Effect of synthesis route on the microstructure, phase stability and properties of the TiNiNbMo high-entropy alloy

In this study, the phase stability of the equiatomic TiNiNbMo system was investigated. Thermodynamic screening via HEAPS suggested competition between entropic stabilization and enthalpic ordering, while CALPHAD simulations explicitly predicted a dual-phase equilibrium (BCC + B2) at 1273 K. To test these predictions, the alloy was synthesized via solid-state mechanical alloying (MA) and liquid-state arc melting (AM). Despite thermodynamic predictions, MA yielded a non-equilibrium, metastable single-phase nanocrystalline BCC solid solution after 10 h of milling. This metastable state was retained because the low-temperature processing restricted the long-range atomic diffusion required for intermetallic precipitation, even as the system accumulated severe lattice microstrain (≈ 1.24%). The metastability was experimentally validated by annealing at 1273 K, triggering the solid solution's decomposition into a BCC matrix and stable intermetallics (NbNi). This was accompanied by significant lattice relaxation and a reduction in thermal conductivity due to phonon scattering at phase boundaries. Conversely, liquid-state AM resulted in a highly segregated multiphase microstructure comprising a primary BCC solid solution and a Laves phase, yielding a microhardness of 869 ± 178 HV. These findings demonstrate that while TiNiNbMo is thermodynamically driven to form ordered phases, solid-state processing provides a unique kinetic pathway to access metastable single-phase microstructures by suppressing precipitation.