Upon ageing of a deformed metal, compositional segregation to dislocations and stacking faults is well known to elevate strength. However, Suzuki segregation effects typically result in a modest strength increase on the order of 10 MPa for many substitutional face-centered-cubic solid solutions. Severe pre-deformation can lead to significant hardening but often at the cost of substantial tensile ductility after subsequent aging. Here we propose a novel strategy to improve the Suzuki hardening effect in a single-phase CoCrNi alloy by meticulously controlling repetitive straining and annealing conditions without compromising ductility. Our findings revealed that multiple stages of annealing along the way of pre-straining significantly increase the fraction of dislocations that trap partitioning species (i.e. Cr), far exceeding the levels achievable through single-shot annealing after straight pre-deformation to the same accumulative strain (40 %). Thermodynamically, the segregation of Cr into stacking faults is driven by reduced local stacking fault energy (SEF) and system energy. The decreased SFE inhibits dislocation cross-slip, promotes partial dislocation nucleation, and facilitates dislocation intersection, leading to a high density of extended stacking fault ribbons in the multi-pass strained and annealed samples. As a result, the yield strength increments of multi-pass treated samples (75 ± 10 MPa) are four times higher than those of single-pass treated samples (18 ± 8 MPa), while retaining an adequate strain hardening rate, thus preserving tensile ductility despite of plastic flow at higher stresses. Our strategy shows promise for broader applications, particularly in scenarios where conventional thermomechanical treatments fail to yield satisfactory results.

Link:Amplifying Suzuki segregation and hardening in a concentrated solid solution alloy - ScienceDirect