Basic Properties and Phase Diagrams

The characteristics of thermal expansion were described by Shashikala et al. (1989). 

Property Ti-base TijAI-base TiAl-base Superalloys 

Ti3Al has an extended composition range, as is visible in the commonly used Ti-Al phase diagram (Fig. 10a), which, however, is still in discussion and has recently been revised, as shown in Fig. 10b (Hellwig etal., 1992 Kainuma etal., 1994). It forms stable equilibria with the two disordered Ti phases, a-Ti with a hexagonal close-packed A3 structure and P-Ti with a b.c.c. A2 structure, and with the other important titanium aluminide TiAl, 

1991 Saunders and Chandrasekaran, 1992 Hoch and Lin, 1993), but the consequences of small oxygen additions on the Tij Al equilibria have not yet been worked out in sufficient quantitative detail. 

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Chapters 7 to 9 apply the thermodynamic relationships to mixtures, to phase equilibria, and to chemical equilibrium. In Chapter 7, both nonelectrolyte and electrolyte solutions are described, including the properties of ideal mixtures. The Debye-Hiickel theory is developed and applied to the electrolyte solutions. Thermal properties and osmotic pressure are also described. In Chapter 8, the principles of phase equilibria of pure substances and of mixtures are presented. The phase rule, Clapeyron equation, and phase diagrams are used extensively in the description of representative systems. Chapter 9 uses thermodynamics to describe chemical equilibrium. The equilibrium constant and its relationship to pressure, temperature, and activity is developed, as are the basic equations that apply to electrochemical cells. Examples are given that demonstrate the use of thermodynamics in predicting equilibrium conditions and cell voltages. 

This chapter introduces additional central concepts of thermodynamics and gives an overview of the formal methods that are used to describe single-component systems. The thermodynamic relationships between different phases of a single-component system are described and the basics of phase transitions and phase diagrams are discussed. Formal mathematical descriptions of the properties of ideal and real gases are given in the second part of the chapter, while the last part is devoted to the thermodynamic description of condensed phases. 

The book begins with a chapter describing the history and growth of CALPHAD. This provides a useful point of departure for a more detailed account of the various strands which make up the CALPHAD approach. Chapters 3 and 4 then deal with the basic thermodynamics of phase diagrams and the principles of various experimental techniques. This is because one of basic pillars of the CALPHAD approach is the concept of coupling phase diagram information with all other available thermodynamic properties. It is a key factor in the assessment and characterisation of the lower-order systems on which the properties of the higher-... 

EIGURE 1.1 Partial phase diagram for ZrOj-YjOj. (From Nowotny, J. et al., Charge transfer at oxygen/zirconia interface at elevated temperatures. Part 1 Basic properties and terms, Adv. Appl. Ceramics 104 (2005) 147-153. With permission.)... 

The constitution of many ferritic steels includes chromium and vanadium as basic alloying elements. Knowledge of the phase diagram and thermodynamic properties of the Cr-Fe-V system is essential to imder-stand the behavior of such iron-based alloys. 

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