J. Kulik

Spinodal ordering beyond the lifshitz point in Cu3Pd observed by high voltage electron microscopy

Abstract Cu-Pd samples of compositions varying from 16 to 26 at.% Pd were irradiated in situ in the 1.5 MeV electron microscope at various temperatures. Low-temperature (−180°C) irradiation produced completely disordered solid solutions, room-temperature irradiation produced steady-state short-range order (SRO), and high-temperature irradiation tended to produce the expected equilibrium long-range order. In particular, the 18 and 20% samples irradiated at room temperature exhibited steady-state modulated SRO although only the simple Ll2 ordered structure was expected at equilibrium. It is suggested that spinodal ordering is responsible for these effects. An f.c.c.-based Cu-Pd phase diagram is proposed incorporating ordering stability loci and a metastable Lifshitz point.

One-dimensional long-period superstructures in Cu3Pd observed by high-resolution electron microscopy

One-dimensional long-period superstructures (LPS) in Cu3Pd have been studied by electron diffraction and high-voltage high-resolution electron microscopy. Use of a high accelerating potential (800 to 1000 kV) for the electron microscope has resulted in micrographs showing well-defined antiphase boundaries. Boundaries contain frequent jogs in specimens of low Pd content. These observations contrast with previous electron microscopy studies which failed to resolve sharp boundaries presumably because of dynamical diffraction effects of lower energy (200 kV) electrons.

Mean-field theory of long-period superstructures on the FCC lattice

An Ising model with long-range competing interactions on the FCC lattice is examined in mean-field theory. The competition of the interaction parameters is characterised by a parameter kappa , and a phase diagram in the (T, kappa ) plane is presented. Long-period superstructures are obtained as ordered states. Results are compared with the ordered structures observed in Ag3Mg.

Models of long-period superstructures

The cause of the stability of long-period superstructures is still something of a mystery. Typically, two very different models have been proposed: according to model I, the period of the superstructure (or modulation) is determined by lowering of the electronic energy resulting from the formation of a new Brillouin zone. According to model II, competing short-range interactions tend to produce long-period structures, the wavelength of which is determined by configurational entropy considerations. Model I is exemplified by the Sato and Toth theory, apparently applicable to long-period superstructures in Cu-Au, for example. Model II is exemplified by the Axial Next Nearest Neighbor Ising Model, for which a low-temperature free energy expansion has recently been given by Fisher and Selke. The latter model appears to apply to long-period superstructures in Ag/sub 3/Mg.

Cu3Pd OBSERVED BY HIGH-VOLTAGE ELECTRON MICROSCOPY

Cu-Pd samples of compositions varying from 16 to 26 at.% Pd were irradiated in situ in a 1.5-MeV electron microscope at various temperatures. Low-temperature (90/sup 0/K) irradiation produced completely disordered solid solutions. Irradiation at room temperature up to as high as about 500/sup 0/K produced steady state short range order (SRO) which, for specimens of 18% or more Pd, is characterized by diffuse intensity at (1 +-q,0) and equivalent positions in reciprocal space (modulated SRO). In general, q is a function of composition, temperature and irradiation dose. High temperature irradiation tended to produce the expected equilibrium long range order - either L1/sub 2/ or a long period superstructure depending on composition and temperature. The 18 to 20% samples irradiated at room temperature exhibited steady state modulated SRO even though the expected equilibrium structure is one of unmodulated order (L1/sub 2/). It is suggested that spinodal ordering is responsible for this latter effect. An f.c.c. based Cu-Pd phase diagram is proposed incorporating ordering stability loci and a metastable Lifshitz point. 37 refs., 12 figs.