Thermodynamics of Liquid Alloys and Metastable Phase Transformations in the Copper - Titanium System

Abstract

536.653:546.56′82We have used solution calorimetry at temperatures of 1573 K and 1873 K over broad concentration ranges to study the mixing enthalpy of Cu − Ti liquid alloys. The molar mixing enthalpies of the system are significant negative values. We have established the temperature dependence of the molar mixing enthalpies of the system: there is an increase in their exothermicity as the temperature is lowered. The significant negative mixing enthalpies of the system allow us to conclude that the chemical bonds are localized in the studied melts and consequently associates form. We tested this conclusion within ideal associated solution theory, which describes well the results obtained with a set of CuTi and CuTi 2 associates. Using the model obtained, we have calculated the excess thermodynamic functions of mixing (enthalpy, Gibbs free energy, heat capacity) for the liquid alloys. We estimated the Gibbs energies of fcc, bcc, and hcp solutions in the system by the CALPHAD method, using data from the initial sections of the phase diagrams and from the corresponding thermodynamic data. We have calculated the metastable phase equilibria between the limiting solid solutions and the liquid or supercooled liquid phase. It was shown that for the supercooled liquid and the amorphous phase, a broad concentration range of relative thermodynamic stability can be obtained. The concentration range of amorphization of Cu − Ti melts corresponds to the position of the metastable liquidus line and the T 0 line at temperatures close to the temperature range of amorphous solidification.The thermodynamic properties of liquid copper − titanium alloys are interesting especially in connection with the possibility of obtaining two-component metallic glasses in this system by quenching from the liquid [1-4] and the prospects for using this system as the basis for development of multicomponent compositions with low critical glassforming rates [5,6]. The possibility of melts passage in the system to the amorphous state can not be explained without analyzing the nature of the temperature-composition dependence of thermodynamic properties of competing phases. Furthermore, data on the thermodynamics of the process of copper and titanium alloy formation is limited and ambiguous.The formation enthalpies of liquid alloys of copper and titanium have been studied by calorimetric methods [7][8][9]. In [7,8], the integral mixing enthalpies of solid titanium with liquid copper were studied at 1373 K in the composition range 0.01 < x Ti < 0.70. Using these data, in [8] the integral mixing enthalpies were calculated for supercooled liquid titanium (Fig. 1a). The values of the first mixing enthalpy of titanium were found: −7.65 kJ/mole [7] and −9.0 kJ/mole [8], and also the minimum of integral mixing enthalpy: -3.8 kJ/mole [8].In [9], the partial mixing enthalpy of titanium with copper was studied in the composition range x Ti = 0-0.55 at a temperature of 1873 K. These data were obtained by recalculating the experimentally determined partial enthalpi...

Thermodynamic assessment of the copper-hafnium system

“…We previously showed [29,30] that CALPHAD calculations can be successively used to predict transformations with involvement of supercooled melts. The paper [29] uses binary systems as an example to show that the concentration boundaries of the decrystallization region may be assessed based on the metastable phase diagram between the supercooled liquid and terminal solid solutions.…”

Phase equilibria and thermodynamics of binary copper systems with 3d-metals. VII. Concentration-temperature dependences of the thermodynamic functions of mixing for liquid alloys of copper and transition metals

“…2. Isotherms of integral enthalpy of mixing copper with transition metals (kJ/mole) obtained experimentally at different temperatures shown in [1,8,[40][41][42][43][44][45] that for the purpose of phenomenological modeling of systems with negative deviations of the excess thermodynamic properties from the ideal behavior, it is possible to employ the ideal associated solution (IAS) model. Mathematical models were used in [2][3][4][5] to describe the temperature dependence of the thermodynamic functions of mixing for melts with positive deviations from the ideal behavior.…”

“…The value of these results is in that they have been obtained using the same calorimetric technique [46], which made it possible to avoid bias errors that often arise in comparing results produced by different experimental techniques and smear the effects associated with the temperature dependence of a thermodynamic property. Such studies have been performed for partial and integral mixing enthalpies of scandium (first mixing enthalpy) [10,47], yttrium [48], titanium [8,12], zirconium [12,44], hafnium [49], and ferrum [5,22,50]. Moreover, as will be shown below, important conclusions on the behavior of the temperature dependence of the mixing enthalpy have been drawn by comparing our experimental data and published data for the copper-lanthanum [51][52][53], coppermanganese [14,15], and copper-gold [54] systems.…”

“…The model parameters were obtained in [1] for Su-Sc, in [8] for Cu-Ti, and in [55] for Cu-Zr and CuHf, while the model parameters for the Cu-Y, Cu-La, Cu-Zn, and Cu-Au systems were found from experimental data on the thermodynamic properties in Table 1.…”

“…The isotherm of the integral mixing enthalpy for the copper-titanium system achieves a minimum in the region of alloys with x Ti ≈ 0.65 [8,12], which corresponds to the congruently melting intermetallic compound CuTi 2 on the phase diagram. It has the highest melting temperature among the IMCs of the system.…”