Search

Biodegradable Mg–Zn–Ca-based metallic glasses (MGs) present improved strength and superior corrosion resistance, compared to crystalline Mg. In particular, in vivo and in vitro attempts reveal that biodegradable Mg–Zn–Ca-based MGs possess excellent biocompatibility, suggesting that they are ideal candidates for temporary implant materials. However, the limited size and severe brittleness prevent their widespread commercialization. In this review, we firstly summarize the microstructure characteristic and mechanical properties of Mg–Zn–Ca-based MGs. Then, we provide a comprehensive and systematic understanding of the recent progress of the biocorrosion and biocompatibility of Mg–Zn–Ca-based MGs. Last, but not least, the outlook towards the fabrication routes, composition design, structure design, and reinforcement approaches of Mg–Zn–Ca-based MGs are briefly proposed.

In this study, two biodegradable Mg-Zn-Ca alloys with alloy content of less than 1 wt % were strengthened via high pressure torsion (HPT). A subsequent heat treatment at temperatures of around 0.45 Tm led to an additional, sometimes even larger increase in both hardness and tensile strength. A hardness of more than 110 HV and tensile strength of more than 300 MPa were achieved in Mg-0.2Zn-0.5Ca by this procedure. Microstructural analyses were conducted by scanning and transmission electron microscopy (SEM and TEM, respectively) and atom probe tomography (APT) to reveal the origin of this strength increase. They indicated a grain size in the sub-micron range, Ca-rich precipitates, and segregation of the alloying elements at the grain boundaries after HPT-processing. While the grain size and segregation remained mostly unchanged during the heat treatment, the size and density of the precipitates increased slightly. However, estimates with an Orowan-type equation showed that precipitation hardening cannot account for the strength increase observed. Instead, the high concentration of vacancies after HPT-processing is thought to lead to the formation of vacancy agglomerates and dislocation loops in the basal plane, where they represent particularly strong obstacles to dislocation movement, thus, accounting for the considerable strength increase observed. This idea is substantiated by theoretical considerations and quenching experiments, which also show an increase in hardness when the same heat treatment is applied.

Mg-Zn-Ca amorphous alloys are considered as potential bone implants. A large number of works have focused on the alloys under free corrosion environment. However, the real service environment of bone implants is a kind of chemistry-mechanics interactive environment in which the materials not only suffer corrosion by body fluids but also bear applied force induced by body movement. In order to imitate the real service environment, surface morphologies and mechanical properties of Mg-Zn-Ca amorphous alloys were studied under different chemistry-mechanics interactive environments in this paper. It was found that cracks and Ca/Mg phosphates formed on the surface of amorphous alloys. The compressive strength of the alloys decreased seriously but could still reach an acceptable value to avoid material failure. Fan-shaped patterns found on all the samples implied that brittle fracture was the main fracture form. Moreover, vein-like patterns could still be found in some areas, showing a locally plastic deformation. This was the reason why the alloy could maintain a high compressive strength after severe and interactive treatments. The study could guide related works in the establishment of experimental environments in the future, which will facilitate a more accurate biomedical evaluation of bone implants.

Mg‒Zn‒Ca metallic glasses are regarded as promising biodegradable materials. Previous studies on this alloy system have mostly focused on the composition regions with a large critical size (Dc) for the formation of metallic glasses, while this paper investigates the composition regions with a small Dc, which has been overlooked by researchers for a long time. The effects of the addition of Ag, Nd, and Yb elements on the microstructure and mechanical properties of Mg‒Zn‒Ca metallic glasses were studied. It was found that the Mg‒Zn‒Ca metallic glass exhibits a single and uniform amorphous structure with a compressive strength of 590 MPa. After the addition of a small amount of Ag into the alloy, the amorphous matrix is retained and new precipitate phases that lead to the decrease of the compressive strength are formed. The addition of the rare earth elements Nd and Yb changes the microstructure from a single amorphous matrix to a large number of quasicrystal phases, which results in an increase in compressive strength. The compressive strength of the Mg‒Zn‒Ca‒Yb alloy increases to 606.2 MPa due to the formation of multi-layered swirling solidified structure and a large number of small quasicrystals with high microhardness. Moreover, this study can be considered as a useful supplement to the existing studies on the Mg‒Zn‒Ca alloy system; it also provides new ideas for designing the microstructure and spatial structure of quasicrystal containing alloys with high performances.

Mg-Zn-Ca metallic glass (MG) is effective for degrading azo dyes; however, the related surface evolution and degradation mechanisms are little known. We comparatively investigated the initial surface corrosion morphologies of melt-spun Mg66Zn30Ca4 MG in deionized water and artificial seawater. It was found that the basic corrosion behavior of the MG was the same, except that the corrosion process was accelerated in seawater. The presence of NaCl obviously promotes the formation of nano-ZnO on the surface of ribbons, causing the rapid degradation of azo dyes due to the photocatalytic effect. The degradation efficiency when combined with 3.5 wt % NaCl was over 100 times higher than that without NaCl. This indicates that Mg-Zn-Ca MG ribbons are effective additives for the degradation of azo dyes in seawater.

New Mg-3Zn-2Ca magnesium alloy was prepared using different processing techniques: gravity casting as well as squeeze casting in liquid and semisolid states. Materials were further thermally treated; thermal treatment of the gravity cast alloy was additionally combined with the equal channel angular pressing (ECAP). Alloy processed by the squeeze casting in liquid as well as in semisolid state exhibit improved plasticity; the ECAP processing positively influenced both the tensile and compressive characteristics of the alloy. Applied heat treatment influenced the distribution and chemical composition of present intermetallic phases. Influence of particular processing techniques, heat treatment, and intermetallic phase distribution is thoroughly discussed in relation to mechanical behavior of presented alloys.

Rod samples of Mg60Zn35Ca5 bulk metallic glass composites (BMGCs) dispersed with Ti particles have been successfully fabricated via injection casting. The glass forming ability (GFA) and the mechanical properties of these Mg-based BMGCs have been systematically investigated as a function of the volume fraction (Vf) of Ti particles. The results showed that the compressive ductility increased with Vf. The mechanical performance of these BMGCs, with up to 5.4% compressive failure strain and 1187 MPa fracture strength at room temperature, can be obtained for the Mg-based BMGCs with 50 vol % Ti particles, suggesting that these dispersed Ti particles can absorb the energy of the crack propagations and can induce branches of the primary shear band into multiple secondary shear bands. It follows that further propagation of the shear band is blocked, enhancing the overall plasticity.