The mechanism of crystallographic ordering in CuPt

The present paper is primarily a study of the ordering characteristics within the CuPt system, near the CuPt composition, using X-ray diffraction, optical microscopy (in conjunction with polarized light), high-voltage electron microscopy and dilatometry.In platinum-rich off-stoichiometric alloys, a wide two-phase region consisting of ordered + disordered platelets was established and the phase boundaries were accurately located. For isothermally ordered stoichiometric alloys, in general two categories of diffraction sequences were observed, depending on the annealing temperature. For anneals in the range 620°C<T<815°C (=T_c), a series of broad, asymmetric X-ray line profiles were obtained during the early part of the ordering cycle: this represents a continuous reaction. However, when samples were annealed a temperatures lower than 620°C, there was unmistakable micrographic evidence of the coexistence of both the ordered and disordered phases: this represents a discontinuous reaction. After making allowances for a number of side-effects which had broadened X-ray reflections at high temperatures, a nucleation-and-growth model is proposed for CuPt at all ordering temperatures.The parallel microscopic studies also exhibited quite contrasting morphologies above and below 620°C: a lamellar structure is the product at high temperatures, whereas a grain-boundary reaction, generating very coarse domains, is observed at lower temperatures. A modified microstructure was observed for samples annealed at T<475°C, when ordered spherulites were seen to grow within the grains. Samples cooled slowly through T _c order by a diffusion-controlled shear process.