Determination of thermodynamic stability of FeB monoboride

Abstract

In this paper we investigate the phase composition and phase transformations in the Fe-B system alloys with boron content in the range of 9.0–15.0 wt.%. We use microstructural, X-ray diffraction, differential thermal and durometric analyzes to determine the physical properties of the alloys. The experimental findings show that in the as-cast alloy structure there is Fe₅B₃ phase in small quantities along with FeB monoboride and Fe₂B boride. The Fe₅B₃ phase is formed as a result of the peritectic reaction L+FeB→Fe₅B₃ at the temperature of 1680 K. The eutectic transformation L→Fe₅B₃ +Fe₂B occurs in the boron concentration range of 8.8–10.5 wt.%. After annealing of the Fe-B alloys at the temperature of 1473 K and cooling with the rate of 10² K/s we observe the occurring of the Fe₅B₃ phase. To spot an opportunity of the secondary monoboride formation in the alloys under consideration, we calculate the thermodynamic characteristics of stability of the system.

Accounting for the contribution of the first degree approximation of high-temperature expansion of thermodynamic potential of FeB iron monoboride in a Fe-B binary alloy enables us to study its thermodynamic stability. It is shown that stability decrease of FeB at 1423 K allows suggesting that at this temperature the phase transformation occurs and this fact correlates to the differential thermal analysis results.