Exposure of Metastable Phases by High Pressure

It was proposed ninety years ago, that after condensation of vapor it is necessary for the condensation to pass through possible high temperature phases until the equilibrium phase is reached. However, it was in general impossible for metals and alloys to freeze a structure of liquid or metastable phase for the reasons of technologies at that time. After then, by the other way, high pressure method has been used to synthesize the metastable phase in which diamond is one of the succeeded examples as known is to all, with energy situated between liquid and equilibrium states. During the past decade, the studies on amorphous and other metastable alloy were carried out intensively, because of the improvements of techniques to solidify liquid alloys at large undercooling, such as small droplet processing and liquid quench. In the former case, formation of a metastable phase is dominated by static undercooling. From thermodynamic studies, it was shown that the nucleation of a metastable phase becomes more likely than that of the stable phase. With an increase in undercooling, some metastable phases with lower melting points, which have been exposed at high pressure, were solidified under atmospheric pressure by using small droplet processing. But for alloys with higher melting points the metastable phases have never been prepared in the same way as carrier medium is limited for droplets. In the case of liquid quenching the metstable phases are formed by a kinetic process. Although the quenching rate to freeze liquid into the metallic glasses is usually lower than to transform which into a crystalline metastable phase, the latter is more difficult to exposure owing to its strict quenching condition. Similar to the solidification of liquid, the crystallization of an amorphous alloy may yields some metastable phases before the equilibrium state was formed. However, the metastable phases are not able to discover due to the fast kinetics of crystallization in many cases. Recently, the idea to expose metastable phase kinetically by high pressure was proposed on the basis of the investigations on crystallization processes of the amorphous alloys under high pressure. According to the generd transformation diagram for amorphous alloys heated under pressure, there are three types for transformation mode: a process to decompose multiphases at lower pressure, a process form single phase metallic compound at higher pressure and a process to yield disorder solid solution at ultra pressure. Differences in the mode were attributed to an effect of high pressure on the atomic rearrangements occurred in the interfaces between amorphours and crystalline phases. In general, diffusion for rearranging atomic positions is suppressed by pressure, so the metastable phase accompanied with smaller entropy change and atomic rearrangement during its forming should be preferred to form kinetically.