Cuboid-like nanostructure strengthened equiatomic Ti–Zr–Nb–Ta medium entropy alloy

This study investigates the effect of annealing treatment on the phase transformation and mechanical properties of the equiatomic TiZrNbTa MEA from room temperature to 1200 °C. After annealing at 1200 °C for 24h, the single solid-solution body-centred cubic (BCC) phase in the as-cast Ta₂₅Zr₂₅Nb₂₅Ti₂₅ transformed into an extremely high number density (~10³/μm²) of Ta–Nb-rich BCC nanocuboidal phase (28 ± 10 nm square, BCC1) and a nanostrip-like Zr-rich BCC phase (3 ± 2 nm thick, BCC2). The phase separation from BCC to BCC1 and BCC2 arises from two primary reasons: (i) the high positive mixing enthalpy of both Ta–Zr and Nb–Zr (strong tendency to separation between each pair), and (ii) the 3–4 orders of magnitude higher mobility of Zr than Ta, Nb and Ti in these MEAs (kinetically driven). Detailed CALPHAD simulations of phase formation in this MEA agreed with experiments and provided insightful phase transformation details. The calculated diffusion distance of Zr (~4.1 nm) from the CALPHAD data corresponds to the measured Zr-rich nanostrip thickness (3 ± 2 nm). The nanocuboidal BCC1-BCC2 structure exhibited 112<111>-type of twinning deformation under compression at room temperature. The Ta₂₅Zr₂₅Nb₂₅Ti₂₅ MEA retained yield strength of ~410 MPa at 1000 °C and ~210 MPa at 1200 °C. The phase transformation during cooling after annealing and the microstructural evolution during compression at temperatures from 600 °C to 1200 °C were characterized and discussed in detail.