Abstract
High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centred cubic solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature, the alloy shows tensile strengths of almost 1 GPa, failure strains of ∼70% and KJIc fracture-toughness values above 200 MPa m1/2; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa m1/2. Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning.
Highlights
High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures
The experimental results (X-ray diffraction and backscattered electron, back-scattered electron (BSE), images)[23] are consistent with the CrCoNi ternary phase diagrams[25], which indicate that the equiatomic composition is a single-phase solid solution at elevated temperatures
The CrCoNi MEA was produced from high-purity elements (499.9% pure) which were arc-melted under argon atmosphere and drop-cast into rectangular cross-section copper moulds followed by cold forging and cross rolling at room temperature into sheets of roughly 10 mm thickness (Fig. 1a)
Summary
High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Between room temperature and 77 K, the alloy displays fracture toughness, KJIc, values at crack initiation that remain well above 200 MPa m1/2 associated with an increase in tensile strength (763-1,280 MPa) and ductility (0.5-0.7), making it not an ideal material for cryogenic applications but putting it among the most damage-tolerant materials in that temperature range[8] This excellent combination of properties can be related to progressively increasing strain hardening with hardening exponents above 0.4 (refs 7,8), it remains unclear why this particular combination of elements with very different crystal structures produces a single-phase microstructure[7,10,11,12,20,21], whereas many others with comparable configurational entropies do not[5]. Threedimensional atom probe tomography on the five-component CrMnFeCoNi alloy in the cast/homogenized state[21] and after severe plastic deformation[20] have shown that it retains its true single-phase character at the much finer atomic scale
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