Designing solid solution hardening to retain uniform ductility while quadrupling yield strength

Single-phase metals can be strengthened via cold work, grain refinement, or solid solution hardening. But the yield strength elevation normally comes at the expense of ductility, i.e., a conspicuous decrease of the uniform elongation in uniaxial tension. This strength-ductility trade-off is often a result of inadequate strain hardening rate that can no longer keep up with the elevated flow stress to prevent plastic instability. Here we alleviate this dilemma by designing oxygen interstitial solution hardening in body-centered-cubic niobium: the strain hardening rate is exceptionally high, such that most of the uniform tensile ductility of Nb can be retained despite of quadrupled yield strength. The oxygen solutes impose random force field on moving dislocation line, promoting the formation of cross-kinks that dynamically accumulate vacancy-oxygen complexes. These obstacles enhance the trapping/multiplication of screw dislocations as well as cross-slip, all promoting strain hardening and strain de-localization. This approach utilizes only a low concentration of interstitial solutes to achieve effective strengthening and strain hardening simultaneously, and is an inexpensive and scalable route amenable to the processing of bulk samples. © 2019 Acta Materialia Inc.