Body-centered cubic (BCC) refractory metals such as Mo are indispensable materials for micro/nano-electromechanical systems and integrated circuits owing to their high strength and thermal stability, and their interconnects and components must withstand complex mechanical loads such as vibration, impact, and electromigration- or thermally induced stress during service. However, the intrinsically low ductility of BCC metals severely limits the reliable performance of these components; in particular, Mo nanowires face widespread deformation localization and premature crack initiation at the nanoscale. Traditional ductility-enhancement strategies such as grain refinement or the introduction of second-phase particles are not applicable at the nanoscale: excessive refinement induces the inverse Hall–Petch effect and grain-boundary cracking, while the size and distribution of second-phase particles in low-dimensional structures are also difficult to control precisely.

Therefore, achieving high uniform ductility in nanoscale refractory metals to ensure the structural integrity and reliability of microdevices remains a key challenge. Designing dual-phase structures is a potential strategy to overcome the strength–ductility trade-off and achieve large uniform elongation: a soft FCC phase can accommodate plastic deformation while a hard BCC phase provides strength, and stress-induced martensitic transformation of a metastable phase can also trigger a transformation-induced plasticity (TRIP) effect to dissipate mechanical energy and significantly improve toughness. However, the high thermodynamic stability of the BCC phase makes such dual-phase nanostructures difficult to fabricate. Recent studies have shown that the incorporation of interstitial H can stabilize nonequilibrium phases in refractory metals at the nanoscale, providing a new pathway to enhance the ductility of refractory metals through H-mediated phase control while avoiding conventional hydrogen embrittlement (HE).

To address the above issues, a joint team comprising Wuhan University and other institutions conducted in-depth research using ZEPTOOLS' in-situ TEM measurement system. The team constructed a hydrogen-stabilized FCC/BCC dual-phase structure on the surface of molybdenum nanowires via high-density current or electron-beam irradiation, achieving approximately 41.4% uniform elongation along the [011] direction, and elucidated from the atomic scale the physical mechanism by which hydrogen-mediated two-step phase transformation and dislocation nucleation synergistically enhance ductility.

Title: Hydrogen-stabilized dual-phase architecture enables ductility in refractory molybdenum nanowires 
Journal: Nature Communications 
URL:https://doi.org/10.1038/s41467-026-79244-8

This study first focuses on the construction of the dual-phase structure and characterization of the phase-transformation process. Using single-crystal BCC Mo nanowires prepared by FIB, the research team formed an FCC phase on the nanowire surface by applying an electrical bias of approximately 400 mV to induce electromigration, or by electron-beam irradiation with a beam current density of approximately 10²–10³ A·cm⁻². Under continuous electron-beam irradiation, the FCC layer thickness is limited to within about 10 atomic layers at the surface and can be repeatedly modulated by the irradiation conditions. Real-time HAADF-STEM captured the angle between two sets of {110}BCC planes in the near-surface region gradually changing from 90° to 70.5°, corresponding to the angle between {111}FCC planes; the measured FCC phase lattice constant is a ≈ 4.17 Å, about 3% larger than the theoretical value for stress-free FCC-Mo (4.03 Å). The phase transformation proceeds layer by layer along the (011)BCC//(11−1)FCC semi-coherent interface, following the Nishiyama–Wassermann (N–W) orientation relationship of (011)BCC//(11−1)FCC and [100]BCC//[110]FCC. Geometric phase analysis indicates that this phase transformation is not primarily driven by large strain or curvature-related surface stress.

The study then focuses on the physical origin of FCC phase stability. Annular bright-field imaging shows no obvious contrast at octahedral interstitial sites in the FCC region, whereas clear contrast appears at these positions in the corresponding dCoM images, suggesting that interstitial impurity doping stabilizes the FCC phase. Experimental EELS spectra show that the plasmon peak energy E_p in the FCC region shifts by only about −0.1 to +0.3 eV relative to the BCC region, while DFT calculations combined with the Drude model show that the experimentally obtained E_p is much larger than that of pure FCC-Mo and falls exactly within the range from FCC-MoH₀.₅ to FCC-MoH. Atom probe tomography further ruled out significant enrichment of C, N, and O. Although the H signal in APT cannot serve as direct quantitative evidence for the presence of hydrogen, phonon calculations evaluated the stability of FCC-MoH under tensile strain and predicted that FCC-MoH₀.₅ is dynamically unstable under tensile loading, thereby favoring stress-induced FCC→BCC phase transformation. Combined experimental and theoretical evidence supports that the incorporation of interstitial hydrogen is the key factor stabilizing the FCC surface phase.

In addition, the study focuses on the plastic deformation mechanism of the dual-phase nanowires. Using ZEPTOOLS' in-situ sample holder, the team applied a quantitative uniaxial tensile load at a strain rate of 10⁻³ s⁻¹ along the axial direction (i.e., [011]) to dual-phase Mo nanowires with a diameter of about 15 nm inside the transmission electron microscope, and observed in real time that the surface FCC structure gradually transformed into BCC; a representative dual-phase sample exhibited crack initiation at the surface after about 45.2% uniform elongation, with the crack propagating along the (010) plane. A representative single-phase sample underwent only about 12.4% uniform elongation before crack initiation and catastrophic failure. Time-resolved HRTEM revealed that this phase transformation is not the single-step shear predicted by the classical N–W model, but rather a two-step shear path consisting of two consecutive T/4 fractional dislocation slips, with the interface angle α gradually increasing from 80.6° to nearly 90°. DFT calculations show that the energy cost of the first T/4 shear in the MoH₀.₅ system is about 0.09 eV·atom⁻¹, comparable to that of the subsequent step, highly consistent with the discrete phase-transformation process observed experimentally.

The study ultimately elucidates the intrinsic mechanism of ductility enhancement from the perspectives of energetics and dislocation dynamics. DFT calculations show that without hydrogen the BCC phase is the most stable; with increasing H content, the FCC phase becomes energetically favorable in the range 0.48<x<0.6, and H incorporation not only lowers the phase-transformation barrier but also reshapes the phase-transformation energy landscape into a non-monotonic profile resembling a generalized stacking fault energy curve. Analysis of the modified ductility parameter D=√(γ_s/γ_usf) based on the Rice criterion shows that the FCC phase has a lower unstable stacking fault energy γ_usf than the BCC phase and thus acts as a compliant soft phase; at low interfacial hydrogen content, its D value is significantly higher than that of the BCC matrix, thereby suppressing surface crack initiation in the early stage of deformation. Peierls barrier calculations show that the migration barrier of a screw dislocation in pure Mo is 65 meV·b⁻¹, decreases to 34 meV·b⁻¹ at low local H concentration, promoting dislocation motion, and increases to 83 meV·b⁻¹ at high concentration, exerting a pinning effect; however, the stress applied to the nanowire far exceeds the Peierls stress of screw dislocations, so mobility regulation is not the dominant factor for ductility. The synergistic effect of the TRIP effect and hydrogen-enhanced dislocation nucleation enables dual-phase Mo nanowires to achieve an average uniform elongation of about 41.4%, approximately 300% higher than that of single-phase BCC nanowires, and a similar BCC-to-FCC transformation is also observed in tungsten nanowires, providing a new strategy for the reliability design of refractory metal interconnects in advanced micro/nanodevices.

As a leading Chinese high-end precision instrument company, ZEPTOOLS is a first-class supplier of in-situ electron microscopy characterization solutions. Its PicoFemto series of in-situ transmission electron microscopy characterization solutions have successively provided technical support for landmark research achievements by users both domestically and internationally. The image below shows the ZEPTOOLS in-situ TEM sample holder product used in this research: