Experiment finds CrMnFeCoNi hardens as nanograins grow under torsion

Severe plastic deformation usually makes a metal harder by driving its grains down to nanometre dimensions, but an experiment on the CrMnFeCoNi high-entropy alloy produced the opposite trend within its nanocrystalline range. Researchers subjected the alloy to high-pressure torsion at temperatures from 77 to 673 kelvin and found that room-temperature microhardness generally increased as the deformation temperature rose and the average grain size grew. The investigators attribute the anomalous hardening primarily to sliding along grain and phase boundaries, consistent with inverse Hall–Petch behavior, but note that short-term annealing after deformation may also have contributed.

The material was the equiatomic, face-centered-cubic alloy commonly called the Cantor alloy, a five-element solid solution containing chromium, manganese, iron, cobalt and nickel. The study cites a stacking fault energy of approximately 30 ± 5 millijoules per square metre at room temperature, a comparatively low value associated with the material’s capacity for twinning and, under sufficiently severe deformation, transformation to a hexagonal close-packed structure. High-pressure torsion combines high pressure with large shear strain, allowing the researchers to vary deformation temperature while tracking changes in phase stability, microstructure, crystallographic texture and microhardness.

For the experiments, the team arc-melted and drop-cast the alloy under high-purity argon. The ingots were homogenized for 48 hours at 1,473 kelvin, after which disks with a radius of 4 millimetres and an initial thickness of about 0.8 millimetres were cut from them. The disks were deformed under a quasi-hydrostatic nominal pressure of 7.8 gigapascals at 77, 293, 473, 573 or 673 kelvin.

Following five rotations, the estimated maximum shear strain was about 160. Synchrotron X-ray diffraction and Rietveld refinement were used to identify phases and estimate their volume fractions, while diffraction peak broadening was used to estimate average crystallite sizes. The mean error in the refined phase fractions was approximately 0.4 percent.

At 77 kelvin, high-pressure torsion partially transformed the original face-centered-cubic alloy into a hexagonal close-packed phase, described as ε-martensite. Quantification indicated that the transformed phase accounted for 67 percent of the material at a shear strain near 100. The transformation was therefore a major part of the low-temperature microstructure rather than a minor secondary effect.

Diffraction measurements also showed that the mean grain size of the surviving face-centered-cubic phase increased from approximately 35 nanometres after deformation at 77 kelvin to about 50 nanometres at 473 kelvin. The hexagonal phase formed at 77 kelvin had a mean size of roughly 45 nanometres, showing that both phases were refined to comparable nanoscale dimensions. At the highest deformation temperature, 673 kelvin, the alloy decomposed into body-centered-cubic and tetragonal σ phases.

Their combined fraction was approximately 1.9 percent at a shear strain near 50 but rose to about 11 percent near a strain of 100, demonstrating that decomposition was strongly dependent on accumulated strain. Severe deformation creates vacancies, dislocations and grain boundaries that enhance atomic diffusion and provide nucleation sites, allowing precipitation to proceed more rapidly than in coarse-grained, undeformed material. Despite this precipitation, the mean grain size at 673 kelvin reached about 130 nanometres by diffraction and 120 nanometres in electron backscatter diffraction.

The latter analysis showed a globular structure consistent with continuous dynamic recrystallization and a bimodal grain-size distribution. Additional measurements helped distinguish the proposed explanation from other microstructural changes. Above room temperature, the dislocation density in the face-centered-cubic phase decreased with increasing deformation temperature, consistent with enhanced dynamic recovery and greater atomic mobility.

The dislocation density was also lower after deformation at 77 kelvin, possibly because of self-annealing and because the hexagonal phase became unstable during unloading or reheating and reversed toward the face-centered-cubic structure. These recovery effects would ordinarily be associated with reduced hardness, so they did not account for the observed increase. The crystallographic texture remained weak and changed little across most processing temperatures; accordingly, the researchers judged its influence through variations in the Taylor factor to be negligible.

Texture randomization was consistent with a contribution from boundary sliding, although it was not direct proof of that process. Room-temperature microhardness increased along the radial shear-strain gradient within each disk and generally saturated above a shear strain of about 60. The annealed, undeformed alloy had a Vickers microhardness of 123, while the values measured after torsion were higher.

When saturation hardness was plotted against deformation temperature, it followed an almost linear upward trend, apart from additional hardening caused by precipitation at 673 kelvin. After an approximate subtraction of that precipitation contribution, the 673-kelvin point also fell close to the trend. The alloy processed at room temperature was harder than the material processed at 77 kelvin, reversing the expectation that the more transformed and more highly refined low-temperature specimen would be harder.

Measurements at 77 and 673 kelvin had relatively large scatter, associated with inhomogeneous deformation at low temperature and continuing precipitation at high temperature. The central interpretation is an inverse Hall–Petch regime. Conventional Hall–Petch strengthening predicts that smaller grains impede dislocation motion more effectively and therefore make a polycrystalline material harder.

In this study, however, microhardness increased with grain size across the face-centered-cubic phase, from roughly 35 to 130 nanometres. The authors connect this reversal to sliding along grain and phase boundaries in an exceptionally fine, highly defective structure. Boundaries generated by severe deformation are far from equilibrium and can support relatively rapid boundary-mediated deformation.

Shear transformation zones, comprising clusters of closely packed atoms that rearrange cooperatively to accommodate local shear, were discussed as a plausible mechanism for nanoplasticity. The interpretation contrasts with ultrafine-grained materials governed by conventional Hall–Petch strengthening, where hardness decreases as grain size increases and recovery becomes more influential. The experiment has an important limitation arising from the time between deformation and measurement.

Warm specimens remained at or near their deformation temperature for roughly 30 to 60 seconds while being unloaded and removed from the anvils, making some static annealing unavoidable. Earlier work cited in the study found that a 30-second heat treatment at 723 kelvin after room-temperature high-pressure torsion could increase hardness by about 30 Vickers units, although phase decomposition may also have contributed to that increase. Because the present microhardness measurements were made only after deformation and cooling to room temperature, they cannot cleanly separate deformation-state hardening from subsequent annealing effects.

Deformation at 77 kelvin was also heterogeneous, as indicated by torque behavior reported in related work and by the wide scatter in indentation measurements. Such localization can produce regions with different strain, microstructure and hardness, adding uncertainty to comparisons among processing temperatures. Overall, the study shows that reducing a high-entropy alloy to nanometre scale does not automatically produce stronger material in the conventional sense.

In CrMnFeCoNi processed by high-pressure torsion, larger grains within the relevant nanostructured range were associated with greater room-temperature hardness rather than lower hardness. The combined phase, grain-size, dislocation-density and texture measurements point to grain- and phase-boundary sliding as the principal explanation, while short-term annealing may also have contributed. These findings apply to the tested Cantor alloy and processing conditions rather than establishing a universal rule for high-entropy alloys.

The authors propose future work using in situ torque measurements during torsion to determine flow stress directly and better distinguish deformation mechanisms from annealing effects.

Subject of Research: Temperature-dependent nanostructuring and anomalous microhardness in high-pressure-torsion-deformed CrMnFeCoNi high-entropy alloy

Article Title: Anomalous hardening of CrMnFeCoNi high-entropy alloy deformed by high-pressure torsion

Article References: Chulist, R., Hohenwarter, A., Pippan, R., Schell, N., & Skrotzki, W. (2026). Anomalous hardening of CrMnFeCoNi high-entropy alloy deformed by high-pressure torsion. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13649-0

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13649-0

Keywords: CrMnFeCoNi, high-entropy alloy, high-pressure torsion, inverse Hall–Petch effect, nanostructured metals, grain-boundary sliding, severe plastic deformation, phase transformation, microhardness, alloy hardening, Anomalous, hardening