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New research at the University of Oxford reveals how disordered materials – substances with amorphous or glassy structures, as opposed to ordered crystals – accommodate deformation. The authors revealed that inhomogeneity of deformation in these materials is present right down to the scale of interatomic bonds that undergo rotation. This paves the way to a better understanding of how materials deform and fracture under load.

The Harwell Campus from the air (Courtesy of Harwell Campus)
The Harwell Campus and Diamond Light Source from the air (Courtesy of Harwell Campus)

The study, led by Prof Alexander Korsunsky from the Department of Engineering Science in collaboration with colleagues from CERN and Diamond Light Source, used the powerful X-ray facilities at Diamond to examine the deformation of amorphous silica, a material that is present in many systems and devices, and is the main component of many glasses and other materials, such as porcelain used in dentistry to make life-like looking veneer on artificial teeth. The researchers looked deep into the material to discover how it responds to loading at scales so small it can only be ‘seen’ by X-rays. At this level, intricate deformation effects take place that are not observable at the macro scale until a catastrophic failure (such as shattering) takes place.

A vast list of chemical, physical and mechanical properties of materials depend on their deformability – the ways in which external influences force the atoms to change their relative positions, often in subtle ways. Strain is a crucial measure of materials deformation for evaluating and predicting the mechanical response, strength, and fracture of a material.

Like many natural and man-made materials, fused silica is ‘disordered’; it does not have the crystal lattice structure found in metals and ceramics, and hard materials such as diamond. It is relatively easy to predict how crystalline materials reacts to strain, because the well-defined structure that persists right down to the nanoscale imposes orderly collective deformation modes. The presence of regular atomic planes means that they move in an organised, systematic and coherent way; in response to strain the planes ‘pull apart’ or get closer by stretching or compressing the interatomic bonds.

However, the behaviour of amorphous materials such as silica is much less predictable. They contain regions that are not organised in a uniform way, and so have no ‘long-range order’, making it almost impossible to predict the location of atoms in the material. Different regions, down to atomic scales, may have more or less flexibility, which means that strain may be accommodated in a different manner to crystalline materials.

It is a huge challenge to investigate behaviour at this level without destroying the material, even with advanced atomic resolution methods such as aberration-corrected STEM. However, synchrotron X-Ray facilities at Diamond Light Source offer a way to do this by looking at the material’s scattering signature known as the Pair Distribution Function (PDF). The researchers discovered that even under moderate loads silica was able to deform under strain not only by stretching interatomic bonds, but also by changing the local angles between them – something that the researchers refer to as ‘scissoring action’. At short ranges below about 10Å this proved to be the main mechanism by which this material was able to undergo deformation. Careful interpretation of the X-ray scattering data revealed that this must happen more in some locations than others, so that strain was inhomogeneous right down to the atomic level. It became clear that the near-neighbour bond lengths change on average much less than the distances between atoms that are further apart – a crucial distinction between short range and long range modes of strain accommodation.

The powerful technology available at Diamond Light Source was a key factor in the success of this research. Whilst well-known microscopy techniques exist to examine arrangements of atoms which are orderly (for example, in crystals), it is much harder to understand disordered atomic arrangements in materials that are amorphous, such as bulk metallic glasses, polymers, and other systems. This requires using cutting edge large scale facilities at Harwell, and complementing them with state-of-the-art electron microscopy instruments in Professor Korsunsky’s MBLEM lab at Oxford. "Oxford has a long tradition of ‘mechanical microscopy’ which goes back almost 400 years ago, when Robert Hooke was laying down the foundations of elasticity, whilst at the same time developing high resolution imaging approaches that we continue to build on to this day," Professor Korsunsky said.

Read the full study, On the origins of strain inhomogeneity in amorphous materials, in Nature Scientific Reports at https://www.nature.com/articles/s41598-018-19900-2

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