亚稳相的高压暴露

 很久以前,便有人指出,气态冷凝成固态时,要连续经历液相及各种高温相,才达到平衡结晶相。但是,液态及高温相往往需靠很大的冷却速度才能冻结下来,这在当时对绝大多数合金,是不可能的。近些年,随着超急冷等技术的进步,关于非晶等亚稳相得研究十分活跃。当超过一定临界冷却速度时,液态合金可固化为非晶态。虽然,亚稳结晶相较非晶应更容易冻结,但是,由于产生各种亚稳相所需的过冷条件各不相同,以及对冷却速度的选择不能是任意的,因此有时它们较非晶还难于形成。与液相凝固过程相似,非晶合金的晶化也服从构型最小重排原理,即在晶化完成之前,存在某些亚稳相变态阶段。但是,限于热力学上的不稳定性及动力学因素,在常压下这些亚稳相同样是难以发现的。作者根据对多种合金系的研究,提出高压暴露亚稳相的设想,并利用非晶等亚稳相的高压变态过程,将进行液态急冷时的速度控制方式,改为便于掌握的高压退火方式,来获得新亚稳相。本文对压力暴露亚稳相的原理和实践,加以论述。

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.