Assessment of the Fe-Ti system
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I.
INTRODUCTION
THE Fe-Ti system has been calculated or assessed several times in the past.[1–4] However, the work by Kaufman and Nesor[1] was based on very limited information and is merely a calculation of the phase diagram. Similarly, the thermodynamic treatment by Murray[2] is incomplete. She modeled the Fe-rich and Ti-rich sides of the bcc phase as two separate phases, and she did not calculate several phase boundaries involving the FeTi compound. An unpublished work by Balasubramanian[3] was used by Ohtani and Hillert[5] to calculate the Fe-Ti-N system; but, unfortunately, it is not known upon which data the binary assessment is based. Finally, the present author found that the most recent description of the Fe-Ti system, by Hari Kumar et al.,[4] could not be combined with a recent assessment of the TiC-N system[6] without unacceptable changes for stoichiometric TiC and TiN. As a consequence, it was decided to reassess the Fe-Ti system with an effort to establish good extrapolations into the ternary Fe-Ti-C and Fe-Ti-N systems without deteriorating the reproduction of the binary experimental data. The present work took into account the following phases: (1) (2) (3) (4) (5) (6) (7)
the the the the the the the
solution of Ti in fcc(Fe); solution of Ti in bcc(Fe) and of Fe in bcc(Ti); solution of Fe in hcp(Ti); Fe2Ti Laves phase, denoted as Lav; FeTi compound; liquid phase, denoted as L; and gas phase. II.
EXPERIMENTAL INFORMATION
A. Thermochemical Information 1. Information on FeTi and Fe2Ti The heat capacity of FeTi and Fe2Ti has been measured by Wang et al.[7] from 120 to 700 K using differential scanning calorimetry. The chemical composition was reported as FeTi and Fe2Ti, respectively, and a metallographic analysis showed that the samples were singlephased. On each sample, the results of four measurements were reported. However, for simplicity, the average values were calculated
STEFAN JONSSON, Group Leader-Heat Treatment, is with the Department of Technological Properties, Swedish Institute for Metals Research, Drottning Kristinas va¨g 48, S-114 28 Stockholm, Sweden. Manuscript submitted June 5, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
and are shown in Figure 1. As seen, the data for Fe2Ti show a peak which is caused by a magnetic transition reported to occur between 260 and 315 K. The heat of formation of FeTi has been measured by Kubaschewski and Dench[8] and Gachon et al.[9] using direct reaction calorimetry. In addition, Dinsdale et al.[10] (quoted in Reference 11) reports a value, but no experimental details are available. In the two formal cases, the reaction products were checked with X-ray diffraction. Kubaschewski and Dench[8] reported it to consist almost entirely of FeTi with a small amount of Fe2Ti. Similarly, Gachon et al.[9] found traces of Ti2O, Ti, and Fe2Ti, only. They also made a microprobe analysis, giving a composition of 50.9 5 0.7 at. pct Ti. The experimental results are collected in Table I. The heat of formation of Fe2Ti has been measured by Gachon et al.[9] and by D
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