By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Dec 12 2024A recent article published in Nature Communications described a simple one-step in-rotating-water spinning method. This method, which incorporates grain coarsening, was used to fabricate Fe34Co29Ni29Al3Ta3Si2 high-entropy alloy (HEA) fibers containing ordered coherent nanoprecipitates with minimal lattice misfit. This soft-magnetic HEA demonstrated reduced domain wall pinning and improved dislocation mobility.
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Background
Soft-magnetic fibers (SMFs) with small coercivity (Hc), typically made from Co and Fe, are used in actuators, sensors, and electromagnetic shielding devices. These also show potential for applications in tactile sensing, human-computer interaction, and geomagnetic navigation.
However, large-scale manufacturing and use of SMFs face challenges due to their limited stretchability and inability to withstand torsional, tensile, and shear loads during long-term operations.
Traditional methods to reduce Hc in amorphous SMFs often involve annealing, which can induce brittleness. On the other hand, improving the mechanical strength of SMFs via precipitation typically compromises their plasticity and Hc. Therefore, achieving a good balance between strength and plasticity is difficult.
HEAs can help overcome this tradeoff because their soft magnetic properties allow for both low coercivity and improved mechanical strength. However, research on soft-magnetic HEAs has primarily focused on bulk materials, powders, and thin films. This study proposes the fabrication of micro-diameter HEA fibers that combine optimal mechanical and soft-magnetic behavior.
Methods
The ingots of Fe34Co29Ni29Al3Ta3Si2 were prepared by arc-melting pure metals and metalloids in an argon atmosphere. The in-rotating-water spinning method was then used to prepare HEA fibers with a diameter of approximately 180 μm.
The hysteresis loops of the prepared fibers were measured using a Physical Properties Measurement System (PPMS) equipped with standard vibrating sample magnetometry (VSM) in an external magnetic field of ± 10000 Oe. Additionally, tensile tests were conducted on the fibers using a universal testing machine at a strain rate of 5×10-3 per second. In-situ scanning electron microscopy (SEM) was also used for micro-tensile testing.
The phase composition of the fibers was analyzed using X-ray diffraction (XRD), while their fracture surfaces were examined with scanning electron microscopy (SEM). Morphological analysis of the grains within the fibers was conducted using electron backscatter diffraction (EBSD) on the SEM. Additionally, the fibers were observed using a transmission electron microscope (TEM).
In-situ dynamic observation of the magnetic viscosity of the HEA fibers was carried out using Lorentz TEM (LTEM) and a self-made horizontal magnetic in-situ sample holder. Three-dimensional atom probe tomography experiments were conducted on the fibers at a temperature of 50 Kelvin.
Results and Discussion
The magnetic hysteresis loops of the HEA fibers showed significant changes in Hc before and after annealing, but no significant variation in magnetization. The Hc decreased from 8.1 Oe in the as-spun fibers to 2.7 Oe in the fibers annealed at 800 °C for 120 minutes and further reduced to 1.1 Oe in the 300-minute annealed fibers.
The Curie temperature of the as-spun fibers was 770 Kelvin, while the 800 °C-120 min annealed fibers had a Curie temperature of 690 K, indicating the HEA’s high-temperature tolerance.
The ultimate tensile strength (σu) of the as-spun fibers was 674 MPa, with a fracture elongation (εf) of 23.4 %. For the 800 °C-10 min annealed fibers, σu increased to 835 MPa with εf of 9.9 %. For the 800 °C-120 min annealed fibers, σu reached 1029 MPa with a plasticity of approximately 20 %. The yield strength improved significantly at higher annealing temperatures, though plasticity decreased.
The tensile elongation of the prepared HEA fibers surpassed that of other commonly reported SMFs. Additionally, the Hc of the prepared fibers was lower than that of FeSi, FeCo, and metal-deposited carbon fibers.
EBSD images exhibited no apparent difference between the as-spun and annealed HEA fibers; both showed a nearly equiaxed grain morphology. TEM images of the as-spun fibers revealed a Ta-rich phase (face-centered cubic (FCC) structure) with an average size of approximately 310 nm. After annealing at 800°C for 120 minutes, the Ta-rich phase content decreased, and an ordered L12 phase with an average precipitate size of approximately 18 nm became dominant (confirmed via XRD).
The ordered L12 precipitates were much stronger than the FCC solid solution matrix, significantly increasing fiber strength. These nanosized hard particles reduced the dislocation storage capacity of the fiber matrix by shortening the mean free path of dislocations, thereby decreasing the plasticity of the HEA.
Conclusion
The researchers successfully synthesized micron-scale SMFs with excellent strength and flexibility using Si microalloying and a one-step in-rotating-water spinning method in Fe34Co29Ni29Al3Ta3Si2 HEA. By coarsening the grains of the HEA fiber, which contains ordered coherent nanoprecipitates with small lattice misfit, they produced abundant dislocation proximity, enhancing fiber strength.
Additionally, the smaller domain wall pinning, compared to the domain wall width in the coarse grains, reduced the Hc of the HEA. The plasticity, strength, and Hc of the SMFs could be further modified by annealing.
This study, therefore, established a method for preparing large, flexible SMFs, overcoming the challenge of insufficient plasticity in traditional SMFs. This advancement could facilitate the use of HEAs in flexible electronics and multifunctional composites.
References and Further Reading
Ma, Y. et al. (2024). A one-step fabrication of soft-magnetic high entropy alloy fiber with excellent strength and flexibility. Nature Communications. DOI: 10.1038/s41467-024-54984-7, https://www.nature.com/articles/s41467-024-54984-7
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