Other examples of long-range order

Before considering the ordering transformation in AuCu, which is rather complex, we might examine the behavior of j5-brass. This alloy is stable at room temperature over a composition range of about 46 to almost 50 atomic percent zinc, and so may be represented fairly closely by the formula CuZn. At high temperatures its structure is, statistically, body-centered cubic, with the copper and zinc atoms distributed at random. Below a critical temperature of about 460°C, ordering occurs the cell corners are then occupied only by copper atoms and the cell centers only by zinc atoms, as indicated in Fig. 13-6. The ordered alloy therefore has the CsCl structure and its Bravais lattice is simple cubic. Other alloys which have the same ordered structure are CuBe, CuPd, and FeCo. 

By calculations similar to those made in the previous section, the structure factors of ) -brass, for the ideal composition CuZn, can be shown to be 

In other words, there are fundamental lines, those for which (/i + fc + /) is even, which are unchanged in intensity whether the alloy is ordered or not. And there are superlattice lines, those for which (h + k + 1) is odd, which are present only in the pattern of an alloy exhibiting some degree of order, and then with an intensity which depends on the degree of order present. 

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