Thermodynamic Study of Liquid Mg-In-Sn Alloys

Liquid Mg-In-Sn alloys were studied by the emf method using concentration cells with liquid electrolyte. Experiments were performed for 30 alloys along three sections on the Gibbs triangle with constantt = 0.25, 0.50, 0.75 [t = X_Sn/(X_Sn + X_In)] and for magnesium concentration from 0.1 to 0.65 at temperatures from 950 to 1100 K. From the change of the slope of the emf versusT, points on the liquidus surface for sixteen alloys were detected. Calculated partial excess Gibbs energies δG_exMg and partial enthalpies δH_Mg were compared with the results of previous work.     

Al-Cu-Li system electromotive force and calorimetric studies—Phase diagram calculations of the Al-Rich part

Electromotive force (emf) studies were made for solid and liquid AI-Cu-Li alloys. Measurements were conducted on four sets of alloys at temperatures and composition ranges as follows (where X_Li is mole fraction):

Densities of solid aluminum-magnesium (Al-Mg) alloys

Densities of solid Al and Al-Mg alloys were measured by the dilatometric method for seven compositions at mole fractions of magnesium: 0.015, 0.05, 0.1, 0.125, 0.15, 0.20, and 0.25. A curvilinear dependence of density on temperature (room temperature up to 800 K) was observed for all investigated alloys. Results are described by polynomials of the second degree. The molar volumes of Al-Mg alloys were calculated from the density measurements. It has been found that the densities of solid Al-Mg alloys show negative deviations from linearity and the molar volumes exhibit positive deviations from ideal behavior for all samples in the experimental concentration range. The density of the β(Al₃Mg₂) phase along the (α(Al) + β)/β boundary was calculated and is described by a temperature-dependent polynomial.     

Densities of solid aluminum-magnesium (Al-Mg) alloys

Densities of solid Al and Al-Mg alloys were measured by the dilatometric method for seven compositions at mole fractions of magnesium: 0.015, 0.05, 0.1, 0.125, 0.15, 0.20, and 0.25. A curvilinear dependence of density on temperature (room temperature up to 800 K) was observed for all investigated alloys. Results are described by polynomials of the second degree. The molar volumes of Al-Mg alloys were calculated from the density measurements. It has been found that the densities of solid Al-Mg alloys show negative deviations from linearity and the molar volumes exhibit positive deviations from ideal behavior for all samples in the experimental concentration range. The density of the β(Al₃Mg₂) phase along the (α(Al) + β)/β boundary was calculated and is described by a temperature-dependent polynomial.     

Thermodynamic mixing properties and solid-state immiscibility in the systems Pd-Rh and Pd-Rh-O

The thermodynamic activity of rhodium in solid Pd-Rh alloys is measured in the temperature range 950 to 1350 K using the solid-state cell: Pt-Rh, Rh + Rh₂O₃/(Y₂O₃)ZrO₂/Pd_{1_x}Rh_x + Rh₂O₃, Pt-Rh. The activity of palladium and the free energy, enthalpy, and entropy of mixing are derived. The activities exhibit strong positive deviation from Raoult’s law. The activities obtained by the electrochemical technique, when extrapolated to 1575 K, are found to be significantly lower than those obtained from vapor pressure measurements. The mixing properties can be represented by a pseudosubregular solution model in which excess entropy has the same type of function dependence on composition as the enthalpy of mixing: ΔH- X_Rh(1-X_Rh,)(31 130 + 4585X_Rh,)J/mol, and ΔS^ex = X_Rh(1-X_Rh)(l0.44 + 1.51X_Rh) J/mol. K. The positive enthalpy of mixing obtained in this study in qualitative agreement with predictions of semiempirical models. The results predict a solid-state miscibility gap withT _c = 1210 (±5) K atX _Rh = 0.55 (±0.02). The computed critical temperature is approximately 100 K higher than that reported in the literature. The oxygen chemical potential for the oxidation of Pd-Rh alloys under equilibrium conditions is evaluated as a function of composition and temperature. The Gibbs energy of formation of PdO is measured as a function of temperature. At low temperatures, the alloys are in equilibrium with Rh₂O₃, and PdO coexists with Pd and Rh₂O₃. At high temperatures, PdO is unstable and Pd-rich alloys are in equilibrium with diatomic oxygen gas.     

Surface tension and thermodynamic properties of liquid Ag-Bi solutions

With the maximum bubble pressure method, the density and surface tension were measured for five Ag-Bi liquid alloys (X _Bi=0.05, 0.15, 0.25, 0.5, and 0.75), as well as for pure silver. The experiments were performed in the temperature range 544–1443 K. Linear dependences of both density and surface tension versus temperature were observed, and therefore the experimental data were described by linear equations. The density dependence on concentration and temperature was derived using the polynomial method. A similar dependence of surface tension on temperature and concentration is presented. Next, the Gibbs energy of formation of solid Bi₂O₃, as well as activities of Bi in liquid Ag-Bi alloys, were determined by a solid-state electromotive force (emf) technique using the following galvanic cells: Ni, NiO, Pt/O ⁻²/W, Ag _X Bi _(1−X), Bi ₂ O _3(s). The Gibbs energy of formation of solid Bi₂O₃ from pure elements was derived: =−598 148 + 309.27T [J · mol⁻¹] and =−548 008 + 258.94T [J · mol⁻¹]; the temperature and the heat of the α → δ transformation for this solid oxide were calculated as 996 K and 50.14 J · mol⁻¹. Activities of Bi in the liquid alloys were determined in the temperature range from 860–1075 K, for five Ag-Bi alloys (X _Ag=0.2, 0.35, 0.5, 0.65, 0.8), and a Redlich-Kister polynomial expansion was used to describe the thermodynamic properties of the liquid phase. Using Thermo-Calc software, the Ag-Bi phase diagram was calculated. Finally, thermodynamic data were used to predict surface tension behavior in the Ag-Bi binary system.     

Surface tension and thermodynamic properties of liquid Ag-Bi solutions

With the maximum bubble pressure method, the density and surface tension were measured for five Ag-Bi liquid alloys (X _Bi=0.05, 0.15, 0.25, 0.5, and 0.75), as well as for pure silver. The experiments were performed in the temperature range 544–1443 K. Linear dependences of both density and surface tension versus temperature were observed, and therefore the experimental data were described by linear equations. The density dependence on concentration and temperature was derived using the polynomial method. A similar dependence of surface tension on temperature and concentration is presented. Next, the Gibbs energy of formation of solid Bi₂O₃, as well as activities of Bi in liquid Ag-Bi alloys, were determined by a solid-state electromotive force (emf) technique using the following galvanic cells: Ni, NiO, Pt/O ⁻²/W, Ag _X Bi _(1−X), Bi ₂ O _3(s). The Gibbs energy of formation of solid Bi₂O₃ from pure elements was derived: =−598 148 + 309.27T [J · mol⁻¹] and =−548 008 + 258.94T [J · mol⁻¹]; the temperature and the heat of the α → δ transformation for this solid oxide were calculated as 996 K and 50.14 J · mol⁻¹. Activities of Bi in the liquid alloys were determined in the temperature range from 860–1075 K, for five Ag-Bi alloys (X _Ag=0.2, 0.35, 0.5, 0.65, 0.8), and a Redlich-Kister polynomial expansion was used to describe the thermodynamic properties of the liquid phase. Using Thermo-Calc software, the Ag-Bi phase diagram was calculated. Finally, thermodynamic data were used to predict surface tension behavior in the Ag-Bi binary system.     

Effect of solid solution treatment on in vitro degradation rate of as-extruded Mg-Zn-Ag alloys

The degradation behaviors of the as-extruded and solution treated Mg-3Zn-xAg (x=0, 1, 3, mass fraction, %) alloys, as well as as-extruded pure Mg, have been investigated by immersion tests in simulated body fluid (SBF) at 37 °C. The as-extruded Mg-Zn(-Ag) alloys contained Mg₅₁Zn₂₀ and Ag₁₇Mg₅₄. While the quasi-single phase Mg-Zn(-Ag) alloys were obtained by solution treatment at 400 °C for 8 h. The quasi-single phase Mg-Zn(-Ag) alloys showed lower degradation rate and more homogeneous degradation than corresponding as-extruded Mg alloys. Degradation rate of solid-solution treated Mg-3Zn-1Ag and Mg-3Zn-3Ag was approximately half that of corresponding as-extruded Mg alloy. Moreover, the degradation rate of solid-solution treated Mg-3Zn and Mg-3Zn-1Ag was equivalent to that of as-extruded pure Mg. However, heterogeneous degradation also occurred in quasi-single phase Mg-Zn-Ag alloys, compared to pure Mg. So, preparing complete single-phase Mg alloys could be a potential and feasible way to improve the corrosion resistance.     

Thermodynamic evaluation of hypereutectic Al–Si (A390) alloy with addition of Mg

This paper presents the thermodynamic evaluation of A390 hypereutectic Al–Si alloy (Al–17% Si–4.5% Cu–0.5% Mg) and alloys up to 10% Mg, using the Factsage® software. Two critical compositions were detected at 4.2% and 7.2% Mg where the temperatures of the liquidus, the start of the binary and of the ternary eutectic reaction are changed. These critical compositions show differences in the formation of Mg₂Si intermetallic particles during the solidification interval. For compositions up to 4.2% Mg, the Mg₂Si intermetallic phase first appears in the ternary eutectic zone. With Mg contents between 4.2% and 7.2%, Mg₂Si particle appears in both the binary and ternary eutectic reactions. Above 7.2% Mg, it solidifies as a primary phase and also during the binary and ternary reactions. The calculated liquid fraction vs. temperature curves also showed a decrease of the eutectic formation temperature (knee point temperature) with the addition of Mg content up to 4.2% Mg. This temperature becomes almost constant up to 10% Mg. The calculation of eutectic formation temperature shows a good agreement with differential scanning calorimetry (DSC) tests.     

Density and surface tension of the Pb-Sn liquid alloys

The maximum bubble pressure method and the dilatometric method were used, respectively, in measurements of surface tensions and densities of Pb-Sn liquid alloys. The experiments were carried out in the temperature range from 573 to 1200 K for the pure Pb, pure Sn, and 7 alloys of the compositions 0.1, 0.2, 0.26, 0.36, 0.5, 0.7, and 0.9 mole fraction of Pb. A straight-line dependence on temperature was observed and fitted by the method of least squares both for the densities and the surface tensions. The calculated density isotherm at 673 K showed a positive deviation from the linearity over the entire range of composition, and the same tendency was seen at 1173 K for compositions higher than X _Pb=0.26. At the lower concentration of Pb, a nearly linear character of 1173 K isotherm was noted. In the case of surface tensions, both at the lowest and the highest temperatures (673 and 1173 K), the deviation from linearity with composition was negative, but deviation decreased with increasing temperature. The isotherms of the compositional dependence of surface tension calculated from the Butler model exhibit good agreement with experimental data.     

Density and surface tension of the Pb-Sn liquid alloys

The maximum bubble pressure method and the dilatometric method were used, respectively, in measurements of surface tensions and densities of Pb-Sn liquid alloys. The experiments were carried out in the temperature range from 573 to 1200 K for the pure Pb, pure Sn, and 7 alloys of the compositions 0.1, 0.2, 0.26, 0.36, 0.5, 0.7, and 0.9 mole fraction of Pb. A straight-line dependence on temperature was observed and fitted by the method of least squares both for the densities and the surface tensions. The calculated density isotherm at 673 K showed a positive deviation from the linearity over the entire range of composition, and the same tendency was seen at 1173 K for compositions higher than X _Pb=0.26. At the lower concentration of Pb, a nearly linear character of 1173 K isotherm was noted. In the case of surface tensions, both at the lowest and the highest temperatures (673 and 1173 K), the deviation from linearity with composition was negative, but deviation decreased with increasing temperature. The isotherms of the compositional dependence of surface tension calculated from the Butler model exhibit good agreement with experimental data.     

The role of crystallography and thermodynamics on phase selection in binary magnesium–rare earth (Ce or Nd) alloys

In-depth understanding of secondary-phase selection during solidification and ageing is a crucial factor underpinning microstructure design and alloy development. Experimental observations show significantly different behavior of secondary phase selection between the Mg–Nd and Mg–Ce systems. Mg–Nd alloys show a significant degree of metastability of secondary phase selection during solidification, with NdMg₁₂ forming upon slow cooling and NdMg₃ upon fast cooling. However, at high heat treatment temperatures and long enough times, Nd₅Mg₄₁ forms. In contrast, Mg–Ce alloys form CeMg₁₂ under a wide variety of casting conditions, and this phase remains stable even after long annealing times at high temperatures. However, both Mg–Ce and Mg–Nd alloys undergo a similar precipitation sequence during ageing. Based on recently developed thermodynamic data and crystallographic matching calculations, the phase selection in these two alloy systems under different processing regimes is well explained by the competition between the driving force (reduction in bulk Gibbs energy) and the nucleation energy barrier (increment of surface energy). The approach used here can be also applied to similar phase selection problems in other alloy systems.     

Nanomechanics of Mg-Al intermetallic compounds

Cold spraying of pure Al powder on a pure Mg substrate together with subsequent post-spray annealing treatment produced Mg₁₇Al₁₂ (β-phase) and Mg₂Al₃ (γ-phase) intermetallic layers on the surface of the substrate. These layers showed significantly better nanomechanical properties, including the reduced elastic modulus and nanohardness, which were determined using nanoindentation, than commercial purity Mg and AZ91 alloys. Combined with their improved corrosion resistance, it is believed that both the γ-phase and the β-phase layers can provide effective protection of Mg alloys from wear and corrosion. The effect of post-spray annealing process on the formation of thick, uniform and dense intermetallic layers on pure Mg substrate was also investigated.     

Effect of Melt-to-Solid Insert Volume Ratio on Mg/Al Dissimilar Metals Bonding

Compound casting is used as a process to join various similar and dissimilar metallic couples. The ratio of melt-to-solid volume is one of the main factors that can affect the contact time between melt and the solid insert. In this investigation, magnesium and aluminum metals (magnesium as the cast metal and aluminum as the solid insert) having melt-to-solid volume ratios (V _m/V _s) of 1.25, 3, and 5.25 were successfully bonded via compound casting. Results demonstrated that by increasing the ratio of V _m/V _s from 1.25 to 5.25, the thickness of the reaction interface between Al and Mg varies within the range of 200 to 1800 μm. X-ray diffraction, scanning electron microscopy, and Vickers microhardness study of the bonding of these two metals showed that the interface consisted of three separate sub-layers within reaction layer. These sub-layers had higher hardness than those of the Al and Mg bulk metals. In all specimens, composition of the sub-layer adjacent to Al (layer I) was Al₃Mg₂ and that adjacent to Mg (layer III) was Al₁₂Mg₁₇/(Mg) eutectic structure. The intermediate layer composition (layer II) in specimens with volume ratio of 1.25 and 3 was a single-phase Al₁₂Mg₁₇, while for the case of volume ratio 5.25 this sub-layer consisted of Al₁₂Mg₁₇/(Mg) eutectic dispersed in Al₁₂Mg₁₇ intermetallic. The results of this research showed that in low melt/solid volume ratios, diffusion-reaction was the dominant mechanism for formation of Al-Mg intermetallic. However, when V _m/V _s and the melt/solid insert contact time increased, the dominant mechanism of Al-Mg intermetallics changed to fusion-solidification due to increase in surface melting of the solid insert. Also the results of push-out tests showed that shear strengths of the interface decrease from 27.1 to 15.1 and 8.3 MPa for the Al/Mg couples prepared at 1.25, 3, and 5.25 V _m/V _s respectively.     

Mechanisms of enhanced heterogeneous nucleation during solidification in binary Al-Mg alloys

The mechanisms involved in the grain refinement of Al-Mg alloys through varying the Mg content and applying intensive melt shearing were investigated. It was found that the oxide formed in Al-Mg alloys under normal melting conditions is MgAl₂O₄, which displays an equiaxed and faceted morphology with {1 1 1} planes exposed as its natural surfaces. Depending on the Mg content, MgAl₂O₄ particles exist either as oxide films in dilute Al-Mg alloys (Mg < 1 wt.%) or as naturally dispersed discrete particles in more concentrated Al-Mg alloys (Mg > 1 wt.%). Such MgAl₂O₄ particles can act as potent sites for nucleation of α-Al grains, which is evidenced by the well-defined cube-on-cube orientation relationship between MgAl₂O₄ and α-Al. Enhanced heterogeneous nucleation in Al-Mg alloys can be attributed to the high potency of MgAl₂O₄ particles with a lattice misfit of 1.4% and the increased number density of MgAl₂O₄ particles due to either natural dispersion by the increased Mg content or forced dispersion through intensive melt shearing. It was also found that intensive melt shearing leads to significant grain refinement of dilute Al-Mg alloys by effective dispersion of the MgAl₂O₄ particles entrapped in oxide films, but it has marginal effect on the grain refinement of concentrated Al-Mg alloys, where MgAl₂O₄ particles have been naturally dispersed into individual particles by the increased Mg content.     

Experimental Study of Phase Relations in the Mg-Er-Si System Focused on Mg-Rich Region at 300 and 400 °C

The phase equilibria and compositions in the magnesium (Mg) rich part of the Mg-Er-Si ternary system at 300 and 400 °C were systemically investigated through the equilibrated alloy method by using x-ray diffraction and scanning electron microscopy assisted with energy dispersive spectroscopy of x-ray. The results show that in the composition range of our study the equilibrium of Er₂MgSi₂ with the (Mg) solid solution, Er₅Mg₂₄, and Mg₂Si exists at both temperatures. A certain amount of Mg atoms can be replaced by Er and Si element, but they cannot replace Mg atom simultaneously. The homogeneity ranges of Mg solid solution as well as the solid solubilities of the intermediate phases at different temperatures are reported.     

Phase transformations in mechanically milled and annealed single-phase β-Al3Mg2

Mechanical deformation drastically affects both microstructure and thermal stability of polycrystalline β-Al₃Mg₂. Upon milling, the β-Al₃Mg₂ phase transforms into a nanoscale supersaturated Al(Mg) solid solution with 40 at.% Mg. Upon heating, the milled powders display a complex thermal behavior characterized by four distinct exothermic events. At low temperatures, an increasing amount of Mg is rejected from the solid solution with increasing temperature. At higher temperatures, the β′-phase, a hexagonal phase with composition Al₃Mg₂, is formed and no traces of the solid solution can be detected, indicating that the solid solution is metastable and transforms into more stable phase(s). Finally, the subsequent exothermic events reveal the formation and growth of the β-Al₃Mg₂ phase.     

Microstructure and corrosion resistance of magnesium alloy ZE41 with laser surface cladding by Al-Si powder

The microstructure and corrosion behavior of commercial alloy ZE41 modified by surface laser cladding with Al-Si powder mixture was studied by SEM, TEM, X-ray diffraction and electrochemical methods. The coating is composed of an Al-Mg matrix and dendrite precipitates of Mg₂Si. In function of the laser speed, the matrix is formed by a Mg solid solution in Al or by the intermetallic phase Mg₁₇Al₁₂. The presence of different matrixes is responsible for galvanic corrosion and decrease of corrosion resistance in interfacial area between coats. Isolated samples of the bulk coatings material showed similar corrosion potentials inspite of different matrix composition. This interpreted in terms of a mechanism involving two steps: (1) an initial dissolution of anodic Mg₂Si particles followed by (2) pitting in the formed crevices. The proposed mechanism corresponds well with the experimental observations and the mechanisms of localized corrosion observed for aluminium alloys in the chloride media described in the literature. Improved corrosion resistance can be achieved by the microstructure homogenization through the optimization of laser parameters and/or following heat treatment.     

Calorimetric Determination of the Enthalpies of Formation of Liquid Ni-Zr Alloys

The enthalpies of formation of liquid Ni-Zr alloys have been measured through the composition range 0 < X_zr < 0.4, where X_zr is the mole fraction of Zr. Four independent runs were made at temperatures ranging from 1740 to 1743 K and one at 1838 K. The compositional dependence of the enthalpies of formation could be fitted quite well with polynomial representations according to the subregular as well as to the sub-subregular solution approximations. The influence of chemical short-range order was found to differ from the earlier Ni-Ti results.