When twins collide: Twin junctions in nanocrystalline nickel

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Thomas, Spencer L.
Srolovitz, David J.
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King, Alexander
Professor Emeritus
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Materials Science and Engineering

The Department of Materials Science and Engineering teaches the composition, microstructure, and processing of materials as well as their properties, uses, and performance. These fields of research utilize technologies in metals, ceramics, polymers, composites, and electronic materials.

The Department of Materials Science and Engineering was formed in 1975 from the merger of the Department of Ceramics Engineering and the Department of Metallurgical Engineering.

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Ames National Laboratory

Ames National Laboratory is a government-owned, contractor-operated national laboratory of the U.S. Department of Energy (DOE), operated by and located on the campus of Iowa State University in Ames, Iowa.

For more than 70 years, the Ames National Laboratory has successfully partnered with Iowa State University, and is unique among the 17 DOE laboratories in that it is physically located on the campus of a major research university. Many of the scientists and administrators at the Laboratory also hold faculty positions at the University and the Laboratory has access to both undergraduate and graduate student talent.

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We present the results of large-scale molecular dynamics simulations of grain growth in polycrystalline nickel with nanoscale grains. The simulations show that grain growth is accompanied by coherent twin boundary (CTB) generation. As the grains grow, twins collide; such collisions result in twin junctions. We catalog all possible twin junctions and show examples of each from the simulations. These include junctions of 2–4 CTBs with grain boundaries and five-fold twin junctions (penta-twins). We elucidate the mechanisms by which all of these junctions form and their relative frequencies. Penta-twins, which are rare in coarse microstructures, occur frequently in nanocrystalline metals. Their absence in macro-scale samples can be traced to the wedge-disclination character (and, consequently, an elastic energy that diverges with sample size). In the nanocrystalline case, the presence of penta-twins can be traced to this twin collision formation mechanism, which is responsible for their wedge-disclination dipole character (relatively small elastic energy). We demonstrate how all CTB junctions, especially penta-twins, retard grain growth.
This is a manuscript of the article Published as Thomas, Spencer L., Alexander H. King, and David J. Srolovitz. "When twins collide: Twin junctions in nanocrystalline nickel." Acta Materialia 113 (2016): 301-310. doi: https://doi.org/10.1016/j.actamat.2016.04.030. © 2016 Elsevier. This manuscript is made available under the Elsevier user license (https://www.elsevier.com/open-access/userlicense/1.0/).