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Single-crystal shape memory CuAlMn bar produced by unique heat treating process

September 13, 2017
Source: ASM International

Tohoku University, Japan, reports that its researchers and colleagues have found an economical way to improve the properties of copper-aluminum-manganese 'shape memory' alloys by a unique heat treating method. The method could make way for the mass production of these improved metals for a variety of applications, including earthquake-resistant construction materials.

The alloy is made of copper, aluminum, and manganese, and is a well-known shape memory metal that is easy to cut with machines. Increasing the size of the material's crystals drastically enhances its elasticity. Altering its shape also makes it quite strong. These combined features make it very attractive for building structures that can withstand earthquakes.

Most metals are made of a large number of crystals but, in some cases, their properties improve when they are formed of a single crystal. However, single-crystal metals are expensive to produce. Metallurgists have developed a cheaper production method that takes advantage of a phenomenon known as 'abnormal grain growth.' This process causes the metal's multiple 'grains', or crystals, to grow irregularly, some at the expense of others, when it is exposed to heat.

The team's technique involves several cycles of heating and cooling that result in a single-crystal metal bar 70 cm in length and 15 mm in diameter. According to Toshihiro Omori, the lead researcher in the study, this is very large compared to the sizes of current shape memory alloy bars, making it suitable for building and civil engineering applications.

To produce the large single-crystal bar, the alloy is heated to 900°C, then cooled to 500°C, five times. This is followed by four cycles of heating to 740°C then cooling to 500°C. Finally, the metal is heated one last time to 900°C. Omitting the lower temperature heating (740°C)/cooling (500°C) cycles does not lead to the single-crystal result.

"Since the present technique is advantageous for mass production of single crystals because of the simplicity of the process, this finding opens the way for applications of shape memory single crystals for structural materials, such as for seismic applications in buildings and bridges," conclude the researchers in their study published in the journal Nature Communications.


Subject Classifications

Industries and Applications | Marine Applications

Materials Processing and Treatment | Heat Treating

Materials Properties and Performance | Physical Properties

Metals and Alloys | Shape Memory Alloys

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