Figure 7 - TEM micrograph of an Al-Mg-Si-Cu sheet sample (overaged) at the [001] foil orientation. (a) BF (bright field), (b) SADP (selected area diffraction pattern), (c) schematic of the four end-on variants of the Q phase. Arrows indicate some of the variants in the BF image in (a).

Figure 8a is a diffraction pattern taken along [001] zone axis of Aluminum. Faint rings can be seen in the pattern (see schematic of rings in Figure 8b). The ratio of the radii of the rings is 1: 1.16: 1.52: 1.73: 2.06: 2.63 which fits perfectly to a primitive hexagonal lattice. These rings come from Q' precipitates where the c axis is parallel to [001] of aluminum. The precipitates with their c axes along [100] or [010], give rise to the extra reflections near the {110} positions in Figure 8a. An indexed pattern for Q'//(200)Al is shown in Figure 9. From this it can be seen that the (210)Q' is parallel to the (020)Al.

The orientation relationship of (210)Q' // (020)Al is within 2° of that reported by (17). Their relationship was derived from the habit planes of the precipitate. Their orientation relationships can be rationalized on the basis of a good fit of lattice spacing. The perfect match in one direction (viz, the c axis of Q' along the <100> Al) is what gives rise to the long dimension of the lath parallel to one of the <100> Al directions (25). The repeat distance along the <150> directions of the aluminum matrix is nm. This is nearly the same as the lattice parameter of the Q' phase, see Figure 10. Hence during the solid state precipitation Q' minimizes its surface energy by maximizing its area on the {150} planes. Overall then, the Q' phase is lath shaped.

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