Ordering instabilities and pretransitional effects
- PDF / 920,917 Bytes
- 8 Pages / 594 x 774 pts Page_size
- 61 Downloads / 267 Views
THE synonymous expressions Pretransformation,
Pretransitional, Premonitory phenomena designate effects occurring in a stable (or metastable) system which indicate that a phase change must eventually take place at some lower temperature. Important questions to be answered are: what is the relationship between premonitory effects and the corresponding phase transformation, are these effects related to first-order, or to second-order phase transformations, or to instabilities, and what is the relationship between instabilities and fluctuations, the latter also being regarded as a pretransitional phenomenon? The object of this communication is to give at least partial answers to these questions and to illustrate the concepts by means of examples mainly chosen among metallic solid solutions. For a more detailed treatment of some of these topics, the reader is referred to the author's recent review article? 2. TYPES OF PHASE T R A N S I T I O N S At any temperature, a given phase of a substance may be a) completely stable, b) inherently unstable, c) less stable than some other phase. In case b), if the transformation from the metastable state to a stable state occurs at equilibrium, the transition is second-order, i.e. it is found that the order parameter increases continuously from its value zero at the transition temperature as the temperature (say) is lowered. At the transition, a cooperative phenomenon occurs, true critical behavior is observed. Hence the transition temperature is called the critical temperature, usually denoted T C. Symmetry rules for second-order transitions have been given by Landau and Lifshitz in their classic text "Statistical Physics. ''2 In case c), another phase has lower free energy below D. DE FONTAINE, formerly with the Materials Department, University of California, Los Angeles, CA 90024, is now Professor, Department of Materials Science & Mineral Engineering, University of California at Berkeley, CA 94720. This paper is based on a presentation made at a symposium on "Pretransformation Phenomena, Fluctuations and Related Effects" held at the annual meeting of The Metallurgical Society of AIME, New Orleans, Louisiana, February 18-22, 1979, under the sponsorship ship of the Structures Activity, Materials Science Division, ASM. METALLURGICAL TRANSACTIONS A
some transition temperature, denoted here T t, and defined as that at which the two phases, the "old" and the "new", have same free energy. There is no "critical" behavior per se at T,, only a discontinuous change in properties (enthalpy, specific volume, order parameter if one can be defined). Such transitions arefirst-order; they are in fact simply those which are not second-order. Often, it is possible to suppress a first-order phase transition to produce a metastable state, at least momentarily. Usually, if the temperature is further reduced, the metastable state will become inherently unstable, and will spontaneously decompose, in a continuous fashion, to more stable forms. In some ways, this continuous transformation resembles a s
Data Loading...
