Article: Controlling dislocation near material surfaces with atomic segregation

Article: Controlling dislocation near material surfaces with atomic segregation

Surface dislocation. The surface structures of Cu-Au alloys (Cu in yellow and Au in blue), show a dislocation in the middle of the figure.

In an article recently published in Nature Materials, Assistant Professor Liang Qi and MSE Ph.D. candidate Chaoming Yang detail a new scientific principle study on how to control the important materials defect dislocation near material surfaces.

“Many materials achieve their structural or functional properties due to their large areas of surface and interfaces, so learning how to control the material defects near these surfaces and interfaces is very important in our overall ability to control material properties,” said Qi.

Among the most important material defects is dislocation – a linear or one-dimensional defect around which some of the atoms are distorted. Dislocations that occur near  material surfaces and interfaces impact properties such as mechanical strength, ductility and toughness, corrosion resistance, and catalytic properties, which is why learning how to control them is critical.

Using transmission electron microscopy (TEM), Liang’s experimental collaborators observed in real time that, when heated, the changes of surface compositions in CuAu alloys from atomic segregation were accompanied by the nucleation and motions of dislocations in the subsurface. “We were able to observe in situ – while it was happening -- how an individual dislocation was generated and moved around just about 1 nm blew the surface. Usually, dislocations in such an area should be very unstable because of their strong interaction with the surfaces, and very difficult to see,” said Qi.

By applying atomistic simulations and theoretical analyses, Qi and Yang illustrated the motion mechanisms of these dislocations. They also explained why such dislocations close to surfaces were so abnormally stable. The intrinsic reason is that several atomic layers on the top surface formed a chemically ordered structure due to atomic segregation. It increased the difficulties for a dislocation from the subsurface to penetrate the top-ordered layers and annihilate. Qi added that controlling atomic segregation to create and manipulate dislocations at alloy surfaces and interfaces could have many applications, including lightweight metals, solar cells, catalysts and functional oxide. For example, the density of these dislocations determines the strain status of the top surface layers, which results in significant changes in surface electronic structures and chemical properties. 

The study was a collaboration with a myriad of researchers at State University of New York (Binghamton, N.Y.), the University of Pittsburgh, and the Center for Functional Nanomaterials, Brookhaven National Laboratory (Upton, N.Y.).