Enhanced strength-ductility synergy of medium-entropy alloys via multiple level gradient structures

The microstructures, mechanical properties, and deformation substructures of gradient Mo0.3NiCoCr medium-entropy alloys (MEAs) with very coarse grain size created by pre-torsion have been investigated. The strength of MEAs increases with the increase of torsion angle, while the tensile elongation nearly remains the same, suggesting the enhanced strength-ductility synergy. The initial dislocation density gradient structure after torsion and the following deformation sub-structure under tension are uncovered by means of electron backscatter diffraction (EBSD), electron channeling contrast imaging (ECCI), and transmission electron microscopy (TEM). The crystal plasticity finite element method (CPFEM) is employed to quantitively evaluate the evolution of dislocation densities and mechanical twinning volume fraction. The combination of experimental characterization and theoretical modeling enables to clarify the underlying strengthening and strain hardening mechanisms. The gradient distribution of dislocations created by the torsion leads to the rise of yield strength. Moreover, the high order of microbands, which arise from the activation of multiple slip systems during torsion, and additional mechanical twinning form in the gradient MEAs upon loading, constituting multiple level gradient structures. As the plastic strain goes on, the microbands can propagate and refine continuously, along with the interactions with the nano twins, in these MEAs with very coarse grain size up to similar to 500 mu m, which produce progressively high strain hardening and stabilize the plastic deformation over the whole deformation regime. This study thus offers guidance for optimizing the mechanical performance of structural materials via tuning the design of gradient structure.