On the crystallographic features of Mn precipitates in a Mg–Sn–Mn alloy

• A novel orientation relationship between α-Mn and Mg matrix is reported.
• The type of the [0 0 0 1]α Mn rods in a Mg–Sn–Mn alloy is clarified.
• Good matching interfacial structure between α-Mn rods and Mg is provided.
• The preferences of different ORs are discussed in terms of interface match.

The crystallographic features of the Mn precipitates in a Mg–Sn–Mn alloy have been investigated using transmission electron microscopy (TEM). All the Mn precipitates are identified as α-Mn. The precipitates have two different morphologies characterized by their different orientation relationships (OR) with the Mg matrix. One is the α-Mn rod with a growth direction of 〈1 1 1〉Mn//[0 0 0 1]Mg, and the OR for this type of α-Mn is expressed as 〈1 1 0〉Mn//〈1 0 1¯ 0〉Mg, {1 1 1}Mn//(0 0 0 1)Mg, which is consistent with that reported in binary Mg–Mn alloys. The other is the blocky α-Mn with a habit plane of {1¯ 1 0}Mn//(0 0 0 1)Mg, and the corresponding OR is novel, which is described as 〈1 1 3〉Mn//〈1 0 1¯ 0〉Mg, {1¯ 1 0}Mn//(0 0 0 1)Mg. An atomically flat and nearly fully coherent lattice planes between the [0 0 0 1]Mg rods and Mg matrix was observed, which account for their preferred morphology. The preferences of the α-Mn precipitates with different ORs have also been discussed in terms of the lattice misfit in interfaces.