Abstract: Adding Al is an important strategy to obtain ultrahigh specific strength in BCC refractory high-entropy alloys (RHEAs). However, the main structure typically transitions from disordered BCC to ordered B2 with increasing Al concentration, leading to poor ductility. In the present study, a phase inversion in a high-Al-content B2-RHEA (Zr₄₀Ti₂₈Nb₁₂Al₂₀) was systematically studied through thermo-mechanical treatment. The grains of the single B2 phase transformed inversely to the BCC + B2 microstructure with a dispersion of spherical B2 precipitates in the BCC grains. The evolution of the microstructure began with the decomposition of the B2 phase into Al-rich and Al-poor regions. The subsequent coarsening of the Al-rich B2 precipitates continuously consumes Al and Zr atoms from the solution. The depletion of Al and Zr in the matrix drives it to gradually form the disordered BCC structure and eventually transform to a single BCC phase matrix. This phase inversion enhanced tensile ductility of the RHEA while still maintaining its high specific strength. The current study provides a novel idea for inhibiting Al-induced brittleness of RHEAs at high Al content.

Key words: High-entropy alloys, Tensile properties, Ordered B2 phase, Strengthening mechanism, Phase inversion