Platinum Metals Rev., 1972, 16, (2), 48
The Equiatomic Cu-Pt Superlattice
We have been trying to elucidate the mechanisms of ordering of the lattice structure in various copper alloy systems, among which the copper-platinum system is especially interesting because of its unique features.
The random f.c.c. solid solution in the equiatomic copper-platinum system shown in Fig. 1a is replaced below the critical temperature of 812°C by a long-range ordered rhombohedral lattice. The latter superlattice configuration consists of alternate layers of pure copper and pure platinum atoms arranged parallel to the (111) planes, there being the same number of nearest neighbours in both the ordered and disordered states, as shown in Fig. 1b.
CuPt is the only known superlattice with this structure but in fact surprisingly little work has been performed on the equiatomic alloy. The fact that there is no change in the number of nearest neighbours upon ordering has meant that most existing theories fail to account for the ordering in CuPt. However, Clapp and Moss (1) have attempted a formulation in terms of first and second nearest neighbours, the approach being referred to as the central pair-wise (CPW) model. The analysis predicts the long-range order behaviour from short-range order data, obtained by Walker (2) from diffuse X-ray scattering experiments on powdered samples, but there is no mention of why CuPt orders at all. The theory could be more rigorously tested using diffuse X-ray scattering from single-crystal samples of CuPt.
Our recent experimental study (3) has shown that CuPt conforms to the earlier established thermodynamic criteria (4) that order-disorder transformations are first-order phase reactions. Moreover, the structural change involved (a rhombohedral lattice replacing a cubic one) means that substantial internal stresses are created which subsequently strengthen the material during the course of transformation, as shown in Fig. 2. Such hardening behaviour, coupled with the corrosion and oxidation resistance of the alloy, can be usefully employed when the alloy has been annealed for intermediate times below the critical temperature (812°C). A thermal polishing technique (5) for producing strain-free surfaces without recourse to electrolytic polishing, which is impracticable, has meant that it is now possible to perform polarised light metallography on bulk samples of CuPt (see Fig. 3) and thus interpret the mechanical characteristics in terms of the ordered morphologies.
Further work is in progress and a forthcoming publication will provide full details of our studies.
- 1P. C. Clapp and S. C. Moss, Phys. Rev., 1966, 142, 418; 1968, 171, 754; 1968, 171, 764
- 2C. Walker, J. Appl. Phys., 1952, 23, 118
- 3R. S. Irani and R. W. Cahn, Nature, 1970, 226, 1045
- 4F. N. Rhines and J. B. Newkirk, Trans. A.S.M., 1953, 45, 1029
- 5R. S. Irani and R. W. Cahn, Metallography, 1971, 4, 91