Thermodynamic calculations were carried out to predict the spinodal solvus in Cu-Ni system. A rapid calculation method using available thermodynamic data already obtained at 973K based on the concept of Cahn's spinodal theory of anisotropic materials was carried out to calculate the spinodal in Cu-Ni system. The chemical spinodal could be obtained by using the expression for $\frac{∂^2f}{∂{X^2}}$ derived by Rundman and Hilliard. To do this, we obtained the analytical relations by least square method from the data of excess partial free energy and entropy of Cu at 973K. As a result, the chemical spinodal showed the asymmetry present in the available miscibility gap. The critical coherent spinodal (620K) obtained assuming that spinodal decomposition occurs in the <100> direction is in good agreement with the experimental value (598K). It is concluded that the spinodal decomposition occurs in the elastically soft <100> direction in Cu-Ni system. Spinodal decomposition and concurrent continuous ordering reaction that occur in rapidly solidified Ni-14 at. % Al alloy were reexamined theoretically and experimentally. In as-rapidly solidified state, the superlattice spots of $Ll_2$ type structure were observed in TE diffraction pattern but no modulation was observed. As aging progresses at 773-873K, development of sidebands in the vicinity of the X-ray diffraction main peaks and a typical modulated structure along the elastically soft <100> direction in TE micrograph were observed. Such observations tend to indicate that rapidly solidified Ni-14 at. % Al alloy may decompose by spinodal decomposition in the limited temperature and composition range, not by nucleation and growth mechanism. The value of 1/3 obtained from the time exponent of the modulation wavelength in the coarsening stage is also obtained as it was obtained in the growth stage of precipitate particles at higher temperature by Ardell and Nicholson. From the observations thus made, it is concluded the supersaturated Ni-14 at. % Al first lowers its free energy by a homogeneous ordering process. Then, this process may be followed by concomitant spinodal decomposition. High supersaturation caused by RSP is thought to make Ni-14 at. % Al decompose spinodally. The sequence of reactions can be understood in terms of a thermodynamic model of the free energy of a solution that use the 1st neighbor interaction energies(V) and the 2nd neighbor interaction energies (U). Especially, the value of U/V =-0.4 with V(-4000J/mol) yielded the coherent spinodal solvus in good agreement with that obtained by using X-ray diffraction and TEM in this study. In addition, the order parameter of homogeneous solution increases with increasing aluminum contents over the critical value. In addition to the theoretical and diffraction studies, the elastic stiffness constants were experimentally measured by the ultrasonic pulse echo method. The value of elastic strain energy $2η^2Y$ in <100> was calculated as 6140 J/mol.
Phase changes that occur in melt spun high carbon Fe-Ni-Al-C alloys are investigated by X-ray diffraction and transmission electron microscopy. A typical modulated crystal structure along the elastically soft $<100>$ directions was observed in the transmission electron micrographs coinciding with the development of sidebands in the vicinity of the X-ray diffraction main peaks. The modulation wavelength increases with the increasing ribbon thickness, indicating that the modulated domains coarsen to the larger extent in the more slowly cooled specimens. The intensity of odd (h+k+1) superlattice spots is much stronger than that of even (h+k+1) superlattice spots. This is attributed to the $L'1_2$ type ordering in carbon rich domains of the modulated structure of as-solidified specimens. After brief heat treatment, these alloys developed an unusual lamellar duplex microstructure made up of perovskite carbide and ferrite plates. The microstructure of melt-spun Fe-27.5 Ni-7.14 Al-0.8C alloy was characterized by X-ray diffraction and TEM. During the initial stage of aging in $\gamma$-phase Fe-27.5 Ni-7.14 Al-0.8C, the striation parallel to the traces of {110} was observed in TE micrograph. The peculiar electron diffuse scattering in the form of rel-rods in <110> directions in reciprocal space of TE diffraction pattern was observed together with streaks in <110> directions. It was also found that the appearance of the extra spot was not due to an ordered phase but due to the intersection of the <110> streaks with the reflecting sphere. It is concluded that the modified microstructure more closely resembles a tweed microstructure than a classical modulated structure. The high elastic anisotropy of nickel is given as the reason for the formation of tweed microstructure. In conclusion, the tweed microstructure formed during aging at 723K is attributed to the pretransformation resistant to $\gamma$-α transformation.
