Hydrogen-stabilized dual-phase architecture enables ductility in refractory molybdenum nanowires

Enhancing the ductility of refractory body-centered cubic (BCC) metals at the nanoscale remains challenging because limited dislocation mobility can lead to premature brittle fracture. Here, we show that a face-centered cubic (FCC)/BCC dual-phase structure can be formed in Mo nanowires (NWs) via non-equilibrium electron-stream excitation (electric current or electron-beam irradiation). Atomic-scale characterizations and first-principles calculations indicate that interstitial hydrogen stabilizes the FCC phase at the NW surface. The dual-phase NWs exhibit ~41.4% uniform elongation along the [011] direction, corresponding to a ~300% increase relative to single-phase BCC NWs. The observed deformation involves two hydrogen-mediated processes: (1) during early plastic deformation, a two-step FCC-to-BCC transformation proceeds through fractional dislocation slip and relieves local stress concentrations, thereby suppressing surface crack initiation; (2) hydrogen promotes dislocation nucleation by modifying surface energy and unstable stacking-fault energy, thereby sustaining plastic flow and delaying failure. These results show that hydrogen-mediated phase transformation and dislocation nucleation enhance the deformability of nanoscale Mo.