overcoming the trade-off between strength and ductility in high entropy alloys

Overcoming the strength-ductility trade-off in metals and alloys is one of the long-standing challenges of materials science. this talk presents some routes for solving this problem, focusing on cocrnialti and cocrfemnni multicomponent high entropy alloys (heas) subjected to different processing routes for property optimization. one strategy for resolving the strength-ductility trade-off utilizes tailored annealing-regulated precipitation strengthening combined with cold-working, although precipitation control and grain refinement are usually mutually contradictory due to the pronounced phase stability of multicomponent metallic alloys. however, proper high-temperature extrusion combined with annealing allows to tune and optimize the microstructures and mechanical properties of multicomponent hea alloys. thereby, hot extrusion effectively reduces the grain sizes of the matrix and simultaneously accelerates the precipitation of coherent l12 nanoparticles inside the fcc matrix and at grain boundaries, resulting in a strongly reciprocal interaction between dislocation slip and hierarchical-scale precipitates. subsequent annealing allows tuning grain sizes, dislocations, twins, and precipitates, further allowing to tailor the deformation characteristics and the overall mechanical properties of the material. the observed high yield strength is attributed to the coupled precipitation strengthening effects from nanoscale coherent l12 particles inside grains and from submicron grain boundary precipitates under the support of pre-existing dislocations. the excellent ductility results from the synergistic activation of dislocations, stacking faults, and twins during plastic deformation. alternatively, surface gradient nanostructuring through shot-blasting also allows to improve the mechanical properties of multicomponent high-entropy alloys. the severe plastic deformation (spd) of surfaces layers induced by shot-blasting creates a multi-scale hierarchical microstructure accompanied by a high density of stacking faults, along with deformation via nano-twinning and even the creation of local amorphous domains in the material. the depth of the spd layer steadily rises with increasing shot-blasting duration. the microhardness as well as the strength of the shot-blasted surface layer increase significantly while still retaining large elongation. nano-grain boundaries, together with stacking faults and twin strengthening within the gradient nanostructured surface layer are responsible for the significant strength increase. during tensile deformation, strain concentration initiates at the sample surface, and gradually spreads into the sample interior. thereby, the nanostructured surface layer with improved strain hardening can prevent early necking and assures steady plastic deformation which is essential for reaching high toughness. altogether, these results provide some examples for the different approaches that can be utilized for optimizing phase constitution and microstructures, thereby providing means for tailoring the deformation mechanisms and the overall mechanical properties of multicomponent alloys through proper alloy design and different processing routes.