Biodegradable metals have been of great interest in making gastrointestinal implants these years. The most researched biodegradable metal is magnesium (Mg), followed by zinc (Zn) and iron (Fe). However, due to the limitations of in vivo experiments and the complex component of the gastrointestinal fluid, their degradation mechanisms in such an environment are still ambiguous. In this work, the human duodenal fluid (HDF) was used to investigate their in vitro degradation behaviors, with a simulated duodenal fluid (SDF) prepared for the control group based on the HDF ionic composition. After immersion of these metals for 7 days, it is found that HDF shows a stronger pH buffering effect than SDF due to the presence of organics. These organics can also hinder the degradation of metals by affecting their product formation in different ways. On the one hand, the adsorption of organics and their effects on the fluid dominate their degradation inhibition effect on Mg and Zn in HDF. On the other hand, they can hinder the further oxidation of the degradation products of Fe, which is the main mechanism resulting in a lower degradation rate of Fe in HDF rather than in SDF. Among the three metals, Mg unsurprisingly shows the highest degradation rate in both fluids. Interestingly, Zn is nearly immune to degradation in HDF, while it presents typical pitting corrosion in SDF. Compared to their degradation rates in popular pseudo-humoral media (e. g. Hanks’ Balanced Salt Solutions, Dulbecco’s modified Eagle’s medium) reported previously, Mg degrades faster, and Zn and Fe more slowly in HDF. The higher in vitro degradation rate of Fe than that of Zn is influenced by oxygen and ions in the degradation environment.

Magnesium and its alloys have such advantages with lightweight, high specific strength, good damping, high castability and machinability, which make them an attractive choice for applications where weight reduction is important, such as in the aerospace and automotive industries. However, their practical applications are still limited because of their poor corrosion resistance, low high temperature strength and ambient formability. Based on such their property shortcomings, recently degradable magnesium alloys were developed for broadening their potential applications. Considering the degradable Mg alloys for medical applications were well reviewed, the present review put an emphasis on such degradable magnesium alloys for structural and functional applications, especially the applications in the environmental and energy fields. Their applications as fracture ball in fossil energy, sacrificial anode, washing ball, and as battery anodes, transient electronics, were summarized. The roles of alloying elements in magnesium and the design concept of such degradable magnesium alloys were discussed. The existing challenges for extending their future applications are explored.

The present work reports the creep behavior and microstructural evolution of the sand-cast Mg–14Gd–0.4Zr alloy (wt.%) prepared by the differential pressure casting machine. Their compressive creep tests at 250 ℃ were performed under various applied stresses (i.e., 60, 80 and 100 MPa). Among them, the sand-cast Mg–14Gd–0.4Zr samples examined under 250 ℃/80 MPa for 39 and 95 h, respectively, were chosen to systemically analyze their creep mechanisms using high-angle annular dark field-scanning transmission electron microscopy (HAADF-STEM). The obtained results showed that the enhancement of creep resistance can be mainly attributed to the coherent β’ and β’_F phases with an alternate distribution, effectively impeding the basal <a> dislocations movement. However, with the creep time increasing, the fine β’+β’_F precipitate chains coarsened and transformed to semi-coherent β₁ phase and even to large incoherent β phase (surrounded by precipitate-free areas) in grain interiors. The precipitate-free zones (PFZs) at grain boundaries (GBs) were formed, and they could expand during creep deformation. Apart from the main cross-slip of basal and prismatic <a> dislocations, <c + a> type dislocations were activated and tended to distribute near the GBs. The aforementioned phenomena induced the stress concentrations, consequently leading to the increment of the creep strain.

To have a better understand on the change of microstructure via kinetics, the diffusion behavior of Mg alloys is of special interest to researchers. Meanwhile, diffusion coefficients of Mg based alloys can explain and represent their diffusion behavior well. The evolution of experimental and calculated methods for detecting and extracting diffusion coefficients was discussed briefly. The reasonable diffusion data, especially self-diffusion coefficients, impurity diffusion coefficients and inter-diffusion coefficients of Mg alloys, were reviewed in detail serving to design the Mg alloys with higher accuracy. Then the practical applications of diffusion coefficients of Mg alloys were summarized, including diffusional mobility establishing, precipitation simulation and mechanical properties prediction.