Volume 88, Issue 11

A thermodynamic description of the Cu–Mg–Zn system is obtained by thermodynamic modeling of the Gibbs Energies of all the phases in this system. The model parameters are optimized based on the experimental phase equilibrium and thermodynamic data available in the literature. The agreement obtained between several calculated isopleths as well as the thermodynamic values of the σ-Cu 2 Mg phase and experimental data indicates that the current description of this system is reasonable. The calculated phase equilibria close to the Cu–Mg side are believed to the reliable for practical applications, while those in the Zn-rich region are subjected to large uncertainty. Additional experimental data particularly in the Zn-rich corner and near the Mg–Zn side are required in order to firmly establish the phase equilibria of this ternary over wide ranges of composition and temperature.

Metastable alloys have been synthesized in a planetary ball mill by mechanical alloying of elemental crystalline powder blends between Ta and one of the 3d-transition metals Co, Ni, and Cu. The progress of alloying and the milling products are characterized by X-ray diffraction, optical metallography, electron microprobe analysis, and by differential scanning calorimetry. The deformation during the milling process reduces the crystallite size within the powder particles to nanometer scale after a few hours of ball milling. For longer milling times super-saturated solid solutions have been formed in all systems for Ta-lean powder blends. Formation of amorphous phases has been observed in all systems: For Ta–Co and Ta–Ni wide glass forming ranges have been found, whereas Ta–Cu shows (only) incomplete amorphization for Ta–Cu. Contaminations of Fe and Cr from the milling tools are particular important for Ta-rich starting compositions or long milling times. The thermal stability of the amorphous phases has been investigated. Crystallization temperatures, crystallization enthalpies, and crystallization products are reported. The thermodynamic conditions for the systems Ta–Ni and Ta–Co, which both exhibit a negative heat of mixing and form several intermetallic compounds in equilibrium, are different from those of the system Ta–Cu, which is immiscible with a positive heat of mixing. The alloy formation is discussed with respect to thermodynamic and kinetic aspects.

Compositions relevant to the processing of aluminum-modified MoSi 2 composites were investigated by electron microscopy and wavelength dispersive spectroscopy (WDS) after equilibrating at 1000 and 1400 °C. The results of these experiments together with literature data were used to determine a set of analytical descriptions of the Gibbs energy of the stable phases in the Al–Mo–Si system relevant to the processing of MoSi 2 composites. These results were then used to explain the thermodynamic aspects of three schemes of in-situ processing of MoSi 2 composites: (a) silica elimination through aluminum additions, (b) the generation of reinforcements using silica precursors and (c) the formation of coatings on niobium reinforcements. The importance of the application of thermodynamics to the analysis of in-situ reactions is highlighted.

The partial and integral enthalpies of mixing of Cu–Si and Cu–Zr melts have been measured by high temperature calorimetry at 1900 ± 10 and 1480 ± 5 K, respectively. In dilute copper solutions (Cu–Si system) and dilute zirconium solutions (Cu–Zr system) anomalous variations of the enthalpy of mixing with compositions have been observed. In the Cu–Si system, in the temperature range from 1600 to 1900 K, a negligibly small temperature dependence of the enthalpy of mixing is observed. A least squares treatment of the data results in the following expressions for the molar partial and integral enthalpy of mixing (in kJ mol −1 ). The activities are estimated on the basis of the experimental enthalpy of mixing data using an empirical relation and show a reasonable agreement with experimental activity data. For the Cu–Si system ( x = x Si ; 0 < x < 1; T = 1900 K) ΔH¯Cu=x282.1−3738.0x+16817.0x2−30709.0x3           +25479.1x4−7941.5x5ΔH¯Si=1−x2−86.4−337.0x+5101.5x2−15620.0x3           +18861.1x4−7941.5x5ΔH=x1−x−86.4−168.5x+1700.5x2−3905.0x3         +3772.3x4−1323.6x5$$\eqalign{ & \Delta {\overline H _{{\rm{Cu}}}} = {x^2}\left( {82.1 - 3738.0x + 16817.0{x^2} - 30709.0{x^3}} \right. \cr & \;\;\;\;\;\;\;\;\;\;\;\left. { + 25479.1{x^4} - 7941.5{x^5}} \right) \cr & \Delta {\overline H _{{\rm{Si}}}} = {\left( {1 - x} \right)^2}\left( { - 86.4 - 337.0x + 5101.5{x^2}} \right. - 15620.0{x^3} \cr & \;\;\;\;\;\;\;\;\;\;\;\left. { + 18861.1{x^4} - 7941.5{x^5}} \right) \cr & \Delta H = x\left( {1 - x} \right)\left( { - 86.4 - 168.5x + 1700.5{x^2}} \right. - 3905.0{x^3} \cr & \;\;\;\;\;\;\;\;\;\left. { + 3772.3{x^4} - 1323.6{x^5}} \right) \cr} $$ For the Cu–Zr system ( x = x Zr ; 0 ≤ x ≤ 0.7; T = 1480 K) ΔH¯Cu=x2220.2−3247.8x+13017.7x2−24416.8x3            +21601.4x4−7215.6x5ΔH¯Zr=1−x2−51.0−542.4x+4058.2x2           −11946.1x3+15588.4x4−7215.6x5ΔH=x1−x−51.0−271.2x+1352.7x2−2986.5x3         +3117.7x4−1202.6x5$$\eqalign{ & \Delta {\overline H _{{\rm{Cu}}}} = {x^2}\left( {220.2 - 3247.8x + 13017.7{x^2}} \right. - 24416.8{x^3} \cr & \;\;\;\;\;\;\;\;\;\;\;\;\left. { + 21601.4{x^4} - 7215.6{x^5}} \right) \cr & \Delta {\overline H _{{\rm{Zr}}}} = {\left( {1 - x} \right)^2}\left( { - 51.0 - 542.4x + 4058.2{x^2}} \right) \cr & \;\;\;\;\;\;\;\;\;\;\;\left. { - 11946.1{x^3} + 15588.4{x^4} - 7215.6{x^5}} \right) \cr & \Delta H = x\left( {1 - x} \right)\left( { - 51.0 - 271.2x + 1352.7{x^2} - 2986.5{x^3}} \right) \cr & \;\;\;\;\;\;\;\;\;\left. { + 3117.7{x^4} - 1202.6{x^5}} \right) \cr} $$

A vertical section through the ternary Fe–Ni–Sb phase diagram with an Fe:Ni ratio of 1:1 was studied by thermal analyses and X-ray powder methods in the composition range between 42 and 90 at.% Sb. Antimony vapor pressures were determined in the ternary phase (Fe 0.5 Ni 0.5 ) 1± x Sb between 45.5 and 51.5 at.% Sb and between 1020 and 1310 K by an isopiestic method. The partial molar properties of antimony were derived at 1173 K. A theoretical model based on a statistical distribution of interstitial metal atoms and metal vacancies over their respective lattice sites was applied to the experimental activity data at 1173 K.

In the atmospheric corrosion of metals, the adsorption of water affects the corrosion rates and, hence, the knowledge of water adsorption and metal oxyhydroxide formation is important in understanding the atmospheric corrosion process. Since gold does not form surface oxides and therefore retains its surface characteristics throughout adsorption experiments, it provides a model system. The purposes of the present research were (i) to measure the adsorption of water on gold as a function of relative humidity (RH) and temperature and (ii) to assess the Brunauer-Emmett-Teller (BET) and the Frenkel-Halsey-Hill (FHH) model for water adsorption. In the present research, the quartz-crystal microbalance (QCM) technique was used to measure the mass change of gold at 0 to 100% relative humidity and 7 to 90 °C under nitrogen environments. BET and FHH models could not fit the data points with single functional relationships. Δ H° ads values were calculated using a modified BET method and were found to decrease with temperature.

The transformation stress and strain anisotropy of single crystal shape memory alloys (SMA) is well known. Models based on the crystallography of the martensitic transformation are successful in predicting the single crystal behavior. Transformation stress and transformation strain anisotropy have also been reported in polycrystals. In the present article, previously developed unconstrained and constrained single crystal models are extended to the textured polycrystalline case in which stress-induced martensite (SIM) forms in a continuous fashion. These models are evaluated using the experimentally determined stress–strain curves for a NiTi sheet which is observed to exhibit continuous growth of stress-induced martensite. The sheet has a moderate texture with three dominant components; however, approximately isotropic transformation stress and strain are predicted. The slope of the superelastic plateau, as well as the transformation strain were used for model evaluation. The model results compare reasonably well with the present experimental results, as well as those published previously by other researchers.

The change of the optical properties of the ordered intermetallic Hume-Rothery phase NiGa as a function of the concentration of lattice defects was measured by differential reflectometry. The lattice defects were produced by changing the composition and quenching from high temperatures. The electron bandstructure of Ni 50 Ga 50 was calculated with the augmented-plane-wave method. The comparison between measured and calculated results shows that the changes in the spectra could be explained with a rigidband model. The existence of free electrons in NiGa was confirmed.