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For tailoring the mechanical and magnetic properties of dual-phase high-entropy alloys (HEAs), it is crucial to understand the effect of each phase on the overall properties. In this paper, the effects of individual FCC and BCC phases on the mechanical and magnetic properties of the FeCoNi(CuAl)0.8 HEA are investigated by nanoindentation and first-principles calculations. The nano-hardness of the BCC phase is 8.73 GPa, which is nearly double the 4.60 GPa of the FCC phase, which ascribes to spherical nanoprecipitates that are only observed in the BCC phase leading to precipitation hardening. First-principles calculations on the electronic structure show that calculated saturation magnetization (Ms) of the BCC phase is 0.81 T, higher than 0.77 T of the FCC phase. An approximate yield strength and Ms can be estimated by summing the volume-fraction-weighted contributions from each phase, and are in good agreement with experimental values. It indicates that the overall mechanical and magnetic properties of the dual-phase HEAs can be tailored by tuning the volume fraction of the individual phase. Our findings are helpful to design prospective dual-phase HEAs with both good mechanical properties and soft magnetic performance by adjusting the content of each phase. [ABSTRACT FROM AUTHOR]

• Tuning ferromagnetic intergranular phase overcomes coercivity-remanence trade-off. • Both increase of coercivity and remanence is obtained in Ga-doped Nd-Fe-B alloy. • Magnetic field annealing increases remanence with a minor decrease of coercivity. The high-performance of a Nd-Fe-B magnet needs both large coercivity and remanence. However, the coercivity – remanence trade-off leads to a dilemma that the coercivity and the remanence cannot be simultaneously increased, and subsequently reduces the maximum energy product ((BH) max). Here, we use atom probe tomography (APT) and demonstrate a strategy that can simultaneously improve the coercivity (H c i) and the remanence (B r) by tuning the concentration of rare earths (Nd + Pr) and ferromagnetic elements (Fe + Co) of the ferromagnetic intergranular phase in (Nd 0.1023 Pr 0.0256 Fe 0.6832 Co 0.1308 B 0.0581) 99.7 Ga 0.3 melt-spun ribbons. In comparison to the Ga-free alloy, the H c i and the B r both increase to 1369 kA/m by 7.0% and to 0.85 T by 2.4%. Interestingly, the magnetic field annealing treatment (1 T, 623 K for 15 min) further improves the B r to 0.90 T with a minor decrease of the H c i. The (BH) max of 134 kJ/m3 is obtained, 12.6% higher than that of the 0.3 at% Ga-doped as-spun sample. Our findings provide an idea to overcome the coercivity-remanence trade-off and subsequently to design prospective permanent alloys with excellent magnetic properties by modifying the content of rare earths and ferromagnetic elements in the ferromagnetic intergranular phase. [ABSTRACT FROM AUTHOR]

The (Nd 10.23 Pr 2.56 Fe 68.32 Co 13.08 B 5.81) 100-x Nb x (x = 0, 0.5, 1, 2, 3) ribbons were prepared by melt-spinning at a wheel speed of 15 m/s. The effects of Nb addition on the microstructure and the magnetic properties of the ribbons were investigated. The X-ray diffraction (XRD) patterns of the ribbons show that the Nd 2 (Fe, Co) 14 B (2:14:1) phase is observed in all samples indicating that Nb addition does not change the phase constitution. The transmission electron microscopy (TEM) images show that Nb addition can dramatically reduce the average grain size from 52 ± 5 nm of the Nb-free alloy to 12 ± 2 nm of the Nb-containing x = 3 alloy. Based on the atom probe tomography (APT) result, we identify different mechanisms for grain refinement at different Nb content. When x = 0.5, the distribution of Nb is uniform at the intergranular phase and Nb addition promotes the segregation of rare-earth Nd and Pr at the thick ferromagnetic intergranular phase (≈ 10 nm), which leads to decreasing the average grain size to 38 nm. With further increasing Nb addition to x = 2, the existence of Nb-rich precipitations (2–4 nm in width) at the grain boundaries effectively reduces the grain size to 16 nm. The remanence (B r) and the maximum energy product ((BH) max) are obviously increased by Nb addition and the maximum of the B r and the (BH) max is obtained in the (Nd 10.23 Pr 2.56 Fe 68.32 Co 13.08 B 5.81) 98 Nb 2 (x = 2) alloy. The values of the B r and the (BH) max are 0.90 T and 138 kJ/m3, 8.4% and 22.1% higher than that of the Nb-free alloy. It ascribes to the grain refinement, the changes of the distribution and the composition of the ferromagnetic intergranular phase by Nb addition. Our findings provide a new method of designing prospective permanent alloys with increased magnetic properties by reducing the grain size and tuning the distribution and chemical composition of the intergranular phase. • Different mechanisms for grain refinement at different Nb content is proposed. • Nb addition changes the distribution and composition of the intergranular phase. • Nb addition increases the remanence and the maximum energy product. [ABSTRACT FROM AUTHOR]

The FeCoNi(CuAl) 0.8 high-entropy alloy (HEA) consists of both face-centered cubic (FCC) phase and body-centered cubic (BCC) phase which allows for a fine tuning of the magnetic properties by controlling the concentration, morphology, and distribution of each phase. In this work, we report the effect of Ho addition on the microstructure, magnetic domains, and alternating current (AC) magnetic properties. FeCoNi(CuAl) 0.8 Ho 0.05 (x = 0.05) HEA exhibits the optimum AC magnetic property with the minimum AC coercivity, hysteresis loss, eddy current loss and total loss among samples studied at both 50 Hz and 950 Hz. Ho addition increases the volume fraction of BCC phase. SEM-EDX results show that Cu and Ho elements are aggregated together at boundaries in Ho-containing alloys. Magnetic domain structure shows that stripe domains are observed in FCC phase and BCC phase with different domain width. Finely spaced domain in BCC phase leads to low eddy current loss. No domain pattern at boundaries is observed indicating that boundaries can hinder motion of domain resulting in an increase of the energy loss. Our findings provide a valid approach toward optimizing the desired physical properties of HEA magnets by adding a small fraction of rare earth element. [ABSTRACT FROM AUTHOR]

Cu-rich precipitates were observed in the FeCoNiCuAl high-entropy alloy (HEA). In this paper, the microstructure of Cu-rich precipitates in the as-cast and annealed HEA was studied. A novel mechanism for the formation and microstructural evolution of Cu-rich precipitates has been discovered. In the solidification of alloy preparation, sphere-like precipitates with 9R close-packed (9R) or body-centred cubic (BCC) structures were formed. It was noteworthy that several sphere-like precipitates self-assembled into a large rod-like precipitate when they were close to each other. After the annealing treatment, the Cu-rich precipitates grew and part of them formed a core-shell structure. Cu atoms in the matrix adsorbed on the surface of the original precipitates to form the 9R structure shell, and the original precipitates underwent a phase transformation to form the twin face-center tetragonal (FCT) structure core. In addition, a large number of nano-clusters with sizes of about 3–8 nm were dispersed in the BCC matrix. The magnetic and mechanical properties of HEA were greatly affected by the annealing treatment. The σ max , σ 0.2 , H V , and B r of the alloy were improved 22.5%, 76.7%, 14.2% and 17.7% by annealing treatment, respectively. This work lays the foundation for further improving the mechanical and magnetic properties of HEAs by adjusting the amount, distribution, and structure of the precipitates. • Some precipitates were self-assembled by 9R or BCC precipitates. After annealing, core-shell precipitates and nano-clusters formed. • After annealing treatment, some precipitates grow into core-shell structure. • After annealing treatment, a lot of nano-clusters were dispersed in the BCC matrix. • The magnetic parameters B r and Vickers hardness of the alloy were improved by annealing treatment. [ABSTRACT FROM AUTHOR]