Bonding Strength and Basic Properties

A basic property is the melting temperature since it is known that materials parameters which characterize the deformation behavior are well correlated with the melting temperature (Frost and Ashby, 1982). Examples are the elastic moduli which not only control the elastic deformation, but are also important parameters for describing the plastic deformation, and the diffusion coefficients which control not only the kinetics of phase reactions, but also the kinetics of high-temperature deformation, i.e. creep. Furthermore, the melting temperature is intuitively regarded as a measure of the phase stability since it limits the application temperature range. 

However, a phase melts when the Gibbs energy of the solid phase, which is a thermodynamic state function and which controls the phase stability, is higher than that of the liquid phase [see e.g. Denbigh (1971) and chapter by Pelton in Volume 5 of MST]. The Gibbs energy G of a phase is given by 

It may be concluded that the phase formation enthalpy may be a better parameter for characterizing bonding strength and phase stability, and for correlating this with the basic properties, e.g. elastic moduli. Formation enthalpies have been determined experimentally (Hultgren, 1963), 

The correlation between the total phase enthalpy and the Young s modulus, which is one of the three elastic moduli, is shown in Fig. 6 for various cubic phases, i.e. for the f.c.c. elements Al and Ni (Al), the f.c.c.-ordered NijAl (LI 2), the b.c.c. ordered FeAl, NiAl and CoAl (B2), and the cubic Laves phases CaAlj, YAI2, LaAl2, NbCr2, Zr i o2 and HfCo2 (Cl 5). The correlation is quite good for the Laves phases, i.e. there seems to be a common scatter band and the data of some other phases are near this scatter band. It has to 

It has to be concluded that there are physically justified correlations between parameters which characterize the bonding strength and the phase stability on the one hand, and the macroscopic phase behavior on the other. However, such correlations represent complex functional relationships, and thus they are useful only for order-of-magnitude predictions. More quantitative predictions have to consider the character and strength of bonding in a more detailed way, i.e. they have to rely on quantum-mechanical calculations which are cumbersome and time consuming. 

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