Re-recognition of the aging precipitation behavior in the Mg–Sm binary alloy

Generally, the Mg–Sm system is one of the magnesium light rare-earth alloys (Mg-LRE), and its aging precipitation behavior is similar to that of the Mg–Nd or Mg–Ce alloys. For a long time, the β phase, precipitated after the β1 intermediate phase, has been considered to be Mg12Sm, although this has never been experimentally verified. In this study, however, we clearly confirm that the β phase is Mg5Sm with a face-centered cubic structure (space group: F4¯3m; a = 22.34 Å) by aberration-corrected scanning transmission electron microscopy (STEM) combined with quantitative STEM simulation. This is similar to the case of magnesium heavy rare-earth alloys (Mg-HRE). The lenticular-shaped β plates have a {101¯0}α habit plane and is fully coherent with the α-Mg matrix, with the orientation relationships among the β phase, β1 phase, and α-Mg matrix being (11¯1)β//(11¯1)β1//(112¯0)α and [110]β//[110]β1//[0001]α. This finding improves our understanding of the precipitation/strengthening mechanism in the Mg–Sm based alloys and is expected to guide the future design of novel high-strength heat-resistant Mg-RE based alloys. •The β phase in the Mg–Sm system is determined to be the Mg5Sm, instead of Mg12Sm.•The β-Mg5Sm phase is of a face-centered cubic structure.•The β plates has a {101¯0}α habit plane and fully coherent with the α-Mg matrix.