Control of dislocation density maximizing precipitation strengthening effect

The strength-ductility trade-off has been the most challenging problem for structural metals for centuries. Nanoprecipitation strengthening is an ideal approach to enhance the strength without significantly sacrificing the ductility. Stable nanoprecipitates have been successfully acquired by nanostructural design, but the number density of nanoprecipitates cannot be further increased. Researchers attempted to enhance number density by introducing highly potent nucleation sites (e.g., dislocations). However, there remains controversy over the influence of dislocations on the nucleation and growth of coherent nanoprecipitates with minimized nucleation barrier. Here, Cu-rich nanoprecipitates in an HSLA steel, as a typical type of coherent nanoprecipitates, are investigated. By combining analytical calculation and experiments, we show that dislocations are harmful for the formation of large numbered Cu-rich nanoprecipitates in a certain density range. Insufficient dislocations deprive solute atoms which decrease homogenous precipitation that cannot be compensated by the increase in heterogeneous precipitation. Under such circumstance, Cu-rich nanoprecipitates have smaller number density but larger size and higher fraction of incoherent structures due to rapid Ostwald ripening. As a result, by controlling dislocation density, the yield strength is increased by 24% without obvious loss in ductility as compared with traditional solution-quench-age process. Our work would help to optimize composition and processing routes that fully exploit the nanoprecipitation strengthening effect.