The team began at the source, manufacturing their own lightweight aggregates from a blend of clay and volcanic ash. By tuning the sintering regime—heating granules at 5 °C per minute, preheating at 400 °C for 20 minutes, then firing between 1100 and 1250 °C—they could dial in the pore structure, density, and strength of each batch. Raising the sintering temperature from 1100 to 1200 °C melted the glassy phase progressively, forming a continuous surface glaze and a uniform closed-pore structure that lowered both density and water absorption while boosting strength. Pushing the temperature to 1250 °C backfired: excessive softening of the glassy phase allowed bubbles to coalesce, enlarging pores and thinning their walls until the aggregates weakened. Holding time followed a similar pattern, with the best cylinder compressive strength of 10.7 MPa achieved after 20 minutes at temperature. Longer grinding of the raw materials made aggregates lighter but also more porous and fragile, since finer feedstocks foamed more vigorously during firing.

With a library of aggregates spanning density grades from 600 to 1200 kg/m³, the researchers turned to the fresh state. They developed a simple but powerful evaluation protocol: after vibrating a concrete specimen, partition plates divided it into upper, middle, and lower thirds, and the aggregates recovered from each layer were weighed. From these layer-by-layer mass fractions they computed a floating index, which measures the excess of aggregates in the top two layers relative to the bottom, and a coefficient of variation, which captures overall spatial non-uniformity. For a perfectly homogeneous mix, each layer would hold roughly one third of the aggregates; any deviation signals redistribution. Vibration duration proved critical—at 20 seconds of vibration both indices rose sharply as the paste’s flocculated structure broke down, so the team standardized on 10 seconds of vibration at 50 Hz to keep the comparisons meaningful.

The aggregate experiments delivered a clear hierarchy of influence. As density grade increased from 600 to 1200, both the floating index and the coefficient of variation fell substantially, confirming that the density difference between aggregate and paste is the primary driving force for upward migration. Particle size came next: larger aggregates, fewer in number at the same volume fraction and presenting less surface area to the paste, floated more readily. Morphology and gradation played secondary but measurable roles. Spherical aggregates, with their smooth surfaces and low rolling resistance, drifted upward more easily than irregular ones, while continuously graded mixes outperformed single-sized ones because smaller particles fill the gaps between larger ones, creating mechanical interlock that resists migration. The worst case—low-density, large, spherical, single-sized aggregates—produced floating indices as high as 47.9 percent.

Paste parameters told an equally instructive story. Increasing the water-to-binder ratio raised both segregation indices roughly linearly, because additional free water thins the paste and erodes its capacity to suspend particles. The researchers also introduced an elegant volumetric metric, the excess filling rate, defined as the volume of mortar beyond what is needed to fill the voids between packed aggregates, normalized by that void volume. At low filling rates, frequent aggregate-to-aggregate contacts provide mechanical restraint; as the rate climbs, those contacts thin out and floating intensifies. Most striking was the effect of supplementary cementitious materials. Fly ash, with its spherical ball-bearing particles, lubricated the paste and worsened segregation, while slag—with its high specific surface area and hydraulic activity—thickened the paste and suppressed floating. The binary fly ash–slag blend proved the champion, cutting the floating index to 5.68 percent and the coefficient of variation to 0.90 by balancing flowability with cohesiveness.

A crucial insight emerged from linking these results to concrete slump, the industry’s standard workability measure. Slump alone cannot predict segregation. Two mixes with identical slump can behave entirely differently depending on whether the flowability gain came from added water, extra paste volume, or mineral admixtures. Water addition destroys suspension capacity; extra paste mainly reduces particle contacts without sacrificing viscosity; slag raises flow while preserving cohesiveness. The fly ash–slag system achieved high slump with minimal floating, demonstrating that flowability and segregation resistance can coexist when the chemistry is right. This finding cautions against mix-design shortcuts that judge fresh concrete by a single workability number.

To explain these observations mechanistically, the team built a kinetic model based on the force balance on a rising aggregate particle: buoyancy drives it up, gravity pulls it down, and Stokes-type viscous drag resists motion, with an additional term accounting for interparticle restraint. In the steady state, the migration velocity scales with the square of particle radius and the density difference, divided by paste viscosity and modified by the restraint factor. This simple proportionality reproduced every major experimental trend—why bigger and lighter aggregates float fastest, why thinner pastes accelerate migration, and why gradation and shape act through particle contacts rather than viscosity. The model offers a predictive framework that can be calibrated for other lightweight aggregate systems, giving mix designers a quantitative tool rather than trial-and-error folklore.

The hardened concrete results tied distribution to performance in unexpected ways. Raising aggregate cylinder compressive strength from 2.4 to 18.7 MPa lifted 28-day compressive strength of the concrete from 23.0 to 59.5 MPa, because weak aggregates shift from the composite’s weakest phase to genuine load-bearers. Compressive strength correlated most strongly with the water-to-binder ratio (R² = 0.91) and aggregate strength (R² = 0.81), while flexural strength responded far more to paste parameters, with R² values of 0.98 for the water-to-binder ratio and 0.96 for the excess filling rate—consistent with flexural failure being governed by crack propagation through the matrix and interfacial transition zone rather than bulk crushing. Intriguingly, the floating index correlated only weakly with compressive strength (R² = 0.75) and barely at all with flexural strength (R² = 0.11), but strongly with specific strength—strength per unit density—with a correlation coefficient of −0.94. Segregation, in other words, is best detected not by strength loss alone but by the efficiency metric that couples load capacity to weight.

The practical payoff is substantial. The optimized mixes achieved specific strengths of 25 to 33 MPa·m³/t, roughly double the 8.5 to 16.5 typical of ordinary concrete, meaning structures can shed self-weight without proportional strength sacrifice. Drying shrinkage told a favorable story as well: all lightweight mixes shrank less than conventional counterparts, thanks to internal curing as porous aggregates release stored water into the hydrating paste, though very porous low-grade aggregates partially offset this benefit by restraining the skeleton less. Microstructural analysis of the best-performing fly ash–slag system showed pores below 20 nanometers accounting for about 95 percent of fine-pore volume at 28 days, with a continuous C-S-H gel network penetrating the open pores at aggregate surfaces and knitting paste to particle. For engineers racing to build lighter, taller, and more sustainable structures, the message is clear: controlling where lightweight aggregates sit in the fresh state—through density matching, gradation, paste viscosity, and blended binders—is as important as the aggregates themselves.

Subject of Research: Floating-driven segregation of lightweight aggregates and its effects on the hardened performance of lightweight aggregate concrete

Article Title: Floating-driven spatial distribution of lightweight aggregates and its effects on the hardened performance of lightweight aggregate concrete

Article References: Zhang, W., Qian, C., Wang, Z., Jia, F., & Zhao, S. (2026). Floating-driven spatial distribution of lightweight aggregates and its effects on the hardened performance of lightweight aggregate concrete. Case Studies in Construction Materials, 25, Article e06533. https://doi.org/10.1016/j.cscm.2026.e06533

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06533

Keywords: lightweight aggregate concrete, aggregate floating, segregation, sintered aggregates, floating index, excess filling rate, specific strength, drying shrinkage, fly ash, slag, interfacial transition zone, internal curing

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