PHYSICAL REVIEW B 103, L140107 (2021) Letter Origin of strain hardening in monolithic metallic glasses X. Yuan , 1 D. ¸ Sopu , 1, 2, * and J. Eckert 1, 3 1 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstraße 12, A-8700 Leoben, Austria 2 Technische Universität Darmstadt, Institut für Materialwissenschaft, Fachgebiet Materialmodellierung, Otto-Berndt-Straße 3, D-64287 Darmstadt, Germany 3 Department of Materials Science, Chair of Materials Physics, Mountanuniversität Leoben, Jahnstraße 12, A-8700 Leoben, Austria (Received 9 January 2021; accepted 13 April 2021; published 29 April 2021) To overcome the brittleness of metallic glasses (MGs), their structure, chemistry, or loading conditions are usually controlled. Here, the local stress state in MGs was modulated without affecting their structure. The elastically designed MG heterostructures provide enhanced ductility together with strain hardening during loading. The stress heterogeneity leads to shear band multiplication that consequently enhances the macroscopic ductility of MGs. In addition, the residual compressive stress significantly increases the strength of the glass and is responsible for the observed strain hardening. DOI: 10.1103/PhysRevB.103.L140107 Enhancing the plasticity [1–4] and overcoming the strain softening of metallic glasses (MGs) remain long-standing issues in materials physics. A common strategy to prevent brittle catastrophic failure at room temperature is the synthe- sis of MG composites with soft crystalline phases [5,6]. In general, a secondary phase improves the ductility of MG com- posites but compromises their strength [1]. To overcome the inverse strength-ductility relationship, MG composites con- taining shape memory crystals were successfully developed. Here, hardening through deformation-induced martensitic transformation of the incorporated crystals overwhelms the shear-banding-induced softening of the glassy matrix [7]. Additionally, the suppression of shearing through size or ge- ometric constraints, as demonstrated for nanosized samples [8,9] or notched rods [10], may also lead to the apparent strain hardening. Recently, it has been shown that strain hardening and enhanced ductility can be also achieved in monolithic MGs even without limitations in size or mechanical constraints when they are highly rejuvenated [11]. Generally, extreme rejuvenation of MG structures by predeformation techniques lowers their yield stress and can in special cases enable strain hardening. This exceptional strain hardening was associated with structural relaxation, which can be regarded as a return from the rejuvenated state. Nevertheless, if so, then a similar phenomenon must be observed in MG structures subjected to rejuvenation by thermal cycling, elastostatic loading, or other rejuvenation techniques that leads to an even stronger rejuvenation effect comparable to that seen after heavy plastic deformation [12]. Furthermore, structural rejuvenation in- volves plastic strain and shear band formation that is usually confined to a thickness of 10-20 nm [13]. Consequently, even the proliferation of a large number of shear bands would result in a low volume fraction of rejuvenated material with respect to the total volume of the sample. Hence, considering the * Daniel.Sopu@oeaw.ac.at possibility of returning from a highly rejuvenated state to an even more aged state could not fully explain the observed strain hardening. However, while the formation of a high den- sity of shear bands can safely explain the observed extensive homogeneous flow, we assume that the strain hardening arises not from intrinsic changes to the glassy structure, but rather from residual stresses imparted during the predeformation protocol. During triaxial tests the formation of a large number of intersecting shear bands is actually the driving force for the local stress modulation between these shear bands that could span over several micrometers [14]. This, in turn, results in the generation of a stress gradient within the sample [14] and the change in the sign of stress from compressive to tensile across shear bands [15]. Consequently, all these could alter the further shear-banding process and enable strain hardening in uniaxial tensile tests. Here, we rely on computational modeling to provide an atomistic model to clarify what causes enhanced tensile duc- tility and strain hardening in monolithic MGs. By modulating the internal stress state and the local structure of the glass, we differentiate between the role of stress heterogeneities, residual stresses, and structural heterogeneities in improving the mechanical properties of MGs. Additionally, we focus our attention on clarifying the origins of strain hardening in monolithic MGs as one of the most debated aspects of the last decades. To provide an atomistic picture of the deformation mech- anisms of stress and structurally modulated MG heterostruc- tures, large-scale molecular dynamics (MD) simulations were carried out using LAMMPS software [16]. The Cu 64 Zr 36 MG was used as a prototype material, and the interatomic inter- actions were described by the Finnis-Sinclair-type potential developed by Mendelev et al. [17]. The starting liquid struc- ture was obtained by randomly distributing 0.9 × 10 6 atoms in a box of 5.6 × 23 × 113 nm 2 with periodic boundary condi- tions (PBCs) in all directions. Uniform-acceptance force bias Monte Carlo [18] and MD simulations in an NPT ensemble were used alternately, in order to deal with the overlapping 2469-9950/2021/103(14)/L140107(5) L140107-1 ©2021 American Physical Society