Compositive Role of Refractory Element Mo in Improving Strength and Ductility of Face-Centered-Cubic Complex Concentrated Alloys

Complex concentrated alloys (CCAs) containing multiprincipal elements have been investigated extensively for the last two decades due to the potential for extended mechanical properties from their wide range of compositional space. Particularly, CCAs having a face-centered-cubic (FCC) structured solid-solution containing various 3d-transition metal elements exhibit improved mechanical properties attributed to their enhanced solid solution strengthening, grain boundary strengthening, and strain hardening capability. Due to their distinctive mechanical properties, great efforts have been conducted to demonstrate the origin of improved mechanical properties. Most of these investigations, however, have been focused on the FCC CCAs composed of 3d-transition metal elements. The addition of other elements with a larger atomic size compared to 3d-transition metal elements usually forms secondary phases. Especially, even a small addition of Mo in FCC CCAs generally causes the formation of topologically close-packed phases such as m and s phases, inducing embrittlement of the alloys. Therefore, it is a lack of systematic study on the effect of refractory element Mo on the mechanical response in single-phase FCC CCAs. In this study, three alloys with different Mo content of the CoNiMo system were fabricated, which is predicted to maintain a single FCC with a wide range of Mo content. Interestingly, solid-solution strengthening and grain-boundary strengthening improve simultaneously with increasing Mo content. In particular, the highest Mo alloy exhibits a high yield strength of ~1 GPa attributed to their significant solid-solution strengthening of 229 MPa and Hall–Petch coefficient of 1028 MPa×mm^1/2. The effect of Mo element on solid-solution strengthening is demonstrated via increasing lattice distortion with increasing Mo content computed by <i>ab initio</i> calculation. The improved grain-boundary strengthening is also correlated to lattice distortion, and grain-boundary segregation of Mo leads to a further increase of the Hall–Petch coefficient. In addition to the strengthening contributions, the addition of Mo contributes to improving the strain hardening capacity, which is crucial for sustaining plastic deformation. The transition of dislocation and deformation twin substructures depending on the decrease of stacking fault energy on increasing Mo content makes the dislocation glide more difficult and leads to a higher strain hardening capability of the alloys.