Atypical homogeneous rheology of a high-entropy metallic glass challenges standard free volume models

Metallic glasses (MGs) exhibit exceptional mechanical properties, but their application is often limited by brittleness. At elevated temperatures near the glass transition (T_g), they undergo homogeneous viscoplastic deformation, a regime commonly described using free volume (FV) theory. Despite its prevalence, the quantitative accuracy and applicability of FV models, particularly for transient behaviors, remain under investigation. This study examines the homogeneous rheology of a LaCeYNiAl high-entropy MG (HEMG) between 475 K and 490 K, and critically assesses the relevance of two prominent FV model formulations. Experimental characterization includes dynamic mechanical analysis and uniaxial tensile tests across various strain rates. The tensile data are subsequently analyzed using two elasto-viscoplastic constitutive frameworks incorporating distinct FV evolution kinetics: Spaepen’s original formulation (model 1), and the bimolecular annihilation kinetics proposed by Van den Beukel/Sietsma (model 2). Our analysis reveals that model 1, when applied to steady-state flow, yields physically inconsistent negative parameters, calling its validity for homogeneous deformation into question. Model 2 demonstrates better qualitative agreement with the experimental stress-strain curves but still fails to accurately reproduce the stress overshoot features. Moreover, fitting model 2 requires unphysically low Young’s modulus values and produces unusual negative apparent activation energies for key kinetic parameters, suggesting limitations in the model structure (e.g., neglecting explicit viscoelasticity) or possibly unique behavior in HEMGs. These findings highlight significant shortcomings of standard FV models in quantitatively capturing the homogeneous deformation of this HEMG, particularly its transient characteristics, and underscore the need for more refined constitutive descriptions.