The Study On Microstructure And Corrosion Behavior Of Biomedical Mg-bi Based Alloys

Magnesium alloys have attracted domestic and foreign scholars studying on the biological magnesium alloy due to their excellent mechanical properties and good biocompatibility. More importantly, magnesium is one of the essential trace elements in human body. At present, the main biomedical magnesium alloy mainly focused on Mg-Al, Mg-Zn, Mg-Ca and Mg-RE alloy. However, considering the biological safety, Al is neurotoxic element, damage to the body’s neurons, may cause dementia or Alzheimer’s disease. The hemolysis rate of Mg-Zn alloy is high and it is not suitable for directly contact with blood. The RE elements easily enriched in the brain may cause liver poisoning. In addition, the as-cast Mg-5Ca（or higher） alloy is very brittle at room temperature.From the point of view of the biological effects of alloying elements, in this paper, Bi and Sn elements are chosen as alloying elements to develop new biological magnesium alloy. The addition of Sn is expected to modify the microstructure and the corrosion resistance of the Mg-Bi alloys. The microstructure and corrosion surface of Mg-Bi based alloy were analyzed by OM、XRD、SEM and EDS. And the corrosion behavior was studied by immersion test,hydrogen evolution and electrochemical tests. The results show that:1) With the increase of Bi, the secondary dendrite spacing（SDAS） of as-cast Mg-xBi alloy decreased, the volume fraction of Mg₃Bi₂ phase increased and resulted in the potential difference between Mg₃Bi₂ phase and α-Mg, thereby accelerated the micro-galvanic corrosion.2) Mg-6Bi-xSn alloys were mainly composed of α-Mg and Mg₃Bi₂ phase. With the further increase of Bi up to 3 wt.%, a new Mg₂ Sn phases were observed. With the addition of Sn, the SDAS of Mg-6Bi-xSn alloy decreased from 29.7 μm to 22.3 μm, and the distribution of second phase changed from continuous to discontinuous.3) With the increase of Sn content, the corrosion resistance of Mg-6Bi-xSn alloy decreased. This is mainly due to: grain refinement provided more grain boundaries, which can provide physical barriers to hinder the corrosion process. Moreover, The alloying Sn improved the potential of the α-Mg matrix, reduced the potential difference between the anode and cathode, weaken the galvanic corrosion, thus, the corrosion resistance of Mg-6Bi-xSn alloy improved.4) Mg-6Bi-xSn alloy can be molding in relatively high rolling temperature（400 ℃） and low speed（20m/min）. The corrosion resistance of Mg-6Bi-2Sn alloy after rolling improved, this is mainly because of grain refinement. After rolling processing, grain refinement brings increasing grain boundaries and formed a physical barrier in the process of corrosion.