Strengthening mechanism of CoCrFeMnNi high-entropy alloys with dual-gradient nanostructures: Grain size and twin boundary spacing

Both a grain size gradient and twinning-induced plasticity can achieve a synergistic effect of high strength and plasticity in metals/alloys. To investigate whether this effect is still produced at the nanoscale, a series of CoCrFeMnNi high-entropy alloy (HEA) atomic models with equiatomic ratios was designed, including homogeneous nanostructured (HNS), single-gradient nanostructured (SGNS), and dual-gradient nanostructured (DGNS) models, and others. The mechanical response of the CoCrFeMnNi HEA under uniaxial tensile loading was simulated by molecular dynamics (MD). Research has shown that, compared to the HNS samples, the mechanical properties of both the HEA samples with SGNS and DGNS are significantly improved, with DGNS producing better strengthening. The grain size gradient yields a certain stress/strain and dislocation density gradient distribution, stimulating the deformation potential of almost all the grains. Furthermore, it also suppresses mechanically driven grain boundary (GB) migration and grain rotation, promotes grain growth, weakens strain softening, and enhances the strengthening effect of GBs, all of which improve the mechanical properties of nanocrystalline HEAs. Twinning also weakens strain softening, and a reasonable twin boundary (TB) spacing and gradient can increase the number of dislocations and affect the direction and resistance to dislocation motion to achieve higher strength. The MD simulation shows that the DGNS improves the mechanical properties of nanocrystalline HEA on the basis of the SGNS, and the combination of a larger grain size gradient and smaller TB spacing gradient has the best strengthening effect. • Grain size gradient induces stress/strain and dislocation density gradient distribution • Both gradient nanostructures contribute to weakening strain softening • Both gradient nanostructures help to increase the dislocation density • GB migration and grain rotation are the main softening mechanisms of nanocrystalline HEA