H. J. Seifert

Experimental investigation and thermodynamic calculation of the Al–Mg–Zn system

Abstract On the basis of a critical assessment, experimental investigations by EPMA on ternary Al–Mg–Zn alloys were specifically performed to provide missing data of the ternary solubilities of the Al–Mg and Mg–Zn phases as well as to improve the knowledge of the extensions of the homogeneity ranges of the ternary τ- and Φ-phases. A thermodynamic description for the Al–Mg–Zn system was obtained taking into account those experimental data together with constitutional, thermodynamic and crystallographic literature information. The binary intermetallic phases are modelled to have ternary solubilities. The ternary τ-phase is modelled according to its crystal structure with cubic symmetry as (Mg) 26 ( Mg , Al) 6 ( Al , Zn , Mg) 48 (Al) 1 in the compound-energy-formalism. The Φ-phase is described by the sublattice formula Mg 6 ( Al , Zn ) 5 .

PHASE EQUILIBRIA AND CRYSTAL CHEMISTRY IN THE Y2O3-AL2O3-SIO2 SYSTEM

In order to clarify inconsistencies in the literature and to verify assumed ternary solubilities, the phase equilibria in the Y 2 O 3 –Al 2 O 3 –SiO 2 system at 1600, 1400, and 1300 °C were experimentally determined using x-ray diffraction (XRD), scanning electron microscope with attached energy-dispersive detector system (SEM-EDX), and electron probe microanalyzer (EPMA). Six quasibinary phases were observed: Y 4 Al 2 O 9 (YAM), YAlO 3 (YAP), Y 3 Al 5 O 12 (YAG), Y 2 SiO 5 , Y 2 Si 2 O 7 (C and D modifications), and ˜ 3 Al 2 O 3 · 2SiO 2 (mullite). Y 4 Al 2 O 9 forms an extended ternary solid solution with the formula Y 4 Al 2(1- x ) Si 2 x O 9+ x ( x = 0 2 ˜0.31). The lowest ternary eutectic temperature was determined at 1371 ± 5 °C by high-temperature differential scanning calorimetry (DSC). The results were compared with previous data available for the Y 2 O 3 –Al 2 O 3 –SiO 2 system and with data for other RE 2 O 3 –Al 2 O 3 –SiO 2 (RE = rare earth element) systems.

Phase equilibria and thermal analysis of Si-C-N ceramics

Abstract The phase reactions, crystallization behaviour and thermal degradation of two Si–C–N ceramics derived from precursors VT50 and NCP200, respectively, were studied by means of CALPHAD type thermodynamic calculations and experimental investigations by DTA/TG, XRD and SEM/EDX. The phase reaction Si3N4+3C=3SiC+2N2 proceeds during the thermal degradation of both ceramics. Additionally, the phase reaction Si3N4=3Si+2N2 occurs during the thermal degradation of the NCP200 ceramic. To explain quantitatively the high temperature behaviour of Si–C–N ceramics, thermodynamic functions, the reaction scheme, isothermal sections, isopleths, phase fraction diagrams and phase composition diagrams (for gas partial pressures) were calculated. The computer simulations were confirmed by the experiments for both ceramics.

Experimental investigation of the MgAl phase diagram from 47 to 63 at.% Al

The purpose of this work was to investigate the MgAl phase diagram in the composition range from 47 to 63 at.% Al which has not previously been consistently determined. The rhombohedral e phase n ear 56 at.% Al is found to form at 410°C following a peritectoid reaction (γ + β → e) and not a peritectic reaction as reported by Schumann and Voss (Giessereiforschung. 33 (1981) 43). The respective homogeneity ranges of the γ phase in the Al-rich part and of the β phase have been determined. Besides the γ, β and e phases, the existence of a high temperature phase (named λ) between 435 and 445°C is suggested. The existence of the ξ phase and of the relevant invariant reactions reported by Schumann and Voss are not confirmed by the present results.

Phase relationships in the system Gd2O3-Al2O3-SiO2

Abstract Previously unknown phase equilibria in the system Gd2O3-Al2O3-SiO2 at 1400 and 1300 °C were determined using X-ray diffraction, SEM-EDX and EPMA. The following six quasibinary phases are observed: Gd4Al2O9, GdAlO3, Gd2SiO5, Gd9.33(SiO4)6O2, Gd2Si2O7 and 3Al2O3·2SiO2 (mullite). Additionally, there are two extended ternary solid solutions, Gd9.33+2x(Si1−xAlxO4)6O2 (with x=0−∼0.33) and Gd4Al2(1−x)Si2xO9+x (with x=0−∼0.3), that are based on the quasibinary phases Gd9.33(SiO4)6O2 and Gd4Al2O9, respectively. No quasiternary compound was detected. The ternary eutectic temperature was measured at 1306±5 °C by high-temperature STA and DSC. The results are compared with the scarce data available for other RE2O3-Al2O3-SiO2 (RE=rare earth element) systems.

Phase equilibria and diffusion paths in the Ti-Al-O-N system

Abstract To provide a basis for the calculation of high-temperature phase reactions in the Ti–Al–O–N system a thermodynamic dataset using the CALPHAD method (CALculation of PHAse Diagrams) was developed. Analytical Gibbs energy descriptions for the system phases were taken from literature or optimized in this work. The solid solution series Al 2 TiO 5 –Ti 3 O 5 (tialite) was modeled with the sublattice description (Al +3 ,Ti +3 ,Ti +4 ) 2 (Al +3 ,Ti +3 ,Ti +4 ) 1 (O −2 ) 5 . The nine energy parameters resulting from the applied Compound Energy Formalism could be reduced to four independent parameters. Reciprocal reactions and the condition of electroneutrality were taken into account. Based on the optimized binary and ternary phase diagrams extrapolating calculations on quaternary compositions were carried out and compared with experimental work. The results show good agreement with the phase assemblages in the microstructure of ceramics in the Ti–Al–O–N system. Additionally, the oxidation behavior of γ-TiAl can be explained qualitatively.

Thermodynamics of liquid and undercooled liquid Al-Ni-Si alloys

Abstract The partial and the integral enthalpies of mixing of liquid Al–Ni–Si alloys have been determined by high-temperature isoperibolic calorimetry for three sections with constant concentration ratios of Ni and Si at 1575±3 K. With a view to a direct use of the thermodynamic properties, the values of the integral enthalpy of mixing of the ternary alloys together with the literature values of the constituent binaries Al–Ni, Ni–Si and Al–Si are analytically described by the thermodynamically adapted power series according to the right-hand Colinet geometry using a least-squares regression analysis. A regular association model adequately describes the thermodynamic properties of liquid and undercooled liquid Al–Ni–Si alloys. The Δ H ( x Al , x Ni , x Si ), Δ S ( x Al , x Ni , x Si ) and Δ G ( x Al , x Ni , x Si ) functions show the existence of essential contributions of ternary interactions among the alloy components. The strongest tendency toward chemical short-range ordering in the liquid state occurs near to compositions corresponding to the intermetallic compound Ni 2 AlSi.

Enthalpy of mixing of liquid Al-Cu-Si alloys

Abstract The partial and the integral enthalpies of mixing of liquid Al–Cu–Si alloys have been measured by high temperature calorimetry at 1575±3 K. Results for three sections with constant concentration ratios of Al and Si are given in tabular form. A least-square regression analysis of the integral enthalpy of mixing data results in the following relationships (in kJ mol−1; T=1575±3 K): Δ H =x Al x Cu α Al–Cu +x Al x Si α Al–Si +x Cu x Si α Cu–Si +x Al x Cu x Si α Al–Cu–Si α Al–Cu =(−75.6±6.5)+(−63.8±8.8)x Al +(232±33)x 2 Al +(−131±28)x 3 Al α Al–Si =−12.6+2.5x Si α Cu–Si =(−16.2±2.2)+(97.7±37.1)x Cu +(−1138±208)x 2 Cu +(3514±489)x 3 Cu + (−4748±515)x 4 Cu +(2216±199)x 5 Cu α Al–Cu–Si =(−241.7±8.8)+(1165±32)x Al +(891±34)x Si +(−935±30)x 2 Al +(−640±35)x 2 Si + +(−1563±52)x Al x Si . The minimum of ΔH changes with composition from −14.5 kJ mol−1 (Cu–25at.%Si) to −17.3 kJ mol−1 (Al–60at.%Cu). The presence of additional ternary interactions or ternary associates in the liquid state could not be observed.

THE STANDARD MOLAR ENTHALPIES OF FORMATION OF ALPHA -SI3N4 AND BETA -SI3N4BY COMBUSTION CALORIMETRY IN FLUORINE, AND THE ENTHALPY OF THE ALPHA -TO-BE TA TRANSITION AT THE TEMPERATURE 298.15 K

The standard molar enthalpies of formation ΔfH°m of α−Si3N4 and β−Si3N4 have been determined by fluorine combustion calorimetry: ΔfH°m (Si3N4, cr, α, 298.15 K) = − (828.9 ± 3.4) kJ · mol−1 and ΔfH°m (Si3N4, cr, β, 298.15 K) = – (827.8 ± 2.5) kJ · mol−1. These results indicate that the enthalpy of the α-to-β transition, approximately (1 ± 4) kJ · mol−1, is negligible within experimental uncertainty.

Enthalpy of mixing of liquid and undercooled liquid ternary and quaternary Cu-Ni-Si-Zr alloys

Abstract The partial and the integral enthalpies of mixing of liquid Si–Zr, Cu–Si–Zr, Ni–Si–Zr and Cu–Ni–Si–Zr alloys with mole fractions of zirconium in the range from 0 up to 30 at.% have been determined by high-temperature isoperibolic calorimetry at 1575–1720 K. The data of the constituent ternaries were described using a regular association model. The enthalpy of mixing of liquid and undercooled liquid Cu–Ni–Si–Zr alloys were estimated for all compositions on the basis of the model description of the constituent binary and ternary systems.