Sr Alloy Research Articles

The effect of strontium in plasticity of magnesium alloys

The objective of this article is to quantify relationships between microstructural changes and the room temperature mechanical behavior of novel extruded Mg–Al–Zn–Sr alloys. To this aim, targeted mechanical testing coupled with full-field strain measurements and simultaneous acoustic emission recordings is further combined with grain-scale observations. To date, the precise effect of alloying elements on the plasticity of Mg alloys is still largely unknown. Strontium in wrought Mg alloys has already been shown to reduce grain size and texture anisotropy, as well as improve their creep resistance, replicating the effects of rare earth elements. To further investigate the effects of Sr on the mechanical properties and plasticity of Mg alloys compression tests were performed and were coupled with nondestructive measurements provided by digital image correlation and acoustic emission monitoring. The results presented herein demonstrate the effect of the extrusion temperature and provide direct correlations between Sr content and bulk mechanical properties, overall plasticity and texture evolution in these novel alloys. Furthermore, the addition of Sr was found to be related to distinct strain localizations near yielding, which were linked to twinning activity, providing in this way a route for quantitative control of their microstructural design.

Development and evaluation of a magnesium–zinc–strontium alloy for biomedical applications — Alloy processing, microstructure, mechanical properties, and biodegradation

A new biodegradable magnesium–zinc–strontium (Mg–Zn–Sr) alloy was developed and studied for medical implant applications. This first study investigated the alloy processing (casting, rolling, and heat treatment), microstructures, mechanical properties, and degradation properties in simulated body fluid (SBF). Aging treatment of the ZSr41 alloy at 175°C for 8h improved the mechanical properties when compared to those of the as-cast alloy. Specifically, the aged ZSr41 alloy had an ultimate tensile strength of 270MPa, Vickers hardness of 71.5HV, and elongation at failure of 12.8%. The mechanical properties of the ZSr41 alloy were superior as compared with those of pure magnesium and met the requirements for load-bearing medical implants. Furthermore, the immersion of the ZSr41 alloy in SBF showed a degradation mode that progressed cyclically, alternating between pitting and localized corrosion. The steady-state average degradation rate of the aged ZSr41 alloy in SBF was 0.96g/(m2·hr), while the pH of SBF immersion solution increased. The corrosion current density of the ZSr41 alloy in SBF solution was 0.41mA/mm2, which was much lower than 1.67mA/mm2 for pure Mg under the same conditions. In summary, compared to pure Mg, the mechanical properties of the new ZSr41 alloy improved while the degradation rate decreased due to the addition of Zn and Sr alloying elements and specific processing conditions. The superior mechanical properties and corrosion resistance of the new ZSr41 alloy make it a promising alloy for next-generation implant applications.

Histological analysis of cells and matrix mineralization of new bone tissue induced in rabbit femur bones by Mg–Zr based biodegradable implants

The biological efficacy of bone inducing implant materials in situ can be assessed effectively by performing histological analysis. We studied the peri-implant bone regeneration around two types of biodegradable magnesium–zirconium alloys, Mg–5Zr and Mg–Zr–2Sr, using histological, histochemical and immunohistochemical methods in the femur of New Zealand White strain rabbits. Our study includes three animal groups: (a) Mg–5Zr, (b) Mg–Zr–2Sr and (c) control. In each group three animals were used and in groups ‘a’ and ‘b’ the respective alloys were implanted in cavities made at the distal ends of the femur; control animals were left without implants to observe natural bone healing. Qualitative assessment of the cellularity and matrix mineralization events of the newly formed bone tissue was done at three months after implantation by histological methods in methyl methacrylate embedded tissue without decalcifying the bone. Quantitative mineral content and density of the new bone (NB) were evaluated by the statistical analysis of dual energy X-ray absorptiometry (DXA) data obtained from three animals in each experimental group. Based on our analysis we conclude that Mg–Zr–2Sr alloy showed better osseointegration of the newly formed bone with the implant surface. Our methodology of studying peri-implant osteoinduction of degradable implants using low temperature methyl methacrylate embedding resin can be useful as a general method for determining the bio-efficacy of implant materials.

The Microstructure and the Degradation Behaviour of a Novel Biomaterial Mg–Zn–Sr Alloy Sheet in Simulated Body Fluid (SBF)

The Microstructure and the Degradation Behaviour of a Novel Biomaterial Mg–Zn–Sr Alloy Sheet in Simulated Body Fluid (SBF)

The Microstructures and Biocompatibility of a Novel Biomaterial Mg–4.0Zn–1.5Sr Alloy Sheet After Aged Treatment

The Microstructures and Biocompatibility of a Novel Biomaterial Mg–4.0Zn–1.5Sr Alloy Sheet After Aged Treatment

Acoustic emission study of Mg–Al–Sr alloy reinforced with short Saffil® fibers deformed in compression

The acoustic emission (AE) method has been used for the in-situ investigations of the mechanical properties of an AJ51 magnesium alloy, reinforced with short Saffil® fibers, in compression mode and at room temperature. It is shown that the frequency analysis of the AE signals could reveal changes in the deformation processes caused by the presence of reinforcing fibers, as well as by the onset of cracks in them.

Microstructures and Mechanical Properties of Mg-8Al-1Nd-0.5Zn-xSr Alloys

The influence of trace strontium (Sr) addition on the microstructure and mechanical properties of Mg-8Al-1Nd-0.5Zn was investigated with OM, SEM, and XRD etc. The results show that the lamellar eutectics and divorced β-Mg17Al12 were reduced or refined with trace Sr addition. Among the as-cast Mg-8Al-1Nd-0.5Zn-xSr alloys, the Mg-8Al-1Nd-0.5Zn-0.05Sr alloy exhibited the best mechanical properties in which the tensile strength, the yield strength reached to 244.9 MPa and 111.7 MPa respectively. In addition, with the increase of Sr addition, the ductility was improved and it was observed that the number of cleavage steps and secondary cracks decreased on the fracture surfaces of tensile samples. It was also observed that the fractures occurred in the coarse β-Mg17Al12 phase instead of the Mg/Mg17Al12 interface or Al11Nd3 phase.

Corrosion Resistance of Mg-xAl-3Ca-0.8Sr Magnesium Alloys

The microstructure of Mg-xAl-3Ca-0.8Sr alloys consists of the α-Mg solid solution, irregular-shaped (Al,Mg)2Ca, bulky Al3Mg13Sr and lamellar (Al,Mg)4Sr phases. The results showed that aluminum has a positive effect on the corrosion resistance of Mg-xAl-3Ca-0.8Sr alloys. The main corrosion product is magnesium hydroxide Mg (OH)2 which forms a cracked layer on the surface of Mg-xAl-3Ca-0.8Sr alloys.

Effect of extrusion temperature on texture evolution and recrystallization in extruded Mg–1% Mn and Mg–1% Mn–1.6%Sr alloys

Alloys Mg–1% Mn (M1) and Mg–1% Mn–1.6Sr (M1–1.6Sr) were subjected to three extrusion temperatures of 300, 350 and 400°C at 8mm/s ram speed in order to investigate the texture evolution and recrystallization mechanisms. M1 alloy exhibits weaker texture after extrusion at 300°C, but develops strong basal texture at 350 and 400°C. The weaker texture of M1 at 300°C is partially due to random orientations created by CDRX mechanism. M1–1.6Sr alloy develops weaker textures at all extrusion temperatures as a result of particle stimulated nucleation promoted by Mg–Sr intermetallics which forms grains with random orientations.

Influence of Extrusion on the Microstructure and Mechanical Behavior of Mg-9Li-3Al-xSr Alloys

Mg-9Li-3Al-xSr (LA93-xSr, x = 0, 1.5, 2.5, and 3.5 wt pct) alloys were cast and extruded at 533 K (260 °C) with an extrusion ratio of 28. The microstructure and mechanical response are reported and discussed paying particular attention to the influence of extrusion and Sr content on phase composition, strength, and ductility. The results of the current study show that LA93-xSr alloys contain both α-Mg (hcp) and β-Li (bcc) matrix phases. Moreover, the addition of Sr refines the grain size in the as-cast alloys and leads to the formation of the intermetallic compound (Al4Sr). Our results show significant grain refinement during extrusion and almost no influence of Sr content on the grain size of the extruded alloys. The microstructure evolution during extrusion is governed by continuous dynamic recrystallization (CDRX) in the α-Mg phase, whereas discontinuous dynamic recrystallization (DDRX) occurs in the β-Li phase. The mechanical behavior of the extruded LA93-xSr alloy is discussed in terms of grain refinement and dislocation strengthening. The tensile strength of the extruded alloys first increases and then decreases, whereas the elongation decreases monotonically with increasing Sr; in contrast, hardness increases for all Sr compositions studied herein. Specifically, when Sr content is 2.5 wt pct, the extruded Mg-9Li-3Al-2.5Sr (LAJ932) alloy exhibits a favorable combination of strength and ductility with an ultimate tensile strength of 235 MPa, yield strength of 221 MPa, and an elongation of 19.4 pct.

Microstructure and texture evolution in Mg–1 %Mn–Sr alloys during extrusion

Microstructure and texture evolution in Mg–1 %Mn–Sr alloys during extrusion has been investigated. At 350 °C, the extrusion of Mg–1 %Mn (M1) alloy exhibits the progressive formation of basal texture from the undeformed zone to the die opening. The extruded microstructure of M1 consists of recrystallized grains nucleated by grain boundary bulging and elongated parent grains along with extensive twinning. At 350 °C, the extrusion of M1–1.6Sr alloy results in progressive elongation of Mg–Sr precipitates in the form of stringers from the undeformed zone to the die opening. The final extruded microstructure of this alloy shows extensive recrystallization occurring at the intermetallic stringers by particle-stimulated nucleation (PSN). M1–(0.3–1.6)%Sr alloys display weaker textures due to PSN which creates new grains with random orientations. At 250 °C, the extrusion of M1 creates necklace of small recrystallized grains around large elongated parent grains. M1–1.6Sr alloy extruded at 250 °C exhibits continuous dynamic recrystallization (CDRX) in the Mg matrix and PSN at Mg–Sr precipitates. PSN is less extensive at lower temperature. Both CDRX and PSN grains have random orientations, and therefore, alloy develops random texture.

Behavior of Double-Oxide Film Defects in Al-0.05 wt pct Sr Alloy

The change in the composition of oxide layers and the possibility of the formation of bonding between the two layers of a double-oxide film defect when held in an Al-0.05 wt pct Sr melt was investigated. The defect was modeled experimentally by maintaining two aluminum oxide layers in contact with one another in the liquid metal at 1023 K (750 °C) for times ranging from 5 seconds to 50 hours. Any changes in the composition and morphology of these layers were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results showed that the A12O3 layers started to transform to SrO gradually from the moment that they submerged into the melt. The transformation caused the two layers to bond with each other gradually. The results illustrated that the composition of the oxide layers of a double oxide film defect submerged in Sr-treated melt is different from that of pure Al, and this might affect the mechanical properties and the behavior of the defect in the melt significantly.

Biocompatibility and biodegradability of Mg–Sr alloys: The formation of Sr-substituted hydroxyapatite

Magnesium is an attractive material for use in biodegradable implants due to its low density, non-toxicity and mechanical properties similar to those of human tissue such as bone. Its biocompatibility makes it amenable for use in a wide range of applications from bone to cardiovascular implants. Here we investigated the corrosion rate in simulated body fluid (SBF) of a series of Mg–Sr alloys, with Sr in the range of 0.3–2.5%, and found that the Mg–0.5 Sr alloy showed the slowest corrosion rate. The degradation rate from this alloy indicated that the daily Sr intake from a typical stent would be 0.01–0.02mgday−1, which is well below the maximum daily Sr intake levels of 4mgday−1. Indirect cytotoxicity assays using human umbilical vascular endothelial cells indicated that Mg–0.5 Sr extraction medium did not cause any toxicity or detrimental effect on the viability of the cells. Finally, a tubular Mg–0.5 Sr stent sample, along with a WE43 control stent, was implanted into the right and left dog femoral artery. No thrombosis effect was observed in the Mg–0.5 Sr stent after 3weeks of implantation while the WE43 stent thrombosed. X-ray diffraction demonstrated the formation of hydroxyapatite and Mg(OH)2 as a result of the degradation of Mg–0.5 Sr alloy after 3days in SBF. X-ray photoelectron spectroscopy further showed the possibility of the formation of a hydroxyapatite Sr-substituted layer that presents as a thin layer at the interface between the Mg–0.5 Sr alloy and the corrosion products. We believe that this interfacial layer stabilizes the surface of the Mg–0.5 Sr alloy, and slows down its degradation rate over time.

Compressive Creep Behavior of Mg-4Al-1RE-1Ca-0.2Sr Alloy

The compressive creep behavior of Mg-4Al-1RE-1Ca-0.2Sr (AECJ411002) alloy was investigated at temperatures in the range of 125~175°C and different compressive stress in the range of 88~112MPa with special apparatus. The results show that the creep deformation increases with the increasing of temperature and compressive stress. There is linear logarithmic relationship between the steady creep rate and all the temperature and stress used. The steady creep rate obeys an empirical equation. The stress exponents are similar at different temperatures and the appearance activation energies are not greatly different under different stresses. Their average value is respectively 6.19 and 39.05kJ/mol. Material constant A is 4.18×10-14. The steady creep rate is controlled by a dislocation climb led by grain boundary sliding. The creep resistance enhances because of the heat-resistant phases Al2Ca and Al4Sr distributing at grain boundary.

Synthesis and characterization of Mg‐Ca‐Sr alloys for biodegradable orthopedic implant applications

Magnesium has recently received an increased amount of interest due to its potential use in biodegradable implant applications. The rapid degradation of conventional Mg is, however, a major limitation that needs to be addressed in the design of these materials, along with consideration of toxicity in selection of alloying elements. In this study, five alloys in the Mg-xCa-ySr system (x = 0.5-7.0 wt %; y = 0.5-3.5 wt %) were prepared and characterized for their suitability as degradable orthopedic implant materials. The alloys were characterized using optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, degradation measurements in Hanks' solution at 37°C, compression testing, and in vitro cytotoxicity testing with a mouse osteoblastic cell line. The results indicate that the Mg-1.0Ca-0.5Sr alloy is the most promising alloy for orthopedic implant applications since it showed the lowest degradation rate in Hanks' solution (0.01 mL cm(-2) h(-1)) along with no significant toxicity to MC3T3-E1 osteoblasts and a compressive strength of 274 ± 4 MPa.

Optimization of the Mg–Al–Zn–Ca–Sr alloy composition based on the parameter A′ in the constitutive equation for the climb-controlled dislocation creep including the stacking fault energy

Abstract In this study, calcium, Ca-, and strontium, Sr-added AZ91 magnesium alloys, which were composed of magnesium, Mg–9 wt.% aluminum, Al–0.8 wt.% zinc, Zn–x wt.% Ca–y wt.% Sr, were formulated and designated as AZXJ91xy alloys. The optimum composition of die-cast AZXJ alloys has been studied by micro-structural analysis and through the application of the constitutive equation of deformation behavior at elevated temperatures that includes the stacking fault energy. The normalized plots of the tensile test results of AZXJ alloys at 448 K by the constitutive equation of deformation behavior indicated that the deformation mechanism of the alloys was climb-controlled dislocation creep. The value of the constant A′ in the constitutive equation differs in the different AZXJ alloys, even though the effect of the solid solution of added elements, which is considered in the stacking fault energy term, was eliminated from A′. Using A′ as an indicator parameter of the creep resistance with the tensile strength at ambient temperature, the optimum composition of the creep-resistant Mg–Al–Zn–Ca–Sr alloy has been determined, and its creep resistance was comparable to those of known heat-resistant magnesium alloys.

Mg–Zr–Sr alloys as biodegradable implant materials

Novel Mg–Zr–Sr alloys have recently been developed for use as biodegradable implant materials. The Mg–Zr–Sr alloys were prepared by diluting Mg–Zr and Mg–Sr master alloys with pure Mg. The impact of Zr and Sr on the mechanical and biological properties has been thoroughly examined. The microstructures and mechanical properties of the alloys were characterized using optical microscopy, X-ray diffraction and compressive tests. The corrosion resistance was evaluated by electrochemical analysis and hydrogen evolution measurement. The in vitro biocompatibility was assessed using osteoblast-like SaOS2 cells and MTS and haemolysis tests. In vivo bone formation and biodegradability were studied in a rabbit model. The results indicated that both Zr and Sr are excellent candidates for Mg alloying elements in manufacturing biodegradable Mg alloy implants. Zr addition refined the grain size, improved the ductility, smoothed the grain boundaries and enhanced the corrosion resistance of Mg alloys. Sr addition led to an increase in compressive strength, better in vitro biocompatibility, and significantly higher bone formation in vivo. This study demonstrated that Mg–xZr–ySr alloys with x and y ⩽5wt.% would make excellent biodegradable implant materials for load-bearing applications.

Effect of strontium on the texture and mechanical properties of extruded Mg–1%Mn alloys

Abstract Magnesium–manganese, M1, alloy is preferred for extrusion applications due to its extrudability. It is mainly used as a sacrificial anode or as a creep resistant alloy at elevated temperatures in the nuclear industry. Since Mn does not provide a significant strengthening effect, the alloy is not considered for structural applications. The basal texture which forms after extrusion orients the basal planes parallel to the extrusion direction causing anisotropy in mechanical properties. This basal texture, as well as the low strength of the alloy are the main challenges in its widespread applications. In this study, the effect of Sr addition on the texture and mechanical properties of M1 alloy was studied. M1–Sr alloys showed weakened texture by developing random texture components during extrusion. The texture randomisation is attributed to particle stimulated nucleation (PSN) around Mg–Sr intermetallics during recrystallisation. M1–Sr compositions are found to show improved strength and ductility as well as reduced yield asymmetry.

Effect of pre-deformation anneal on the microstructure and texture evolution of Mg–3Al–1Zn–0.7Sr alloy during hot extrusion

The effect of pre-deformation annealing on the microstructure and texture of an AZ31 + 0.74 wt% Sr alloy has been investigated. As-cast samples as well as three samples that have been annealed at 400 °C for 10, 30, and 120 min were extruded at 300 °C. Results indicate that annealing transforms the bulky non-equilibrium Al–Mg–Sr precipitates to stable Al4Sr spheroids. As the extent of this transformation increases before extrusion, there is seen an increase in the amount of uniformly dispersed intermetallic stringers in the extruded material. Texture measurements reveal the alignment of basal poles with the compression axis (perpendicular to the circular cross section of the extruded bar) and the formation of the basal ring texture in all the samples. However, an increase in the duration of the pre-deformation anneal switches the plane facing the extrusion direction from first order prismatic (10-10) to second order prismatic planes (11-20). Annealing decreases the Al solute concentration in Mg and lowers the lattice resistance against dislocation movement. Consequently, the more favorable (0002)[11-20] slip system is activated in grains that see low basal resolved shear stress (τ). As a result, those grains work harden and are consumed by dynamic recrystallization (DRX). However, the (0002)[-1100] slip system with high τ still avoids basal dislocation movement. Hence, the grains with high τ(0002)[-1100], which need to move dislocations in the (0002)[-1100] system to fulfill the strain compatibility conditions across the microstructure would be prevented from work hardening and DRX. This specific orientation has a (11-20) plane facing the extrusion direction.

The Corrosion Resistance of the Mg-Al-Ca-Sr Sand Casting Magnesium Alloys

In this paper, the corrosion resistance of two sand-casting creep resistant magnesium alloys Mg-9Al-1.5Ca-0.3Sr and Mg-9Al-2.2Ca-0.8Sr in the salt environment has been investigated. Specimens of each alloy has been immersed in 3.5% NaCl solution at room temperature and successively taken out after 1, 2, 4, 5 and 9 days. After immersion test, the microstructure and the appearances of the corroded structure were examined. The corrosion rates of both investigated alloys increased lineally with increasing the exposure time in both solutions. Mg-9Al-1.5Ca-0.3Sr alloy exhibits the higher corrosion rate during the immersion test than Mg-9Al-2.2Ca-0.8Sr. The corrosion layer of both alloys consists of MgO, MgOH and phases containing Cl, Na, Al and Ca. The increase of Ca content in the Mg-9Al-2.2Ca-0.8Sr alloy improved the corrosion resistance due to the formation of the reticular (Mg,Al)2Ca phase, which acted as an effective barrier against corrosion.