Enhanced Elevated-Temperature Strength in Refractory Complex Concentrated Alloys via Temperature-Induced Transition from Screw-to-Edge Dislocation Control

Refractory complex concentrated alloys (RCCAs) show promise for high-temperature applications but often lose strength due to screw-dislocation-controlled plasticity. We demonstrate a temperature-driven transition from screw- to edge-dislocation-controlled deformation in a single-phase NbTaTiV RCCA. Tensile tests from 298-1573 K reveal a pronounced intermediate-temperature strength plateau and yield strengths surpassing other ductile RCCAs and the Ni-based superalloy CMSX-4 above 1273 K. In-situ neutron diffraction, TEM, and molecular dynamics identify a crossover near ~900 K, where edge dislocation glide stabilized by V-induced lattice distortion dominates, enabling enhanced strength retention and a clear design strategy for ultrahigh-temperature applications.

💡 Research Summary

The authors investigate the high‑temperature mechanical performance of a refractory complex concentrated alloy (RCCA) composed of equiatomic NbTaTiV, focusing on the underlying deformation mechanisms that govern strength retention at elevated temperatures. The alloy was produced by vacuum arc melting, multiple remelts, cold rolling to 1 mm thickness, and a high‑temperature anneal at 1473 K for 3 h, resulting in a fully recrystallized, single‑phase body‑centered cubic (BCC) microstructure with an average grain size of ~62 µm and a lattice parameter of 3.239 Å, as confirmed by SEM‑BSE, EDS, and neutron diffraction.

High‑temperature tensile tests were performed from 298 K to 1573 K in 200 K increments at a strain rate of 5 × 10⁻⁴ s⁻¹. The 0.2 % offset yield strength exhibits three distinct regimes: (i) a low‑temperature regime (≤ 573 K) where strength drops sharply with temperature, (ii) an intermediate‑temperature plateau (≈ 773–1173 K) where the yield strength remains relatively constant, and (iii) a high‑temperature regime (> 1173 K) where strength again declines due to diffusion‑controlled mechanisms such as dislocation climb and dynamic recrystallization. Notably, the alloy’s yield strength surpasses that of several ductile RCCAs (NbTaTi, Nb₄₅Ta₂₅Ti₁₅Hf₁₅, HfNbTaTiZr) and exceeds the Ni‑based single‑crystal superalloy CMSX‑4 above ~1273 K, maintaining useful strength up to 1573 K.

To uncover the deformation mechanism responsible for the plateau, the authors combined transmission electron microscopy (TEM), in‑situ neutron diffraction, and molecular dynamics (MD) simulations. TEM of specimens deformed to <2 % strain revealed that at 298 K and 573 K the microstructure is dominated by long, straight <111> screw dislocations on {110} slip planes. At 773 K, mixed‑character dislocations begin to appear alongside the screws, indicating the onset of a transition. By 973 K and 1173 K, the dislocation network becomes highly tangled with curved, edge‑dominant segments, showing that plastic flow is increasingly governed by edge dislocation glide.

In‑situ neutron diffraction was used to extract dislocation densities via a modified Williamson–Hall analysis that separates size and strain broadening contributions. At 298 K the data fit well to a screw‑dislocation model, whereas at 1173 K the edge‑dislocation model provides a superior linear fit. Intermediate temperatures (673 K and 873 K) display a gradual shift in the goodness‑of‑fit, confirming a continuous transition rather than an abrupt switch.

MD simulations employed a customized embedded‑atom method (MEAM) potential for NbTaTiV. Separate simulations of a/2 <111>{110} screw and edge dislocations were performed over 300–1200 K. The calculated flow stresses show a clear crossover near ~900 K: below this temperature screw dislocations require higher stress to glide (due to a high Peierls barrier), while above it edge dislocations become the easier carriers of plasticity because the lattice distortion introduced by vanadium reduces the edge‑dislocation barrier more than that for screws. This crossover aligns with the experimentally observed strength plateau.

The authors attribute the transition to the large atomic size misfit of V (≈ 8 % smaller radius than the other constituents), which generates substantial local lattice distortion. This distortion preferentially impedes screw dislocation motion (which relies on core reconstructions) while having a comparatively milder effect on edge dislocation glide. Consequently, as temperature rises, thermally activated processes enable edge dislocations to dominate, leading to a weaker temperature dependence of flow stress and thereby preserving strength at intermediate and high temperatures.

Beyond the primary screw‑to‑edge transition, the authors acknowledge additional contributions to the plateau, such as short‑range chemical ordering, cross‑core diffusion, and superjog nucleation, which may further stabilize the microstructure. Nevertheless, the dominant mechanism is convincingly demonstrated to be the temperature‑driven shift in dislocation character.

The paper concludes by proposing a “lattice‑distortion‑based alloy design” strategy: incorporating elements that create substantial size misfit (e.g., V) can be used to tailor the dominant dislocation type at service temperatures, enabling RCCAs that retain high strength where conventional Ni‑based superalloys fail. This insight opens a pathway toward ultrahigh‑temperature structural materials for aerospace, energy, and defense applications.