Experimental and thermodynamic investigation of the microstructural evolution of a boron-rich Fe-Cr-Nb-B alloy

Wear and corrosion resistant amorphous/nanocrystalline Fe-Cr-Nb-B coatings have been successfully synthesized by high velocity oxygen fuel in previous works. These coatings show excellent wear resistance and superior corrosion resistance. However, the phase evolution during cooling of the new alloys in the Fe-Cr-Nb-B system is not well understood. In addition, commercial thermodynamic databases for Fe-based alloys do not fully describe the system at the composition ranges of interest. In the present work, the phase evolution of as-melted Fe 51 Cr 10 Nb 20 B 19 alloy was investigated by combining detailed microstructural characterization with thermodynamic evaluation using a database developed for the Fe-Nb-B system. The solidification path starts with the formation of (Fe,Cr,Nb) 3 B 2 diboride phase, followed by the formation of austenite at lower temperature. On further cooling, the remaining liquid transforms eutectically to (Fe,Cr)NbB boride and austenite. In the solid-state region, the austenite transform allotropically to ferrite. At even lower temperature, there exists a peritectoid-like transformation, where the (Fe,Cr)NbB boride nucleates both at ferrite grain boundaries and around the existing (Fe,Cr,Nb) 3 B 2 diboride. Thermodynamic calculations showed good agreement with experimental observations, opening new possibilities to guide future alloy development in the Fe-Cr-Nb-B quaternary system.