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Abnormal Grain Growth Induced by Cyclic Heat Treatment

Science  27 Sep 2013:
Vol. 341, Issue 6153, pp. 1500-1502
DOI: 10.1126/science.1238017

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Making the Grain

Most metals contain a large number of ordered crystalline regions that are separated by disordered grain boundaries. If the material is annealed at elevated temperatures, the larger grains will grow uniformly at the expense of the smaller ones. This process slows down over time, making it hard to create very large grains. Abnormal grain growth, in which a few of the crystalline regions grow much faster and larger than the others, can occur if the material is put through a complex annealing process involving straining of the samples. Omori et al. (p. 1500; see the Perspective by Taleff and Pedrazas) find that a much simpler and shorter annealing process can trigger abnormal grain growth in copper-based shape-memory alloys. Thermal cycling between a high-temperature single-phase region and a lower-temperature two-phase region generated dislocations at low temperatures and grain growth on heating. Because this method does not require external straining of the sample, it is not limited to thin sheets or wires.

Abstract

In polycrystalline materials, grain growth occurs at elevated temperatures to reduce the total area of grain boundaries with high energy. The grain growth rate usually slows down with annealing time, making it hard to obtain grains larger than a millimeter in size. We report a crystal growth method that employs only a cyclic heat treatment to obtain a single crystal of more than several centimeters in a copper-based shape-memory alloy. This abnormal grain growth phenomenon results from the formation of a subgrain structure introduced through phase transformation. These findings provide a method of fabricating a single-crystal or large-grain structure important for shape-memory properties, magnetic properties, and creep properties, among others.

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