Aging tunes cryogenic strength in Fe-based medium-entropy alloy

Five hours of aging produced the highest ultimate tensile strength among the cryogenic conditions examined for a cold-rolled Fe-based medium-entropy alloy: 1,437 megapascals at 77 kelvin, with a total elongation of 53.5%. The treatment created a balance between two competing changes: Cu-rich precipitation made the face-centered cubic matrix more susceptible to transformation into martensite, while recovery removed dislocations that can serve as heterogeneous nucleation sites. After 72 hours of aging, the alloy reached 1,313 megapascals and 67.7% total elongation, showing how the same chemical partitioning can produce a different mechanical response when the defect structure changes.

The study examined the transformation-induced plasticity of Fe64Ni15Co8Mn8Cu5, a metastable alloy whose deformation can involve conversion of its face-centered cubic, or FCC, matrix into body-centered cubic, or BCC, martensite. This transformation provides sustained strain hardening and delays necking, while its activation depends on the thermodynamic stability of the FCC matrix and the kinetics of martensite formation. Cooling generally destabilizes metastable FCC phases, but the available nucleation environments also influence when and how rapidly the transformation proceeds.

The researchers therefore investigated both effects rather than treating aging as a single strengthening treatment. The alloy was fabricated by vacuum-induction melting from pure metals greater than 99.99 weight percent under argon, followed by homogenization at 1,200 degrees Celsius for two hours. It was cold-rolled from a thickness of 7 millimetres to 1.5 millimetres, corresponding to a total reduction of 78.5%.

Samples were then aged at 550 degrees Celsius for 5, 12, 48 or 72 hours and water-quenched. The temperature was selected to promote Cu-rich phase separation with sufficient diffusion kinetics while minimizing recrystallization, helping the researchers distinguish Cu partitioning and recovery from more extensive replacement of the cold-worked microstructure. Three tensile tests were conducted for each condition at 298 and 77 kelvin.

Total elongation was not directly compared between temperatures because the ambient- and cryogenic-temperature specimens had different geometries. Electron backscatter diffraction showed that the cold-rolled material and the two representative aged conditions remained overwhelmingly FCC, with only about 0.1–0.2% thermally induced BCC martensite before deformation. Partial recrystallization began preferentially in strain-localized regions such as grain boundaries and shear bands, but remained limited.

Fine recrystallized grains approximately 2 micrometres in size represented about 1% of the 5-hour-aged sample and 5.5% of the 72-hour-aged sample, leaving both microstructures predominantly cold-worked. The estimated density decreased nonlinearly from about 1.60 × 10^15 per square metre in the cold-rolled state to 7.83 × 10^14 after 5 hours and 1.74 × 10^14 after 72 hours, indicating progressive recovery through dislocation rearrangement and annihilation. At the same time, transmission electron microscopy and energy-dispersive X-ray spectroscopy identified Cu-rich FCC precipitates.

Following 5 hours of aging, the enriched regions were only a few nanometres across, and their phase fraction could not be determined reliably. After 72 hours, well-developed precipitates were approximately 10 nanometres in diameter, occupied about 16.4% of the phase fraction, and contained an average of 7.28 ± 0.48 atomic percent Cu. Cu partitioned from the supersaturated FCC matrix into these particles, leaving the surrounding matrix depleted in Cu.

Calculations using the estimated matrix compositions found that this depletion made the FCC-to-BCC transformation more thermodynamically favorable at both 298 and 77 kelvin. The authors noted that changes in stacking-fault energy may also have contributed, although their calculation directly addressed the relative thermodynamic stability of the two phases. Aging for 5 hours lowered yield strength to 730 megapascals but raised ultimate strength to 798 megapascals and uniform elongation to 7.5%.

After 72 hours, yield strength fell further to 686 megapascals, ultimate strength remained similar at 794 megapascals, and uniform elongation increased to 10.4%. All three conditions exhibited post-yield softening at room temperature, although it became less pronounced after aging. A quantitative strengthening model attributed most of the change in yield strength to the loss of Taylor dislocation strengthening, estimated at 412, 338 and 159 megapascals for the cold-rolled, 5-hour-aged and 72-hour-aged conditions, respectively.

Cryogenic testing produced a markedly different mechanical balance. The cold-rolled specimen reached a yield strength of 1,014 megapascals, an ultimate tensile strength of 1,192 megapascals, a uniform elongation of 50.4% and a total elongation of 65.7%. The 5-hour-aged specimen combined a lower yield strength of 905 megapascals with the highest ultimate tensile strength among the investigated conditions, 1,437 megapascals, while retaining 49.6% uniform and 53.5% total elongation.

With 72 hours of aging, yield and ultimate strengths declined to 833 and 1,313 megapascals, but uniform and total elongations increased to 58.3% and 67.7%. Peak cryogenic strain-hardening rates followed a related pattern: approximately 2.5 gigapascals for the cold-rolled material, 4.0 gigapascals after 5 hours and 3.5 gigapascals after 72 hours. The 5-hour condition reached maximum hardening at about 15% true strain, whereas the 72-hour condition did not peak until roughly 35%.

Electron backscatter diffraction mapping showed that the different hardening curves corresponded to different transformation kinetics. At about 10% true strain, the 5-hour-aged sample contained approximately 46% BCC martensite, compared with about 15% in both the cold-rolled and 72-hour-aged samples. At 25% strain, the 5-hour material had reached about 55% BCC, while the cold-rolled sample was at approximately 22.8%.

By 40% strain, the cold-rolled fraction had risen to about 32.1%, and the 72-hour sample had approached the martensite fraction of the 5-hour material. Because digital image correlation could not be applied reliably during cryogenic testing, the researchers used finite-element analysis to estimate local strains for the EBSD observations. The model incorporated a small locally reduced-property region to reproduce experimentally observed off-center necking; the authors stated that this imperfection had a negligible effect on the overall tensile response.

The results distinguish two roles that can be combined under the broad description of aging. Cu partitioning provides a thermodynamic push toward martensitic transformation by reducing the relative stability of the FCC matrix, whereas dislocations provide heterogeneous environments in which martensite can nucleate rapidly. The cold-rolled alloy contained abundant dislocations but retained a relatively Cu-rich, stable FCC matrix, so its transformation proceeded gradually and its peak strain-hardening rate was the lowest of the three conditions.

Five hours of aging combined Cu depletion with a still-high dislocation density of 7.83 × 10^14 per square metre, enabling an early burst of transformation and sustained hardening. Seventy-two hours of aging further altered the matrix composition but reduced dislocation density to 1.74 × 10^14 per square metre, shifting extensive transformation toward higher strains and distributing it over a wider strain interval. The findings therefore indicate that aging can control not only the amount of deformation-induced martensite but also the strain range over which transformation occurs.

Within the cold-rolled Fe-based medium-entropy alloy studied, short-term aging provided the most favorable balance between chemical destabilization of the FCC matrix and retained defect density, producing the highest cryogenic ultimate tensile strength and strain-hardening rate examined. Prolonged aging delayed early martensite nucleation and produced a more gradual hardening response with greater ductility. The study is limited to one alloy composition, a single aging temperature and the tested cold-rolling, aging and tensile conditions, so its results establish a materials-design mechanism within this system rather than a general performance limit for Fe-based medium- and high-entropy alloys.

Subject of Research: Aging control of cryogenic transformation-induced plasticity in a Cu-added Fe-based medium-entropy alloy

Article Title: Aging-controlled cryogenic TRIP kinetics in a Cu-added Fe-based medium-entropy alloy via Cu-rich precipitation and dislocation density reduction

Article References: Park, H., Kwon, J., Lee, D. W., Koo, B. W., Lee, J. A., Choi, J., Kim, E.-C., Lee, B.-J., Haftlang, F., & Kim, H. S. (2026). Aging-controlled cryogenic TRIP kinetics in a Cu-added Fe-based medium-entropy alloy via Cu-rich precipitation and dislocation density reduction. Journal of Materials Science. https://doi.org/10.1007/s10853-026-13889-0

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13889-0

Keywords: Fe-based medium-entropy alloy, cryogenic mechanics, transformation-induced plasticity, martensitic transformation, Cu-rich precipitation, dislocation recovery, metastable alloy, strain hardening, strength–ductility balance, Aging-controlled, cryogenic, TRIP