Strength and flexibility are two opposites that usually need to be balanced in steel. But now engineers at Purdue University and Sandia National Labs have developed a new treatment that can be applied to steel alloys to make them both stronger and more ductile at the same time, which could have a range of uses in energy and aerospace.
Strength is a measure of how large a load a material can withstand before it fails, while ductility measures how easily it can be extended or elongated into different shapes. Those two properties are usually at odds with each other, leading to a trade-off that needs to be made depending on the application at hand. In metals, it all comes down to the tiny grains that make up the material. Larger grains are better at deforming to allow better ductility, while smaller ones can handle more strain, boosting strength.
In a new study, scientists have developed a treatment for steel that can better balance strength and ductility by adjusting these grains. The team treated a steel alloy known as T-91 to produce a new material they call Gradient T-91 (G-T91), which, as the name suggests, has a gradient of grain sizes throughout.
The treatment forms a thin layer of ultra-fine metal grains from the surface down to about 200 micrometers into the material. The grains on the outside measure less than 100 nanometers long, while those in the center are up to 100 times larger. This grants G-T91 a yield strength of 700 megapascals – a 36% improvement over that of untreated T-91 – and a plasticity that was 50% better than T-91.
“This is the beauty of the structure; the center is soft so it can sustain plasticity but, by introducing the nanolaminate, the surface has become much harder,” said Zhongxia Shang, lead author of the study. “If you then create this gradient, with the large grains in the center and nanograins in the surface, they deform synergistically. The large grains take care of the stretching, and the small grains accommodate the stress. And now you can make a material that has a combination of strength and ductility.”
To see how this was working, the team took scanning electron microscope images of the material at different stages of applied strain. Normally, the ultra-fine grains near the surface are oriented vertically, but as more strain is applied they begin to take on a more globular shape, then turn and stretch out horizontally. This allows the steel to deform more effectively.
But exactly how and why the grains move remains a mystery, the team says. Future work will investigate this, which may help uncover better ways to arrange the grains to make materials with different properties.
The research was published in the journal Science Advances.
Source: Purdue University
