Bifunctional nanoprecipitates strengthen and ductilize a medium-entropy alloy

Abstract

Single-phase high- and medium-entropy alloys with face-centred cubic (fcc) structure can exhibit high tensile ductility^1,2 and excellent toughness^2,3, but their room-temperature strengths are low^1-3. Dislocation obstacles such as grain boundaries⁴, twin boundaries⁵, solute atoms⁶ and precipitates^7-9 can increase strength. However, with few exceptions^8-11, such obstacles tend to decrease ductility. Interestingly, precipitates can also hinder phase transformations^12,13. Here, using a model, precipitate-strengthened, Fe-Ni-Al-Ti medium-entropy alloy, we demonstrate a strategy that combines these dual functions in a single alloy. The nanoprecipitates in our alloy, in addition to providing conventional strengthening of the matrix, also modulate its transformation from fcc-austenite to body-centred cubic (bcc) martensite, constraining it to remain as metastable fcc after quenching through the transformation temperature. During subsequent tensile testing, the matrix progressively transforms to bcc-martensite, enabling substantial increases in strength, work hardening and ductility. This use of nanoprecipitates exploits synergies between precipitation strengthening and transformation-induced plasticity, resulting in simultaneous enhancement of tensile strength and uniform elongation. Our findings demonstrate how synergistic deformation mechanisms can be deliberately activated, exactly when needed, by altering precipitate characteristics (such as size, spacing, and so on), along with the chemical driving force for phase transformation, to optimize strength and ductility.