­­­ Thermodynamic investigation of Fe-Ti-Y ternary system

GONG Wei-ping(龚伟平), CHEN Teng-fei(陈腾飞), LI Da-jian(李大建), LIU Yong(刘 咏)

State Key Laboratory for Powder Metallurgy, Central South University, Changsha 410083, China

Received 5 March 2008; accepted 9 September 2008

Abstract: An extensive investigation on the Fe-Ti-Y system was performed via experimental measurement and thermodynamic calculation. The Fe-Ti-Y ternary couples at 1 273 K were prepared with a desire to provide accurate phase relationships needed for the refinement of this ternary phase diagram. And a tentative isothermal section of Fe-Ti-Y at 1 273 K was built based on the experimental information. In the thermodynamic modeling, the thermodynamic parameters for the Ti-Y binary system and the ternary phase in the Fe-Ti-Y system were evaluated. Those for the Fe-Ti and Fe-Y systems from literature were slightly modified for the compatibility. The isothermal sections of Fe-Ti-Y ternary system at 873 K and 1 273 K were calculated. The ternary compound Fe₁₁TiY and Fe₂(Ti, Y) solid solution formed from Fe₂Ti and Fe₂Y are detected, which is in good agreement with the literature information.

Key words: Fe-Ti-Y; thermodynamic calculation; Fe₁₁TiY; diffusion couple

1 Introduction

A promising approach for achieving high creep strength and radiation damage resistant alloys is to create a very high density of fine-scale features that act as dislocation obstacles, serve as the dominant nucleation site for small helium bubbles and can promote vacancy- interstitial recombination. A high density of coherent nanometer-scale Y-O-Ti nanoclusters(NCs) are produced by mechanical alloying (MA) Fe-Cr-Ti powders with Y₂O₃ followed by hot consolidation[1-8]. Its high- temperature strength, radiation response, lattice Monte Carlo simulations of nanocluster formation in nanostructured ferritic alloys were studied[5-7].

In the targeted technological application, basic knowledge is required for process development and optimization, in particular the thermodynamic and transport properties of Fe-Cr-Ti powders and their mixtures with Y₂O₃ as well as in the addition of  elements. However, these data are scarce and not easily accessible in literatures. As a consequence, intensive efforts are made both on research and development aspects and on data development.

The Fe-Ti-Y system is investigated in this work because it is the subsystem of the Fe-Cr-Ti-Y-O steel and it has not been thermodynamically evaluated up till now.

2 Evaluation of data from literatures

The information on phase diagram for the binary sub-systems of Fe-Ti-Y is available in Refs.[9-14]. MURRARY[9-10], KUBASCHEQSKI[11] and HARI KUMAR et al[12] evaluated the Fe-Ti system respectively and indicated that there are two stable intermediate phases, FeTi and Fe₂Ti. The Fe-Y system has been studied quite thoroughly by various investigators and the latest assessment was carried out by ZHANG et al[13]. In the work of ZHANG et al[13], four intermediate compounds Fe₁₇Y₂, Fe₂₃Y₆, Fe₃Y and Fe₂Y were detected. According to Ref.[14], the Ti-Y system is a simple eutectic-type phase diagram with limited mutual solubility of components. This work adopted the thermodynamic data from Refs.[10, 13] for Fe-Ti, Fe-Y system, respectively, but made a thermodynamic assessment on the Ti-Y system with the available information.

For Fe-Ti-Y system, a ternary compound Fe_10.8Ti_1.2Y was observed by DE MOOIJ and BUSCHOW[15] in the alloys with composition of Fe_12-xTi_xY homogenized and equilibrated at 1 123 K for 14-21 d. In the work of LIU et al[16], the compound Fe_10.8Ti_1.2Y was confirmed.

3 Experimental

3.1 Experimental procedure

Prior to the present experiment, a preliminary thermodynamic calculation for the Fe-Ti-Y system is performed by using the available thermodynamic database. The computed Fe-Ti-Y phase diagram is used to guide the choice of the experimental temperatures. Another reason to conduct the experiment is that even the most detailed measurement[15-16] failed to obtain accurate data on the ternary phases.

Iron (99.95%, mass fraction), titanium (99.5%, mass fraction) and yttrium (99.5%, mass fraction) were used as the starting materials. The binary Fe-Ti couples were first prepared by diffusion welding the elemental bars in a silica capsule back-filled with high purity argon at    1 273 K for 72 h. One of the Fe-Ti couples and a Y block were ground, polished, cleaned, and then swathed together with Mo wires to make a Fe-Ti-Y diffusion couple, which was wrapped in Mo foil. The Fe-Ti-Y diffusion couples as shown schematically in Fig.1 were annealed in a diffusion furnace at (1 273±1) K for 1200 h and subsequently quenched in water.

After standard metallographic preparation, all of the Fe-Ti-Y couples were examined by optical microscopy and scanning electron microscopy (SEM).

Fig.1 Construction of Fe-Ti-Y diffusion couple

3.2 Experimental results

The microstructure of a Fe-Ti-Y couple annealed at 1 273 K for 1 200 h is shown in Fig.2. The backscattered electron images (BSE) of the Fe-Ti, Fe-Y and Fe-Ti-Y couple annealed at 1 273 K for 1 200 h are given in Figs.3-5, respectively. The isothermal section of Fe-Ti-Y system at 1 273 K is constructed in Fig.6 according to the experimental data.

Fig.2 Microstructure of ternary couple annealed at 1 273 K for 1 200 h

As shown in Figs.3 and 4, the data close to the boundary binary systems are in accord with the literature ones and the reported binary compounds Fe₂Ti, FeTi in the Fe-Ti system, Fe₂Y, Fe₃Y, Fe₂₃Y₆, Fe₁₇Y₂ in the Fe-Y system are confirmed by the present experiments. The experimental results show that Fe₂Ti and Fe₂Y form a continuous solid solution, namely Fe₂(Y, Ti). In the ternary triple, a ternary phase with the composition of approximate Fe₉TiY is detected. Several works have reported the ternary phase Fe_12-xTi_xY, so in this work the detected ternary compound is treated as Fe_12-xTi_xY. More experimental measurements are required.

4 Thermodynamic models

In the present modeling, thermodynamic assessments on Ti-Y and Fe-Ti-Y system are carried out.The thermodynamic parameters in the Fe-Ti and Fe-Y systems are taken from Refs.[10, 13], respectively. In this work, only the analytical expressions for Gibbs energies of the ternary phases are briefly presented.

Fig.3 Backscattered electron image (BSE) of Fe-Ti binary couple annealed at 1 273 K for 1 200 h

Fig.4 Backscattered electron images (BSE) of Fe-Y binary couple annealed at 1 273 K for 1 200 h

Fig.5 Backscattered electron image (BSE) of Fe-Ti-Y ternary couple annealed at 1 273 K for 1 200 h

Fig.6 Scheme of Fe-Ti-Y diffusion triple at 1 273 K

4.1 Liquid phase

The Gibbs energy of the ternary liquid phase is described by the Redlich–Kister polynomial[17]:

                          (1)

where R is the gas constant, x(Fe), x(Ti) and x(Y) are the mole fractions of Fe, Ti and Y, respectively. The standard element reference(SER) state[18], i.e. the stable structure of the element at 298 K and 10⁵ Pa, is used as the reference state of the Gibbs energy. The parameters denoted as (i, j=Fe, Ti, Y) are the interaction parameters from the binary systems.

The excessive Gibbs energy  is expressed as

=x(Fe)x(Ti)x(Y)[x(Fe)L^L(Fe)+

x(Ti)L^L(Ti)+ x(Y)L^L(Y)]                  (2)

where L^L(Fe), L^L(Ti) and L^L(Y) are the ternary interactive parameters to be optimized.

4.2 Model used for ternary compound

Due to the very few experimental data, the ternary phase Fe_12-xTi_xY is treated as a stoichiometric compound with the formula Fe₁₁TiY. The Gibbs energy for Fe₁₁TiY relative to the pure elements is expressed by

 (3)

where the coefficients A and B are to be evaluated in the present work. The parameters   and  are the Gibbs energies of BCC-Fe, HCP-Ti and HCP-Y, respectively.

Through taking into account of crystal structure and experimental homogeneities, sublattice models are employed to describe the solid solution phase Fe₂(Ti, Y). In accordance with the formula for the sublattice model, the molar Gibbs enrgy of Fe₂(Ti, Y) can be expressed as

           (4)

where x″(Ti) and x″(Y) are the site fractions of Ti and Y in the second sublattice. The two parameters denoted asandare relative to the Gibbs energies of BCC-Fe, HCP-Ti and HCP-Y, respectively, at the same temperature. (i=0, 1) is ternary parameters to be evaluated.

5 Parameter evaluation

The optimization was conducted using the Thermo-calc software package[19]. The critically selected experimental data were processed with a specific weight factor reflecting the experimental uncertainty. The optimization process consists of five steps.

In the first step, the Gibbs energies of the phases in the Ti-Y system were evaluated by reproducing the experimental phase diagram information. In the next step, the thermodynamic parameters for Fe-Ti and Fe-Y systems were slightly modified from Refs.[10, 13], for the compatibility. In the third step, the Gibbs energy of liquid Fe-Ti-Y mixtures as well as the ternary solid solution phases was calculated, to reproduce the experimental information. An excellent agreement between calculations and experimental data from Refs.[15-16] and this work is obtained. In the fourth step, the Gibbs energy functions of ternary compound Fe₁₁TiY were constructed by using the present measurement as well as the literature information. In the last step, all model parameters were assessed simultaneously in a least square optimization to represent the key experimental data within experimental uncertainty.

6 Results and discussion

By adopting the thermodynamic parameters in Refs.[10, 13] for the Fe-Ti and Fe-Y binary systems, and combining the present optimized parameters for Ti-Y system and Fe-Ti-Y ternary phases, the thermodynamic properties and phase diagram of Fe-Ti-Y ternary system can be calculated. Figs.7-9 show the calculated phase diagrams for Fe-Ti, Fe-Y and Ti-Y systems, respectively. Fig.10 and Fig.11 show the present calculated isothermal sections of Fe-Ti-Y system at 873 K and 1 273 K, respectively. Compared with the experimental phase diagram at 1 273 K (Fig.6), the good agreement between the experiments and thermodynamic calculation is obtained. Moreover, the present calculation indicates that the ternary compound Fe₁₁TiY can exist at temperature between 873 K and 1 273 K.

Fig.7 Calculated Fe-Ti phase diagram with thermodynamic parameters from Ref.[6]

Fig.8 Calculated Fe-Y phase diagram with thermodynamic parameters from Ref.[9]

Fig.9 Calculated Ti-Y phase diagram

Fig.10 Calculated isothermal section of Fe-Ti-Y at 873 K

Fig.11 Calculated isothermal section of Fe-Ti-Y at 1 273 K

7 Conclusions

On the basis of the literature information, the thermodynamic parameters for the Ti-Y binary system are optimized. On the basis of the thermodynamic parameters for the Fe-Ti and Fe-Y binary systems, a slight modification is made for compatibility. The Fe-Ti-Y ternary diffusion couples at 1 273 K are prepared and analyzed by optical metallurgy and SEM. The thermodynamic optimization and calculation on the Fe-Ti-Y system are carried out to reproduce the information. The ternary phases Fe₂(Ti, Y) and Fe₁₁TiY are detected by both the present experiments and thermodynamic calculation and the good agreement between the calculation and experiments is obtained.

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Foundation item: Project(50634060) supported by the National Natural Science Foundation of China; Project(50721003) supported by the Creative Research Group of National Natural Science Foundation of China

Corresponding author: CHEN Teng-fei, Tel: +86-731-8830067; E-mail: tengfei@mail.csu.edu.cn

DOI: 10.1016/S1003-6326(08)60252-6

(Edited by YANG Bing)