Abstract
The Solubility of nitrogen and the equilibrium of the Ta-nitride forming reaction in liquid Fe-Ta alloys under the presence and absence of Ta-nitride were studied by the sampling method at 1600, 1650 and 1700°C in the concentration range up to 25% Ta. Some thermodynamic interpretations of the atomic interactions in the liquid Fe-Ta-N system were also studied. The results obtained are summarized as follows.
1) Tantalum decreased the activity of nitrogen in liquid iron and the interaction coefficient of tantalum could be expressed as the following quadratic function of tantalum concentration:
2) When an improved model of statistical thermodynamics, which may be applicable for actual solution involving short range order was applied to the results, the theoretical values could be estimated to be in good agreement with the experimental values.
3) In ternary iron alloy systems where the alloying constituents were transition elements j, a definite correlation was found between eO(j) and eN(j) asfollows.
eO(j)times;102=0.278[eN(j)]2×104+0.994
4) The tantalum nitride which appeared in the liquid Fe-Ta-N system was identified as hexagonal TaN (η) by Debye-Scherrer X-ray diffraction analysis. The standard free energy of decomposition was determined as
TaN(s)=Ta+ N
ΔG=70500-35.7T±260(cal)
5) The values of γX(0) in liquid Fe-X binary system were derived by thermodynamic calculation and found a tendency to being more negative for the smaller atomic number elements in IV-a and V-a groups.
6) It was found that there was a correlation between the bonding characteristics of nitrogen with V-a group elements in liquid iron and those in austenite
