In our search for an optimum soft magnet with excellent mechanical properties which can be used in applications centered around “electro mobility”, the nanocrystalline CoCrFeNiGax (x = 0.5, 1.0) bulk high entropy alloys (HEAs) have been successfully produced by high energy ball milling (HEBM) followed by spark plasma sintering (SPS).

At first, nanocrystalline (~10 nm) singe fcc high entropy alloy (HEA) CoCrFeNiGax (x = 0.5, 1.0) particles with excellent compositional homogeneity at micro- and nanoscale were produced by short-term (190 min) HEBM in Ar. Based on differential scanning calorimetry (DSC) results, the HEA powders show a thermal stability of up to 1273 K despite the low melting point of Ga (302.9K).

Then, using these HEA CoCrFeNiGax (x = 0.5, 1.0) powders we produced homogeneous nanocrystalline bulk HEAs by SPS at 1073K, which is not possible starting with elemental powders in a direct SPS process or by using other melting routes.

SPS at 1073 K of the CoCrFeNiGa0.5 powder increases the crystallinity of the fcc phase, while for the CoCrFeNiGa powder a partial fcc→bcc transformation is observed. 3D local compositional mapping at atomic resolution by Atom probe tomography (APT) of CoCrFeNiGa0.5 HEA shows a truly homogeneous distribution of all elements.

The nanocrystalline HEA CoCrFeNiGax (x = 0.5, 1.0) powders show a paramagnetic behaviour at 300 K with a Curie temperature (Tc) in the range of 130 K – 150 K. The SPS consolidation leads to an enhancement of magnetic properties for the equiatomic CoCrFeNiGa HEA, and increases the Tc up to 735 - 750 K, and saturation magnetization Ms (300K) by 10 times (55 Am2/kg) compared to the HEA powder, while the magnetic nature of CoCrFeNiGa0.5 bulk HEA remains unchanged.

The Vickers microhardness (Hv) of the SPS-consolidated CoCrFeNiGa and CoCrFeNiGa0.5 HEAs is 3.7 GPa and 6.32 GPa respectively, which is 3 and 5 times higher than for bulk samples sintered from elemental powder blends.