Scientists blow hot and cold to produce single-crystal metal

A new heating method for certain metals could lead to improved earthquake-resistant construction materials.

A Japanese team led by researchers from Tohoku University has found a cost-effective way to improve the properties of some 'shape memory' metals, known for their ability to return to their original shape after being deformed. The method could pave the way for the mass production of these improved metals for a variety of applications, including earthquake-resistant construction materials.

Most metals are made of a large number of crystals. In some cases, however, the properties of metals improve when they are formed of a single crystal, but single-crystal metals are expensive to produce.

The researchers have now developed a cheaper production method that takes advantage of a phenomenon known as 'abnormal grain growth'. Using this method, a metal's multiple 'grains', or crystals, grow irregularly, some at the expense of others, when it is exposed to heat.

The team's technique employs several cycles of heating and cooling to induce the growth of a single-crystal metal bar 70cm in length and 15mm in diameter, which is very large compared to the sizes of current shape-memory alloy bars. This makes it suitable for building and civil engineering applications, says Toshihiro Omori from Tohoku University, the lead researcher of the study.

To produce the large single-crystal metal bar, a metal 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. All these heating/cooling cycles are required to produce the single-crystal metal.

The alloy used by the researchers, which is made of copper, aluminium and manganese, is a well-known shape memory metal that is easy to cut with machines. Increasing the size of the metal's crystals drastically enhances its elasticity, while altering their shape makes the metal quite strong. With these combined features, the metal could prove of use for building structures that can withstand earthquakes.

"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 a paper on this work in Nature Communications.