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We investigate the Johari-Goldstein (JG) $\beta$-relaxation process in amodel metallic glass-forming (GF) material (Al90Sm10), previously studiedextensively by both frequency-dependent mechanical measurements and simulationstudies devoted to equilibrium properties, by molecular dynamics simulationsbased on validated and optimized interatomic potentials with the primary aim ofbetter understanding the nature of this universal relaxation process from adynamic heterogeneity (DH) perspective. The present relatively low temperatureand long-time simulations reveal a direct correspondence between the JG$\beta$-relaxation time $\tau_{JG}$ and the lifetime of the mobile particleclusters $\tau_M$, defined as in previous DH studies, a relationship dual tothe corresponding previously observed relationship between the$\alpha$-relaxation time $\tau_\alpha$ and the lifetime of immobile particleclusters $\tau_{IM}$. Moreover, we find that the average diffusion coefficientD nearly coincides with D$_{Al}$, of the smaller atomic species (Al), and thatthe hopping time associated with D coincides with $\tau_{JG}$ to withinnumerical uncertainty, both trends being in accord with experimental studies.This indicates that the JG $\alpha$-relaxation is dominated by the smalleratomic species and the observation of a direct relation between this relaxationprocess and rate of molecular diffusion in GF materials at low temperatureswhere the JG $\beta$-relaxation becomes the prevalent mode of structuralrelaxation. As an unanticipated aspect of our study, we find that Al90Sm10exhibits fragile-to-strong (FS) glass-formation, as found in many othermetallic GF liquids, but this fact does not greatly alter the geometricalnature of DH in this material and the relation of DH to dynamical properties.

Dynamic Heterogeneity, Cooperative Motion, and Johari-Goldstein  $\beta$-Relaxation in a Metallic Glass-Forming Material Exhibiting a Fragile  to Strong Transition