ANÁLISIS E INTERPRETACIÓN DE RESULTADOS 1 Introducción

In the present work, phase equilibria of binary and ternary systems containing Laves phase have been determined by experiment and/or modeling. The knowledge of phase equilibria is of great importance when developing materials with specific properties.

Many technically relevant materials are composed of about 10 to 15 elements, whereas the study of phase equilibria is often restricted to a maximum of four elements (as it is not easy to visualize with more elements). Nevertheless, it is often possible to predict the formation of certain phases by studying the binary, ternary or quaternary phases as each individual phase will be formed with less than 5 elements. The studies of sub-systems like Cr-Nb, Fe-W, Cr-Fe-Nb and Fe-Nb-Si in the present study are compared with the development and studies of model alloys which can be then in successful cases being produced at an industrial scale.

During the development of new alloys, different compositions of elements are tried or the additions of new elements are studied.

I. Development of laves phase strengthened ferritic steel

During the development of the high chromium ferritic steel Crofer 22H, different additions of elements have been tested in order to check their effect on the properties of new steels, especially oxidation behavior and creep strength. Froitzheim et al. [142] have proposed different additions of element in order to promote the formation of strengthening Laves phase particles. In this study, the direct influence of sub- system phase diagram was pointed out. In the alloy reference Crofer 22 APU, where the ferritic matrix is composed of α–(Fe,Cr), the addition of Nb to Crofer 22 APU results in the precipitation of Nb with Fe and Cr and form Laves phase particles as can be seen on the Cr-Fe-Nb phase diagram (Figure 73).

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Figure 73 – Comparison of the microstructure of model alloy [142] with the Cr-Fe-Nb phase equilibria (present work). The two microstructure images are taken from Froitzheim et al. [142], the left microstructure is the reference alloy (model alloy with a similar composition as Crofer APU) and on the right microstructure of the model alloy with the addition of Nb and Si.

The Laves phase in the newly developed alloy as given by Froitzheim et al. [142] has a similar composition as expected from the Cr-Fe-Nb phase equilibria. Si dissolves in the Laves phase. The formation of the Laves phase is promoted by Si, the comparison with the ternary Cr-Fe-Nb system from the present work is done in a lower order system. As mentioned by the authors [142] “There are limited thermodynamic data available for the Fe-Cr-Nb-Si system” which limits the understanding of the chemical driving forces behind the formation of the different phases. The only article related to the quaternary phase diagram is given by Vilasi et al. [179] but they do not provide a lot of information about Si dissolving in the ternary Laves phase. Nevertheless, according to several studies on the Laves phase, Si promotes the formation of the Laves phase, stabilized it and occupies the position of Fe and Cr in the crystal structure. The overall Laves phase formula is written as (Fe,Cr,Si)2Nb.

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In a similar way, Laves phase strengthening particles can be formed with tungsten (W). Abe et al. [180] have studied the effect of Fe2W on the creep properties of ferritic-martensitic steel. Furthermore in long

term application, the effect of W forming a Fe2W Laves phase allows an improvement of solid solution

strengthening of ferritic-martensitic steel [2].

II. Development of Nb-silicide

Even if the goal of the present study is an improvement of the thermodynamic database for steel, this database could also be used for the development of other materials (i.e. Ni-superalloys, Nb-silicides) as soon as they contain the same elements and the phases of interest as in the present study.

For the replacement of Ni-superalloys, Nb is one of the materials which is attractive for high temperature structural materials but has a poor oxidation resistance.

In order to develop further Nb alloys, several authors have tested different alloy composition as well as their casting process. Zhang et al. [181] have studied the addition of Fe on such alloys in order to improve the oxidation resistance. The addition of Fe into Nb-silicides has promoted the formation of Nb4FeSi

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Figure 74 –Comparison of the Fe-Nb-Si phase diagram (reproduced from Wang et al. [15]) with Fe+Nb- silicide microstructure (micrograph reproduced from [181]). The three phase field region Nb+Nb4FeSi+Nb5Si3 is observed on the micrograph.

III. Phase diagram vs ferritic steel alloy

The present comparison between model alloys and sub-systems of binary, ternary and quaternary alloys is of great importance for the development of new materials. Nevertheless, in addition to the different systems studied in the present work, other systems containing Laves phases are of great importance but are not well studied in the literature. For more clarity, the different sub-systems of Laves phase containing systems are reported in Figure 75. A color code is used to show which systems have been already studied and which systems would need to be studied for better understanding of the Laves phase containing systems in Crofer 22H and similar ferritic steels.