Le Chatelier's principle has been the subject of much discussion and controversy because it sometimes seems to give the wrong predictions. A classic example is the ammonia gas reaction. The principle is now re-examined, starting from a strict and general derivation given recently. A chemical reaction in a gas mixture is then considered and the conditions are derived under which a chemical reaction will increase the amount of a type of molecule but decrease its chemical potential, a case which has caused confusion. It is demonstrated that there are no exceptions to Le Chatelier's principle if applied to infinitesimal changes of an equilibrium system. However, finite changes may violate the principle. The conditions are given under which this may occur in a gas mixture. The mathematical formulation of the principle is transformed to a form lending itself to quantitative calculations.

On the microscopic scale the physical models and their mathematical counterparts available today are much more detailed and elaborate than the relatively coarse approach discussed in the previous section for macroscopic processes. Phase transformation between condensed phases for example can be treated by a method which combines an explicit description of diffusion processes with thermochemical calculations assuming local equilibrium. A short description of this method is discussed below.

Dissolution of spheroidal cementite in austenite at 910°C has been studied by means of scanning and transmission electron microscopy. Three different morphologies of transformation have been observed. Most of the particles undergo a more or less homogeneous shrinkage. However, some of the particles transform to austenite by a Widmanstätten type of reaction, whereby austenitic plates form inside the cementite. It is also observed that some of the cementite transforms to M7C3 carbide and austenite by a eutectoid reaction. The results can be understood by considering the phase diagram and assuming that local equilibrium prevails at the phase interface during the reaction.

If the different components in a compound material are chemically incompatible the heat treatment of such a material will present some difficulties. There may be strong driving forces for the transfer of atoms by means of diffusion between the two component materials. This transfer may result in a drastic change in the properties. Here we will present a detailed study of diffusion in a compound steel by means of numerical solution of the diffusion equations. We will also present a more approximate calculation where the mobile carbon is equilibrated between the steels and the less mobile substitutional elements are only equilibrated within each steel but not between steels. Thus, complete equilibrium is established within each steel. In the latter calculation both steels are multicomponent and multiphase materials.

Hillert often used the combined first and second law of thermodynamics to discuss and derive the principles of equilibrium as well as non-equilibrium thermodynamics. His method will now be reviewed and applied to reactions in homogeneous systems as well as transport processes As an example of homogeneous systems undercooled liquids and glasses is discussed.

The carbon potential of a furnace atmosphere is of utmost practical importance during heat treatment of steels. The atmosphere is called active if the carbon potential differs from that of the steel and in that case there will be a transfer of carbon from the atmosphere to the steel or vice versa. This is the situation, for example, during the carburising in the case hardening process. In the other processes one rather requires an atmosphere that is inactive with respect to the steel, for example during austenitising of complex tool steels. In order to gain a deeper understanding of the behaviour of furnace atmospheres and to make possible optimisation of heat treatment operations a cooperation was started between AGA Innovation and the Royal Institute of Technology. The project involves thermodynamic calculations of the equilibrium composition and carbon potential of furnace atmospheres used in practice and the equilibrium properties of multicomponent steels.