Studying Thermodynamics of Metastable States

TL;DR

This paper presents a classical thermodynamic framework for analyzing metastable states, enabling calculations of free energy, heat capacity, and stability, and explores their behavior under various conditions including mechanical loading and heating rates.

Contribution

It introduces a comprehensive thermodynamic approach to describe metastable states, including their stability, relaxation dynamics, and phase transitions, with applications to alloys.

Findings

Dependence of heat capacity on heating rate

Calculation of relaxation time from phase shift

Derivation of Ostwald stage rule

Abstract

Simple classical thermodynamic approach to the general description of metastable states is presented. It makes possible to calculate the explicit dependence of the Gibbs free energy on temperature, to calculate the heat capacity, the thermodynamic barrier, dividing metastable and more stable states, and the thermal expansion coefficient. Thermodynamic stability under mechanical loading is considered. The influence of the heating (cooling) rate on the measured dynamic heat capacity is investigated. A phase shift of the temperature oscillations of an ac heated sample is shown to be determined by the relaxation time of the relaxation of the metastable nonequilibrium state back to the metastable equilibrium one. This dependence allows one to calculate the relaxation time. A general description of the metastable phase equilibrium is proposed. Metastable states in AB3 alloys are considered. Reasons for the change from the diffusional mechanism of the supercritical nucleus growth to the martensitic one as the heating rate increases are discussed. The Ostwald stage rule is derived.