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• The intricate composition of the alloy plays a crucial role in generating chemical inhomogeneities, which are pivotal for creating the complex phase structure at the grain boundary. • To harness intergranular precipitation strengthening without compromising ductility, the intergranular nanoprecipitates can consist of multiple phases with varying compositions and structures. Despite differences in crystalline structure and orientation, the transport of dislocation plasticity can be sustained if the crystal planes conducive to dislocation glide are well-matched. • The complex structure of intergranular precipitates can create an undulated stress field near grain boundaries, enhancing the strengthening effect and facilitating multiple slip and cross-slip mechanisms during deformation. • Our material demonstrates a yield strength of approximately 1010 MPa and an ultimate tensile strength of around 1500 MPa, with a notable fracture elongation of 41 %. Precipitation at grain boundaries is typically not regarded as an efficient method for strengthening materials since it can induce grain boundary embrittlement, which detrimentally affects ductility. In this research, we developed a multi-principal element alloy (MPEA) with the composition Cr 30 Co 30 Ni 30 Al 5 Ti 5 (at.%), incorporating both intragranular and intergranular nanoprecipitates. Utilizing multiscale, three-dimensional, and in-situ electron microscopy techniques, coupled with computational simulations, we established that intergranular nanoprecipitation in this material plays a crucial role in enhancing strength and promoting dislocation plasticity. The structure of intergranular nanoprecipitation comprises multiple phases with varying composition and structure. Despite the diversity, the crystal planes conducive to the easy glide of dislocations are well-matched, allowing for the sustained continuity of dislocation slipping across different phase structures. Simultaneously, this structure generates an undulated stress field near grain boundaries, amplifying the strengthening effect and facilitating multiple slip and cross-slip during deformation. Consequently, it promotes the proliferation and storage of dislocations. As a result, our material exhibits a yield strength of approximately 1010 MPa and an ultimate tensile strength of around 1500 MPa, accompanied by a significant fracture elongation of 41 %. Our findings illuminate the potential for harnessing intergranular nanoprecipitation to optimize the strength-ductility trade-off in MPEAs, emphasizing the strategy of leveraging complex compositions for the design of sophisticated functional microstructures. [Display omitted] [ABSTRACT FROM AUTHOR]

• Dislocation behavior in a polycrystalline Mg-5Y alloy were quantitatively studied by in situ tensile test, visco-plastic self-consistent (VPSC) modeling, and TEM. • The critical resolved share stress (CRSS) values of basal , prismatic , pyramidal I , pyramidal I , and pyramidal II dislocation slip were measured. • The mobility of dislocations in Mg- Y alloy experimentally proved better than that in Mg-Ca alloy. • The reason behind the high ductility of Mg- Y alloys was clarified. In this study, the dislocation behavior of a polycrystalline Mg-5Y alloy during tensile deformation was quantitatively studied by an in-situ tensile test, visco-plastic self-consistent (VPSC) modeling, and transmission electron microscopy (TEM). The results of the in-situ tensile test show that dislocations contribute to most of the deformation, while a small fraction of dislocations are also activated near grain boundaries (GBs). The critical resolved shear stresses (CRSSs) of different dislocation slip systems were estimated. The CRSS ratio between prismatic and basal dislocation slip in the Mg-Y alloy (~13) is lower than that of pure Mg (~80), which is considered as a major reason for the high ductility of the alloy. TEM study shows that the dislocations in the alloy have high mobility, which also helps to accommodate the deformation near GBs. [Display omitted] [ABSTRACT FROM AUTHOR]