Researchers from North Carolina State University and Qatar University have developed a new “high-entropy” metal alloy that has a higher strength-to-weight ratio than any other existing metal material.
High-entropy alloys are materials that consist of five or more metals in approximately equal amounts. These alloys are currently the focus of significant attention in materials science and engineering because they can have desirable properties.
The NC State research team combined lithium, magnesium, titanium, aluminum and scandium to make a nanocrystalline high-entropy alloy that has low density, but very high strength.
The Alloy is about 27% Scandium. The production of Scandium can be boosted from about 5-6 tons per year to 2000 tons if it was regularly recovered from coal and uranium mining. Currently Scandium metal costs $15500 per kilogram.
The high mechanical hardness of these alloys can partly be explained by their nanocrystalline grain sizes. However, their hardness values exceed nanocrystalline Al base alloys, for example, by factors of 2–3 times. A low-density (2.67 g cm−3) high-entropy alloy with composition Al20Li20Mg10Sc20Ti30 has been prepared by mechanical alloying of elemental powders. The as-milled structure is single-phase fcc, with a nanocrystalline grain size of 12 nm, and a mechanical hardness of 5.9 GPa. The sample without O, N contamination transforms to the hcp structure on annealing at 500°C. The sample containing O, N does not transform, but rather exhibits either a slight phase separation or unit cell distortion upon annealing at 800°C. Calculations of the energies of the competing structures are consistent with the experimental observations. This material exhibits a combination of hardness (strength) and low density that is not equaled by any other metallic material.
Ashby plot of strength vs. density for engineering materials. (Yield strength for metals and polymers, tear strength for elastomers, compressive strength for ceramics, and tensile strength for composites.) Reproduced from Elsevier 2010
“The density is comparable to aluminum, but it is stronger than titanium alloys,” says Dr. Carl Koch, Kobe Steel Distinguished Professor of Materials Science and Engineering at NC State and senior author of a paper on the work. “It has a combination of high strength and low density that is, as far as we can tell, unmatched by any other metallic material. The strength-to-weight ratio is comparable to some ceramics, but we think it’s tougher – less brittle – than ceramics.”
There are a wide range of uses for strong, lightweight materials, such as in vehicles or prosthetic devices.
“We still have a lot of research to do to fully characterize this material and explore the best processing methods for it,” Koch says.
At this point, the primary problem with the alloy is that it is made of 20 percent scandium, which is extremely expensive.
“One thing we’ll be looking at is whether scandium can be replaced or eliminated from the alloy,” Koch says.
A low-density, nanocrystalline high-entropy alloy, Al20Li20Mg10Sc20Ti30 was produced by mechanical alloying. It formed a single-phase fcc structure during ball milling and transformed to single-phase hcp upon annealing. The alloy has an estimated strength-to-weight ratio that is significantly higher than other nanocrystalline alloys and is comparable to ceramics. High hardness is retained after annealing.
SOURCES – North Carolina State University, Materials Research Letters, Elsevier
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