Chromium Trade-Off Revealed: Stronger Corrosion Shield, Softer Alloy in High-Entropy Metal

High-entropy alloys have long promised a new era of metals designed not around one dominant element, but around the deliberate chaos of five or more principal components mixed in nearly equal proportions. A new open-access study published in the Journal of Materials Science: Metallurgy has now put one of the most intriguing refractory members of this family under the microscope, asking a deceptively simple question: what happens when you add chromium to the AlMoNbTi high-entropy alloy? The answer, delivered through nanoscale indentation, electrochemical spectroscopy, and detailed microstructural imaging, is a compelling trade-off that materials scientists will be parsing for years. Chromium, it turns out, makes this rugged alloy dramatically better at resisting corrosive attack in saltwater, cutting corrosion current by nearly half and shrinking pitted surface area by almost forty percent. The price, however, is a measurable softening of the material and a shift toward a more chemically and mechanically heterogeneous microstructure.

The research team, led by Nafiz Ahmed Badhan and S M Yeasin Habib of Lamar University together with colleagues at Idaho National Laboratory and Clemson University, synthesized two alloys by vacuum arc melting: the four-element base alloy AlMoNbTi and its five-element counterpart AlCrMoNbTi, with chromium added in equimolar proportion. Both ingots were remelted at least five times to homogenize their chemistry and then subjected to hot isostatic pressing at 1200 degrees Celsius under 100 megapascals of pressure for four hours, a treatment designed to eliminate the casting porosity that plagues arc-melted refractory alloys. By removing such artifacts before testing, the authors ensured that the hardness values and corrosion currents they measured reflected the intrinsic character of each composition rather than flaws introduced during processing.

Microstructural analysis told the first part of the story. Backscattered electron imaging in the scanning electron microscope revealed that both alloys share a three-region architecture: a grey matrix, white island-like features, and black precipitates. Adding chromium enlarged the grey regions and increased the density of black, titanium-rich particles. Energy-dispersive X-ray spectroscopy mapping showed that aluminum dissolves relatively uniformly, while the white regions are enriched in aluminum, molybdenum, and niobium, the grey regions concentrate titanium and chromium, and the black particles are titanium-rich precipitates. Crucially, the alloy remains body-centered cubic with an ordered B2 superlattice, a structure long associated with the room-temperature brittleness of aluminum-containing refractory high-entropy alloys. The chromium addition did not dismantle this framework, but it did intensify elemental segregation within it, a change with profound consequences for how the material deforms and corrodes.

Nanoindentation, performed with a Hysitron TI 980 Triboindenter and a Berkovich tip at a maximum load of 20 millinewtons, captured the mechanical fingerprints of that segregation. The base AlMoNbTi alloy displayed hardness values ranging from 9.97 to 14.41 gigapascals, with a single, well-defined peak in the hardness distribution near 12.25 gigapascals. The chromium-containing alloy behaved very differently: its hardness distribution became bimodal, with one peak near 12.25 gigapascals and a second near 9.25 gigapascals, and its load-displacement curves scattered far more widely. Of 66 analyzed indents, roughly 42 percent landed on the softer phase. The overall average hardness of AlCrMoNbTi fell to 10.81 gigapascals, an 11.68 percent decrease relative to the base alloy, even as the reduced modulus rose modestly by about 3.1 percent to 200.57 gigapascals.

The authors trace this localized softening to a subtle disruption of crystallographic order. In the B2 structure of AlMoNbTi, aluminum and molybdenum preferentially occupy one sublattice while niobium and titanium occupy the other, and this long-range order strengthens the material by forcing dislocations to glide in paired super-dislocations across anti-phase boundaries. Drawing on prior work showing that chromium-enriched, titanium-depleted regions wet B2 domains with a more disordered A2-like phase, the team argues that chromium locally destabilizes the B2 superlattice and promotes a softer, chemically homogeneous A2 body-centered cubic phase. That loss of anti-phase-boundary strengthening, rather than the formation of hard Laves phases, which appear only in small volume fractions, best explains the bimodal hardness and the 11.68 percent softening. Notably, both alloys remain considerably harder than many other body-centered cubic high-entropy alloys reported in the literature.

The corrosion story is where chromium truly earns its reputation. Using electrochemical impedance spectroscopy in a 3.5 weight percent sodium chloride solution, the same brine concentration that approximates seawater, the researchers found that the chromium-containing alloy exhibited a 4.5 percent higher charge transfer resistance, meaning ion exchange at the metal-electrolyte interface slowed. More striking were the changes in the dielectric properties of the surface: effective double-layer capacitance dropped by 75.8 percent, and the phase-shift exponent moved 10.7 percent closer to the ideal capacitive value. Under the Helmholtz model, lower capacitance corresponds to a thicker protective layer, indicating that chromium promotes the growth of a denser, more ideal passive film on the alloy surface.

Potentiodynamic polarization tests reinforced the picture. The corrosion potential shifted positively from minus 403 to minus 356 millivolts versus the saturated silver-silver-chloride reference electrode, and the corrosion current plummeted by 44.2 percent, from 52 to 29 nanoamperes per square centimeter. Pitting potentials exceeded 1 volt versus the reference in both alloys, evidence of excellent resistance to passive film breakdown, though the chromium-bearing alloy showed a distinct secondary passivation region at potentials above 1.7 volts relative to its corrosion potential. This secondary passivation, the authors explain, is the signature of chromium’s celebrated repassivation ability: when the protective chromium oxide film breaks down at high anodic potentials, dissolved trivalent chromium ions hydrolyze inside incipient pits to form a chromium hydroxide barrier that stifles the pit and allows a new chromium-rich passive layer to reform.

Surface imaging after the polarization experiments made the improvement visible to the eye. The base AlMoNbTi alloy corroded in clustered, non-uniform patches, consistent with preferential attack along galvanically coupled, aluminum-rich pathways in the ordered sublattice network. The chromium-containing alloy, by contrast, showed a far more random and even distribution of pits, suggesting that chromium’s disruption of the ordered structure created a chemically more homogeneous surface with fewer weak points. Quantitative image analysis with ImageJ revealed that the average pitted area, as a percentage of the surface, fell from 20.02 percent to 12.28 percent, a reduction of approximately 38.66 percent attributable to chromium addition.

The authors ground these observations in thermodynamics and strengthening theory. Chromium raises the alloy’s valence electron concentration from 4.5 to 4.8, still comfortably within the body-centered cubic regime, and its smallest atomic radius in the five-element group increases lattice distortion and the atomic size mismatch parameter, which helps explain the heightened segregation. Calculations of solid-solution strengthening show that chromium itself contributes the largest single increment, roughly 1112 megapascals, more than aluminum at 705 megapascals, and that the total solid-solution strengthening of the disordered A2 phase reaches about 2060 megapascals. Combined with an estimated 68 megapascals from Orowan-type precipitation strengthening by the titanium-rich particles, the calculated hardness of the soft phase, about 7 gigapascals, lands reasonably close to the measured 9.15 gigapascals, with the residual gap attributed to grain and phase boundary strengthening and impurity effects.

The broader significance of the study lies in its demonstration that alloying additions in high-entropy systems cannot be judged by a single metric. Chromium simultaneously strengthens the passive film, enables self-healing repassivation, redistributes and suppresses pitting, and yet softens the load-bearing matrix by eroding B2 order. For engineers contemplating refractory high-entropy alloys for marine, chemical, or high-temperature service, the message is that composition must be tuned against the full property envelope. The research, funded by the U.S. National Science Foundation under award number 2138674, provides both a rigorous experimental baseline and a mechanistic framework for that tuning, showing that even within a family of famously complex metals, a single element can rewire the balance between durability and strength.

Subject of Research: Chromium alloying effects on the mechanical and corrosion properties of the AlMoNbTi high-entropy alloy

Article Title: Effects of Cr addition on the mechanical and corrosion properties of the AlMoNbTi high-entropy alloy

Article References: Badhan, N. A., Habib, S. M. Y., Fan, Z., Fan, X., Zhang, X., & Sun, C. (2026). Effects of Cr addition on the mechanical and corrosion properties of the AlMoNbTi high-entropy alloy. Journal of Materials Science: Metallurgy, 1(1), Article 18. https://doi.org/10.1007/s44492-026-00018-w

Image Credits: AI Generated

DOI: 10.1007/s44492-026-00018-w

Keywords: high-entropy alloy, AlMoNbTi, chromium addition, nanoindentation, corrosion resistance, electrochemical impedance spectroscopy, potentiodynamic polarization, pitting corrosion, B2 ordering, passive film, refractory alloy, segregation