Metallic glasses are formed by cooling melted alloys under conditions that prevent the melt from crystallizing1. They have remarkable mechanical properties — in particular, they can be subjected to high forces and undergo a large amount of deformation before they stop behaving elastically and start to deform permanently (plastically). However, they have one key weakness: they are prone to catastrophic failure under stress because they soften during plastic deformation, rather than hardening, as crystalline metals do. Writing in Nature, Pan et al.2 report a method for preparing metallic glasses that causes them to harden during plastic deformation, thereby avoiding the instabilities that lead to failure.

Read the paper: Strain-hardening and suppression of shear-banding in rejuvenated bulk metallic glass

If you take a paper clip and bend it, you’ll find that more force is needed as you bend it to an increasingly sharp angle. This is an example of work, or strain, hardening — the strengthening of a material through plastic deformation. At the atomic scale, the plastic deformation of metallic crystals in the wire is caused by the motion of ‘dislocations’. These linear defects in the crystal structure multiply, intersect and entangle as deformation proceeds, thereby getting in each other’s way and strengthening the material3. This makes work hardening one of the most complex problems in science: it needs to be understood at many length scales, from the atomic-scale lengths of the dislocation cores, through the nano- and micrometre scales involved in dislocation interactions and structures, to the macroscale lengths associated with crack propagation and the structural stability of bulk materials.

The mechanical behaviour of metallic glasses is fundamentally different. Because their atomic structure is not periodic, there are no dislocations. Plastic deformation instead occurs through shear, a mode of deformation that affects small groups of atoms (known as shear transformation zones; STZs) throughout the glass4. This shearing loosens (dilates) the atomic structure, and the resulting increase in volume facilitates the formation of new STZs. If the rate of deformation is sufficiently high, the atomic structure does not have time to relax and densify again. As a result, the local deformation rate continues to rise and finally becomes unstable, forming a narrow zone of intense shear strain (a type of deformation) known as a shear band.

Strength ceiling smashed for light metals

Shear bands are macroscopic phenomena. They cause steps to form on the surfaces of materials and can therefore be suppressed by applying appropriate constraints — for example, by sandwiching metallic-glass layers between conventional hard metals5. Pan et al. accomplish this suppression by cutting a deep, narrow notch around the circumference of a cylindrical glass bar, and compressing it in the direction of its axis (see Fig. 1a of the paper). The central region of the bar near the notch undergoes extensive plastic deformation, during which shear bands are suppressed by the constraints exerted by the outer parts of the bar. The authors then cut out the central part and deformed the unconstrained sample under tension or compression. Remarkably, the resulting material exhibits properties similar to those of conventional crystalline metals: it undergoes work hardening and does not form shear bands.

The mechanism responsible for this hardening, however, is far from conventional. To explain why, let’s consider the ground states of crystals and glasses. A crystal in its undeformed ground state has the lowest possible flow stress (a measure of the force needed to sustain plastic deformation). The introduction of dislocations during deformation costs energy, and their entanglement raises the flow stress3. A glass in its ground state, however, has the highest possible flow stress because it has the lowest number of STZs. Deformation of this state costs energy, but through shear-induced dilatation introduces new STZs that lower the flow stress (see ref. 6, for example).

All glasses are in non-equilibrium states. When they are heated (annealed) to a temperature at which their atoms can move, the process tightens up their atomic packing and lowers their energies towards a ground state7. This process is called structural relaxation, or ageing, and it changes the properties of glasses8. For example, it can increase the density by a few tenths of a per cent; raise the elastic stiffness by a few per cent; increase viscosity by many orders of magnitude; and sometimes cause ductile glasses to become brittle.

Atomic-scale hardening mechanisms apply on larger scales in ‘architected’ materials

Reversal of this ageing process is called rejuvenation, and can be achieved in several ways. The simplest is to heat a glass until it becomes a liquid again, and then rapidly cool it1. Another approach is to ‘shake up’ the structure, for example by ion irradiation9 or plastic deformation10. By heavily deforming samples of metallic glasses under constrained conditions, Pan et al. raise the energies of the glasses far above the energy of the ground state, rejuvenating them and loading them up with STZs. When the authors then deform them under the less-constrained conditions of a tensile or compressive test, structural relaxation sets in: the atomic packing increases and the volume introduced by the earlier deformation disappears; the number of STZs drops, causing the flow stress to increase; and work hardening is achieved.

The practical implications of this work are clear: if metallic glasses can be treated so that the threat of shear-band failure is greatly reduced, then they can be more fully exploited for structural applications. However, this will require the development of methods for rejuvenating large volumes of metallic glasses — Pan and colleagues’ rejuvenated samples are only 3 millimetres long and 1.5 mm in diameter. Large-scale rejuvenation will require the deformation of large quantities of alloys under constrained conditions, which could be achieved using methods such as confined cold rolling10 or equal-channel angular extrusion11.

The authors’ rejuvenation technique might also advance glass science. Because glasses are not in equilibrium, their properties depend on the processing path by which a particular state is reached. For example, in their experiments, Pan et al. measured the heat of relaxation of their glasses (a measure of the glasses’ internal energy) after rejuvenation and after various stages of subsequent deformation. It would be interesting to know how the structure and other properties of their glasses compare with those of glasses that have the same heats of relaxation, but which were obtained by the cooling of melted material and annealing. In other words, what makes the authors’ rejuvenation technique attractive is that it opens up many more paths for exploring the complex relationship between structure and properties in glasses.