New high-entropy alloy is as light as aluminum, stronger than titanium alloys

11 December 2014

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.) The low-density HEA is indicated with the star. Youssef et al. Click to enlarge.

Researchers from North Carolina State University and Qatar University have developed a new high-entropy alloy that has a higher strength-to-weight ratio that they say is unmatched by any other metallic material. The researchers used mechanical alloying to combine lithium, magnesium, titanium, aluminum and scandium to make a low-density, nanocrystalline alloy (Al₂₀Li₂₀Mg₁₀Sc₂₀Ti₃₀) with an estimated strength-to-weight ratio that is significantly higher than other nanocrystalline alloys and is comparable to ceramics. An open access paper on their work is published in the journal Materials Research Letters.

High-entropy alloys (HEAs) are a new class of multi-component alloy systems in which the design of the alloys is based not on adding to a single base element, but on choosing elements that will form solid solutions when mixed at near-equiatomic concentrations. (Earlier post.) HEAs

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While there is continuing high interest in development of alloys with low densities along with high strength for energy-saving applications such as in transportation and energy, to date there have only been a few reports of studies of low-density HEAs. For purposes of our discussion we define low density as less than 3 g cm⁻³. … To our knowledge, no single-phase low-density high-entropy alloy (LDHEA) has been reported. In this paper, we report our results on the processing, structure, and mechanical hardness of a low-density HEA.

Because of the high vapor pressures of lithium and magnesium, the team used mechanical alloying to prepare the alloys instead of melting and casting. Powders for the alloy were loaded into a stainless steel vial with stainless steel balls in a high-purity argon atmosphere glove box. Ball milling was performed in a modified SPEX 8000 mixer mill cooled by liquid nitrogen for 2 hours, followed by milling at room temperature for 14 hours. They used liquid nitrogen cooled milling to avoid welding of the powders at the beginning of the mechanical milling process.

The resulting alloy had a low-density of 2.67 g cm⁻³, a nanocrystalline grain size of 12 nm, and a mechanical hardness of 5.9 GPa.

The density is comparable to aluminum, but it is stronger than titanium alloys. 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. We still have a lot of research to do to fully characterize this material and explore the best processing methods for it.

At this point, the primary problem with the alloy is that it is made of 20 percent scandium, which is extremely expensive. The researchers are exploring whether or not scandium can be replaced or eliminated from the alloy.

Lead author of the paper is Dr. Khaled Youssef of Qatar University. Co-authors include Alexander Zaddach and Changning Niu, Ph.D. students at NC State; and Douglas Irving, an associate professor of material science and engineering at NC State. The work was supported in part by the National Science Foundation under grant number DMR-1104930.

Resources

• Khaled M. Youssef , Alexander J. Zaddach , Changning Niu , Douglas L. Irving , Carl C. Koch (2014) “A Novel Low-Density, High-Hardness, High-entropy Alloy with Close-packed Single-phase Nanocrystalline Structures,” Materials Research Letters doi: 10.1080/21663831.2014.985855