Abstract
High entropy alloys (HEA) contain multiple principal alloying elements, and possess unique properties due to the high configurational entropy and lattice strain in the system. Ferromagnetic FeCoNi-based HEAs exhibit dramatic changes in crystal structure and the type of magnetism expressed when adding non-magnetic elements such as Al, Cr, Ga, Ti, etc. Interestingly, Alnico permanent magnets also contain multiple principal alloying elements, such as Fe, Co, Ni, and Al, along with other minor additions. This well-studied system is similar in concept to high entropy alloys (HEAs). In this paper, we investigate the hard magnetic properties of FeCoNiAl-based HEAs with additions of Cu/Ti. The addition of Cu/Ti to an equimolar FeCoNiAl alloy is effective at enhancing coercivity, due to spinodal decomposition, but at the expense of saturation magnetization. By varying the ratio of Fe and Co, however, with respect to the other alloying elements, the saturation magnetization is increased, while generally retaining or improving the coercivity. In particular, the Fe2CoNiAlCu0.4Ti0.4 HEA shows promising hard magnetic properties as an isotropic cast magnet, with an HC of 1,078 Oe and (BH)max of 2.06 MGOe, slightly better than the performance of isotropic cast Alnico 2 magnets. The thermal stability is also sufficient for use at elevated temperatures over 200 °C. There was also an interesting increase in high temperature coercivity observed at temperatures from ∼650-800 °C, where these alloys often exhibited higher coercivity than that measured at RT.
Highlights
Neodymium-based permanent magnets (PMs) have many advantages over rare-earth free PMs in electronic devices and machines
We investigate the effects of annealing temperature and dopant additions on the structural and hard magnetic properties of FeCoNiAlCuXTiX High entropy alloys (HEA), while varying the atomic ratio of the Fe/Co ferromagnetic elements
The coercivity of alloys containing Cu and Ti is heavily dependent on the annealing condition, with all alloys exhibiting a maximum value after an anneal at ∼700 ○C, which might be due to spinodal decomposition, as is often observed in Alnico magnets (see Figure 1(b))
Summary
Neodymium-based permanent magnets (PMs) have many advantages over rare-earth free PMs in electronic devices and machines. Among the commercialized RE-free magnets, Alnico magnets, comprised of Fe, Co, Ni, Al and other minor additions, show great potential for replacing RE based magnets in specific applications. These alloys can be made large enough for use in devices such as motors and generators and can be used at elevated temperatures (> +180 ○C) due to their high Curie temperatures of ∼750 – 870 ○C.2. The maximum energy product, (BH)max, in Alnico magnets is fairly temperature independent up to ∼300 ○C, while rare earth, Nd-based magnets show a significant drop-off from ∼45 MGOe at room temperature to below 10 MGOe at 200 ○C.1. These alloys can be made large enough for use in devices such as motors and generators and can be used at elevated temperatures (> +180 ○C) due to their high Curie temperatures of ∼750 – 870 ○C.2 The maximum energy product, (BH)max, in Alnico magnets is fairly temperature independent up to ∼300 ○C, while rare earth, Nd-based magnets show a significant drop-off from ∼45 MGOe at room temperature to below 10 MGOe at 200 ○C.1 While these REfree alloys possess a more stable temperature performance, their room temperature (BH)max values are comparatively quite low (∼1 – 10 MGOe) the residual induction (Br) is similar to that of the Nd-based magnets
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