Variations in grain size and density strengthen steel while making it more ductile.
By Blaine Brownell, AIA
Materials have an implicit, performance-based hierarchy. Metal is almost always preferred over bamboo, for example, when it comes to brute strength or longevity. This partiality results not only from research and testing but also from cultural biases. The so-called Age of Steel—made possible by advancements during the Industrial Revolution—is still viewed as an era of material innovation compared with previous epochs in which wood was a dominant building material. Recent discoveries have led to a reversal in perspective, however, as metallurgical scientists increasingly look to bamboo and other organic materials for clues to improving the performance of metal.
Researchers at North Carolina State University (NC State) and the Chinese Academy of Sciences’ (CAS) Institute of Mechanics in Beijing have found a way to enhance metals' mechanical characteristics by modifying their internal structure to emulate bamboo. The secret lies in each material's gradient structure. The cells in bamboo vary in size and density, imparting both strength and flexibility. In comparison, metals have small, tightly packed homogeneous grains that deliver high strength at the expense of reduced ductility. "In short, the gradual interface of the large and small grains makes the overall material stronger and more ductile, which is a combination of characteristics that is unattainable in conventional materials,” said Xiaolei Wu, a professor of materials science at the CAS institute and lead author of two papers (here and here) related to the research, in a press release.
Wu and his team created an industrial, interstitial free (IF) steel with a gradient structure—a test that returned notable results. Conventional IF steel has a stress limit of 65.3 kilo-pounds per square inch (ksi) and may only be stretched up to 5 percent of its length before failure. By contrast, the gradient-structured steel can withstand 72.4 ksi and may be stretched up to 20 percent of its length.
Such a promising finding points to the significant potential improvements in metal technologies that can be achieved by looking to biological materials long thought to be inferior. The researchers are also investigating gradient structures to improve materials' corrosion resistance, wear, and fatigue. "We think this is an exciting new area for materials research because it has a host of applications and it can be easily and inexpensively incorporated into industrial processes," Wu said.
This article was originally published in ARCHITECT July 2014.
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