Overcoming strength-ductility trade-off of nanocrystalline metals by engineering grain boundary, texture, and gradient microstructure

The strength-ductility trade-off has been a long-standing dilemma for polycrystalline metals. Though many strategies such as introduction of gradient and twinned microstructure have been proposed to overcome the strength-ductility trade-off of nanocrystalline (NC) metals, a higher strength-ductility synergy is always called for. Here schemes taking advantage of texture engineering with incorporation of grain boundary (GB) strengthening and gradient microstructure design are proposed to achieve a more desirable strength-ductility synergy. Taking into account the size-dependent storage of dislocations in grains, GB sliding, and evolution of the void volume fraction, crystal plasticity finite element simulations incorporating the Gurson-type model are performed to reveal the mechanisms underlying the low ductility and strength-ductility trade-off of NC metals. Low GB strength and nonmonotonic variation of dislocation storage ability with respect to the grain size are revealed as the dominant factors responsible for the low ductility and the strength-ductility trade-off in NC metals. The speculation that there exists a most brittle grain size for NC metals is confirmed. It is also found that the ductility of NC metals is governed by the GB damage and the ability of intragranular dislocation storage at high GB strength. Moreover, it is shown that the failure tensile strain for cube-textured NC copper at a certain grain size could be more than twice the failure tensile strain for untextured one at high GB strength.