Atomic stresses in materials visualised
Microscopic atomic-scale stresses are crucial in determining the hardness and mechanical properties of materials. While this effect is empirically known since the fabrication of weapons and agricultural tools in the middle ages, Peter Schall from the University of Amsterdam and colleagues of Cornell University have now found a way to directly visualise and measure these fine atomic-scale stresses.
Their findings, published in Nature Materials, improve our understanding of microscopic defects in materials, and help to develop better materials with ever improved mechanical properties.
Already the old blacksmiths in the middle ages knew that hammering the glowing iron during fabrication of spearheads or picks hardened the material, largely improving its performance. We know nowadays that this hardening is due to the introduction of atomic-scale defects within the material, whose interactions lead to increased stiffness of the material. A popular example is steel, in which carbon atoms act as defects that make the material orders of magnitude harder than the original iron itself.
Responsible for this hardening are the internal atomic-scale stresses that – by superposition – determine the mechanical behaviour of the material on a larger scale. While the three-dimensional stress fields of isolated atomic defects are largely known, it is less clear how their complex interaction determines the material properties. This is because the superposition of atomic-scale stresses governing the interaction of these defects remains challenging to measure.
Micrometer-scale model material
Researchers of Cornell University together with Peter Schall of the UvA Institute of Physics have now succeeded to directly visualize and measure these microscopic stresses in a model material. They used tiny, micrometer-size spheres suspended in a solvent that form all states of matter – gas, liquid, and especially solids – and in many respects resemble the behaviour of atoms. Previous work of some of the researchers had shown that these model materials exhibit defects that propagate and interact just like defects in atomic materials do. Because the particles are several orders of magnitude larger than atoms, they can be imaged conveniently in three dimensions, giving a rich atomic-scale picture of material processes.
New nano-textured materials
In these model materials composed of micrometer-size hard spheres, stresses are transmitted via collisions: due to their thermal motion, the particles collide with each other in the dense material; the closer the particles are, the more often they collide. The authors devised a protocol to convert the imaged particle separations into collisions and hence into particle-scale stresses. The resulting finely resolved stresses give a detailed lively picture of complex stress fields in materials, and show how also atomic defects interact to determine material properties on a larger scale. The work, while giving new insight into one of the oldest and most profound problems of material science, can be used to develop new nano-textured materials, in which the superposition and interaction of defects is designed to yield specific desired material properties.
Neil Y. C. Lin, Matthew Bierbaum, Peter Schall, James P. Sethna and Itai Cohen: ‘Measuring nonlinear stresses generated by defects in 3D colloidal crystals’, in: Nature Materials, 1 August 2016.