Strain hardening in an interstitial high entropy alloy under cryogenic conditions

Cryogenic Deformation Response and Mechanical Properties

We investigated the cryogenic deformation response and underlying mechanisms of a carbon-doped interstitial high-entropy alloy (iHEA) with the nominal composition Fe49.5Mn30Co10Cr10C0.5 (at.%). By evaluating coarse-grained (6 μm, containing dispersed M23C6 nano-precipitates) microstructural variants, we observed a substantial enhancement in mechanical performance at 77 K. The ultimate tensile strength increased sharply to approximately 1000 MPa for the coarse-grained alloy and 1300 MPa for the fine-grained alloy, while maintaining an excellent tensile elongation of around 50%. This represents a massive improvement in strength compared to room temperature behavior, achieved with virtually no sacrifice in ductility.

Microstructural Evolution and Phase Transformation Mechanisms

Prior to loading, both microstructural variants exhibited a nearly single face-centered cubic (FCC) structure, with the HCP phase fraction remaining below 1%. Upon cryogenic tensile deformation, we detected a massive, deformation-driven phase transformation. The HCP martensite fraction increased continuously, reaching up to 75% in fractured coarse-grained specimens and 67% in fine-grained ones. Our transmission electron microscopy observations revealed that this transformation proceeds via the overlapping of wide stacking faults, driven by the cross-slip of stair-rod type stacking faults and mutual dislocation interactions.

Notably, mechanical twinning, which coexists with the transformation at room temperature in this alloy system, was entirely suppressed at 77 K. This mechanistic shift is dictated by the temperature-dependent reduction in stacking fault energy, which strongly favors direct FCC-to-HCP martensitic transformation over twinning under cryogenic conditions. While grain boundaries and precipitates initially hinder HCP plate growth at low strains, the overwhelming thermodynamic driving force at 77 K ensures that both coarse and fine-grained variants ultimately achieve similarly high HCP phase fractions at fracture.

Origin of the observed Extraordinary Strain Hardening

The extraordinary strain hardening observed at 77 K stems from two synergistic mechanisms operating across different strain regimes. At low strains (<10%), dislocation slip and stacking fault formation dominate the FCC matrix. The fine-grained variant exhibits higher initial strain hardening here due to dislocation pile-ups at grain boundaries and pinning by M23C6 precipitates.

As deformation progresses to medium and high strains, dynamic strain partitioning takes over. The gradually transformed HCP phase and the retained FCC matrix undergo efficient, damage-free co-deformation and load-sharing. The sustained, high-level transformation-induced plasticity (TRIP) effect, fueled by the low stacking fault energy and elevated flow stress at cryogenic temperatures, continuously delays necking. Consequently, the strain hardening rate does not decay as it does at room temperature; instead, it remains exceptionally high or even increases throughout the deformation regime. This unique combination of mechanisms establishes interstitially doped, metastable high-entropy alloys as a highly promising structural material class for demanding cryogenic applications.