Chapter 2 Phase Equilibria in One-Component Systems.pptVIP

Chapter 2 Phase Equilibria in One-Component Systems 2.1 General Phase equilibria are determined by the interaction of state variables, such as temperature, pressure, volume and composition. The direction of equilibria: smallest force. Clausius-Clapeyron equation: 2.2 Transformation Equilibria in the Solid State Polymorphism: With the variation of the pressure and temperature and composition in a multi-component system , the crystal structure of a solid substance transforms into another lattice with different symmetry relationships as well as different distances between the atoms or molecules. Usually, this transformation is proceeding by diffusion processes within the phase boundary or in its immediate neighborhood. Enantiotropic transformation: Phase transformations between two stable solid phases. It can be carried out on a reversible manner. Monotropic transformation: A metastable solid phase transforms into a stable one. It is not reversible. The solid phase transformation are connected with volume changes. Thus, the critical temperature depends on pressure. Some examples of polymorphic transformation: iron, titanium, uranium. Generally, solid phases existing at high temperatures have less dense structures than those existing at low temperatures. An exception is the most common metal iron. 2.3 Monotropic Transformation Metastable phases are not formed by equilibrium reactions, but they are often found in real systems. We can not heat treat steels to reach different properties without the metastable phase martensite. Several metastable phases can be observed in a real system. Generally, on non-equilibrium reactions the phases with the lowest stability is formed. Ostwald’s rule of steps: On a monotropic transformation, the metastable phase with the lowest stability do not transition immediately into the stable phase. On the contrary, a gradual reduction of the Gibbs energy takes place so that the metastable phase is formed which is the next one