University of Toronto researchers have designed a new composite material that is both very light and extremely strong – even at temperatures up to 500 Celsius.
The material, which is described in a paper published in Nature Communications, is made of various metallic alloys and nanoscale precipitates, and has a structure that mimics that of reinforced concrete – but on a microscopic scale.
These properties could make it extremely useful in aerospace and other high-performance industries.
“Steel rebar is widely used in the construction industry to improve the structural strength of concrete in buildings and other large structures,” says the study’s senior author Yu Zou, an associate professor in the department of materials science and engineering in U of T’s Faculty of Applied Science & Engineering. “New techniques such as additive manufacturing, also known as 3D metal printing, have now enabled us to mimic this structure in the form of a metal matrix composite. This approach gives us new materials with properties we’ve never seen before.”
While steel is still the major structural material in trains and automobiles, aluminum has some advantages in airplanes due to its lower weight.
Lightweighting – reducing the weight of components while retaining their strength – means that less power is needed to move the vehicle, which in turn improves fuel efficiency. It is particularly important in aerospace, where every gram counts.
But aluminum alloys also have their downsides, explains Chenwei Shao, a research associate in Zou’s lab and lead author on the new paper.
“Until now, aluminum components have suffered from performance degradation at high temperatures,” says Shao. “Basically, the hotter they get, the softer they get, rendering them unsuitable for many applications.”
To overcome this problem, the team aimed to build a composite of various metals that would have the same structure as reinforced concrete: a cage or mesh composed of steel rebar, surrounded by a matrix of cement, sand and aggregate.
“In our material, the ‘rebar’ is a mesh made of titanium alloy struts,” says Shao. “Because we use a form of additive manufacturing in which we fire lasers at metal powders to heat them into solid metal, we can make this mesh any size we want. The struts can be as small as 0.2 millimetres in diameter.”
To fill in the spaces between these struts, the team used a technique known as micro-casting to create a matrix of other elements such as aluminum, silicon and magnesium. This matrix acts like the cement to hold it all together.
Further strength is provided by micrometre-sized particles of alumina and silicon nanoprecipitates embedded in the ‘cement’ matrix. These particles are much like the gravel or aggregate found in concrete.
The team then subjected their new material to a variety of tests to determine its strength.
“At room temperature, the highest yield strength we got was around 700 megapascals; a typical aluminum matrix would be more like 100 to 150 megapascals,” says Shao. “But where it really shines is at high temperature. At 500 Celsius, it has a yield strength of 300 to 400 megapascals, compared to about five megapascals for a traditional aluminum matrix.
“In fact, this new metal composite performs about as well as medium-range steels, but at only about one-third the weight.”
The ability of the material to resist degradation at such high temperatures was surprising, so the team built detailed computer models to understand the underlying mechanisms.
“What we found was that at high temperatures, this composite material deforms via a different mechanism than most metals,” says study co-author Huicong Chen, who led the computer simulations. “We called this new mechanism ‘enhanced twinning,’ and it enables the material to maintain much of its strength, even when it gets very hot.”
Zou says that while it may be some time before the new material begins to be deployed by industry, its discovery underlines the advantages of newly emerging techniques, such as additive manufacturing.
“We wouldn’t have been able to make this material any other way,” he says. “It’s true that it still costs a lot to create materials like this at scale, but there are some applications where the high performance will be worth it. And as more companies invest in advanced manufacturing technologies, we will eventually see the cost come down.
“We think this is an exciting step forward toward stronger, lighter and more efficient vehicles.”
This research was supported by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, Digital Research Alliance of Canada, and the Canada Research Chair in Materials and Manufacturing for Extreme Environments.