Combining laminated bimodal heterostructure and dislocation source limitation for superior strength-ductility synergy

Abstract

Many unremitting pursuits have been paid on designing microstructures to enhance the strength-ductility synergy of metallic materials. In this work, laminated bimodal heterostructure and dislocation source limitation were introduced together into pure Ni with various laminate thicknesses by electrodeposition and subsequent annealing to circumvent the limitations of the strength-ductility trade-off. The specimen, composed of alternative fine-grained and ultrafine-grained laminates with the optimized laminate thickness, had a combination of 601 MPa yield strength which was 4.8 times its Hall-Petch yield strength (124 MPa), and 24.5% reasonable ductility. The combination went beyond the range established by the reported pure Ni. When the fine-grained laminates started dislocation slip first and ultrafine-grained ones remained elastic, incompatible deformation between the two different laminates resulted in a high density of piled-up geometrically necessary dislocations (GNDs) against the interfaces, increasing the yield strength. Deformation incompatibility was also present in each of the individual laminates exhibiting bimodal structure, which further facilitated strengthening and strain hardening. Plentiful dislocation cells and walls were found to form inside the fine-grained laminates during work hardening, which could contribute to the production of GNDs. The carefully constructed heterostructures maximized the role of GNDs. Besides, dislocation source limitation induced further hardening, in the form of the yield-point behaviour, and ductilization which resulted from more room inside grains for the accumulation and interaction of dislocations. The superior strength-ductility synergy here paves a promising paradigm for designing high-performance structural metallic materials.