The peculiarities of formation and thermodynamic functions of τ-phase

Анотація

It is shown that for alloys with boron content of 0.1–6.5% (wt.) and carbon content of 0.3–4.0% (wt.) without pretreatment no formation of cubic boron carbide takes place under crystallization. The cubic boron carbide can be obtained by pre-annealing at a temperature of 1173 K for an hour and further heating to a temperature of 30 K above the liquidus and cooling of alloys with boron content of 2.5–4.0% (wt.) and carbon content of 0.8–3.0% (wt.). Formation of crystals of cubic boron carbide is possible as a constituent of multiphase inclusions for alloys with boron content of 0.1–0.3% (wt.) and carbon content up to 0.4–0.5% (wt). It should be noted that for alloys with boron content of 4.2–6.0% (wt.) and carbon content of more than 3.0% (wt.) the pretreatment does not result in formation of cubic boron carbide. The increase in boron content in the alloy to 0.3–0.5% (wt.) and carbon content to 0.5–0.7% (wt.) leads to formation of the eutectic α-Fe+Fe₂₃(CB)6, which is arranged on the boundaries of pearlite grains.

The thermodynamic functions of Fe₂₃(CB)₆ cubic boron carbide are derived for the first time using the Hillert and Staffonsson model and accounting for the first degree approximation of high-temperature expansion of the thermodynamic potential for binary alloys. We obtain temperature dependences of such thermodynamic functions for Fе₂₃(CB)₆ phase as Gibbs energy, entropy, enthalpy and heat capacity CP , as well as calculate their values at the formation temperature of the phase. The approach used in this paper enables to give the most complete from the thermodynamic point of view description of cubic boron carbide formed from a liquid.