Thermodynamic evaluation of the Fe-Mn-Si system and the γ/ε martensitic transformation

The thermodynamic properties of the Fe-Mn-Si system are analyzed by means of thermodynamic models for the individual phases. Special attention is paid to the γ → ε martensitic transition. A complete set of parameters, from which arbitrary sections of the phase diagram as well as the M_s and A_s temperatures may be calculated, is given.     

Thermodynamic vacancy model of the surface energetic state of solids

Based on the broken-bond concept of metal surface formation, a thermodynamic vacancy model of the surface metal layer (TVM), in which the Gibbs surface energy (ΔG _s ) of a metal is defined as the excess adsorption energy of vacancies (N _V(s)) in the surface layer of the metal ΔG _s = −RTlnN _V(s), is developed. The relation is derived from the Gibbs adsorption equation for a metal, which is treated as a binary system that involves atoms and vacancies. TVM enables one to estimate the concentration of vacancies and adatoms on low-index faces of cubic-lattice metals from the literature surface energies evaluated with the use of the surface physics methods. Examples of the application of TVM in the physical chemistry of surfaces and corrosion of metals are given.     

Thermodynamic assessment of the Hg-Sn system

A thermodynamic assessment of the Hg-Sn system has been carried out using the CALPHAD method. The comprehensive assessment covers the extensive phase diagram data as well as the enthalpy, activity, and vapor pressure data. Two cases of intermetallic compounds in the Hg-Sn system are considered. Case 1 considers the intermetallic compounds β, γ, and HgSn₄ as having no solubility and can thus be treated as the stoichiometric phases β-HgSn₃₈, γ-HgSn₁₂, and HgSn₄. Case 2 uses a sublattice model to more accurately describe a solubility of the γ phase; it also considers the stoichiometric δ-HgSn₇ phase. The ε phase was considered to be metastable and neglected in the thermodynamic assessment. Thermodynamic parameters have been optimized using all the assessed experimental thermodynamic and phase equilibrium data. Both calculated phase diagrams of the Hg-Sn system (Cases 1 and 2) and the thermodynamic data are reasonable and satisfactory when compared with literature data. Future crucial experiments are suggested.     

Thermodynamic assessment of the Hg-Sn system

A thermodynamic assessment of the Hg-Sn system has been carried out using the CALPHAD method. The comprehensive assessment covers the extensive phase diagram data as well as the enthalpy, activity, and vapor pressure data. Two cases of intermetallic compounds in the Hg-Sn system are considered. Case 1 considers the intermetallic compounds β, γ, and HgSn₄ as having no solubility and can thus be treated as the stoichiometric phases β-HgSn₃₈, γ-HgSn₁₂, and HgSn₄. Case 2 uses a sublattice model to more accurately describe a solubility of the γ phase; it also considers the stoichiometric δ-HgSn₇ phase. The ε phase was considered to be metastable and neglected in the thermodynamic assessment. Thermodynamic parameters have been optimized using all the assessed experimental thermodynamic and phase equilibrium data. Both calculated phase diagrams of the Hg-Sn system (Cases 1 and 2) and the thermodynamic data are reasonable and satisfactory when compared with literature data. Future crucial experiments are suggested.     

Thermodynamic calculation of the critical potentials of the selective dissolution of Ag-Au and Cu-Au alloys

A thermodynamic approach to estimating the critical potential of the selective alloy dissolution E _c at a vacancy concentration N _v(s) in the superficial layer is considered. The N _v(s) dependence on the potential and the concentration of gold N _Au in the alloy is calculated. The E _c potential corresponds to the critical concentration of vacancies N _v(s) ≈ 10⁻², and the E _c value itself is determined chiefly by the zero-point potential of the alloy, which is a pronounced function of the surface enrichment in gold. The calculated E _c-N _Au functions for Ag-Au and Cu-Au alloys satisfactorily coincide with the experimental dependences. Original Russian Text ? A.E. Kutyrev, Yu.Ya. Andreev, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 2, pp. 152–159.     

Thermodynamic causes of the high diffusion rate of atoms in the superficial layer of a metallic electrode

Concentration of vacancies N _V(S) in a superficial layer (SL) of fcc lattices of Ag, Cu, and Au metals is thermodynamically estimated and analyzed. The calculations are based on a thermodynamic vacancy model of the metal SLs, by which the N _V(S) value is related to the surface Gibbs energy ΔG _s = −RTlnN _V(S). A strong ΔG _s dependence on the electrode potential and the zero point value of the metal results in the increase in N _V(S) value, which reaches nearly 10⁻² at standard potentials of the above metals. The high surface self-diffusivity of atoms (D ≅ 10⁻¹⁵ cm²/s) calculated from the in situ STM measurements of the electrode surfaces is due to the high concentration of vacancies in the metal SLs. Original Russian Text ? Yu.Ya. Andreev, 2007, published in Zashchita Metallov, 2007, Vol. 43, No. 1, pp. 18–24.     

Kinetics of inhibition of aluminum corrosion in hydrogen chloride electrolytes with binary salt mixtures

The kinetics of inhibition of aluminum corrosion with SnCl₂, CdCl₂, PbCl₂, and their equimolar binary mixtures was studied under contact exchange conditions. Prerequisites for synergism in these mixtures were determined. The parameter γ of mutual influence of the components in the mixture was formally separated into the constituents associated with changes in the aluminum surface coverage with the contact deposit, γ_s, and in the free corrosion potential of aluminum, γ_ΔE . In the mixtures studied, γ_s > 1, while γ_ΔE < 1; however, the overall effect is synergism because γ_s > 1/γ_ΔE . Original Russian Text ? O.I. Barteneva, V.V. Bartenev, 2006, published in Zashchita Metallov, 2006, Vol. 42, No. 5, pp. 532–538.     

A generalized thermodynamic treatment of phase equilibria in coherent multilayers

We present a generalized way of treating phase equilibria in coherent planar multilayers. A correct recognition of elastic stress and strain components as thermodynamic potentials or densities is crucial for the use of the criteria for intrinsic stability as well as for the applicability of the conventional method of common tangent construction and the Gibbs phase rule. It is shown that a method analogous to the conventional common tangent construction exists in thermodynamic density subspaces for which some of density variables are held constant. In a thermodynamic density subspace withm _s density variables fixed, a common tangent construction can be made between the extremized free energies for systems of (m_s+l)-phase coexistence in order to satisfy the thermodynamic equilibrium conditions of systems with more thanm _s+1 coexisting phases. This method is applied to a coherent binary system configured as plane-parallel plates to demonstrate that equilibrium states with more than two coexisting phases cannot be thermodynamically stable in the binary multilayer system under certain mechanical loading conditions.     

Thermodynamic Assessment of the Mn-B System

The phase equilibrium and thermodynamic data on the Mn-B system are critically reviewed. Based on the experimental data, a thermodynamic modeling is performed on this system. A set of self-consistent thermodynamic parameters is obtained and the calculated results are in general agreement with the experimental data. A parameter ξ is introduced to compare the degree of flatness of the δMn/L liquidus with those in other metal-boron systems. It is found that the δMn/L liquidus is the flattest, which causes difficulties in its modeling. An effective approach to evaluate the thermodynamic parameters for o-Mn₂B_0.982 phase is developed. Such an approach is valid for evaluating thermodynamic parameters of the phases with limited phase diagram data.