Thermodynamics of inclusion formation in Fe-Ti-C-N alloys

The thermodynamics of the formation of titanium carbonitride in liquid iron-titanium-carbon-nitrogen alloys were investigated in order to predict under what conditions it will form in liquid steel. A metal-carbonitride equilibration technique was used. Titanium carbonitride of a desired composition was made by mixing and high-temperature sintering of very fine powders of titanium nitride and carbide. The formation of titanium carbonitride was confirmed by lattice parameter measurements on the samples before and after the experiments. The equilibrium concentrations of titanium, carbon, and nitrogen in equilibrium with a specific titanium carbonitride were obtained at 1873 K. Activities of titanium carbide and nitride relative to pure solid titanium carbide and nitride were calculated. It was found that titanium carbonitride solid solution is almost ideal. From the results, calculations were performed to predict at which composition various carbonitrides will form.     

Thermodynamics of formation of th-fe alloys

Electromotive force cells have been used to determine the Gibbs free energies, enthalpies, and entropies of formation for Th₂Fe₁₇, ThFe₅, Th₂Fe₇, ThFe₃, and Th₇Fe₃ over the temperature range 928 to 1164 K. Solid CaF₂ was used as the electrolyte. Comparisons of Th-Fe, Th-Co, and Th-Ni thermodynamic data show patterns which correlate with crystallographic and magnetic behavior. Some inferences have been drawn about the bonding interactions within the phases in these alloy systems. formerly Graduate Students, Department of Metallurgy, Iowa State University, Ames, Iowa 50010,     

Thermodynamics of formation of Y-Ni alloys

The Gibbs free energies, enthalpies, and entropies of phase formation were determined for nine Y-Ni intermediate phases from electromotive force measurements. With CaF₂ solid electrolyte, emf measurements were made over the temperature range 900 to 1225 K. The Gibbs energies of formation per mole of Y reactant were found to be nearly constant for the four Y-poor phases; this is in keeping with the close structural relationship among these four phases. The Gibbs energies of formation of the Y-Ni phases at 973 K have been compared with those of the Th-Fe, Th-Co, Th-Ni, La-Co, La-Ni, Y-Fe, and Y-Co systems, and an increasing bond strength with increasing number of bonding electrons is evident. Comparison of the experimental values for the enthalpies of formation of the Y-Ni phases has been made with values predicted by the Miedema model and by the Watson-Bennett model. Both predictions yield values that are within ±50 pet of the experimental values which is the range of scatter that is normally expected from these simple models. Formerly Graduate Assistant, Department of Materials Science and Engineering, Iowa State University.     

Thermodynamics of formation of Th-Ni alloys

Electromotive force cells have been used to determine the Gibbs free energies, entropies, and enthalpies of phase formation for Th₂Ni₁₇, ThNi₅, ThNi₂, ThNi, and Th₇Ni₃. Solid CaF₂ was em ployed as the electrolyte, and it has previously been shown that CaF₂ is an ionic conductor over the temperature range of measurements, 841° to 1141°K. The experimental results can be correlated with the atomic coordinations in the crystal structures of these five Th-Ni intermediate phases. Formerly Graduate Student, Department of Metallurgy, Iowa State University     

Thermodynamics of formation of Y-Co alloys

The Gibbs free energies, enthalpies, and entropies of phase formation were determined for nine Y-Co intermediate phases from electromotive force measurements. Solid CaF₂ was employed as the electrolyte, and emf measurements were made over the temperature range 850 K to 1200 K. The data indicate that the Gibbs free energies of formation per mole of Y reactant are nearly constant for the four Y-poor phases. Such behavior is associated with the close structural similarity of these phases. The present experimental Gibbs energy data at 973 K are compared with those of the Th-Fe, Th-Co, Th-Ni, La-Co, La-Ni, and Y-Fe systems. It was observed that the Gibbs free energy of formation can be empirically correlated with the total number of valence electrons in these alloy systems. The enthalpy of formation of an equiatomic Y-Co alloy was determined to be −27.1 kJ/mole atoms. This experimental value is in good accord with the theoretical predictions of the Miedema and Watson-Bennett models.     

Thermodynamics of formation of Th-Co alloys

Electromotive force cells have been used to determine the Gibbs free energies, enthalpies, and entropies of formation for Th₂Coi₇, ThCo₅, Th₂Co₇, ThCo, and Th₇Co₃. Solid CaF₂ was employed as the electrolyte, and measurements were made over the temperature range 917‡ to 1233‡K. Comparison of the Th-Co enthalpy values with Th-Ni values shows that the relative bond strengths associated with thorium-transition metal interactions in the two systems invert between the thorium-poor and thorium-rich regions. Formerly Graduate Student, Department of Metallurgy, Iowa State University.     

帘线钢中钛夹杂形成的热力学分析

对帘线钢中TiN夹杂的形成热力学进行了计算,通过理论计算表明,在帘线钢凝固过程后期,固液界面的TiN平衡浓度积较低,易形成钛夹杂.相对碳含量较低的帘线钢而言,碳含量较高的帘线钢在凝固后期由于固液界面的温度较低,导致TiN平衡浓度积也较低,更易于形成钛夹杂.根据理论分析结果,采取了相应的技术措施,使与钛夹杂相关的帘线钢合格率得到明显提高.     

Thermodynamics and kinetics analyses of ZrB2 formation in molten aluminium alloys

Smelter grade aluminium can be used as a source for electrical conductor grade aluminium after the transition metal impurities such as zirconium (Zr), vanadium (V), titanium (Ti) and chromium (Cr) have been removed. Zirconium (Zr), in particular, has a significant effect on the electrical conductivity of aluminium. In practice, the transition metal impurities are removed by adding boron-containing substances into the melt in the casthouse. This step is called boron treatment. The work presented in this paper, which focuses on the thermodynamics and kinetics of Zr removal from molten Al–1?wt-%Zr–0.23?wt-%B alloy, is part of a broader systematic study on the removal of V, Ti, Cr and Zr from Al melt through boron treatment carried out by the authors. The thermodynamic analyses of Zr removal through the formation of ZrB₂ were carried out in the temperature range of 675–900°C using the thermochemical package FactSage. It was predicted that ZrB₂ is stable compared to Al–borides (AlB₁₂, AlB₂) hence would form during boron treatment of molten Al–Zr–B alloys. Al–Zr–B alloys were reacted at 750?±?10°C for 60 minutes, and the change in the chemistry and microstructure were tracked and analysed at particular reaction times. The results showed that the reaction between Zr and AlB₁₂/B was fast as revealed by the formation of boride ring at the early minutes of reaction. The presence of black phase (AlB₁₂), i.e. the original source of B, after holding the melt for 60 minutes advocated that the reaction between Zr and AlB₁₂/B was incomplete, hence still not reached the equilibrium state. The kinetics data suggested a higher reaction rate at the early minutes (2 minutes) of reaction compared to at a later stage (2–60 minutes). Nevertheless, a simple single-stage liquid mass transfer controlled kinetic model can be used to describe the overall process kinetic. The analysis of integrated rate law versus reaction time revealed that the mass transfer coefficient (k_m) of Zr in molten alloy is 9.5?×?10^?4?m?s^?1, which is within a typical range (10^?3 to 10^?4?m?s^?1) observed in other metallurgical solid–liquid reactions. This study suggests that the overall kinetics of reaction was predominantly controlled by the mass transfer of Zr through the liquid aluminium phase.     

Thermodynamics of segregation in alloys

The segregation process is described in terms of thermodynamic equations that use concepts of excess partial molar free energies and activity coefficients. Using such generalized equations, simplified equations are developed that can be used to check the segregation behavior in terms of thermodynamic quantities that are generally available in the literature. These simplified equations can be used to represent segregation semiquantitatively in many situations, including those where the elements form highly stable chemical compounds, such as sulfides, borides, and phosphides. The validity of our equations is checked by comparing the calculated segregation values with those observed on the surface of Ni, Cr, Al, Y, and NiCrAl(Y) alloys containing small concentrations of sulfur.     

Thermodynamics of Ca-Ga alloys

The enthalpies of formation of the intermetallic compounds CaGa₄, Ca₃Ga₈, and CaGa₂, at 298.15 K, were determined by high-temperature liquid gallium solution calorimetric measurements to be −24.9 ± 4.9 kJ·g at.⁻¹, −25.4 ± 2.4 kJ·g at.⁻¹, and −38.8 ± 4.8 kJ·g at.⁻¹, respectively. The enthalpies of formation of CaGa₄ at 988 K and that of Ca₃Ga₈ at 1070 K were determined, using precipitation calorimetry, to be −28.2 ± 1.7 kJ·g at.⁻¹ and −22.5 ± 1.4 kJ·g at.⁻¹, respectively. The integral enthalpy of mixing of the (Ca-Ga) liquid alloys (ΔH ⁰) measured at 1309 K are described by the following Redlich-Kister equation:

Thermodynamics of Ti in Ag-Cu alloys

The thermodynamic activities of Ti at dilution in a series of Ag-Cu alloys and eutectic Ag-Cu alloys containing In or Sn were measured using a galvanic cell technique employing a ThO₂-8 pct Y₂O₃ electrolyte. The equilibrium oxide phase formed by the reaction of Ti (X_Ti > 0.004) in the Ag-Cu alloy melts with an A1₂O₃ or ZrO₂ crucible was Ti₂O (s). The free energy of formation of Ti₂O (s) was estimated from available thermodynamic data. Titanium activities were calculated from measured oxygen potentials and the free energy of formation of Ti₂O (s). Titanium in the eutectic Ag-Cu melt showed a positive deviation from ideal solution behavior at 1000°C, and its activity coefficient at infinite dilution was about 6.5 relative to pure solid Ti. Indium and Sn did not increase the activity coefficient of Ti in eutectic Ag-Cu melts. Silver increased the Ti activity coefficient in the Ag-Cu-Ti melts significantly. The Ti activity coefficient value in liquid Ag was about 20 times higher than in eutectic Ag-Cu melt at 1000 °C.     

Thermodynamics of molten Li-Sn alloys

The activity of Li in molten Li-Sn alloys was continuously varied and monitored electrochemically in cells of the type Al-LiAl/glass electrolyte/Sn/glass electrolyte/Al-LiAl. The temperature (320 to 380 °C) and compositional dependence of the Li activity coefficient, γ_Li, was found to follow a quadratic expression of the form In γ_Li = A + B(1 − X_Li)² up to 30 mole pct lithium. Further, the liquidus temperature, T_L, was found to follow TL(°C) = 642 X_Li + 188 for 0.20 X_Li 0.44 over the temperature range 320 to 470 °C. The partial and integral molar heats of solution were calculated and the results indicate that strong attractive forces exist between Sn and Li. These forces are strong enough to induce substantial ordering in the melt to an extent that the integral molar entropy of mixing at high Li contents (36 mole pct) is negative.     

Thermodynamics of austenitic Fe-C alloys

New data on the activity of carbon in austenite have been obtained by CO₂/CO equilibration of Fe-C alloys in the temperature range 900° to 1400°C. Equations for the thermodynamic properties of carbon and iron in austenite are obtained from the data combined with selected data from the literature. A slightly modified phase diagram is presented. The stability of cementite is also determined from data published earlier and the results of the present study. Parameters of the various models of the behavior of carbon in austenite taken from the literature are also calculated from the data.     

Thermodynamics and kinetics of the formation of widmanstätten ferrite plates in ferrous alloys

Experimental data on the formation of Widmanstätten/bainitic ferrite in ferrous alloys(i.e., the Widmanstätten start temperature, partition of alloying elements, incomplete transformation, lengthening kinetics,etc.) are examined on the basis of thermodynamic calculations and kinetic analyses. A morphological change of ferrite from grain-boundary allotriomorph to Widmanstätten plate occurs well above theT ₀ temperature, except in high Mn and Ni alloys, but does so in the regime of carbon diffusion control in all alloys. Under the assumption that the plate tip consists of a pair of ledges of the height equal to the tip radius, the reported lengthening kinetics of ferrite plates can be accounted for very well by the diffusion-controlled motion of these ledges in a wide range of carbon supersaturation. It is also shown that the transformation stasis (incomplete transformation) observed below the kinetically definedB _s in some iron alloys cannot be unequivocally attributed to either the completion of the precipitation of no-partitioned ferrite or the loss of the driving force for subsequent shear transformation.     

Thermodynamics of phosphorus in carbon-saturated manganese-based alloys

The interaction coefficient of phosphorus with silicon in a Mn-Si-C_sat alloy has been measured at 1573 K, equilibrating CaC₂, C, and Ca₃P₂ in a quartz capsule to keep the phosphorus partial pressure as high as 33.7 Pa. The activity coefficient of phosphorus in the melts increases with increasing silicon content, and the interaction parameter between silicon and phosphorus in the melts at carbon saturation ε^Si_P, C_sat was found to be 10.4. The activity of phosphorus in a carbon-saturated Fe-Mn alloy was also determined at temperatures of 1573 to 1673 K using a chemical equilibration technique between BaO-BaF₂ fluxes and Fe-Mn-C_sat with manganese mass contents ranging from 0 to 85.3 %. A slight decrease in the activity coefficient of phosphorus in Fe-Mn-C_sat alloys was observed with increasing manganese content, as a reflection of a stronger interaction between manganese and phosphorus than that between iron and phosphorus. The value for e^Mn_P,Csat was found to be -0.0029 between 1573 and 1673 K.     

Thermodynamics of oxygen behaviour in cobalt-nickel alloys

In this study the concentration and chemical potential of oxygen in liquid Co-Ni alloys equilibrated with cobalt-nickel aluminate spinel solid solutions and alumina have been determined at 1773, 1823 and 1873K as a function of nickel concentration. The oxygen content of the melt has been measured by suction sampling and inert gas fusion analysis. The corresponding oxygen potential has been determined with the following solid state cell: Mo, Mo+MoO₂ | (MgO)ZrO₂ | (Co, Ni) melt + AI₂O₃ + (Co, Ni)O·(1+x)Al₂O₃, Mo. The effect of nickel on the activity coefficient of oxygen in Co-Ni alloys has been determined. The results for the activity coefficient have been modelled with Wagner's interaction parameters and also the more recent exponential method of St. Pierre et al. at the three temperatures.     

Thermodynamics of dilute bcc Fe-Si alloys

The thermodynamic properties of silicon in the α-phase of the Fe-Si system in the region 0.028 <x _Si < 0.084 and 1100 < °C < 1370 has beem measured by the emf cell Mo, Si(s) | SiO₂-Li₂O | (Si-Fe)(s), Mo. The results are expressed in the form $$\log {\text{ }}\gamma _{Si}^\alpha {\text{ = 1}}{\text{.19 }} - {\text{7070/}}T + [ - 6.30 + 18,300/T]x_{Si} $$