Ductile-brittle-ductile transition in an electrodeposited 13 nanometer grain sized Ni-8.6Â wt.% Co alloy

A ductile-brittle-ductile transition in the fracture mode of the Ni-8.6 wt.% Co alloy with an average grain size of 13 nm was observed through increasing the strain rates from 1.04 × 10−5 to 1.04 s−1 at room temperature (RT). The Ni-Co alloy exhibited a limited plastic strain (about 1%) at the intermediate strain rates of 2.08 × 10−3 to 4.17 × 10−2 s−1, which was attributed to that in this strain rate range less dislocations or GB atoms would be activated. However, a gradual brittle-ductile transition occurred with the strain rate decreasing from 2.08 × 10−3 to 1.04 × 10−5 s−1. The lower strain rates allow the GB atoms diffuse easily, which would relax the stress concentration and hence enhance the ductility. Another brittle-ductile transition happened with increasing the strain rates from 4.17 × 10−2 to 1.04 s−1. The enhanced ductility at high strain rate can be explained by stress-assisted activation of GB atoms.