X-ray determinations of order and atomic sizes in Co-Pt solid solutions

Abstract

The equilibrium long-range order in the alloy CoPt has been determined by means of X-ray intensity and lattice tetragonality measurements on powder briquettes quenched from various annealing temperatures. The critical temperature, T c is 833 ± 2° C, and the long-range order vanishes discontinuously on heating, at a value of S c = 0.78. Local order and size-effect coefficients were also obtained for a series of disordered Co-Pt alloys by means of diffuse X-ray scattering measurements from quenched powder briquettes. A preference for unlike nearest-neighbors was found with magnitudes comparable to those observed in the Au-Cu system. A more general method of interpreting the size-effect coefficients in terms of atomic radii is given. An elastic theory is used to determine the average atomic dimensions in solution from the lattice parameters and the bulk moduli. This result is then combined with the measured size-effect coefficients to provide information on the distortion of the shape of the atoms in solid solution. These methods are applied to Co-Pt disordered solutions and the results are also used to predict the solc a ratio for the perfectly ordered alloy CoPt in good agreement with experiment.