Architecting unusual dual-gradient structures to overcome the strength-ductility trade-off in metallic materials

Abstract

It has been a longstanding challenge for metallic materials with homogeneous structures to escape the common strength-ductility trade-off dilemma. In this work, an unusual dual-gradient structure, consisting of both grain size gradient and strain gradient, was introduced into commercial pure titanium wires by mediating torsion strain and annealing parameters to address this issue. Microstructural characteristics of the annealed and tensile samples were systematically characterized by electron backscattered diffraction and transmission electron microscope to unveil the origin and influence of dual-gradient structure. The results show that the strain and grain size distributions can be controlled by tuning torsion strain followed by partial recrystallization annealing. Specifically, when the torsion angles are 40π and 50π, the corresponding annealed samples are characterized with a gradient decline of both grain size and residual strain from the center to the surface, while the samples with a 20π torsion angle exhibit the opposite gradient characteristics. Compared with homogeneously structured pure titanium with a yield strength of 268 MPa, the samples with a 40π torsion angle shows a remarkable yield strength of 379 MPa, which increases by 40 %, while without sacrificing the uniform elongation. This improved strength-ductility synergy can be attributed to the combined effects of fine grain strengthening, dislocation strengthening, and back stress strengthening, endowed by dual-gradient structure. Dynamic strain adjustments throughout the deformation process help to minimize the hardness difference between soft and hard zones in the heterostructures, promoting better deformation coordination between various layers and ultimately inducing a decent ductility of dual-gradient structure. This study not only offers a new insight and guidance to solve the strength-ductility trade-off dilemma, but also provides an attractive method for industries to fabricate dual-gradient structure in a low-cost and simple way.