On the stability of the V5B6-phase

The currently assessed V–B phase diagram indicates a peritectoid formation at 1727 °C (V₃B₄+VBV₅B₆) for the V₅B₆ boride. The observation of this phase in several as-cast V–B alloys has lead us to a systematic evaluation of its stability, specially its possible formation from the liquid. In this work, V–B alloys (52–56 at.% B range) were produced through arc melting and heat-treated under high vacuum at 2000 °C for 2 h. The materials in both as-cast and heat-treated conditions were characterized through scanning electron microscopy (SEM) (backscattered electron (BSE) images) and X-ray diffraction (XRD). The features observed in the as-cast samples allowed us to conclude that this phase should be formed from the liquid through the L+V₃B₄V₅B₆ peritectic reaction. In spite of not reaching equilibrium condition during heat-treating, the results from the characterization of heat-treated samples have indicated the stability of the V₅B₆-phase at 2000 °C, in disagreement with the currently accepted V–B phase diagram.