The Transition from Strain Softening to Strain Hardening in Metallic Glasses

Despite their excellent mechanical properties, metallic glasses (MGs) are significantly hindered by poor plasticity and toughness, which are essential for structural applications. The brittleness arises from the rapid propagation of shear bands (SBs), leading to strain softening and catastrophic failure. Recent advancements in microstructural engineering, particularly boundary engineering, such as nano-glass, focus on the utilization of heterogeneous structures to promote the proliferation and delocalization of SBs. Various attempts have been made experimentally to address these issues, but with very limited improvement in tensile strength and toughness. Under tensile loading, micro- or nano-pillar samples exhibit strain softening and continue to undergo plastic deformation after reaching yield or peak stress, especially the nano-glass micro-pillar. Reports on tensile strain-hardening in MG micro-pillars are rare. In this finite element simulation study, we optimize appropriate statistical and spatial distributions of free volume within the microsamples. Both the post-yield strength and the mean tangent modulus increase with progressive gradient structural modifications, thereby inducing a transition from strain-softening to strain-hardening behavior, as well as a concurrent transition from plastic fracture to brittle fracture. We systematically investigate the deformation mechanisms and transition mechanisms of fracture modes, which are closely associated with heterogeneous microstructures and their evolution in MGs. These insights into the transition mechanism could significantly facilitate the design and optimization of MGs to achieve enhanced toughness and strain hardening.