The basic phase diagrams

Abstract

The reader is assumed to be familiar with the interpretation of binary phase dia-grams and the chapter begins with a brief description of the specific features invol-ved in the graphical representation of ternary and even higher order systems. Six ternary systems are analysed in detail by way of example : Fe-Cr-C, Fe-Ni-Cr, Fe-Mn-S, Fe-Co-Cu, Fe-Mo-Cr and Fe-C-V. They have been chosen because they include all the typical phase reaction configurations encountered in iron-base alloys, and especially in steels. Calculated phase diagrams 3 are extensively employed in this section, since they enable large numbers of isothermal sections and isopleths to be plotted, facilitating the detailed study of their variation with temperature or composition. 4-1 Equilibria between condensed phases Basic rules Our knowledge of the metallurgy of steels, particularly as regards the effects of compo-sition and temperature on microstructure, is largely based on experimental data obtained using a wide range of physical and chemical techniques. The thermodynamics of phase equilibria provides the only unifying framework that enables these data to be compared and validated. When considering a particular system, a given quantity of matter is treated, that is a fixed total number of molecules (usually gram molecules or moles). The nature of the molecules is determined by the composition, i.e. by the concentrations of the different constituents, either elements or compounds. If a system comprises N constituents, the composition will be fully defined when N-1 concentrations are fixed. The concentra-tions may be given in terms of atom or mole fractions or atomic or weight percentages. In practice, metallurgical phase diagrams are usually represented in terms of weight percentages. This approach will be applied in most of the diagrams considered, atomic 3. The majority of the calculations were performed using the Thermocalc or Pandat softwares, with data available in the SGTE bank in 2002.