Enhanced strength and ductility in a high-entropy alloy via ordered oxygen complexes

Abstract

Oxygen, one of the most abundant elements on Earth, often forms
an undesired interstitial impurity or ceramic phase (such as an oxide
particle) in metallic materials. Even when it adds strength, oxygen
doping renders metals brittle1–3. Here we show that oxygen can take
the form of ordered oxygen complexes, a state in between oxide
particles and frequently occurring random interstitials. Unlike
traditional interstitial strengthening4,5, such ordered interstitial
complexes lead to unprecedented enhancement in both strength and
ductility in compositionally complex solid solutions, the so-called
high-entropy alloys (HEAs)6–10. The tensile strength is enhanced
(by 48.5 ± 1.8 per cent) and ductility is substantially improved
(by 95.2 ± 8.1 per cent) when doping a model TiZrHfNb HEA
with 2.0 atomic per cent oxygen, thus breaking the long-standing
strength–ductility trade-off11. The oxygen complexes are ordered
nanoscale regions within the HEA characterized by (O, Zr, Ti)-rich
atomic complexes whose formation is promoted by the existence of
chemical short-range ordering among some of the substitutional
matrix elements in the HEAs. Carbon has been reported to improve
strength and ductility simultaneously in face-centred cubic HEAs12,
by lowering the stacking fault energy and increasing the lattice
friction stress. By contrast, the ordered interstitial complexes
described here change the dislocation shear mode from planar slip
to wavy slip, and promote double cross-slip and thus dislocation
multiplication through the formation of Frank–Read sources (a
mechanism explaining the generation of multiple dislocations)
during deformation. This ordered interstitial complex-mediated
strain-hardening mechanism should be particularly useful in
Ti-, Zr- and Hf-containing alloys, in which interstitial elements
are highly undesirable owing to their embrittlement effects, and
in alloys where tuning the stacking fault energy and exploiting
athermal transformations13 do not lead to property enhancement.
These results provide insight into the role of interstitial solid
solutions and associated ordering strengthening mechanisms in
metallic materials.