Specific Heat Capacities of Some Ternary Aluminides

Journal of Phase Equilibria and Diffusion, 2014

Specific heat capacities of the c-TiAl and s phases in the Al-Ti-Cr system were determined in the temperature range between 298 and 773/873 K based on one binary (51.2Al-48.8Ti) and three ternary alloys (53Al-44Ti-3Cr, 64.6Al-30.2Ti-5.2Cr, 65.3Al-26.7Ti-8Cr). The specific heat capacity measurements in this system were performed for the first time. The measurements were carried out using heat-flux DSC devices employing the classical three step method. The measured heat capacities were lower than the approximation using the Neumann-Kopp rule. These differences in the heat capacity data arise probably due to the variation of the chemical bonding state between the intermetallic phases and the pure elements which are not taken into account by Neumann-Kopp's rule. In order to check the reliability of the obtained results, the enthalpy increment was measured by using drop calorimetry. The heat increments determined using the drop calorimetry was in an excellent agreement with the enthalpy increments calculated using the analytic description of the heat capacities from DSC.

Intermetallics, 1999

The Al±Ni±Ti phase diagram has been thermodynamically assessed and a consistent set of thermodynamic functions has been developed. The thermodynamic modeling is based on an experimental investigation of the phase equilibria in the composition range of 0.14x Al 40.7. Alloys were prepared by argon-arc or vacuum-electron beam melting of elemental powder blends. X-ray powder diraction, metallography, SEM and EMPA-techniques were employed to analyze the samples in the as-cast state as well as after annealing at 800, 900 and 1000 C. The existence of the four ternary compounds, (1 to (4 , has been con®rmed, although homogeneity regions dier signi®cantly from reports in the literature. The homogeneous phase, previously claimed at ``Al 23 Ni 26 Ti 51 '', is shown by high resolution microprobe and X-ray diraction measurements to be an extremely ®ne-grained eutectic structure. The congruent melting behavior of (4 elxi 2 i is con®rmed, but, in contrast to earlier reports, primary crystallization and congruent melting have been observed for (1 el 13 xi 2 i 5 and (3 el 3 xii 2. In contrast to earlier assessments, (1 Y(2 and (3 are experimentally found to be stable at 800, 900 and 1000 C. The thermodynamic modeling of the ternary phases (2 and (3 is done with sim-pli®ed sublattice models, considering their crystal structure and homogeneity ranges. The sublattice model for (4 is taken from an earlier asessment of the nickel-rich ternary phase equilibria. The present assessment covers the entire composition range. An application to the solidi®cation behavior of ternary alloys is also exempli®ed.

Intermetallics, 1999

Phase relations in the ternary system Al±Ni±Ti have been experimentally established for the isothermal section at 900 C for concentrations 0.1 4 x Al 4 0.7. The investigation is based on X-ray powder diraction, metallography, SEM and EMPA-techniques on about 40 ternary alloys, prepared by argon-arc or vacuum-electron beam melting of proper elemental powder blends. The existence of four ternary compounds, t 1 to t 4 , is con®rmed, however, in contrast to earlier investigations at signi®cantly dierent compositions and with dierent shape of the homogeneity regions. This is particularly true for the phase regions of t 3-Al 3 NiTi 2 with the MgZn 2-type structure ranging from Al 30 Ni 28 Ti 42 (composition lowest in Al) to Al 50 Ni 16 Ti 34 (composition richest in Al) and for t 2-Al 2 NiTi. The complex atom site substitution mechanism in t 3 changing from Ti/Al exchange at Al-poor compositions towards Ni/Al replacement for the Al-rich part was monitored in detail by quantitative X-ray powder diraction techniques (Rietveld analyses). In contrast to earlier reports, claiming a two-phase region Ni{Al x Ti 1-x } 2 +t 3 , we observed two closely adjoining three-phase equilibria: a 2-AlTi 3 +Ni{Al x Ti 1-x } 2 + t 4-AlNi 2 Ti and a 2-AlTi 3 +t 3-Al 2 NiTi 2 +t 4-AlNi 2 Ti. The earlier reported ``homogeneous phase at Al 23 Ni 26 Ti 51 H '' was shown by high resolution microprobe and X-ray diraction measurements to be an extremely ®negrained eutectic. The experimental results are in ®ne agreement with the thermodynamic calculation.

In the framework of its 6th Framework Programme, the European Union funds the Integrated Project IMPRESS, related to industrial applications of Ti-Al and Ni-Al alloys. One central task of this project is the precise determination of the relevant thermophysical properties of selected alloys for both the solid and liquid phases. The properties to be measured include thermal data such as heat of fusion, specific heat, and thermal conductivity, as well as thermophysical and transport properties such as density, surface tension, and viscosity. In addition to conventional high-temperature equipment, containerless methods are used. This article introduces the IMPRESS project, and discusses the first results obtained to date.

Journal of Alloys and Compounds, 2008

Thermodynamic analysis of three binary Ti-based alloys: Ti-Al, Ti-V, and Al-V, as well as ternary alloy Ti-Al-V, is shown in this paper. Thermodynamic analysis involved thermodynamic determination of activities, coefficient of activities, partial and integral values for enthalpies and Gibbs energies of mixing and excess energies at four different temperatures: 2000, 2073, 2200 and 2273 K, as well as calculated phase diagrams for the investigated binary and ternary systems. The FactSage is used for all thermodynamic calculations.

Acta Metallurgica et Materialia, 1995

The solid-state phase transformations and phase equilibria in a series of AI-Ti-Ta alloys in the range 38 55%A1, 19 30%Ti and 20-38%Ta have been investigated. It is shown that a variety of phases exist over this composition range and that the microstructures vary dramatically with prior thermal history. For example, the disordered fl phase exists at high temperatures over a large composition range and its decomposition depends strongly on both composition and cooling rate in a manner similar to that reported in titanium rich Ti AI Nb alloys. In addition, the ~ phase is shown to extend to higher AI contents than originally reported and its decomposition is similar to that observed in binary Ti-A1 alloys (i.e. it forms "massive" y at high cooling rates and ~ + 7 or a2 + "/at slower rates). The ~-(Ta~Ti~ +)2A1 phase becomes stable at lower temperatures in the tantalum-rich alloys studied and its formation is also shown to be a strong function of cooling rate and composition. The various results are used to suggest revisions to the ternary AI Ti-Ta isothermal sections at both 1723 and 1623 K. a~ 4~ I 2625 2626 WEAVER and KAUFMAN: TERNARY Al Ti Ta SYSTEM composition. Meanwhile, the a reportedly transformed to a + ?, via a cellular reaction sequence. Boettinger et al. [16] used differential thermal analysis (DTA), high temperature XRD, scanning electron microscopy (SEM), and TEM to evaluate the phase equilibria and microstructural stability of an AI-25Ti 25Ta alloy over a wide temperature range. They found that this alloy consists of 100% /~ above 1798 K and :c + fl between 1723 and 1773 K. Between 1573 and 1673 K, this alloy was observed to consist of various combinations of~, fl, ?, and a, while, below 1573 K, the alloy consisted of 7 and a only. This data was used to estimate a partial isothermal section at 1603 K. Although both the studies of Boettinger et al. [16] and McCullough et al. [11, 12] accurately indicated the phases present at elevated temperatures in the AI-Ti Ta system, no detailed accounting of the phase equilibria at high temperatures and the solid-state transformation sequences during cooling is available. The purpose of this paper is to better define (1) the solid-state phase transformations that occur during cooling, particularly near the composition AI 25Ti-25Ta and (2) the phase equilibria in the AI Ti Ta system at 1623 and 1723 K. In order to accomplish these goals, 28 ternary alloys were selected, isothermally annealed, and either water quenched (WQ), air cooled (AC) or furnace cooled (FC). Selected samples were then characterized using combinations of light optical microscopy (LOM), SEM, TEM, XRD, electron probe microanalysis (EPMA), and DTA in order to provide information concerning the phase transformations during solidstate cooling.

Journal of Mining and Metallurgy, Section B: Metallurgy, 2008

Thermodynamic calculations of three binary Ti-based alloys: Ti-Al, Ti-Fe, and Al-Fe, as well as ternary alloy Ti-Al-Fe, is shown in this paper. Thermodynamic calculations involved thermodynamic determination of activities, coefficient of activities, partial and integral values for enthalpies and Gibbs energies of mixing and excess energies at different temperatures: 1873K, 2000K and 2073K, as well as calculated phase diagrams for the investigated binary and ternary systems. The FactSage is used for all thermodynamic calculations.

Scripta Metallurgica et Materialia, 1991

Scripta Metallurgica et Materialia, 1992

Ternary a_d,'J_itions of the beta isomorphous elements, Nb, V, Me and Ta, have long been known to improve the ambient tempcratare ductility and fracture toughness of 0r2 (Ti3A1) and"/(TiA1) intermetallic alloys [1 -4]. In the former the increase has begn related to the stabilization of the ductile [3 phase, while the latter enhancement has been related to changes in tetragonality, unit cell volume, twin density and/or electronic structure. Control of the mechanical properties of ternary and more complex tz2(Ti3 A1)and ?(Ti AI) intermetallic alloys will require optimization of their prior thermomechanical history. Such control can be achieved through a knowledge of relevant high temperature phase equilibria; indeed, establishment of appropriate phase equilibria in the Ti-A1-Nb ternary system continues to be the subject of an extensive investigation at the University of Wisconsin .

International Journal of Thermophysics, 2007

The thermal diffusivities of some industrially important alloys have been measured as a part of the EU funded Intermetallic Materials Processing in Relation to Earth and Space Solidification (IMPRESS) project which is coordinated by the European Space Agency (ESA). The thermal diffusivities of the alloys were measured by the Laser flash method with a carefully designed gas cleaning system to remove traces of oxygen from the argon atmosphere. In the present work, the thermal diffusivity of TiAlNb (Ti46.1Al45.9Nb8 at %) and AlNi alloy (Al-Ni31.5 at %) alloys have been measured independently at Royal Institute of Technology, Sweden (KTH) and National Physical Laboratory, UK (NPL).