. In general, rare earth elements (REEs) are widely used in magnesium (Mg) alloys with the aim of enhancing the mechanical properties of Mg-based alloys. In this study, the REE holmium (Ho) was added to an Mg-1Zr-2Sr alloy at different concentrations of Mg1Zr2SrxHo alloys (x = 0, 1, 3, 5 wt. %) and the microstructure, mechanical properties, degradation behaviour and biocompatibility of the alloys were systematically investigated. The results indicate that the addition of Ho to Mg1Zr2Sr led to the formation of the intermetallic phases MgHo3, Mg2Ho and Mg17Sr2 which resulted in enhanced mechanical strength and decreased degradation rates of the Mg-Zr-Sr-Ho alloys. Furthermore, Ho addition (≤5 wt. %) to Mg-Zr-Sr alloys led to enhancement of cell adhesion and proliferation of osteoblast cells on the Mg-Zr-Sr-Ho alloys. The in vitro biodegradation and the biocompatibility of the Mg-Zr-Sr-Ho alloys were both influenced by the Ho concentration in the Mg alloys; Mg1Zr2Sr3Ho exhibited lower degradation rates than Mg1Zr2Sr and displayed the best biocompatibility compared with the other alloys.

Solidification of cast magnesium alloys

A description of the key solidification steps in the formation of the as-cast microstructure of magnesium alloys is presented. The focus is on the two common magnesium alloy groups: Mg-Al alloys and Mg-Zn-rare earth alloys. The key elements described are: nucleation (including grain refinement), growth of the primary phase and the formation of the eutectic phases. In addition the effect of casting process (e.g. high-pressure diecasting and sand casting) on the outcomes from solidification are discussed. This includes consideration of the formation of banded defects during solidification in the dynamic environment of high pressure die casting.

Zirconium, calcium, and strontium contents in magnesium based biodegradable alloys modulate the efficiency of implant-induced osseointegration

Development of new biodegradable implants and devices is necessary to meet the increasing needs of regenerative orthopedic procedures. An important consideration while formulating new implant materials is that they should physicochemically and biologically mimic bone-like properties. In earlier studies, we have developed and characterized magnesium based biodegradable alloys, in particular magnesium-zirconium (Mg-Zr) alloys. Here we have reported the biological properties of four Mg-Zr alloys containing different quantities of strontium or calcium. The alloys were implanted in small cavities made in femur bones of New Zealand White rabbits, and the quantitative and qualitative assessments of newly induced bone tissue were carried out. A total of 30 experimental animals, three for each implant type, were studied, and bone induction was assessed by histological, immunohistochemical and radiological methods; cavities in the femurs with no implants and observed for the same period of time were kept as controls. Our results showed that Mg-Zr alloys containing appropriate quantities of strontium were more efficient in inducing good quality mineralized bone than other alloys. Our results have been discussed in the context of physicochemical and biological properties of the alloys, and they could be very useful in determining the nature of future generations of biodegradable orthopedic implants.

Collagen type-I leads to in vivo matrix mineralization and secondary stabilization of Mg-Zr-Ca alloy implants

Biodegradable magnesium-zirconia-calcium (Mg-Zr-Ca) alloy implants were coated with Collagen type-I (Coll-I) and assessed for their rate and efficacy of bone mineralization and implant stabilization. The phases, microstructure and mechanical properties of these alloys were analyzed using X-ray diffraction (XRD), optical microscopy and compression test, respectively, and the corrosion behavior was established by their hydrogen production rate in simulated body fluid (SBF). Coll-I extracted from rat tail, and characterized using fourier transform infrared (FT-IR) spectroscopy, was used for dip-coating the Mg-based alloys. The coated alloys were implanted into the femur bones of male New Zealand white rabbits. In vivo bone formation around the implants was quantified by measuring the bone mineral content/density (BMC/BMD) using dual-energy X-ray absorptiometry (DXA). Osseointegration of the implant and new bone mineralization was visualized by histological and immunohistochemical analysis. Upon surface coating with Coll-I, these alloys demonstrated high surface energy showing enhanced performance as an implant material that is suitable for rapid and efficient new bone tissue induction with optimal mineral content and cellular properties. The results demonstrate that Coll-I coated Mg-Zr-Ca alloys have a tendency to form superior trabecular bone structure with better osteoinduction around the implants and higher implant secondary stabilization, through the phenomenon of contact osteogenesis, compared to the control and uncoated ones in shorter periods of implantation. Hence, Coll-I surface coating of Mg-Zr-Ca alloys is a promising method for expediting new bone formation in vivo and enhancing osseointegration in load bearing implant applications.

The effect of chromate on the pitting susceptibility of AA7075-T651 studied using potentiostatic transients

The effect of chromate on metastable pitting of AA7075-T651 as determined via potentiostatic polarisation is reported. A systematic study of metastable pitting and its correlation with stable pits was conducted in various concentrations of sodium chromate (Na2CrO4), revealing the metastable pitting rate was able to provide a quantitative metric for pitting corrosion. The size and number of metastable pits decreased significantly in the presence of chromate. The present study is intended as a general baseline for the assessment of future chromate replacement technologies, as elaborated herein. © 2014 Elsevier Ltd.

Strontium content and collagen-I coating of magnesium-zirconia-strontium implants influence osteogenesis and bone resorption.

Our objective was to study the role of Collagen type-I (Col-I) coating on Magnesium-Zirconia (Mg-Zr) alloys, containing different quantities of Strontium (Sr), in enhancing the in vitro bioactivity and in vivo bone-forming and mineralisation properties of the implants.