Mesoscale martensitic transformation in single crystals of topological defects



A martensitic transformation is a transformation that involves diffusionless, and concerted atomic motion. Martensitic transformations are common in metals; they are key to understanding and controlling phenomena such as shape memory, which is encountered in metallic alloys, or hardening of Fe-C alloys by quenching. A recent study by the de Pablo and Nealey groups, has found that martensitic-like tranformations can also occur in liquids. How can a fluid, where molecules are constantly moving, exhibit a ‘diffusionless’ transformation?  The answer to this question is found in intriguing, liquid-crystalline states of matter known as blue-phases.

Blue-phase liquid crystals (BPs) are states of matter that exhibit fluid properties, but are highly ordered on two distinct sets of length scales. At molecular length scales, they exhibit long-range orientational order. At micrometer length scales, chirality gives rise to networks of topological defects that arrange themselves into a crystalline cubic symmetry. In these materials, the molecules are highly anisotropic, and they are constantly moving, but the preferred molecular orientations depend on their position within the unit cell, whose size is on the order of a few hundred nanometers. In order to analyze the transformation between BPs with different crystalline symmetries, researchers from the Nealey and the de Pablo groups developed a strategy to control the nucleation and growth of BPs; once the ability to manipulate and produce macroscopic single crystals was achieved, they proceeded to study the structural and dynamic details of the BP-crystal transformations.

“Through a combination of theory and experiments, we have discovered that a single-crystal BP transitions into another BP with a different defect network symmetry through a local re-organization of the crystalline structure of topological defects, where the characteristic dimensions are on the order of hundreds of nanometers. Such crytal-crystal transformations occur with little hysteresis and no diffusion of the double twisted cylinders that constitute the main structural feature of the crystals,’’ explained the authors in the manuscript published in the Proceedings of the National Academy of Sciences of USA (2017), DOI: 10.1073/pnas.1711207114. The authors relied on this feature to draw an analogy to the martensitic transformations observed in atomic solid crystals: "We report that BP-transitions exhibit some of the key characteristics of a martensitic transformation, where the diffusion-less feature is associated with the collective behavior of the double twist cylinders, occurring over scales of hundreds of nanometers," explained Xiao Li. "It is fascinating to discover that, under precise conditions, regions that encompass billions of molecules can behave in a manner that mimics processes occurring at atomic scales,’’ commented Jose A. Martinez, who shares with Xiao Li equal contributions to the manuscript.

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