Atomistic Investigation of Dislocations and Deformation Twins in BCC Transition Metals

Abstract

Body-centered cubic (BCC) transition metals (TMs), including Group VB (V, Nb, and Ta), GroupVIB (Cr, Mo, and W), and Fe, are structurally important engineering materials. Except for V, Cr and Fe, Nb, Ta, Mo, and W are refractory metals with melting points above 2700 K, high room-temperature hardness, and excellent chemical inertness, enabling extreme-service applications. Despite their importance, BCC TMs exhibit persistent mechanical anomalies, including low-temperature brittleness and a pronounced brittle-to-ductile transition (BDT). The BDT is believed to be governed mainly by dislocation mobility rather than nucleation. Plasticity in pure BCC metals is dominated by 1/2⟨1 1 1⟩ screw dislocations with gigapascal-level Peierls stresses at low temperatures, whereas edge and mixed dislocations have much lower lattice friction. Increasing temperature enhances dislocation mobility and activates additional slip systems, improving ductility. Polycrystalline BCC metals show weak work hardening, while single crystals exhibit orientation-dependent hardening arising from dislocation accumulation and junctions. However, dislocation reactions in BCC metals remain poorly understood. When dislocation plasticity is suppressed and under high stress condition, such as at low temperatures or in nanocrystalline materials, deformation twinning dominates. Twinning is common in Fe, Group VB metals, and Cr, but rare in bulk Mo and W. Nevertheless, extensive twinning has been observed in nanocrystalline Mo and W at room temperature, and its atomic-scale mechanisms remain unclear. Understanding the activation, interaction, and competition between dislocation slip and deformation twinning is therefore essential for elucidating the mechanical behavior of BCC metals. Addressing these issues requires atomistic models capable of capturing dislocation behaviors, twin nucleation, and defect interactions beyond first-principles scales. Large-scale molecular dynamics (MD) simulations are thus adopted which rely critically on accurate interatomic potentials. However, existing potentials often fail to describe both dislocation and twinning behavior. Motivated by these limitations, this Thesis develops a new interatomic potential for W and, together with other recent potentials, systematically investigates atomic-scale deformation mechanisms in BCC TMs.

The interatomic potential for W is based on the extended modified embedded-atom method (XMEAM). Compared with classical MEAM, XMEAM extends partial electron density functions and introduces distinct screening descriptions for embedding and pair interactions, improving the description of directional bonding and local atomic environments. The W-XMEAM potential reproduces key bulk properties of W and captures defect physics relevant to plastic deformation and fracture. It has the non-degenerate (ND) core structure and Peierls barrier of 1/2⟨1 1 1⟩ screw dislocations in quantitative agreement with density functional theory and accurately describes deformation twinning under general loading. With a computational cost about two to three orders of magnitude lower than machine-learning and foundation models, W-XMEAM enables large-scale MD simulations of extended dislocation networks and twin-dislocation interactions. Accuracy and efficiency make W-XMEAM a robust model for studying deformation mechanisms in BCC W at experimentally relevant hundreds-of-nanometer length and time scales.

Various interatomic potentials are used to investigate twin activation and growth under general stress loading and their competition with dislocation slip. The reduced-constraint (RC) slip method is applied to compute normal-stress-dependent generalized stacking fault enthalpy (GSFH). The restoring stress, the gradient of GSFH, provides a stress-based criterion for twin nucleation and unifies compression- and tension-induced twinning in W. Under compression, twinning is favored when the restoring stress approaches or crosses zero. Under tension, twinning occurs when the first local maximum in restoring stress is lower than the second, indicating a distinct tension-controlled multi-layer nucleation pathway. Compression also induces a twin-boundary (TB) transition from a reflection-symmetric to a near-isosceles structure, whereas tensile loading leaves the TB unchanged. The RC framework is further extended to include coupled normal and shear stresses. For interatomic potentials that correctly reproduce zero-stress generalized stacking fault energies (𝛾_RC-lines), the resulting normal-shear-coupled GSFH provides a quantitative description of twin nucleation under arbitrary direction of arbitrary loading. This framework directly predicts whether deformation proceeds by twinning or dislocation emission, offering a mechanistic criterion for deformation-mode activation in BCC TMs.

In BCC crystals, reactions between 1/2⟨1 1 1⟩ dislocations can form ⟨0 0 1⟩ junctions, yet their stability, strength, and role in work hardening remain unclear. We investigate the formation and stability of ⟨0 0 1⟩ junctions formed by coplanar intersections of 1/2⟨1 1 1⟩ screw dislocations in Group VB (Nb, Ta), Group VIB (Mo, W), and Fe at room temperature. Thirteen interatomic potentials are used, covering EAM, MEAM, and XMEAM formalisms and exhibiting either ND or degenerate (D) screw dislocation cores. Junction formability is found to depend strongly on the elastic properties and shows no correlation with core structure. In Nb, 1/2⟨1 1 1⟩ segments readily act as single-arm dislocation sources, generating additional 1/2⟨1 1 1⟩ dislocations and promoting plastic flow. Surface-induced kink nucleation causes ND-core dislocations to slip on {1 1 0} or {1 1 2} planes with low resolved shear stress, leading to anomalous slip. The analysis is extended to non-coplanar edge, mixed, and screw dislocation interactions, and the shear stresses required to unzip junctions are approximated at finite temperatures. A clear hierarchy in junction strength emerges: Nb shows the strongest resistance to unzipping relative to the critical resolved shear stress of a single 1/2⟨1 1 1⟩ screw dislocation, followed by Ta and Mo, whereas W exhibits negligible junction-mediated hardening. These trends agree with experiments and underscore the role of dislocation junctions in BCC work hardening.

By elucidating atomic-scale dislocation, junction, and deformation-twinning mechanisms in BCC TMs, this Thesis provides a solid foundation for future studies and design of BCC transition alloys.

Date of Award	6 May 2026
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	David Joseph SROLOVITZ (Supervisor) & Zhaoxuan WU (Supervisor)