Scientists in China have uncovered a previously unreported interface-specific transformation in a bulk nanostructured metallic glass.
Study: An anomalous interfacial structural-compositional rearrangement in a bulk granular nanostructured glass. Image Credit: VAlex/Shutterstock.com
This new understanding provides key insights into the engineering of amorphous materials at the nanoscale.
Writing in Communications Materials, the team reports that interfaces within a Pd40Ni40P20 granular nanostructured glass behave as thermodynamically metastable phases that undergo a distinct structural-compositional rearrangement well below the glass transition temperature.
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In crystalline materials, grain boundaries are known to act as defects or even distinct thermodynamic phases. Whether a similar concept applies to amorphous materials has remained an open question.
Granular nanostructured glasses (GNGs), which consist of amorphous nanograins separated by amorphous interfaces, offer a rare opportunity to examine this problem directly.
These interfaces are known to be loosely packed and structurally disordered, making them effective sites for shear-band nucleation. At the same time, the surrounding nanograins impede shear-band propagation, allowing GNGs to exhibit enhanced plasticity compared with homogeneous bulk glasses.
Despite their importance, the structure, stability, and thermodynamic role of amorphous-amorphous interfaces have been difficult to isolate and control.
Producing the Nanoglass
To address this issue, the researchers developed a two-step fabrication strategy.
First, a homogeneous Pd40Ni40P20 bulk glass was produced and used as a target for inert gas condensation, generating amorphous nanoparticles. These nanoparticles were then allowed to relax for approximately 30 days, during which nickel segregated to the surface over a depth of about 3 nm.
The separation of nickel in this way is attributed to entropy-favoured medium-range-order disordering, which lowers the surface Gibbs free energy.
In the second step, the relaxed nanoparticles were consolidated under a triaxial pressure of 8 GPa. This high-pressure process eliminated porosity and fused the particle surfaces into internal amorphous interfaces, producing a fully dense bulk GNG with millimetre-scale dimensions.
A Distinct Thermal Signature Below Tg
Differential scanning calorimetry revealed an unusual thermal response.
In addition to a previously reported broad exothermic feature associated with a polyamorphous transition, the GNG exhibited a pronounced, narrow exothermic peak beginning at approximately 470-480 K and ending near 530 K, well below the glass transition temperature.
This “TS-peak” corresponds to a heat release of roughly 782 J mol-1, comparable to crystallization energies, despite the interfaces occupying only a small fraction of the material volume.
Electron microscopy and in situ synchrotron X-ray diffraction confirmed that the material remained fully amorphous after the transition, ruling out crystallization or conventional structural relaxation.
Inspecting The Interface
Microscopy and atomic-scale analyses showed that, in the as-prepared state, the GNG consists of Pd-rich nanograins separated by nickel-enriched, low-density interfacial regions with highly disordered atomic packing.
Auto-correlation function analysis confirmed that these interfaces exhibit reduced medium-range order compared with the grains.
When heated through the TS range, the system undergoes a coupled structural and chemical rearrangement that is confined to the interfaces. Medium-range ordering increases, atomic packing becomes denser, and the distribution of cluster connection modes shifts toward more efficiently packed configurations.
At the same time, nickel diffuses from the interfaces into the nanograins, while palladium diffuses outward, driven by the alloy’s thermodynamic preference for chemical homogeneity and its negative mixing enthalpy.
Rather than disappearing, the interfaces progressively converge toward the structure and composition of the bulk glass, demonstrating that they act as metastable amorphous phases capable of transitioning into a lower-energy interfacial state.
Mechanical Properties Rewritten By Interface Control
The interfacial rearrangement directly affects mechanical behaviour.
In the as-prepared GNG, highly disordered, compositionally fluctuating interfaces promote shear-band nucleation, resulting in significantly enhanced plasticity without sacrificing strength.
After heating to the TS range, the rearranged interfaces exhibit reduced free volume and structural heterogeneity. As a result, yield strength and hardness increase, but plasticity is reduced.
This change reflects controlled interfacial evolution rather than embrittlement or crystallization, revealing the role of amorphous interfaces as active structural elements rather than passive defects.
Implications For Amorphous Materials Design
The findings provide strong evidence that amorphous-amorphous interfaces can exist as thermodynamically metastable phases with their own structural states and transition pathways.
By controlling interfacial composition, disorder, and stability, researchers may be able to tune the balance between strength and ductility in metallic glasses more precisely than previously possible.
For materials scientists, the work provides an outline for interface engineering in amorphous systems, one that parallels grain-boundary engineering in crystalline materials but operates through fundamentally different structural principles.
Journal Reference
Fu S., et al. (2025). An anomalous interfacial structural-compositional rearrangement in a bulk granular nanostructured glass. Communications Materials. DOI: 10.1038/s43246-025-01057-x