The superlattice in β Brass

Abstract

The transformation in β brass (CuZn) has formed the subject of much experimental work, and a considerable amount of speculation due to the fact that it differs in certain essential characteristics from the more general type of transformation associated with a change of phase. Tammann and Heusler (1926), in a review of the experimental data available in 1926, pointed out that the transformation was accompanied by changes in heat content, thermal expansion and electrical resistance, which were very similar to those occurring in pure iron during the magnetic transformation. They concluded that the β — β' transformation was not a change in phase, and suggested that it was caused by a rearrangement of the atoms in the alloy without a change in lattice type. In the β or high-temperature modification the zinc and copper atoms were supposed to be distributed at random amongst the atomic sites of the body-centred lattice, whereas in the β' or low-temperature modification a more or less perfectly ordered arrangement, with say copper atoms at the cube corners and zinc atoms at cube centres, was supposed to exist. This view is now generally accepted, and the β — β' transformation is often quoted as being typical of order-disorder transformations in general. More recently the thermodynamics of such atomic rearrangement processes have been considered in some detail (Bragg and Williams 1934; Bethe 1935) and the change in energy content calculated. The result obtained is in reasonable agreement with that found experimentally in the case of β brass (Sykes 1935), thus confirming the original suggestion of Tammann and Heusler.