Hybrid Er2O3 and B4C Additions Reshape Gamma-Ray Shielding in Ni–Co–Fe–Cr–Mo High-Entropy Alloys

High-entropy alloys have spent the better part of two decades earning their reputation as a radical rethink of metallurgy. Instead of building a metal around one dominant element with small dashes of additives, researchers mix five or more principal elements in roughly equal proportions, forcing the crystal lattice into a state of controlled chaos. The resulting materials can be extraordinarily strong, corrosion-resistant, and remarkably tolerant of radiation damage, which is precisely why nuclear engineers have begun eyeing them for reactors, radioactive waste containment, and medical shielding. A new study published in the Journal of Materials Science by İskender Özkul of Mersin University and colleagues takes this ambition a step further by asking a deceptively simple question: what happens when you stuff a Ni–Co–Fe–Cr–Mo high-entropy alloy full of two very different ceramic reinforcements at the same time?

The team chose erbium oxide (Er2O3) and boron carbide (B4C) as their additives, and the choice is anything but arbitrary. Erbium is a rare-earth element with a high atomic number and a well-documented appetite for absorbing gamma photons, while boron carbide is famous for capturing thermal neutrons thanks to the exceptional cross section of boron-10. Combining a gamma-absorbing oxide with a neutron-hungry carbide inside a single alloy matrix is an attractive strategy for producing a dual-purpose shielding material, one that could theoretically stand between personnel and mixed radiation fields without the need for layered composites of lead and borated plastics.

To test the concept, the researchers produced five distinct samples, labeled S0 through S4, using vacuum arc melting. This technique involves melting the constituent metals in a water-cooled copper hearth under high vacuum, repeatedly flipping and remelting the button-shaped ingots to ensure chemical homogeneity. The base alloy, S0, contained only the five principal elements in equiatomic proportions. The remaining samples received varying amounts of Er2O3, B4C, or both, allowing the team to isolate the effects of each reinforcement and then compare them against the hybrid combination. Before any melting took place, thermodynamic calculations confirmed that the metallic matrix satisfied the general criteria expected for solid-solution high-entropy alloy formation, including appropriate values of atomic size mismatch, mixing enthalpy, and valence electron concentration.

X-ray diffraction told a reassuring story about the underlying crystal structure. In every sample, regardless of reinforcement content, the face-centered cubic (FCC) matrix remained the dominant phase. This is significant because FCC high-entropy alloys tend to exhibit superior ductility and fracture toughness compared to their body-centered cubic counterparts, and preserving that structure while adding ceramics is a delicate balancing act. Diffraction peaks corresponding to Er2O3 appeared clearly in the oxide-containing compositions, confirming that the ceramic had survived the melting process as a distinct phase rather than dissolving into the matrix. Several weaker secondary reflections were also detected, but their low intensity and overlap with neighboring peaks prevented unambiguous phase assignment, a common challenge in heavily alloyed systems.

Scanning electron microscopy paired with energy-dispersive X-ray spectroscopy (SEM-EDS) provided a more intimate view of the microstructure. The analysis confirmed the presence of regions rich in erbium, oxygen, boron, and carbon, consistent with the successful incorporation of both ceramic phases. However, the maps also revealed local compositional variations within the microstructure, meaning the reinforcements were not distributed with perfect uniformity. Such heterogeneity is a well-known consequence of arc melting, where rapid solidification rates and limited diffusion can trap ceramic particles in clusters or leave compositional gradients across the ingot. These microstructural inhomogeneities would later prove relevant to the mechanical and radiation-shielding results.

On the mechanical front, the reinforcements delivered exactly what materials scientists hope for from ceramic particle strengthening. Microhardness measurements showed a clear and consistent increase across all reinforced samples compared to the unreinforced base alloy. The standout performer was sample S3, which contained the hybrid Er2O3 and B4C combination and achieved a microhardness of 693 ± 51 HV0.5. This value represents a substantial improvement over the baseline and demonstrates that the dual-ceramic approach can simultaneously stiffen the alloy without destroying the FCC matrix. The mechanism likely involves a combination of load transfer from the ductile matrix to the hard ceramic particles, grain refinement induced by the particles acting as nucleation sites, and Orowan-type dislocation pinning around fine dispersoids.

The radiation-shielding experiments, however, told a more nuanced story. The team measured gamma-ray transmission at four discrete energies: 276, 302, 356, and 383 kiloelectronvolts, using a high-purity germanium (HPGe) detector operated under narrow-beam geometry. This setup, in which collimators restrict the photon beam to a well-defined path through the sample, minimizes scattering contributions and allows for accurate determination of the linear attenuation coefficient. Sample S4 exhibited the lowest nominal transmission factor at most energies when measured at its actual physical thickness, suggesting it was the most effective barrier in raw terms.

But raw transmission numbers can be misleading when samples have different thicknesses. To make a fair comparison, the researchers applied the Beer-Lambert relationship to correct for thickness and extract the nominal linear attenuation coefficient for each composition. After this normalization, a different picture emerged: sample S2 generally showed the highest nominal linear attenuation coefficient, while S4 gave closely comparable values at several energies. The researchers were careful to note that some of the differences between samples fell within the propagated counting uncertainties, meaning they could not be interpreted as statistically meaningful distinctions. This level of methodological caution is refreshing in a field where small differences are sometimes oversold as breakthroughs.

Perhaps the most important takeaway from the study is the disconnect between mechanical performance and radiation-shielding performance. The composition that delivered the highest hardness, S3 with its hybrid reinforcement, did not necessarily provide the strongest thickness-normalized gamma-ray attenuation. The attenuation behavior turned out to be governed by a complex interplay of factors including reinforcement composition, sample thickness, overall density, phase distribution, microstructural heterogeneity, and measurement uncertainty. A harder alloy is not automatically a better shield, and optimizing for one property may come at the expense of another.

This finding carries real weight for anyone designing structural materials for nuclear environments. In reactor components, spent-fuel casks, or medical isotope production facilities, engineers must balance mechanical integrity, thermal stability, corrosion resistance, and radiation attenuation simultaneously. The Özkul and colleagues study demonstrates that these objectives cannot be treated as independent variables; the same microstructural features that enhance hardness may or may not enhance photon absorption depending on how the reinforcing phases are distributed and how they interact with the alloy matrix at the atomic scale. The authors conclude that reinforced high-entropy alloy composites must be optimized according to the specific mechanical and radiation-related property being targeted, rather than assuming that a single composition will excel across all metrics. As the field of superfunctional high-entropy alloys matures, studies like this one provide the empirical grounding needed to move from promising concepts to deployable materials, one carefully characterized sample at a time.

Subject of Research: Gamma-ray shielding and mechanical behavior of Er2O3- and B4C-reinforced Ni–Co–Fe–Cr–Mo high-entropy alloy composites

Article Title: Impact of hybrid Er2O3 and B4C additions on structural characteristics and experimental gamma-ray transmission behavior of Ni–Co–Fe–Cr–Mo high-entropy alloys

Article References: Özkul, İ., Güler, Ö., Durmus, H., Camgoz, B., ALMisned, G., Yaprak, G., & Tekin, H. O. (2026). Impact of hybrid Er2O3 and B4C additions on structural characteristics and experimental gamma-ray transmission behavior of Ni–Co–Fe–Cr–Mo high-entropy alloys. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13766-w

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13766-w

Keywords: high-entropy alloys, Er2O3, B4C, gamma-ray shielding, radiation attenuation, microhardness, vacuum arc melting, X-ray diffraction, HPGe detector, Beer-Lambert law, metal matrix composites, nuclear materials