Effect of high energy ball milling, heat treatment and spark plasma sintering on structure, composition, thermal stability and magnetism in CoCrFeNiGax (x = 0.5; 1) high entropy alloys

Abstract

Nanocrystalline (∼10 nm) singe-fcc CoCrFeNiGa_x (x = 0.5, 1.0) high entropy alloy (HEA) particles with excellent structural and compositional homogeneity were prepared from elemental powders using a single-step, short-term (190 min) high energy ball milling (HEBM) at room temperature (RT). Both HEA powders exhibit paramagnetic behaviour at RT with a small ferromagnetic contribution at low fields (the saturation magnetization M_s= 4.5 Am²/kg – 7.5 Am²/kg; the average Curie temperature T_c = 130 K – 150 K). They are thermally stable up to 1295 K–1305 K despite the low melting Ga (302.9 K). Heat treatment up to 1000 K enhances M_s to 59.9 Am²/kg and T_c to 740 K for the CoCrFeNiGa HEA powder due to an irreversible fcc→bcc structural transformation, whereas the magnetic properties of CoCrFeNiGa_0.5 do not show this enhancement. In-situ TEM heating reveals nanosized σ-phase Cr-rich precipitates (< 50 nm) at 875 K only for the CoCrFeNiGa HEA powder. Spark plasma sintering (SPS) of powders produces homogeneous nanocrystalline bulk HEAs. SPS at 1073 K of the CoCrFeNiGa_0.5 powder increased the crystallinity of the fcc phase. Three-dimensional local compositional mapping at atomic resolution by atom probe tomography indicates a homogeneous distribution of all elements. Bulk HEAs exhibit similar magnetic behavior to heat-treated HEA powders. Combining HEBM and SPS yields homogeneous bulk HEAs with low-melting Ga and enhanced structural, composition, thermal stability, as well as improved magnetic properties (M_s = 55Am²/kg and T_c = 750 K), which 45% and 47 K higher, respectively, compared to conventional melting approaches.