Understanding the phase stability and transformation kinetics of multi-principal element alloys (MPEAs) under extreme conditions is critical for optimizing their performance under extreme conditions such as high-temperature and high-pressure environment. In this work the high pressure-temperature (p-T) phase diagram and solid-liquid transition mechanism of an equiatomic NiCoCr alloy are investigated based on embedded atom method (EAM) potential, through advanced molecular dynamics (MD) simulation combined with enhanced sampling techniques. In order to overcome the timescale limitations of traditional MD in capturing phase transitions as rare events, a hybrid approach integrating well-tempered metadynamics (WTMetaD) and the on-the-fly probability-enhanced sampling with expanded ensembles is used in this work. Collective variables such as enthalpy per atom S_H, and two-body entropy S_S are used to explore the polymorphic states of the NiCoCr alloy. The crystallinity s_env, potential energy U, and volume V are utilized to drive phase transitions, and sampling configurations are performed in the range of 1550–1750 K and 0–10 GPa by using multithermal-multibaric-multiumbrella simulation.

Several key results about liquid-solid phase transition in NiCoCr alloy are obtained as follows.

1) Phase diagram prediction. NiCoCr alloy exhibits a stable body-centered cubic (BCC) phase under high-pressure condition (e.g. 10 GPa) at elevated temperatures (up to 1750 K), rather than a face-centered cubic stable (FCC) phase at room temperature and ambient pressure. The solid-liquid coexistence line shifts upward with the increase of pressure, raising the melting temperature from ~1400 K (ambient pressure) to about 1750 K (over 10 GPa).

2) Free energy landscape. The free energy curves corresponding to different thermodynamic conditions are obtained using reweighting techniques and block averaging methods, which reveal that the increase of pressure and decrease of temperature can reduce the free-energy difference ΔG_L→BCC, while simultaneously increasing G_ \mathrmBCC \to\mathrmL^* required for melting. The combined effects of these changes enhance the stability of the BCC phase in NiCoCr under high-temperature and high-pressure condition.

3) Activation parameters and kinetic mechanism. For the activation parameters of solid-liquid dynamic mechanics, S_\mathrmL \to \mathrmBCC^* of NiCoCr alloy decreases with the increase of temperature and the decrease of pressure ( from (–4.32 ± 0.16) J·mol^–1·K^–1 at 1550 K to (–6.71 ± 0.48) J·mol^–1·K^–1 at 1750 K, 0 GPa ), and | V_\mathrmL \to \mathrmBCC^* | increases with temperature increasing and pressure decreasing ( from (–88.21 ± 2.57) ų at 0 GPa to (–26.09 ± 6.35) ų at 10 GPa, 1600 K). At constant temperature, increasing pressure lowers S^* sensitivity to temperature change, whereas higher temperatures amplify pressure’s role in reducing | V_\mathrmL \to \mathrmBCC^* |, the change of pressure has no significant effect on V_\mathrmBCC\to \mathrmL^* .

These results demonstrate that the synergistic effects of pressure and temperature on S^* and V^* dictate the phase stability and transformation kinetics of NiCoCr alloys under extreme conditions. The predicted p-T phase diagram and quantitative activation parameters provide critical ideas for designing MPEAs with tailored microstructures for high-pressure applications. Limitations of the EAM potential in describing magnetic interactions and non-equilibrium states are discussed, and the necessity of of future validation through first-principles calculations and high-pressure experiments is emphasized.