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A fracture-resistant high-entropy alloy for cryogenic applications

Science  05 Sep 2014:
Vol. 345, Issue 6201, pp. 1153-1158
DOI: 10.1126/science.1254581

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A metal alloy that is stronger when cold

Metal alloys normally consist of one dominant element, with others in small amounts to improve specific properties. For example, stainless steel is primarily iron with nickel and chromium but may contain trace amounts of other elements. Gludovatz et al. explored the properties of a high-entropy alloy made from equal amounts of chromium, manganese, iron, cobalt, and nickel. Not only does this alloy show excellent strength, ductility, and toughness, but these properties improve at cryogenic temperatures where most alloys change from ductile to brittle.

Science, this issue p. 1153

Abstract

High-entropy alloys are equiatomic, multi-element systems that can crystallize as a single phase, despite containing multiple elements with different crystal structures. A rationale for this is that the configurational entropy contribution to the total free energy in alloys with five or more major elements may stabilize the solid-solution state relative to multiphase microstructures. We examined a five-element high-entropy alloy, CrMnFeCoNi, which forms a single-phase face-centered cubic solid solution, and found it to have exceptional damage tolerance with tensile strengths above 1 GPa and fracture toughness values exceeding 200 MPa·m1/2. Furthermore, its mechanical properties actually improve at cryogenic temperatures; we attribute this to a transition from planar-slip dislocation activity at room temperature to deformation by mechanical nanotwinning with decreasing temperature, which results in continuous steady strain hardening.

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