Low-temperature deformation mechanism in a work-hardenable body-centered cubic high-entropy alloy with a large uniform elongation

Metals/alloys with a body-centered cubic (BCC) structure usually exhibit a high yield strength but suffer from poor work-hardening capability, leading to limited uniform elongation. This limitation becomes even more pronounced at low temperatures, where a ductile-to-brittle transition is commonplace. High-entropy alloys (HEAs), benefiting from their huge compositional space, provide an opportunity to achieve a large uniform elongation in BCC alloys. However, the lack of in-situ characterization techniques, especially at low temperatures, has challenged the determination of the underlying mechanisms. Here, using in-situ neutron diffraction measurements, in conjunction with microstructure observations, we identified the deformation mechanism responsible for an exceptionally large uniform elongation at liquid nitrogen temperature in a single-phase BCC (TiZrHf)_86.4Al₂Nb_11.6 HEA. We found that the initial plastic deformation is driven by a BCC-to-orthorhombic (known as α″) phase transformation, while twinning of the α″ phase and deformation-induced amorphization contribute to the ductility at the later stage. The cooperation of multiple deformation modes resulting from phase transformation overcomes the undesirable work-softening caused by dislocation-mediated plasticity, enabling a large uniform elongation while maintaining a high yield strength. The mechanism revealed through neutron diffraction demonstrates a feasible strategy by engineering deformation pathways to improve the low-temperature mechanical properties, thus providing guidance for developing advanced structural materials for cryogenic applications.