Showing papers by "Y. Austin Chang published in 1985"

Magnetic contributions to the thermodynamic functions of pure Ni, Co, and Fe

Abstract: An empirical mathematical equation is proposed for the magnetic contribution to the specific heat of pure metals The corresponding functions for enthalpy, entropy, and Gibbs energy are of simple form Two parameters used for each element are the critical temperature,Tc, and the total magnetic entropy The parameters have been determined from a careful separation of magnetic and nonmagnetic contributions to the specific heat Debye temperatures for Ni, Co, and Fe have been determined considering data to much higher temperatures than other studies The magnetic specific heats extracted from experimental data agree very well with the proposed equation over the entire temperature range and for all three elements Comparisons with different mathematical functions found in the literature give agreement only for the case of iron The total magnetic entropy given by a classical relation is found to be high, and a quantitative correction is given Various magnetic standard states are discussed The lattice stabilities of bcc- and fcc-iron are calculated assuming that the difference of the nonmagnetic specific heats is linear from 500 K to 1810 K A simple equation is obtained in which the anomalous temperature dependence is explained by the independently determined magnetic contribution The calculated values agree very well with Orr and Chipman’s assessment The stability of bcc iron at low temperatures is quantitatively rationalized

A thermodynamic study on an associated solution model for liquid alloys

Abstract: Liquid alloys with a rapid increase of component activities over a narrow composition range may be modelled by an associated solution model. Associated species with a fixed stoichiometry in that composition range are postulated in equilibrium with elemental species. Palrwise interactions among all species are described by Margules-type equations. The success of this description is, of course, not a verification of the physical existence of associated species. The thermodynamic background of this model is investigated in this study. For binary liquid alloys, the key equations governing the calculation of activities in internal equilibrium are presented graphically. Formulae for the calculation of the Gibbs energy, enthalpy and entropy of mixing in the postulated species-system are developed and related to the corresponding effects in the binary liquid alloys. The formation of very asymmetric and twin miscibility gaps is discussed. The difference between the Gibbs energy of species and real alloys at stoichiometry is pointed out. Limiting cases of weak and strong association are discussed and formulae for terminal values of activity coefficient and partial enthalpy of solution are developed. Structural related properties of Sn-Te liquid alloys are calculated and compared to experimental data. A quantitative prediction of data in ternary and multicomponent alloys is one major motivation for the precise description of binary alloys and a suitable extension of the associated solution model is presented. The model is compared to other associate models and to sublattice models. Despite the different physical picture, close phenomenological and mathematical resemblance is discovered among the associated solution and the sublattice model.

Thermodynamics and Phase Relationships of Transition Metal-Sulfur Systems: Part V. A Reevaluation of the Fe-S System Using an Associated Solution Model for the Liquid Phase

Abstract: The relevant thermodynamic and phase equilibrium data for the Fe-S binary system have been reevaluated in light of more recent data. An associated solution model is used to describe the thermodynamic properties of the liquid phase as a function of composition and temperature. For the pyrrhotite phase, a statistical thermodynamic model based on the formation of Frenkel defects in the lattice is used. For the austenite and ferrite phases, the solute is assumed to follow Henry’s law, and pyrite is taken to be a stoichiometric compound. The model parameters for the various phases are obtained by evaluating all relevant experimental data reported in the literature. The calculated phase diagram is in good agreement with the experimental data. The calculated sulfur activity values for the liquid phase agree well with experimental values including those at higher sulfur concentrations. This is an improvement of an earlier evaluation by Sharma and Chang using the same model with the same number of parameters.

Relative stability of alloys

Abstract: A mathematical function, the relative stability, is defined which combines advantages of Darken's stability and excess stability function. The relative stability is finite over the entire composition range and approaches the value of one in terminal regions. A simple classification is given whether a solution phase is more stable than an ideal solution, less stable or even unstable in terms of values of the relative stability > 1,

Magnetic Effect on the Phase Equilibria of FCC(Fe,Ni) Alloys at Low Temperatures

Abstract: Phase stabilities of iron alloys at low temperatures are strongly infuenced by magnetic effect. The appearance of certain type of equilibria is often due entirely to magnetic contribution to the Gibbs energy of the pertinent phase. The appearance of the stable and metastable equilibria in fcc(Fe,Ni) alloys are discussed in terms of the magnetic interaction.