Break-down of the relationship between α-relaxation and equilibration in hydrostatically compressed metallic glasses

Glasses encode the memory of any thermo-mechanical treatment applied to them. This ability is associated to the existence of a myriad of metastable amorphous states which can be probed through different experimental pathways. It is usually assumed that this memory can be erased in the supercooled liquid, and that this process occurs on a time scale controlled by the α-relaxation. We find that this assumption does not apply for hydrostatically compressed glasses. Annealing under pressure a prototypical metallic glass can irreversibly modify its dynamics, thermodynamics and structure, reduce the atomic mobility and lead to structural modifications of the first coordination shells which reduce the thermal stability with respect to a glass annealed in absence of pressure. When heated above their glass transition temperature, these compressed glasses do not convert into the pristine supercooled liquid, implying the existence of an additional process, beyond the α-relaxation, contributing to the equilibrium recovery of the material. These results establish pressure as a powerful tool for engineering non-equilibrium glassy materials with tailored properties, while deepening our understanding of relaxation dynamics in disordered systems under extreme conditions.

💡 Research Summary

This study investigates how hydrostatic compression and annealing affect the structure, dynamics, and thermodynamic state of a prototypical metallic glass, Pt₄₂·₅Cu₂₇Ni₉·₅P₂₁. Two distinct high‑pressure processing routes were employed at 5 GPa: (i) high‑pressure quenched glasses (HPQG), obtained by cooling a supercooled liquid under pressure (T_comp > T_g(P)), and (ii) high‑pressure annealed glasses (HPAG), produced by heating the glass below its pressure‑shifted glass transition (T_comp < T_g(P)).

Differential scanning calorimetry using single‑shot flash DSC revealed opposite thermal stabilities. HPQG displayed a large enthalpy‑recovery peak, a ~12 K increase in the onset of the glass transition (T_g,onset), and a lower fictive temperature, indicating a more relaxed, kinetically stable state. This aligns with the expectation that the high‑pressure liquid has a higher viscosity, slowing the α‑relaxation. In contrast, HPAG showed a reduced enthalpy‑recovery peak, a ~9 K decrease in T_g,onset, and a higher fictive temperature, signifying a thermodynamically less stable, rejuvenated state despite being processed near T_g.

Synchrotron X‑ray diffraction and pair‑distribution‑function (PDF) analyses were performed on the as‑compressed samples and during subsequent heating at ambient pressure. The first sharp diffraction peak (FSDP) of HPAG shifted to lower q‑values after compression, reflecting an expansion of the medium‑range order (MRO) associated with the first coordination shell (≈2.5 Å). Simultaneously, the FWHM of the FSDP narrowed, suggesting increased local ordering. Upon heating, the FSDP position first contracted sharply near T_g, then followed the usual thermal expansion of the liquid above ~520 K. PDF peaks up to ~9 Å also expanded, whereas at larger distances the structure became more compact, revealing a coexistence of negative and positive thermal expansion within different length scales.

Analysis of the differential PDF highlighted changes in specific inter‑cluster connections. The intensities corresponding to 2‑atom (edge) and 3‑atom (face) connections decreased with temperature in HPAG, while the 4‑atom (tetrahedral) connection increased, implying a loss of icosahedral and P‑centered trigonal‑prism/dodecahedron motifs and the emergence of new octahedral‑like arrangements. This structural reorganization accompanies a reduction in atomic mobility and a permanent modification of the free‑volume landscape.

Crucially, when HPAG samples were heated above their glass transition under ambient pressure, neither their structure nor dynamics reverted to those of the pristine supercooled liquid. This observation contradicts the widely held assumption that the α‑relaxation alone erases any prior thermo‑mechanical history in the supercooled liquid. The results therefore point to an additional, pressure‑induced relaxation pathway that persists beyond the α‑relaxation timescale.

Overall, the work demonstrates that hydrostatic pressure is a powerful tool for engineering non‑equilibrium metallic glasses with tailored properties. Depending on whether the material is quenched from the high‑pressure liquid or annealed below T_g(P), the resulting glass can become either more stable or rejuvenated, with distinct changes in thermal stability, atomic mobility, and medium‑range structure. These findings deepen our understanding of relaxation dynamics under extreme conditions and open new avenues for designing amorphous alloys with customized performance.