Strengthening by Engineering Chemical Segregation at Dislocations in a Concentrated Solid Solution Alloy

Abstract

Upon ageing of a defect-ridden metal, compositional segregation to dislocations and stacking faults is well known to elevate strength.  However, such Suzuki segregation effects deliver a strength increment merely on the order of 10 MPa for many substitutional face-centered-cubic solid solutions.  Severe pre-deformation can lead to significant hardening but sacrifices large tensile ductility after subsequent aging.  Here we propose a novel strategy to improve the Suzuki hardening effect in a single-phase CoCrNi alloy by carefully controlling repetitive straining and annealing conditions without sacrificing ductility.  It was revealed that multiple stages of annealing along the way of pre-straining increase the fraction of bogged-down dislocations collecting partitioning species (i.e. Cr), to a level well beyond that achievable via one-shot annealing after straight pre-deformation to the same accumulative strain (40%).  Thermodynamically, the segregation of Cr into stacking faults is driven by reduced 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 widened 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 that of single-pass treated samples (18 ± 8 MPa), while retaining an adequate strain hardening rate, such that there is no sacrifice of tensile ductility despite of plastic flow at higher stresses.  Our strategy holds promise for broader application, especially in those cases where routine thermomechanical treatment fails to produce satisfactory results.

Keywords: Chemical Segregation, Solid Solutions, strength and ductility, dislocations, strain hardening