Experimental liquidus map reveals four solidification zones in Cr–Nb–Ta alloys

The first experimental liquidus projection for the chromium–niobium–tantalum alloy system has identified four primary solidification regions, no detected ternary compounds and two proposed class II ternary invariant reactions. Researchers analyzed 19 as-cast alloys using scanning electron microscopy, energy-dispersive spectroscopy and X-ray diffraction. The resulting projection shows a broad body-centered cubic niobium–tantalum phase field, a substantial C14 Laves-phase field, a smaller C15 Laves-phase field near the Cr–Nb binary edge and a more limited chromium-rich BCC field. Because the work was based on as-cast samples, the diagram represents experimentally observed solidification behavior and reaction paths rather than a complete equilibrium phase diagram at every temperature.

The new map is intended to support the development of high-temperature structural materials made with refractory metals. The source study notes that niobium and tantalum can act as strengtheners in refractory high-entropy alloys, while chromium can contribute oxidation resistance and density reduction. It also identifies Cr-based superalloys and AB2-type Laves phases, including NbCr2 and TaCr2, as promising because of their low density, high-temperature strength, corrosion and oxidation resistance, and microstructural stability. Before the reported investigation, the literature contained isothermal sections for the Cr–Nb–Ta system at 1000 °C, a partial section at 1300 °C and a section at 1500 °C, but no published liquidus projection for the complete ternary system.

To construct the projection, the team produced ingots of approximately 2 g from chromium, niobium and tantalum with a minimum stated purity of 99.7 wt. %. The alloys were arc-melted five times in a water-cooled copper crucible under argon with a minimum purity of 99.995%, and the ingot was rotated by 180 degrees between meltings to promote compositional homogeneity. The melting chamber was purged four times before each operation, and a titanium getter was melted before each sample to reduce residual contaminant gases. After melting, one half of each ingot was hot-mounted in epoxy, ground with abrasive papers from P400 to P2400 and polished with a 0.05 µm colloidal silica suspension, while the other half was mechanically ground for diffraction analysis.

Mass loss during melting did not exceed 1.4 wt. %. The investigators used those losses to estimate a possible composition interval for every alloy by assigning the total loss to each of the three elements. Backscattered-electron imaging was used to examine the as-cast microstructures, EDS provided local chemical measurements, and X-ray diffraction identified the crystalline phases. Phase patterns were interpreted with established crystallographic data, and qualitative and quantitative phase analyses were performed through Rietveld refinement, which also yielded cell parameters. This combination allowed the researchers to connect phase composition and microstructure with the locations of primary solidification fields, monovariant lines and ternary invariant reactions.

Four binary alloys were examined to verify liquid eutectic compositions in the Cr–Ta and Cr–Nb systems. Two Cr–Ta samples contained eutectic microconstituents made from BCC and C14 phases; one included tantalum-rich BCC and the other chromium-rich BCC. EDS gave liquid eutectic compositions of 46.6 ± 0.2 at. % Cr and 89.4 ± 0.4 at. % Cr, respectively. In the Cr–Nb system, one alloy had an entirely eutectic microstructure of niobium-rich BCC and C15, with a measured liquid composition of 51.1 ± 0.5 at. % Cr. A second alloy showed primary C15 surrounded by a chromium-rich BCC plus C15 eutectic, with a measured liquid composition of 80.8 ± 0.1 at. % Cr. These measurements were consistent with experimental values reported previously for the same binary eutectics.

The four primary phase fields differ markedly in size. The BCC niobium–tantalum field dominates most of the projected ternary composition space, while the C14 Laves-phase field covers most of the alloy compositions examined. The C15 primary field is restricted to a small area close to the Cr–Nb binary edge, and the chromium-rich BCC field occupies a smaller region than the niobium–tantalum field. Alloys near the Cr–Nb side still produced primary C14, including one measured at 29.6 ± 0.1 at. % Nb and 4.6 ± 0.1 at. % Ta, supporting the extensive reach of the C14 field. No evidence of a ternary compound was detected in the investigated alloys.

Separating the C14 and C15 Laves polytypes required a combined analytical approach. Conventional electron backscatter diffraction, initially considered for phase identification, could not reliably distinguish them because their closely related structures generated similar Kikuchi diffraction patterns. The problem was especially pronounced in fine eutectic constituents, where phase dimensions were comparable to the EBSD interaction volume and signals from neighboring phases overlapped, reducing pattern quality and indexing reliability. The researchers instead combined microstructural observations with EDS point analyses because C14 and C15 had measurably different compositions in the investigated alloys. In alloy #9, the brighter backscattered-electron particles contained more tantalum and less niobium and were identified as C14, while C15 contained less than approximately 1 at. % Ta. Chromium-rich BCC appeared darker than both Laves phases, and niobium–tantalium-rich BCC appeared brighter than either.

Microstructures in which primary C14 was surrounded by C15 indicated that the m–UI and m–UII monovariant lines have peritectic rather than eutectic behavior. That observation supported interpreting the two proposed ternary points as class II transition reactions instead of class I ternary eutectics. The UI reaction was defined as L + C14 ↔ C15 + (Nb,Ta), while UII was defined as L + C14 ↔ C15 + (Cr). Alloys associated with UI developed eutectic microconstituents of C15 and niobium–tantalum-rich BCC, whereas others associated with UII ended with C15 and chromium-rich BCC. The last-solidified regions of several alloys were re-examined by SEM at different magnifications, accelerating voltages and beam currents, and consistently showed only two phases in those regions.

The solidification sequences of the individual alloys determined how the reaction lines connect across the projection. Alloys with primary C14 followed by C15 and then a chromium-rich BCC eutectic advanced toward the maximum point m and the peritectic-type m–UII line. Other alloys with primary C14 enclosed by C15 solidified with a C15 plus niobium–tantalum eutectic and supported the peritectic-type m–UI line. Alloy #16 lay close to UI on the e1–UI eutectic-type line and completed solidification along the UI–e3 line. Alloy #6 had an entirely eutectic structure, placing it on the e1–UI line where liquid solidified into niobium–tantalum-rich BCC and C14 before the solidification path reached UI. Alloy #7 showed primary C14, followed by eutectic formation of niobium–tantalum and C14, and finally simultaneous formation of niobium–tantalum and C15.

The authors report that the resulting topology closely resembles the previously published Cr–Nb–Hf liquidus projection. Their proposed explanation is that tantalum and hafnium have comparable behavior with chromium and niobium: both form stable C14 and C15 Laves phases with chromium and can promote formation of niobium-containing BCC solid solutions. That comparison concerns the arrangement of primary fields rather than a demonstrated equivalence in material properties. Taken together, the measurements provide a first system-wide experimental framework for interpreting solidification in the Cr–Nb–Ta system, showing when chromium-rich BCC, niobium–tantalum-rich BCC, C14 or C15 is expected to form as the primary phase and identifying the transition reactions that can follow. The scope remains limited by the use of 19 as-cast alloys and the need to infer reaction topology from their final microstructures and phase compositions.

Subject of Research: Experimental liquidus projection and solidification reactions in the ternary Cr–Nb–Ta alloy system

Article Title: Liquidus projection of the Cr–Nb–Ta system

Article References: Silveira, V. M., Luiz, J. V., Borowski, K. E., Barros, C. S., Chad, V. M., Gigolotti, J. C. J., Nunes, C. A., & Coelho, G. C. (2026). Liquidus projection of the Cr–Nb–Ta system. Journal of Materials Science, 61(40), 31168-31182. https://doi.org/10.1007/s10853-026-13371-x

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13371-x

Keywords: Cr–Nb–Ta alloys, liquidus projection, phase equilibria, Laves phases, solidification, refractory alloys, high-entropy alloys, scanning electron microscopy, X-ray diffraction, metallurgy, Liquidus, projection