Dual-scale chemical ordering enables exceptional cryogenic mechanical properties of medium-entropy alloys

Abstract The mechanical properties of metallic materials often significantly deteriorate when used under harsh conditions, particularly at cryogenic temperatures. Yet, safe and lasting low-temperature infrastructures are needed to realize the growing global demand for liquid energy carriers such as hydrogen[1]. Traditional microstructure strategies designed for high strength and ductility at room temperature often fail when exposed to such low temperatures due to the unresolved conflict between strength and damage tolerance under such conditions[2, 3]. Here we introduce a new type of dual-scale atomic ordering nanostructure embedded in a massive metallic solid solution matrix to overcome this challenge. This reconciles advantages of two antagonistic mechanisms in one material concept, namely, a well-balanced enthalpy-driven ordering on the one hand and entropy-driven solid solution on the other. We realize the approach in form of an exceptionally high number density of coexisting sub-nanoscale chemical short-range ordered (SRO, ~2.4×10^26 m-3) and nanoscale long-range ordered (NLRO, ~4.5×10^25 m-3) zones in a CoNiV-based compositionally complex alloy. We observe several types of interactions between the chemical ordering and the dislocations, i.e., the defects that carry both, the material’s inelastic deformation and potentially also its damage initiation. We observe an ordering-induced increase in dislocation shear stress (providing strength) as well as a more rapid dislocation multiplication due to the frequent blocking of the planar dislocation flux by NLRO and the associated generation of new dislocations (providing ductility). The latter effect also releases stress concentrations at NLRO obstacles that otherwise would promote damage initiation and failure (a phenomenon observed in materials with larger ordered zones and precipitates). Consequently, a higher strength-ductility combination (strength-elongation product 76.5 GPa% with 1172 MPa yield strength at 87 K) is achieved in comparison to reference materials devoid of such ordering hierarchy (yield strength 794 MPa), containing only SRO (yield strength 931 MPa) or coherent precipitates of a few tens of nanometers (strength-elongation product 38.8 GPa%). Our results thus reveal the effects of different types (and scales) of coexisting chemical ordering on mechanical properties of complex alloys and provide guidelines to control them, which serves as a new design approach for strong and tough materials particularly for cryogenic applications.