Twinning-Dominated Plasticity and Structural Stability of NiCoCrX_0.1 (X= Mo, Ti) MEA under 52 GPa Pressure

Medium-entropy alloys (MEAs) have emerged as promising candidates for extreme high-pressure applications due to their superior structural stability and mechanical performance. Herein, NiCoCrX_0.1 (X = Mo, Ti) MEAs were fabricated by arc melting, and their high-pressure behavior was systematically investigated via diamond anvil cell (DAC) compression, coupled with in situ synchrotron X-ray diffraction (XRD) and transmission electron microscopy (TEM). In situ XRD confirms both alloys retain a stable single-phase face-centered cubic (FCC) structure without phase transformation up to ∼52 GPa, with monotonic unit cell volume reduction upon compression. Bulk moduli, derived by fitting pressure-volume data to the third-order Birch-Murnaghan equation of state, are significantly higher than those of conventional NiCoCr MEAs, indicating enhanced compression resistance. A pressure-driven deformation mechanism transition is observed: from annealing-twinning-dominated microstructure at ambient pressure to deformation-twinning-dominated plasticity under high pressure, rationalized by pressure-induced reduction in intrinsic stacking fault energy (SFE) and formation of nanoscale deformation twins. This study provides critical insights into the high-pressure response of MEAs, guiding the design of high-performance alloys for extreme pressure environments.