Effect of High-Pressure Torsion on the Microstructure and Magnetic Properties of Nanocrystalline CoCrFeNiGax (x = 0.5, 1.0) High Entropy Alloys

Abstract

In our search for an optimum soft magnet with excellent mechanical properties which can be used in applications centered around “electro mobility”, nanocrystalline CoCrFeNiGax (x = 0.5, 1.0) bulk high entropy alloys (HEA) were successfully produced by spark plasma sintering (SPS) at 1073 K of HEA powders produced by high energy ball milling (HEBM). SPS of non-equiatomic CoCrFeNiGa0.5 particles results in the formation of a single-phase fcc bulk HEA, while for the equiatomic CoCrFeNiGa composition a mixture of bcc and fcc phases was found. For both compositions SEM/EDX analysis showed a predominant uniform distribution of the elements with only a small number of Cr-rich precipitates. High pressure torsion (HPT) of the bulk samples led to an increased homogeneity and a grain refinement: i.e., the crystallite size of the single fcc phase of CoCrFeNiGa0.5 decreased by a factor of 3; the crystallite size of the bcc and fcc phases of CoCrFeNiGa—by a factor of 4 and 10, respectively. The lattice strains substantially increased by nearly the same extent. After HPT the saturation magnetization (Ms) of the fcc phase of CoCrFeNiGa0.5 and its Curie temperature increased by 17% (up to 35 Am2/kg) and 31.5% (from 95 K to 125 K), respectively, whereas the coercivity decreased by a factor of 6. The overall Ms of the equiatomic CoCrFeNiGa decreased by 34% and 55% at 10 K and 300 K, respectively. At the same time the coercivity of CoCrFeNiGa increased by 50%. The HPT treatment of SPS-consolidated HEAs increased the Vickers hardness (Hv) by a factor of two (up to 5.632 ± 0.188) only for the non-equiatomic CoCrFeNiGa0.5, while for the equiatomic composition, the Hv remained unchanged (6.343–6.425 GPa).

Microstructure Evolution and Mechanical Properties of AlCoCrFeNi2.1 Eutectic High-Entropy Alloys Processed by High-Pressure Torsion

“…The microhardness reached the first saturation state at an equivalent strain of approximately 10 before gradually rising to 500 HV. Subsequently, the microhardness steadily increased to 550 HV, reaching a second saturation point at an equivalent strain of around 40, which was consistent with the studies of many scholars [22,24,25,33,37,[41][42][43].…”

Magnetic Materials via High-Pressure Torsion of Powders

“…Similarities to phase transitions and refined grains are reported. 129134) Further works on high entropy alloys 135,136) and shape memory materials are found. 137) The possibilities of the high-pressure torsion process applied to magnetic materials is probably an often underestimated technique.…”

Combustion Synthesis and Reactive Spark Plasma Sintering of Non-Equiatomic CoAl-Based High Entropy Intermetallics

“…Most HEAs are obtained by different melting technics [ 2 , 3 , 5 , 8 , 10 , 11 , 12 ] or mechanical alloying [ 4 , 6 , 7 , 9 , 20 , 21 ]. These methods allow destroying the crystalline structure of the starting materials and effectively mix the components.…”