Chemical and mechanical properties of stainless, environment-friendly, and nonflammable Mg alloys (SEN alloys): A review

This review article provides overall understanding of stainless, environment-friendly, and nonflammable Mg alloys (SEN alloys) recently developed at the Korea Institute of Materials Science. SEN alloys are produced by adding small amounts of Ca and Y (each < 1 wt%) into commercial Mg–Al based alloys, resulting in exceptional ignition and corrosion resistances and impressive mechanical properties. Their main advantages of SEN alloys are as follows. (1) A dense multi-oxide layer of SEN alloys comprising MgO, CaO, and Y₂O₃ impedes the outward dispersion of Mg vapor and the inward penetration of O₂ during oxidation, thereby enhancing the oxidation and ignition resistances. (2) The presence of Ca- and Y-based second-phase particles in SEN alloys can enhance their corrosion resistance because Ca-containing particles prevent the spread of corrosion, and the replacement of Al-containing particles with less noble ones containing Y (e.g., Al–Mn–Y or Al–Y particles) retards corrosion. (3) The addition of minor amounts of Ca and Y renders excellent mechanical properties due to improved strengthening effects. These enhanced properties are attributed to more pronounced dynamic recrystallization and grain refining behaviors caused by the second-phase particles during extrusion. (4) Despite the presence of various types of second-phase particles, the fatigue properties of SEN9 alloys are similar to those of commercial AZ91 alloys. (5) Simultaneous introduction of Ca and Y suppresses the formation of Mg₁₇Al₁₂ discontinuous precipitates during aging, leading to the enhanced elongation of aged SEN alloys. (6) Adding mischmetal into the SEN9 alloy leads to a six-fold enhancement in extrudability. Consequently, the studies conducted on SEN alloys demonstrate their excellent ignition and corrosion resistances and mechanical properties, which broaden the industrial applications of Mg alloys by addressing their inherent weaknesses.