It is well known that the lattice waves in alloy can be looked upon as the superposition of a series of plane waves with different wave vectors. Because of these plane wave′s diffraction action for X photon, there are two satellites (sidebands) around X ray main diffraction peak. With the wavelength and asymmetric factor α y of rectangle wave of the distribution of concentration introduced, the amplitude of modulation wave appearing along some crysallographic direction can be expressed clearly in the form of a sum of several diffraction wave vectors in the reciprocal space, and the diffracted intensity can be obtained. The X ray diffraction angle of sidebands strongly depends on the distribution of the wavelength. Fig.1b gives the simulated X ray diffraction profiles. It shows that when we fix the average modulated wavelength and change the distribution of wavelength, the angle difference between the satellite and main diffraction peak varies correspondingly. The simulated diffraction profiles are in good agreement with experimental results [1] (Fig.1a). The more diffuse the distribution of wavelength is, the nearer the sidebands are to main peak, and vice versa. In addition, the intensity and position of satellite are obviously restricted by the asymmetric factor of wave shape. Any lattice wave propagating in crystal can be resolved along coordinate axes. On the basis of the principle of superposition, all compositions of the lattice wave have diffraction profiles of themselves. Add two diffraction patterns perpendicular to each other on the reciprocal plane which is normal to the projected direction, we get the simulated TEM diffraction pattern of spinodal decomposition. Fig.2a is the TEM pattern by Kubo H [2] , Fig.2b and 2c are the simulated TEM patterns by Kubo H [2] and Khachaturyan A G [3] respectively. Our simulated TEM pattern (Fig.2d) is in good agreement with Fig.2a.
