The most stable crystal structure and the formation processes of an order-disorder (OD) intermetallic phase in the Mg–Al–Gd ternary system
The most stable crystal structure for an 18R-type order-disorder (OD) intermetallic phase in the Mg-Al-Gd ternary system and its formation processes by annealing at 525 °C have been investigated by means of transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The most energetically favourable polytype at 525 °C is found to be the structurally simplest one, a maximum degree of order (MDO) polytype (monoclinic, 1M, space group: C2/m) that is described with
... is described with a single stacking vector in stacking sixlayer structural blocks. The formation of this simplest polytype occurs in the sequence of (i) enrichment of Gd and Al in four consecutive close-packed planes while keeping the hcp stacking of the AB-type, (ii) formation of Al 6 Gd 8 clusters in the four consecutive atomic planes, introducing a stacking fault in the middle of the four consecutive atomic planes, (iii) thickening by the formation of Gd and Al-enriched four consecutive planes at a distance of two or three close-packed Mg atomic planes from the pre-existing Gd and Al-enriched four consecutive atomic planes so as to form 6-layer and sometimes 7-layer structural blocks, (iv) in-plane ordering of Al 6 Gd 8 clusters in the four consecutive atomic planes and the stacking of structural blocks in the preferential stacking positions to form the OD structure, and (v) elimination of different structural blocks (other than 6-layer ones) and the long-range ordering in the stacking of structural blocks.