Microstructure Modification by Electron Beam Processing in Magnesium Alloys to Control the Degradation Rate

Electron beam processing (EBP) was applied on two different cast magnesium alloys (AZ91 and ZKX50) followed by subsequent heat treatment (HT). The microstructure and corrosion properties of the alloys in all of the conditions were investigated by standard microscopy and electrochemical measurements. After EBP, a uniform and significantly refined microstructure compared to the as-cast condition is observed. The grain size decreased notably and a more homogeneous distribution of the intermetallic phases is achieved. Electrochemical tests indicated that the AZ91 alloy exhibits higher corrosion resistance in EBP condition compared to EBP-HT and as-cast conditions after 1-day immersion in 3.5wt% NaCl solution. While the ZKX50 alloy shows higher corrosion resistance in EBP-HT compared to as-cast and EBP states after 1-day immersion in 0.5wt% NaCl solution. The corrosion behavior of magnesium alloys can be attributed to the grain size and distribution of the secondary phases. A connected Mg17Al12 network surrounding α-Mg grains in EBP AZ91 provides a barrier for the corrosion propagation from α-Mg grain to the neighbor grains and thus increases the corrosion resistance. Heat treatments at high temperature affect the corrosion resistance due to the dissolution of the β-phase network. In the case of ZKX50 alloy, the dissolution of Ca2Mg6Zn3 secondary phases after heat treatment seems to provide higher corrosion resistance, while distributed secondary phases affect negatively the corrosion resistance of EBP ZKX50 compared to the as-cast condition due to the microgalvanic corrosion caused by these phases.