Some solid metals exist in various structural forms, a phenomenon known as polymorphism. Iron, for example, adopts one type of cubic lattice (α-iron) at room temperature, but transforms into another (γ-iron) above 912 °C. This is an example of a phase transformation — an abrupt change in the atomic structure of a material that occurs during a gradual change in temperature or pressure. The transformation of γ- to α-iron that occurs when iron alloyed with carbon is rapidly cooled from a high temperature has long been used to produce hard and strong steels; by contrast, pure iron is soft and ductile. Writing in Nature, Meiners et al.1 report that polymorphic phase transformations can also occur at the interfaces between the tiny crystals that make up most pure metals. This discovery suggests a fresh approach for processing metallic materials to optimize their properties for applications.

Read the paper: Observations of grain-boundary phase transformations in an elemental metal

The vast majority of solid metals and their alloys are polycrystals — assemblies of billions of minuscule single crystals called grains, which are separated by grain boundaries. These boundaries are often the ‘weak links’ that cause a material to be brittle and to fracture. However, they can also be used to strengthen materials, because some grain boundaries efficiently block dislocation glide (a fundamental mechanism of plasticity in metals that involves the movement of lattice defects).

About a century ago, it was thought2 that grain boundaries were amorphous layers about one micrometre thick. However, modern microscopy tools have revealed3 that the distorted atomic structure in these boundaries is only a few ångströms thick, comparable to interatomic distances. It is also now known that the atomic structures of most grain boundaries can be thought of as periodic arrangements of certain atomic structural units4 — that is, the grain boundaries can be thought of as two-dimensional crystals that have their own atomic structure, which is very different from the structure of the grains that they separate.

A key question is whether these 2D structures undergo phase transformations that are unrelated to those in adjoining grains. For alloys that consist of two or more components, the answer is a resounding ‘yes’. Such phase transformations have been extensively characterized in experiments, and have been described by theoretical and computational models5,6.

The situation was less clear for pure metals: indirect evidence7,8 hinted that phase transformations are possible at grain boundaries in pure tin and copper, but no direct observations had been made. Observing changes in the atomic structure of a grain boundary is a daunting task, because only minor displacements of atoms at the boundary are required to change its structure, and the atoms move much faster in the boundary than in a grain9.

Meiners et al. now report that grain-boundary phase transformations occur in pure copper. The authors studied several grain boundaries in thin copper films that had been deposited on sapphire substrates, under ultra-clean conditions to exclude any potential effects of impurities. Using a scanning transmission electron microscope, they directly imaged the positions of columns of atoms in the thin-film samples. Their atomic-resolution images reveal the coexistence of two distinct atomic structures in two grain boundaries that had similar geometric parameters, as would be expected during phase transformations. The authors refer to these structures as domino and pearl phases, on the basis of the patterns formed by the atoms within them (Fig. 1).

Figure 1 | Dominos and pearls. Most metals are assemblies of billions of tiny single crystals, called grains, which are separated by grain boundaries — layers of atoms that have a different periodic arrangement from that of the grains themselves. Meiners et al.1 used high-resolution microscopy techniques to investigate grain boundaries in thin copper films deposited on sapphire, and observed two coexisting phases (pictured), which they named domino and pearl phases. The authors also observed these phases in atomic computer simulations (not shown) of a grain boundary that had the same geometry as the ones in the films. They conclude that phase transformations occur in the grain boundaries of copper, and therefore probably also in those of other pure metals. Scale bar, 1 nanometre. (Image taken from Fig. 1 of ref. 1.)

However, this observation alone does not constitute proof of a phase transformation, because one of the two phases could be in a highly unstable state that formed during the deposition of copper film, and that was preserved in the solid film on cooling. The authors therefore obtained further proof using a machine-learning approach (an evolutionary algorithm) in atomic computer simulations of a grain boundary that had the same geometry as the experimentally observed boundaries. The simulations showed that the pearl phase corresponds to the lowest-energy state of the grain boundary, whereas the domino phase is in a metastable state. The simulations also showed that the metastable domino phase is stabilized when stress is applied perpendicularly to the plane of the simulated grain boundary, so that its energy matches that of the stable pearl phase — thereby establishing a true thermodynamic equilibrium between the two phases.

Nanoparticle atoms pinpointed

Meiners and colleagues’ work clearly proves that phase transformations occur in the grain boundaries of pure metals, and thus opens up fresh opportunities for materials design. The number of possible polymorphs of bulk metals is generally limited, but the variety of grain-boundary structures and their possible metastable polymorphs (sometimes referred to as complexions6) is essentially boundless10,11. One can therefore envisage a processing technique that optimizes the overall performance of a material by producing grain-boundary phases (either stable or metastable) that maximize the positive effects of boundaries, but minimize their negative effects. For example, if one could produce grain-boundary polymorphs in aluminium that efficiently block dislocation glide (to maximize mechanical strength) and minimize electron scattering (minimizing electrical resistivity), the resulting metal would be a ‘dream material’ for making wire conductors in overhead power lines — eliminating the need for more-expensive aluminium-based composite wires.

However, it remains to be seen whether the full potential of engineering phase transformations at grain boundaries can be realized in practice. One reason is that it is not clear how processing methods could be designed that produce desired grain-boundary phases. Moreover, a similar concept known as grain-boundary engineering12 — the use of processing methods to obtain grain boundaries that have a desired geometry and properties, without using phase transformations — has so far yielded only modest practical results.

Another issue is that the large number of possible grain-boundary polymorphs will make it difficult to systematically determine polymorph properties. High-throughput computational methods based on machine learning and big data will be of help here1,13. Indeed, Meiners and colleagues’ work is a promising example of how the synergistic combination of high-resolution microscopy techniques and computational methods can lead to conceptual breakthroughs in the study of grain boundaries.