Reheated glass does not forget

A metallic glass is an alloy that conducts like a metal, but has an amorphous structure like a glass. It is typically made by fast-cooling a molten alloy, so that it solidifies before its atoms have a chance to arrange themselves with their preferred crystalline order. Metallic glasses are of interest as engineering materials, because they are strong like metals, but have degrees of elasticity and corrosion resistance, and often have unusual electric, magnetic and mechanical properties.

Pressure is one way to explore these properties, but experiments so far have been few, and often with conflicting results. Some studies have found that, when compressed and then released, the atoms in a metallic glass do not all end up denser and more tightly packed, as one might expect, but apparently the opposite – less packed and energetically more “liquid-like”. Conventional X-ray techniques have failed to fully unmask what is going on.

A team including Beatrice Ruta at the CNRS Institut Néel in Grenoble, also an ESRF visiting scientist, has turned to EBS-powered instrumentation to explain the mystery. The researchers began by rapidly cooling molten alloys of platinum, copper, nickel and phosphorous – together known to be excellent glass-formers – to make them a metallic glass, before subjecting them to pressures of 5–10 GPa. While maintaining this fixed pressure, then they heated and cooled two samples differently: one to above the glass transition temperature (the liquid state) and back, and the other to close-but-below the glass transition temperature. Finally, they released the pressure.

As the earlier results suggested, the sample that was not heated beyond the glass transition temperature ended up seemingly more “liquid like”, thermodynamically speaking. Using high-energy X-ray diffraction (XRD) at the ESRF’s ID15A beamline, as well as X-ray photon correlation spectroscopy (XPCS) at ID10 and fast differential scanning calorimetry in the lab, Ruta and her colleagues were able to show that the sample had actually expanded and contracted on different length scales. This is where simple X-ray diffraction analysis would have been misled. “A [single] structural marker obtained from XRD (the position of the first sharp diffraction peak) cannot properly describe the evolution of the macroscopic density,” says Ruta.

But the real surprise came when the researchers reheated the sample at ambient pressure. Usually, scientists expect a glass to equilibrate and relax back into the original supercooled liquid on passing its glass transition temperature, but Ruta and her colleagues found instead that the atoms in their sample kept a memory of the pressure-treatment process, suggesting the existence of a new, much slower relaxation mechanism controlling the equilibration. “This was extremely surprising, as the loss of the glass history upon equilibration is a paradigm of the glass/liquid state,” she says. “Such slow relaxation phenomena have been discovered in polymers, but were not expected to happen in system with a low degree of structural complexity such as a metallic alloy.”

Ruta, whose work is supported by an ERC Starting Grant, believes the result would not have been possible without the EBS. “The high-energy diffraction at ID15A allowed us to extract the structure of the glass with high resolution, which is necessary to uncover the subtle effects showed here,” she says. “And the EBS upgrade was particularly important for the XPCS technique. With an increase of the coherent flux by two orders of magnitude, we could characterise the atomic mobility with unprecedented quality.”

Although the team did not perform any mechanical tests, Ruta believes there is a possibility that pressure treatment could preserve a metallic glass for longer-term applications. “One big problem with metallic glasses is that they exhibit aging – the spontaneous relaxation of the glass with time – which can lead to fracture and crystallisation,” she says. “Making the glass more ‘liquid-like’, by rejuvenating it for instance with pressure treatments, could help to preserve its amorphous structure.”

Reference:

Cornet, A. et al., Mater. Today DOI:10.1016/j.mattod.2025.12.011

Text by Jon Cartwright