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A new metallurgical strategy, high-entropy alloying (HEA), was used to explore new composition and phase spaces in the development of new refractory alloys with reduced densities and improved properties. Combining Mo, Ta, and Hf with "low-density" refractory elements (Nb, V, and Zr) and with Ti and Al produced six new refractory HEAs with densities ranging from 6.9 g/cm^sup 3^ to 9.1 g/cm^sup 3^. Three alloys have single-phase disordered body-centered cubic (bcc) crystal structures and three other alloys contain two bcc nanophases with very close lattice parameters. The alloys have high hardness, in the range from H^sub v^ = 4.0 GPa to 5.8 GPa, and compression yield strength, σ^sub 0.2^ = 1280 MPa to 2035 MPa, depending on the composition. Some of these refractory HEAs show considerably improved high temperature strengths relative to advanced Ni-based superalloys. Compressive ductility of all the alloys is limited at room temperature, but it improves significantly at 800°C and 1000°C.
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INTRODUCTION
Recently, several refractory high-entropy alloys (HEAs) were produced and their microstructures and properties were reported.1-8 Some of these al- loys exhibit high-temperature properties that are competitive with the current Ni-based superalloys. In particular, refractory HEAs have the potential to operate at much higher temperatures; for example, some have considerably higher strengths at tem- peratures above 800^C than Ni superalloys. Unfor- tunately, the balance of properties such as their high density, low room-temperature (RT) ductility and insufficient oxidation resistance are issues that still need to be resolved by subsequent metallurgical studies.
Historically, the alloying element Al is associated with several beneficial properties in many high- temperature (HT) alloys. For example, in Ni- and Ni/Fe-based superalloys Al forms coherent cc (Ni3Al- based) particles, which improve the strength and creep resistance.9 Al also forms a protective oxide scale, which improves oxidation and corrosion resistance. In steels, Al is widely used as a deoxi- dizer, ferrite stabilizer, and a precipitation-forming element, and it is the most effective alloying ele- ment for preventing austenite grain growth.10,11 Finally, in Co-Cr-Fe-Ni-based HEAs, Al stabilizes a body-centered cubic (bcc, ferrite) phase and consid- erably increases strength.12-14 In all these cases, Al addition reduces the alloy density.
In this work, Al was added to several refractory HEAs to reduce alloy density, increase HT...