Multi-BCC Heterostructure Formation in Equiatomic TiNbZrTa Alloy: EBSD/EDS Evidence and CALPHAD Analysis

Refractory multiprincipal alloys offer the possibility to adapt microstructure and phase stability for service in environments that demand excellent performance. These alloys primarily stabilize BCC solid solutions while avoiding brittle intermetallics. Their stabilization, however, depends not only on configurational entropy but also on mixing enthalpy, atomic size mismatch, and non-equilibrium solidification routes. In the present work, the microstructure and phase stability of an equiatomic TiNbZrTa alloy produced in a water-cooled Cu mold were evaluated by XRD/Rietveld, EBSD/EDS (including KAM), and Calphad simulations (equilibrium one-axis sections and Scheil classic). The results revealed the coexistence of three compositionally distinct BCC phases with different morphologies and chemical composition. One-axis phase diagrams were used to identify single-phase BCC regions, whereas Scheil simulations were employed to analyze the non-equilibrium phase formation. In addition, Vickers microhardness tests revealed variations among microstructure, showing positive correlations with KAM and grain refinement, consistent with strengthening mechanisms involving lattice distortion, solid solution, and BCC1/BCC2/BCC3 interfaces. The equiatomic TiNbZrTa alloy forms BCC heterostructures from the liquid state, with the potential to tune phase fractions and microstructural length scales without forming brittle intermetallic second phases.