**P. G. Ovcharenko, E. V. Kuzminykh, V. I. Ladyanov**

**ABSTRACT:**

Concentration dependence of the interaction parameter in melts *ε*^{B}_{B} of the system Fe – B is considered. For calculations we used the experimental data summarized in the phase diagram of Fe – B. The phase diagram was been taken from literature. We received the expression for the boron activity coefficient in the melt: *Y*_{B}=1/K_{Fe2B}*x*^{2}_{Fe}*Y*^{2}_{Fe}*x*_{B} considering the eutectic point (* _{ }x_{B}*= 0.17, Т = 1177°С) where the melt to be in equilibrium with the solid boride Fe

_{2}B. The equilibrium constant of reaction of boride (K

_{Fe2B}) formation was calculated on the Δ G

^{o}

_{T}of reaction. Using the first approximation of Ulica the Δ G

^{o}

_{T}of reaction was estimated according to the standard heat ΔH

^{o}

_{298}and entropy ΔS

^{o}

_{298}of the process with a reduction to the standard conditions “pure liquid iron and liquid (metastable) Bor”. To calculate the activity coefficient of iron in the melt (

*Y*

_{Fe}) we took into account in the eutectic point the melt to be in equilibrium with the solid solution of boron in iron. From the equilibrium condition of the solid and liquid solutions of Fe – B (

*Y*

_{Fe}) is believed to be equal one due to the very low concentration of boron in solid solution we got that in liquid solution Y

_{Fe}=0.95. Using the values Y

_{Fe}and K

_{Fe2B}we calculated value Y

_{B}in the melt of eutectic composition. It is equal 0.017 with standard condition “pure liquid Bor”. To recalculate Y

_{B}with temperature of 1177 at the temperature of 1600 we used the ratio ln

*Y*=T1/T2*ln

_{i,T2}*Y*, getting 0.043, that corresponds to the parameter value

_{i,T1}*ε*

^{B}

_{B}equal to 6.0.

*ε*

^{B}

_{B}is assumed to be liner function of

*x*

^{B}

_{B}and take into account that in an infinitely dilute solution

*ε*

^{B}

_{B}=2.5 so at 1600:

*ε*

^{B}

_{B}=2.5 + 21

*x*

^{B}

_{B}. The getting dependence is proved to be very close to the dependence that known from the literature obtained theoretically on the basis of a statistical model of the solution.

**KEY WORDS: **

melts, state diagram, iron–boron system, iron boride, equilibrium, activity coefficient, interaction parameter