Linking Thermal History to Shear Band Interaction and Macroscopic Ductility in Metallic Glasses

Shear band propagation and interaction are critical to the mechanical performance of metallic glasses and are strongly governed by thermal history, yet their microscopic mechanisms remain unclear. Here, using molecular dynamics simulations combined with a state-of-the-art annealing protocol, we systematically investigate these behaviors in a model metallic glass across effective quenching rates spanning six orders of magnitude. Through a double-notch model, we show that the normalized interaction distance relative to the single shear band width is significantly larger in slowly quenched samples than in rapidly quenched ones. Atomic-scale analysis reveals that rapidly quenched samples exhibit a high density of pre-existing soft regions, which trigger correlated shear transformation zones through local vortex fields, resulting in propagation path locking and weak inter-band coupling. In contrast, slowly quenched samples exhibit enhanced structural heterogeneity and a right-shifted activation energy spectrum, promoting a single large-scale vortex field ahead of the shear band front. This field facilitates long-range stress transmission and induces shear band deflection, convergence, and coalescence, a transition resembling a “shielding effect” in fracture mechanics, where vortex-mediated disturbances destabilize the advancing shear band front. Our findings establish a direct microscopic connection between glass stability and shear-band-mediated plasticity and suggest that regulating shear band interactions offers a promising route to enhance the room-temperature ductility of metallic glasses.

💡 Research Summary

This paper investigates how the thermal history of a metallic glass influences shear‑band propagation, interaction, and ultimately macroscopic ductility. Using large‑scale molecular dynamics (MD) simulations combined with a hybrid thermal cycling (HTC) annealing protocol, the authors generate two distinct structural states of a model Zr₅₀Cu₄₀Al₁₀ alloy: a rapidly quenched “H‑Sample” (effective cooling rate ≈10¹⁰ K/s) and an ultrastable, slowly cooled “L‑Sample” (effective cooling rate ≈10⁴ K/s). The HTC method incorporates swap Monte‑Carlo moves to achieve cooling rates comparable to experimental casting, thereby bridging the gap between simulation and real materials.

Both glass states are fashioned into a double‑notch geometry, allowing two shear bands to nucleate from opposite sides under uniaxial tension. By varying the axial distance D between the notches (0–40 nm), the authors control the initial spacing S of the potential shear bands and thus the strength of their interaction. Tensile loading is applied at a constant strain rate of 4 × 10⁷ s⁻¹, and the evolution of atomic shear strain, von Mises stress, and local structural metrics is recorded. Approximately 150 independent simulations (≈70 H‑Samples, ≈80 L‑Samples) provide statistically robust data.

Mechanical response curves reveal that the L‑Sample exhibits a higher elastic modulus, larger yield stress (τ_y), and a higher critical strain to yield, reflecting its denser atomic packing and greater short‑range order (SRO). The H‑Sample, rich in excess free volume, yields at lower stress and shows a more pronounced stress drop (Δτ) after yielding. The authors introduce a normalized stress‑drop parameter Δτ/τ_y as a proxy for post‑yield instability; smaller values indicate a smoother transition to steady‑state flow and thus better ductility.

When Δτ/τ_y is plotted versus the scaled spacing S/δ (δ = characteristic shear‑band width), a non‑monotonic trend emerges for both glass states. An optimal spacing range (approximately 2–3 × δ) minimizes Δτ/τ_y, implying that moderate shear‑band interaction maximizes energy dissipation and strain delocalization. At very small spacings the two bands essentially merge into a single, highly localized shear zone, offering little additional dissipation. At very large spacings the bands propagate independently, again reducing the beneficial interaction. Notably, the spacing that yields the minimum Δτ/τ_y is larger for the slowly cooled L‑Sample than for the rapidly quenched H‑Sample, indicating that the effective interaction length is not governed solely by geometric band width.

Atomic‑scale analysis clarifies this counter‑intuitive result. In the H‑Sample, pre‑existing soft regions are abundant; they act as nucleation sites for shear transformation zones (STZs) that are spatially isolated and linked by small, transient vortex‑like rotational zones. This “path‑locking” mechanism causes the two shear bands to follow nearly identical trajectories with limited mutual influence. The vortex fields are small, so stress transmission ahead of the band front is short‑ranged, and the bands remain largely independent.

Conversely, the L‑Sample displays enhanced structural heterogeneity and a right‑shifted activation‑energy spectrum, leading to the formation of a large‑scale vortex field ahead of each shear‑band front. This vortex field serves as a “shielding effect” analogous to fracture‑mechanics shielding: it destabilizes the advancing front, promotes deflection, convergence, and eventual coalescence of neighboring bands, and transmits shear stress over longer distances. The result is a broader, more diffuse shear zone that can accommodate larger plastic strains before failure.

Temporal snapshots of von Mises strain illustrate that, for the H‑Sample at a spacing of ~3 nm, several discrete STZs (≈4–5) activate sequentially along the band front, producing a discontinuous, stop‑and‑go propagation pattern. In the L‑Sample under the same spacing, a continuous vortex‑mediated shear field dominates, allowing the bands to merge smoothly and broaden. At larger spacings (~8 nm), the H‑Sample still shows limited interaction, while the L‑Sample maintains significant vortex‑induced coupling, confirming that the vortex field extends the effective interaction range beyond the geometric band width.

The authors thus establish a direct microscopic link between glass stability (set by cooling rate) and shear‑band‑mediated plasticity. In ultrastable glasses, large vortex fields enhance long‑range stress transmission, promote shear‑band interaction, and reduce post‑yield stress drops, thereby improving ductility at room temperature. The study suggests that engineering the vortex‑mediated interaction—through thermal treatment, pre‑compression, surface patterning, or compositional design—offers a promising route to overcome the intrinsic brittleness of metallic glasses and to design tougher, more ductile amorphous alloys.