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 degrees C have been investigated by means of transmission electron microscopy and scanning transmission electron microscopy. The most energetically favourable polytype at 525 degrees C is found to be the structurally simplest one, a maximum degree of order polytype (monoclinic, 1M, space group: C2/m), described with a single stacking vector in stacking six-layer 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 hexagonal close-packed stacking of the AB-type, (ii) formation of Al6Gd8 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 six-layer and, sometimes seven-layer structural blocks, (iv) in-plane ordering of Al6Gd8 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 six-layer ones) and the long-range ordering in the stacking of structural blocks.