Alloy design based on single–principal-element systems has approached its limit for performance enhancements. A substantial increase in strength up to gigapascal levels typically causes the premature failure of materials with reduced ductility. Here, we report a strategy to break this trade-off by controllably introducing high-density ductile multicomponent intermetallic nanoparticles (MCINPs) in complex alloy systems. Distinct from the intermetallic-induced embrittlement under conventional wisdom, such MCINP-strengthened alloys exhibit superior strengths of 1.5 gigapascals and ductility as high as 50% in tension at ambient temperature. The plastic instability, a major concern for high-strength materials, can be completely eliminated by generating a distinctive multistage work-hardening behavior, resulting from pronounced dislocation activities and deformation-induced microbands. This MCINP strategy offers a paradigm to develop next-generation materials for structural applications.

In the present work, low cycle fatigue (LCF) behavior of an equiatomic CoCrFeMnNi high entropy alloy (HEA) is correlated to the microstructural evolution at 550 °C. The fully reversed strain-controlled fatigue tests were conducted in air under strain amplitudes ranging from 0.2 to 0.8%. The measured cyclic stress response showed three distinct stages which include initial cyclic hardening followed by a quasi-stable cyclic response until failure. The rate and amount of cyclic hardening increased with the increase in strain amplitude. In comparison to common austenitic stainless steels, CoCrFeMnNi HEA shows comparable strength and improved LCF lifetimes at similar testing conditions. Electron-microscopy investigations after failure reveal no noticeable change in grain size, texture and annealing twins density. Initial cyclic hardening is attributed to dislocation multiplication and dislocation-dislocation as well as dislocationsolute atom interaction. The quasi-stable cyclic response is associated with the equilibrium between dislocation multiplication and annihilation, which also leads to the formation of heterogeneous dislocation structures such as ill-defined walls and cells, particularly at higher strain amplitudes. Besides this, the material exhibits serrated plastic-flow due to interactions between mobile dislocations and diffusing solute 2 atoms (such as Cr, Mn and Ni). Lastly, segregation in the form of Cr-and NiMn-enriched phases were observed near grain boundaries, which appears to have a detrimental effect on the fatigue life.

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