Newswise – Scientists at the Department of Energy’s Oak Ridge National Laboratory and the University of Tennessee, Knoxville, have found a way to simultaneously increase the strength and ductility of an alloy by adding tiny precipitates to its matrix and adjusting their size and spacing. The precipitates are solids that separate from the metal mixture when the alloy cools. The results published in the journal Nature will open up new avenues for the further development of building materials.
Ductility is a measure of the ability of a material to permanently deform without breaking. Among other things, it determines how far a material stretches before it breaks and whether this break is graceful or catastrophic. The higher the strength and ductility, the tougher the material.
“A holy grail of construction materials has long been: How do you increase strength and ductility at the same time?” Said Easo George, principal researcher of the study and Governor’s Chair for Advanced Alloy Theory and Development at ORNL and UT. “Overcoming the compromise between strength and ductility will enable a new generation of lighter, stronger, and damage-tolerant materials.”
If construction materials could become stronger and more ductile, parts of cars, airplanes, power plants, buildings and bridges could be built with less material. Lighter vehicles would be more energy efficient to manufacture and operate, and a more robust infrastructure would be more resilient.
Co-study director Ying Yang from ORNL conceived and directed the Nature study. Guided by computer-aided thermodynamic simulations, designed and tailored model alloys with the particular ability to undergo a phase transition from a face-centered cubic (FCC) to a body-centered cubic (BCC) crystal structure caused by changes in either temperature or stress.
“We put nanoprecipitates in a transformable matrix and carefully controlled their properties, which in turn controlled when and how the matrix transformed,” said Yang. “In this material, we deliberately enabled the matrix to undergo a phase transition.”
The alloy contains four main elements – iron, nickel, aluminum and titanium – which make up the matrix and precipitates, and three minor elements – carbon, zirconium and boron – which limit the size of the grains, individual metallic crystals.
The researchers carefully kept the composition of the matrix and the total amount of nanoprecipitates the same in different samples. However, they varied the precipitation sizes and distances by adjusting the processing temperature and time. For comparison, a reference alloy without precipitates was also produced and tested, but which had the same composition as the matrix of the alloy containing precipitates.
“The strength of a material usually depends on how close the precipitates are to each other,” said George. “If you do it a few nanometers [billionths of a meter] in their size they can be very close together. The closer they are to each other, the stronger the material becomes. “
Nanoprecipitates in conventional alloys can make them super-strong, but they also make the alloys very brittle. The team’s alloy avoids this brittleness because the precipitates perform a second useful function: by limiting the space of the matrix, they prevent it from transforming during thermal quenching, a rapid immersion in water that cools the alloy to room temperature. As a result, the matrix remains in a metastable FCC state. As the alloy is then stretched (“stretched”) it gradually transforms from metastable FCC to stable BCC. This phase change during stretching increases strength while maintaining sufficient ductility. In contrast, the alloy completely converts to stable FCC with no precipitates during thermal quenching, which precludes further conversion during elongation. This makes it both weaker and more brittle than the alloy with precipitates. Together, the complementary mechanisms of conventional waste reinforcement and strain-induced transformation increased strength by 20% to 90% and elongation by 300%.
“The addition of precipitation to block dislocations and make materials ultra-strong is well known,” said George. “What is new here is that the adjustment of the distance between these precipitates also influences the tendency to phase transformation, whereby several deformation mechanisms can be activated as required to improve ductility.”
The study also showed a surprising reversal of the normal reinforcing effect of nanoprecipitates: an alloy with coarse, widely spaced precipitates is stronger than the same alloy with fine, closely spaced precipitates. This reversal occurs when the nanoprecipitates become so small and densely packed that the phase transformation is essentially shut off during the elongation of the material, similar to the transformation that is suppressed during thermal quenching.
This study relied on complementary techniques performed in the user facilities of the DOE Office of Science at ORNL to characterize the nanoprecipitates and deformation mechanisms. At the Center for Nanophase Materials Sciences, atom probe tomography showed the size, distribution, and chemical composition of precipitates, while transmission electron microscopy revealed atomistic details of local regions. At the High Flux Isotope Reactor, small-angle neutron scattering quantified the distribution of fine precipitates. And at the spallation neutron source, neutron diffraction examined the phase transformation according to different degrees of exposure.
“This research introduces a new family of structural alloys,” said Yang. “Precipitation properties and alloy chemistry can be precisely adjusted to activate deformation mechanisms exactly when they are needed to thwart the trade-off between strength and ductility.”
Next, the team will study additional factors and deformation mechanisms to identify combinations that could further improve mechanical properties.
It turns out there is a lot of room for improvement. “Today’s structural materials are only a small fraction – maybe only 10% – of their theoretically possible strengths,” said George. “Imagine the weight savings that would be possible in a car or airplane – and the resulting energy savings – if that strength could be doubled or tripled while maintaining reasonable ductility.”
The title of the Nature paper is “Bifunctional nanoprecipitates strengthen and ductilize an alloy with medium entropy”.
The DOE Office of Science and the Laboratory Directed Research and Development Program of the ORNL supported the research.
UT-Battelle leads ORNL for the Department of Energy’s Office of Science, the largest single funder of basic research in the physical sciences in the United States. The Science Office is working to address some of the most pressing challenges of our time. More information is available at energy.gov/science.
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