Effects of atomic size mismatch on glass transition decoupling in high-entropy metallic glasses

The calorimetric and dynamical glass transitions are uncoupled in high entropy metallic glasses (HEMGs). That is, the thermal glass-transition or vitrification temperature ( T _g) and α-relaxation temperature ( T_α ) are uncorrelated as a function of alloy stoichiometry. The underlying mechanism that gives rise to this is not understood, nor is the connection to atomic structure. Here, molecular dynamic (MD) simulations of different HEMG model systems are performed to investigate the glass transition behavior. In addition to the high configurational entropy of mixing (for an ideal solution), we show that the effect of mismatch entropy plays a critical role in decoupling vitrification kinetics/ α-relaxation in HEMGs. Microscopically, the atomic size difference contributes significantly to the sluggishness of long-range diffusion and decreased dynamic heterogeneities. This gives rise to a depressed structural α-relaxation. More importantly, we find that HEMGs with the decoupling vitrification kinetics/ α-relaxation can show unique mechanical performance, such as better radiation tolerance and higher resistance to shear banding. Our results provide insights into the high entropy effect of glassy metallic materials, and suggest a means for tuning properties of HEMGs.