Nanoscale precipitates as sustainable dislocation sources for enhanced ductility and high strength

Significance

Precipitates in a material are traditionally thought of as dislocation obstacles that lead to elevated strength and reduced ductility. In contrast, recent experiments suggest that nanoscale precipitates facilitate both high strength and large ductility. To help resolve this apparent paradox, here we reveal that nanoprecipitates provide a unique type of dislocation sources that are activated at sufficiently high stress levels and render uniform plasticity by simultaneously serving as efficient dislocation sources and obstacles to dislocation motion, giving rise to sustained deformability. The findings can guide development of next generation of materials such as multiple-element alloys with precipitate engineering.

Abstract

Traditionally, precipitates in a material are thought to serve as obstacles to dislocation glide and cause hardening of the material. This conventional wisdom, however, fails to explain recent discoveries of ultrahigh-strength and large-ductility materials with a high density of nanoscale precipitates, as obstacles to dislocation glide often lead to high stress concentration and even microcracks, a cause of progressive strain localization and the origin of the strength–ductility conflict. Here we reveal that nanoprecipitates provide a unique type of sustainable dislocation sources at sufficiently high stress, and that a dense dispersion of nanoprecipitates simultaneously serve as dislocation sources and obstacles, leading to a sustainable and self-hardening deformation mechanism for enhanced ductility and high strength. The condition to achieve sustainable dislocation nucleation from a nanoprecipitate is governed by the lattice mismatch between the precipitate and matrix, with stress comparable to the recently reported high strength in metals with large amount of nanoscale precipitates. It is also shown that the combination of Orowan’s precipitate hardening model and our critical condition for dislocation nucleation at a nanoprecipitate immediately provides a criterion to select precipitate size and spacing in material design. The findings reported here thus may help establish a foundation for strength–ductility optimization through densely dispersed nanoprecipitates in multiple-element alloy systems.