Development of “Macroscopic Composition Gradient Method” and its application to the phase transformation

A new characterization method, “Macroscopic Composition Gradient (MCG) Method” is proposed to investigate the phase transformations near the phase boundaries. The MCG method is a new technique to investigate the phase transformations in various composition alloys by utilizing a single specimen having the macroscopic solute composition gradient. Since the macroscopic composition gradient in the MCG alloy is so prepared as to cross over the phase boundary, the morphological transition of critical phenomena at the phase boundary can continuously be investigated by means of analytical transmission electron microscopy. By utilizing the MCG method, the various kinds of phase transformation, such as the coherent and incoherent precipitation boundaries, the order/disorder phase transition and the morphological change at the spinodal line have successfully been investigated. Furthermore, to an important thing, the critical size of precipitate-nucleus and the nucleation rate near the solubility limit can be experimentally obtained for respective nucleus. The phase decomposition of supersaturated solid solution progresses by a mechanism of spinodal decomposition even in the N-G region of phase diagram. On the basis of experimental results, the application limit of the conventional nucleation theory is investigated, and hence the failure of Boltzmann–Gibbs free energy becomes obvious in the early stage of phase decomposition. noteworthy that the present experiment is systematically conducted for the alloy composition range very close to the solubility limit. Such critical phenomena of phase transformation have been scarcely examined in the past. The MCG method proposed here is considered to open a new way to investigate the critical phenomena in the phase boundary.