The sliding of a perfect twin boundary, with mirrored crystal lattices on both sides, was long considered to be impossible at room temperature in metals. Here, authors show that it is possible when a nanoscale twin boundary within a copper nanopillar is compressed along certain orientations, through in-situ transmission electron microscopy (left) and molecular dynamics simulation (right).
Most metals and semiconductors, from the steel in a knife blade to the silicon in a solar panel, are made up of many tiny crystalline grains. The way these grains meet at their edges can have a major impact on the solid's properties, including mechanical strength, electrical conductivity, thermal properties, flexibility, and so on. When the boundaries between the grains are of a particular type, called a coherent twin boundary (CTB), this adds useful properties to certain materials, especially at the nanoscale. It increases their strength, making the material much stronger while preserving its ability to be deformed, unlike most other processes that add strength. Now, researchers have discovered a new deformation mechanism of these twin crystal boundaries, which could help engineers figure out how to more precisely use CTBs to tune the properties of some materials. Contrary to expectations, it turns out that a material's crystal grains can sometimes slide along these CTBs. The new finding is described in a paper published this week by Ming Dao, a principal research scientist in MIT's Department of Materials Science and Engineering; Subra Suresh, the Vannevar Bush Professor Emeritus of Engineering and president-designate of Nanyang Technological University in Singapore; Ju Li, the Battelle Energy Alliance Professor in MIT's Department of Nuclear Science and Engineering; and seven others at MIT and elsewhere.
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